An Apple Pie From Scratch, Part Vb: Tectonics: Alternatives to Plate Tectonics

USGS

As familiar as it seems to us, it’s easy to forget that, as far as we can see so far, Earthlike plate tectonics is a rather unusual state of affairs. No other planet in the solar system shows the same pattern of features, and we really have no idea what exoplanets might be like in this regard. When it comes to talking about habitable worlds, certainly there seems to be a lot of intersection between the requirements for life and plate tectonics, but that doesn’t mean they’re exclusive to each other. In fact, a planet with nearly identical properties to modern Earth could lack plate tectonics¹ just due to a different geological history.

For the rest of this series, I will be building our example world, Teacup Ae, as a planet with very Earthlike plate tectonics and presuming my readers are doing the same with their worlds, but for now I want to take a quick detour to look at other modes of tectonics—observed or hypothesized—and the kinds of geography they might produce. Bear in mind that this is still a field with much uncertainty and scant evidence to work from, and in fact many of the details of how plate tectonics operates on Earth today aren't clearly understood, so don't be surprised that some of the studies I'll be citing directly contradict each other's conclusions, and don't necessarily trust that the output of a computer model perfectly predicts the behavior of real planets; much of this post will be a review of ideas for tectonic regimes that might occur rather than a list of regimes known to exist.

Squishy Lid

Mobile Lid

Back to Part Va

Heat Transfer and Lid Type

All planetary tectonics are ultimately driven by the movement of heat: planets (and moons) have warm interiors, due either to heat left over from the violent process of planet formation or heat produced by differentiation between materials of different density, decay of radioisotopes, or the tidal influence of other bodies. On Earth, this amounts to around 47 TW of total heat production, or 0.092 W/m² when spread over the surface. That pales in comparison to the ~240 W/m² of energy absorbed from sunlight, but when insulated by thousands of kilometers of rock it's enough to warm the interior to thousands of Kelvin. The only way for a planet to lose heat is to radiate it into space, so this creates a gradient between the warm interior and cool surface that encourages motion of heat. Some heat can simply conduct through rock to reach the surface, but if this is insufficient to cool the interior then convection may result: hot rock in the interior expands and becomes less dense and more buoyant, and so rises towards the surface, (this can happen whether the interior is mostly liquid or solid; as mentioned last time, at the scale of a planet even solid rock can "flow" through continuous deformation, helped by the high heat and pressure). Once the rock reaches the surface, it can radiate its heat directly into space, and the low pressure may also cause the rock to melt, spreading out over the surface, and allow volatiles like water or CO₂ to vaporize, dispersing heat into the atmosphere. The rock thus cools and so becomes denser and less buoyant, to the point that it may sink back into the interior. Carrying heat to the surface within rock through convection is faster than conduction, so this helps balance out heat production in the interior, with a hotter interior encouraging more vigorous convection.

This is all largely the same physics as what causes circulation of water in a heated pot. But rock is a more complex material than water, forming various minerals in different chemical and physical conditions that have different mechanical properties and will respond in different ways to heat and pressure. This creates a lot of potential diversity in how convection manifests on the surface and what type of terrain it produces.

One of the most important distinctions between different tectonic modes is the degree to which the crust is horizontally mobile across the surface, like the ever-moving plates of Earth. Convection currents naturally tend to have a horizontal component: as hot material rises to the surface, it pushes other surface material to the sides, and then cold material sinks elsewhere; hence the tendency to form convection cells, where material moves in a closed loop as it warms and cools, which is why you can see a lot of lateral motion on top of boiling water. But rock is not water; though very young planets may have large molten mantles and older ones may retain liquid cores, the bulk of the interior of a mature planet will be solid, and though it can gradually deform enough to behave somewhat like a fluid and convect, there's still a great deal of friction that may resist this sort of lateral motion. to have large moving plates like Earth requires forces great enough to overcome that friction without simply tearing them apart.

Geologists thus broadly divide planets into two types of tectonic regime: stagnant lid, where the crust is largely static and any tectonic motion is predominantly vertical, with hot rock rising directly up through the crust and cold rock sinking directly downwards; and mobile lid, where the crust moves horizontally across the surface between regions of rising hot rock and sinking cold rock, but as we'll see, there may be a fair bit of grey area and nuance between them. The metaphorical "lid" in these cases is the relatively rigid surface layer of the planet above the convecting interior, which includes the crust but can also include a cool portion of the upper mantle rigidly attached to the crust, grouped together as the lithosphere; the distinction being that the crust has a different mineralogical composition to the mantle, but the lithosphere (crust and upper mantle together) have a different physical behavior to the below asthenosphere and lower mantle.

A detailed diagram of Earth's internal structure, distinguishing layers based on mineral structure and mechanical behavior. Gervilla et al. 2019

There's a few important points to be clear about before we start looking at different modes in detail: First, I'll be mostly focusing on planets (or moons) with broadly similar composition to Earth, with a predominantly silicate mantle and crust and usually a metallic core. Planets richer in carbon, metals, or other materials may have their own tectonic tendencies, but almost nothing is known about them, and precious little is even clearly speculated—though we will have a quick look at icy worlds near the end.

Comparisons of the interior structure and surface topography of rocky bodies in the solar system. Plesa et al. 2026

Second, as I've already alluded to a bit, at the scale of planets the behavior of materials can't be divided neatly into rigid solids and free-flowing liquids. Under high heat and pressure, rocks can undergo plastic deformation, changing shape without breaking, and continuous deformation like this can allow for a sort of fluid flow, but quite slowly and still with a great deal of friction. It's perhaps easier to think of rock under these conditions as acting something like putty or dough. Really it wouldn't feel particularly soft if you could touch this rock without burning, and even under the high pressure of the mantle it doesn't deform as quickly, but this gives you some sense of how it moves over time, flowing, sliding, and deforming without necessarily needing hard breaks but not completely losing its shape like a liquid. The cooler rock on a planets surface does tend to be more rigid and brittle, tending to fracture rather than bend, which on Earth helps form large tectonic plates with thin boundaries between them. But within plates there can still be a good deal of bending, squeezing, and stretching, and under other conditions this might be the more standard behavior of the crust.

Finally, I will attempt to give some idea of what the surface should look like in all these cases, many of these tectonic regimes are known only from coarse computer models, and so there's simply not much to go on. Exotectonics is in many ways quite a young field with limited samples to study and a lot of open questions.

An example of the typical terrain features we expect to see with Earth-like, mature plate tectonics. I'll make similar example maps for some of the other tectonic modes we'll discuss, but for many the maps will involve a lot of guesswork and essentially artistic interpretation of very limited information, so don't read too much into the fine details.

Stagnant Lid

Stern et al. 2018

The generally more straightforward case, and to a certain extent a stagnant lid can be regarded as essentially the default tectonic regime. Mobile plates likely require a specific combination of factors which many planets—possibly even many Earthlike habitable planets—may simply fail to ever achieve, and even mobile-lid planets are likely to have periods of stagnant lids when they're young or old. Regardless, as a stagnant-lid planet ages, it still likely progresses through a series of somewhat different tectonic regimes as the interior cools and convection becomes more sluggish. In many of these regimes there may still be some degree of lateral motion of the crust, but it will not be globally ubiquitous or allow for such vast scales of displacement as with mobile-lid planets.

Young Worlds

Every planet necessarily begins in a hot state, as planetesimals violently collide together to form a single body. They also begin at least somewhat undifferentiated, with materials of different densities and chemical behaviors mixed together. So there’s a lot of heat trying to get out, and a lot of material trying to move into a density gradient and releasing more heat as it slides past each other. All of this is a good recipe for action.

We therefore expect an earthlike planet usually starts with a molten surface, with easy internal convection and lots of heat radiating into space. But such a hot surface cools fast; intense sunlight or a thick atmosphere might help retain at least a partially molten surface² for a while, but generally we expect that the surface should cool and solidify within a few million years³, if not even sooner. Some parts of the mantle may remain molten for longer (a molten layer below the crust is often called a "magma ocean" in the literature, which I think creates a bit of confusion in how long surface lava should last), but a solid mantle is generally presumed for any mature planet.

Exactly what Earth looked like after it's crust first cooled (or after it had been melted again by the moon-forming impact and then recooled) is a subject of substantial debate and uncertainty, but different planets elsewhere in the cosmos could possibly resemble different models of early Earth. It's often expected that the planet would have initially formed a dark basalt crust, similar to modern ocean crust (though perhaps even richer in magnesium and iron and so tinged slightly green), but some models⁴ instead suggest a lighter, greyish anorthosite crust, more similar to modern continental crust; later volcanism would tend to be basaltic, forming heavier, darker crust that would sit lower in the mantle. A young planetary system likely still has a great deal of debris flying about, and so impacts would be common, with the largest melting sections of crust such that the craters fill in with upwelling magma. Our moon, having cooled and become largely inactive shortly after forming, gives us something of a snapshot of this phase, with heavily cratered anorthosite highlands and flatter, often round basalt lowlands. A larger planet like Earth likely soon loses these early continents as they're broken apart and buried by large impacts, flood volcanism, and violent early subduction, but perhaps some intermediate worlds could retain them for longer.

Shortly after the crust first forms (within ~100 million years), buildup of heat in the mantle may cause the planet to slightly expand, fracturing the crust⁵ and forming a global network of rifts; possibly these could transition to subduction zones as the expansion stops and the surface begins to cool, but as we'll see most models tend to expect that it would take billions of years for Earth to develop the right conditions for subduction.

Exactly when oceans appeared is also hard to say: Naturally the surface would've been initially too hot for liquid water, but as it rapidly cooled, steam in the young atmosphere or outgassed from volcanoes would have soon rained down⁶ on the surface, and some geological evidence⁷ indicates surface water within as little as 150 million years after forming. Some have still suggested⁴ at least a brief dry period before water was delivered to the surface by comets or emerged from the interior. In any case, once oceans appear they might become quite deep⁸ (as a hot upper mantle without subduction will tend to outgas most of its water to the surface), possibly flooding most of the surface even if there are some continent-like highlands. But there's also evidence⁹ for at least some remaining land, and various subtleties in the formation process and composition of the interior¹⁰ could influence the early water present on other worlds.

Heat Pipe Tectonics

Once a stable crust forms, some smaller bodies like our moon may cool fast enough to promptly stop most tectonic activity, but for larger or hotter worlds simple conduction of heat through the crust won't be enough to cool the interior. Plumes of hot magma will rise through the mantle and burst through the crust here and there across the surface. Magma will flow up through the punctures—“heat pipes”—for a time, flooding over the surface and forming volcanic highlands. Like shield volcanoes on Earth, these volcanoes are likely to be fairly broad and shallow—though still potentially quite high at their peaks just through sheer size—and have frequent eruptions producing wide lava flows. Eventually this activity locally cools the mantle and so the flow stops and the heat pipes are filled in with solid rock, with volcanism shifting elsewhere on the surface.

Io is the archetype of such tectonics in our solar system, driven by it's ~2 W/m² of predominantly tidal heating¹¹, and the effects on surface geography are obvious: the moon is dotted with broad, highly active volcanoes that give it a “pimply” appearance, though they often cluster together on highlands formed by uplift of the crust by mantle plumes. Elsewhere, the surface has few craters despite the absence of an atmosphere or water erosion, because any that do form are soon covered with fresh lava flows, with essentially the entire surface replaced¹² every few hundred thousand years. Io also has a number of pateras, vast lava lakes up to hundreds of kilometers¹³ across continuously fed by a stream of magma from below. They do have a thin solid crust about 5-10 meters thick, but this is replaced every few years or so as the cooling rock becomes dense enough to sink into the lake and the unveiled lava surface cools and solidifies. On a world with a substantial atmosphere, however, the air would likely convect a great deal of heat away from such areas and cool them faster, so they may be significantly less persistent or impressive in scale (though surface replacement may also be more frequent).

An incomplete (~75%) elevation map of Io. Io only has 8% of Earth's surface area, so similar structures might appear smaller and more numerous on the map of an Earth-sized heat pipe world. White et al. 2014

As new lava flows pile up, older ones are pushed down until they melt again at the base of the crust. The crust may thus actually be fairly thick and so rigid enough to resist much lateral motion, but this constant inward flow of material compresses the interior, and so, like a crushed ball of dough squeezing through your fingers, this may force blocks of crust to thrust upwards¹⁴, piercing the crust to form ragged ridgelines.

White et al. 2014

On a more Earthlike world with an atmosphere and oceans, this burial and melting of crust could carry down carbonates with it, allowing for the formation of a climate-stabilizing carbon-silicate cycle. Intermittent lava flows across the whole globe may not sound too hospitable, but they need not be quite as frequent as on Io; Earth may have had¹⁵ similar heat pipe tectonics for over a billion years, possibly lasting until after the origin of life, with a surface heat flux potentially less than 1/10 that of Io and correspondingly lower outflow of lava on the surface. On a larger planet or one that otherwise has slower cooling of the interior, it could perhaps persist for much longer.

Even for the more extreme cases, life may not be out of the question: in the past I've expressed worries that such high rates of volcanism may fill the atmosphere with CO₂ and other greenhouse gasses faster than weathering processes can remove them, but more detailed modelling¹⁶ has suggested that a stable weathering rate and climate may be possible with internal heating rates as high as 30 W/m², if not even higher, and such high internal heating rates still likely allow for a globally solid crust, even if regional volcanism may be very high.

The end of a heat pipe regime may be quite rapid¹⁵; as the production of new lava slows, the burial and sinking of old surface rock also slows, which gives more time for material in the crust to be heated from below; this allows more heat to conduct through the crust and radiate from the surface, and also thins the crust and potentially allows it to break and subduct, or otherwise transition to another tectonic mode that helps dissipate heat from the interior, and so rates of volcanism rapidly decline. It's hard to say exactly how long that transition would take from the existing models though; probably nothing that would be obvious in a single lifetime, but perhaps still quick enough to be disruptive to the evolution of any life dependent on the nutrients and terrain produced by heat pipe volcanism; Mercury shows some indication¹⁷ of a roughly 200-million-year transition from global volcanic activity to regional pockets and then cessation.

Drip-and-Plume

Once a planet has transitioned out of the heat pipe regime, a larger portion of its heat conducts through the crust and so volcanism significantly declines. The crust thins at least initially, though a distinct lithosphere forms in the mantle. For smaller worlds with rapidly cooling interiors, this may be more or less the end of their tectonically active phase; larger planets may potentially transition into some type of mobile lid, but many, if not most, may remain as stagnant lids with active but more sedate volcanism. Rather than piercing the crust piecemeal, mantle convection is more concentrated in large, long-lived plumes, forming broad volcanic highlands, with older, undisturbed lowlands between them.

Venus is the archetype for this mode of tectonics, though bear in mind as we discuss its surface features that there are some different ways to interpret its tectonics that we'll discuss later. Though scattered with shield volcanoes like Io, most are inactive, and there is a clearer division of the surface into highlands and lowlands. There are also some features present that wouldn’t last long on Io: impact craters and ancient lava flow channels from millions of years past. Notably, Venus has similar geothermal heating¹⁸ to Earth, meaning that it gives us a good sense of how Earth's terrain would appear if it happened never to develop plate tectonics, though we do have to bear in mind the different climate and lack of surface water implying different patterns of erosion.

Elevation map of Venus; total range from lowest to highest point is about 14 km, surface area is 90% of Earth. NOAA

The upwelling plumes of hot rock push up the surface and form large volcanoes, but only a minor portion reaches the surface; much of the plume instead spreads outwards under the lithosphere and cools just by conducting heat to the surface. As the plume ages, the rock at its edges becomes cool and dense enough to start sinking back down into the mantle, and can drag parts of the crust and lithosphere with it. On Earth, such a process may help start subduction, but here the crust is too weak or friction too great to form rigid plates, so instead of pulling down more crust with it, these sections of surface rock tear away, forming lithospheric drips that sink into the mantle.

Modern conveyor-belt-like subduction (top) versus more fluid lithospheric dripping (bottom). Arndt 2013

This can still affect surface features, in particular local plumes and dips are likely responsible¹⁹ for coronae, circles of ridges and trenches between 60 and 1,000 km in diameter formed as an initial volcanic bulge subsides back down and forms drips around its edges.

Modelled terrain and observed features on Venus as a volcanic bulge (left) collapses to form a corona (right). Gerya 2014

Old, dense sections of lithosphere may also form drips on their own, thinning the crust and causing it to sink lower in the mantle, forming some of the lowland regions.

But even though the surface is mostly static, there may still be some lateral movement²⁰ where the motion of plumes in the mantle drags the crust above it. At one end, this forms a lowland where the crust is stretched and thinned, crossed by an arc of volcanoes; at the other end, the crust is compressed to form a high plateau like Venus's Lakshmi Planum.

Model for formation of the Lakshmi Planum plateau by deformation of the crust by mantle upwelling. Harris and Bedard 2014

Were Venus’s surface partially submerged in water, there would be large landmasses we could call continents separated by large bodies of water we could call oceans, but they wouldn’t be directly analogous to those features on Earth. Rather than concentrated mountain ranges and volcanic island arcs, the landmasses would mostly have high interiors sloping gradually down to the coasts, and islands would be randomly distributed off the coasts, though long-term fluvial and coastal erosion might help to form flatter coastal plains and wash away small islands.

Possible coastlines of a partially flooded Venus (the shaded-in grid squares were for use in a climate model). Way and Wang 2017

Some type of continental crust might appear²¹ under such conditions, though; the action of mantle plumes could pull water into the mantle and cause the formation of felsic lava that would rise to the surface. Our cratons on Earth may have first formed in something like this way. But without plate motion, they could not have grown into full continents.

Oddly enough those fractal-based terrain generators I dragged in the last post might do a decent job of simulating the appearance of a world with drip-and-plume tectonics—though it would still lack the interesting features like the coronae and uplifted plateaus, as well as erosion and deposition features we'd expect on a wetter planet.

Despite lacking subduction, there is still some potential for a carbon-silicate cycle on a temperate drip-and-plume world, as deep deposits of carbonate materials may eventually be pulled down into the mantle with lithospheric drips, returning that carbon into circulation so it can erupt in later volcanoes. Long-term stability may still be an issue, though; some models²² predict that rather than continuously resurfacing, heat may build up in the mantle over several hundred million years until it bursts through in a catastrophic global resurfacing, after which volcanism dramatically reduces. Such swings in volcanism could cause catastrophic climate variability. But if Earth experienced a stage of drip-and-plume tectonics in transitioning between heat pipe and plate tectonics modes, that would mean that either this cycling is not ubiquitous or it’s survivable.

Carbon outgassing and sequestration on a drip-and-plume world. Stevenson 2019

Presuming a similar size, composition, and orbit to Earth, a stagnant-lid world could remain habitable²³ for up to 5 billion years; shorter than Earth could last (with a slower-evolving sun), but still substantial. It could even²⁴ maintain a magnetic field however important that may or may not be. But if it had a higher CO₂ content or less land area, it would be more prone than a similar mobile-lid world to reach a moist greenhouse and end up like, well, Venus. And without constant CO₂ outgassing at ridges and subduction zones, consistency of volcanism is an issue; a gap without volcanism of 100,000 years would send the planet spiraling into a snowball state. But however intermittent its major volcanic events are, recent evidence²⁵ for ongoing volcanism on Venus is cause for optimism.

Upwelling and Delamination

As a stagnant-lid planet continues to age and cool, internal convection slows and the lithosphere thickens, making it increasingly hard to deform or fracture. Rather than frequent new mantle plumes pushing up through the crust and pulling down lithospheric drips around them, convection in the interior settles into a more stable pattern of a few regions of persistant mantle upwelling and slow sinking of the oldest sections of crust until their lower sections tear away or "delaminate". So much the same basic process of drip-and-plume, but much slower, with regions of volcanic activity or tectonic subsidence remaining static for billions of years.

The archetype here is Mars. Rather than numerous volcanic hotspots, Mars has a few broad highlands where large mantle plumes have pushed up the crust, most notably around Tharsis: this immense bulge has created large faults and cracks around it, like at Valles Marineris, and formed huge volcanoes as magma continues pushing up through the same channels for billions of years.

Elevation map of Mars (with shaded relief); total elevation range 29 km, 28% Earth's surface area. We're not too sure why the northern hemisphere is lower and younger, but it could relate to earlier tectonic motion that stalled before getting fully underway. NASA

Today, the planet is not quite volcanically dead; though there is no evidence of ongoing activity, eruptions may have occurred²⁶ as little as 50,000 years ago, and likely will again in the future. But these are brief, isolated episodes between global periods of quiescence for thousands, even millions of years. Outside of these few hotspot regions, the surface is dominated by craters accrued over billions of years of geological stasis, though the heaviest cratering likely occurred early on when the solar system was more filled with debris, so this may not be as prominent for a world that maintained global tectonic activity for longer. Even on Mars, there's a clear contrast between the older south and younger, less cratered north.

With so little volcanic activity or tectonic motion, the carbon-silicate cycle likely ceases to operate and the climate becomes unstable. On a wet planet like Earth, we'd expect this to ultimately result in limit cycling: declining CO₂ levels would eventually result in a global snowball, but this would stop surface weathering and so allow CO₂ to accumulate until it thaws the ice, and then weathering would resume and so on in a cycle, with longer glacial periods and shorter warm periods, until the planet freezes completely. Mars does appear to have had widespread glaciers at some point, but was too dry to fully snowball, but nevertheless similarly seems to have gone through²⁷ a cycle with long cold periods but intermittently warmer and wetter conditions, perhaps due to limit cycling²⁸ but possibly also triggered by shifts in obliquity or transient volcanic activity. If we can expect this generally for old dry planets is hard to say, as this climate shift was likely also related to the loss of Mars's atmosphere and oceans, due in part to declining volcanism but also the planet's low gravity and escape velocity.

This is meant to evoke an aging but still habitable case, which could potentially come at various points in a planet's tectonic development.

In either case, while the climate eventually becomes hostile to the survival of complex life, there seems to be some prospect for brief habitable episodes long after vigorous tectonic activity has mostly stopped.

Cold Stagnant Lid

Whatever stages it passes through as it ages, every planet eventually runs out of heat. The crust will thicken, the mantle become ever more viscous, and volcanism will cease entirely, though small transient eruptions may carry on a while; even if the mantle cools and becomes rigid quite quickly, early mantle plumes²⁹ may form concentrations of radiogenic material that heat local pockets of melt that may then continue erupting for billions of years. Earth's moon appears to have had little tectonic activity after forming but shows some evidence³⁰ of regional volcanism as recently as 2 billion years ago (claims of even younger activity as recently as³¹ under 100 million years ago have also been made but are more controversial³²).

Even once volcanic ends, there can still be some tectonic motion; as a planet cools, it will shrink slightly, and so the surface is compressed and wrinkles. This may be responsible³³ for some ridges on Earth’s moon and Mercury.

Elevation map of the moon; elevation range 20 km, 7.4% Earth's surface area. The near side of the moon is lower on average, and filled with more basaltic magma flows, probably due to tidal effects. NASA/GSFC/Arizona State University

Such features will become increasingly drowned out by impact craters, though some broad variability of the surface may remain from formation, as both the moon and Mars retain clear highland and lowland regions. If impact rates are fairly low, some features may remain locked in place for billions of years: carbon-silicate cycling is impossible, so sooner or later all surface water will either freeze or escape to space, and surface erosion will largely stopped. The atmosphere too will no longer be replenished by volcanic outgassing, but it may take much longer to escape (and some small amount of gasses can continue diffusing out of the crust).

Cold stagnant-lid planets are dead worlds, and aside from the craters most surface features will be remnants of a more exciting time.

Squishy Lid

This is a recently proposed³⁴ tectonic regime mostly put forward as a new interpretation of Venus's surface features³⁵, but potentially appearing in a range of settings including possibly early Earth³⁶. It forms something of an odd intermediate between stagnant- and mobile-lid regimes, in which there is substantial local horizontal motion of sections of crust, but no potential for large displacements of tectonic plates across the surface.

In this regime, upwelling plumes of hot rock create some surface volcanism, but mostly push magma into the crust (similar to the formation of subsurface plutonic rock on Earth, so this is often called the "plutonic squishy-lid" regime). This weakens the crust and makes it more, well, squishy, squeezing or stretching without fully breaking. The weakened crust above the plume stretches as the plume spreads out and tugs at the above crust, while elsewhere other sections of crust weakened by older plumes may produce sinking drips, that pull on the crust behind them. This creates a set of forces somewhat like those that drive plate motion on Earth (though really it's closer to the sluggish-lid tectonics we'll look at later), and indeed it appears that it can allow for rigid plates of crust to move horizontally across the surface at rates up to a few centimeters per year, similar to slower-moving continents on Earth. Given a sufficient rate of plume and drip activity across the world, essentially the entire surface can be split into moving plates separated by thin boundary regions with convergent, divergent, and transform motion, much like plate tectonics.

Sketch of a cross-section through the lithosphere of a squishy-lid planet; note the shifting convergent zone between A and B and new divergent zone in C. Lourenço et al. 2020

The trouble is that these boundaries are all fairly short-lived; without proper subduction, convergent zones soon lock up, and the plume activity driving motion away from divergent zones is also fairly ephemeral. Convergent zones may also shift as dripping and faulting forms new weak points, and similarly dripping from old sections of crust may thin the crust and encourage new plumes to rise as they sink and displace the surrounding rock. Thus, rather than persistent plates displacing far across the surface, there's more of a constant jostling, with old boundaries frequently shutting down while new ones appear, forming new arrangements of plates that may move in different directions under the shifting balance of forces. Without subduction there's also no good way for one section of crust to get out of the way to let another pass, so there's no potential for the sort of global rearrangement of features like we see with Earth's supercontinent cycle.

In this interpretation, many of Venus's highlands may be due not just to static mantle plumes, but lateral compression at convergent zones, while lowlands may show recent divergence. Much of Venus's surface³⁷ seems to be divided into small flat regions divided by faults, which may record previous patterns of collisions and rifting. There does seem to be some correlation with boundary lines and volcanic activity, especially around diverging boundaries, but a large number of Venus's volcanoes still appear to be essentially random hotspots.

A comparison of Earth's divergent features, forming thin plate boundaries that allow for thousands of km of spreading, with Venus's network of potential rift features, which are overlapping, do not form clear plates, and are short-lived, with under 100 km of spreading. Montiel et al. 2025

In Earth's case, a squishy lid is typically proposed as an intermediate stage between an early heat pipes regime and later mobile lid regimes. However, at that time Earth may have had fairly low topography and deep seas, so this squishy behavior may have mostly been playing out on the seafloor; however, the first continents may have formed³⁸ in this period, sitting much higher in the crust by virtue of their lower density, and these may have been more resistant to deformation and rifting, so there could have been a period with small island continents jostling around in a squishy ocean. Still, as some amount of subduction helped grow and collide these continents, they could have started displaying more squishy behavior³⁹ (and arguably some continental crust still does in areas where repeated collisions have created many fractures or prolonged volcanism has weakened the crust), and other worlds could conceivably have squishy lids with shallower oceans, such that they display more squishy lid features on the surface. It's even possible that something like continental crust could form⁴⁰ on a world even without oceans, and perhaps this accounts for⁴¹ some of Venus's highlands.

If Venus is a squishy-lid world, then are there more truly stagnant drip-and-plume worlds? The key requirement for a squishy-lid regime is most rising magma intruding into and weakening the crust rather than punching narrow passages through to erupt onto the surface. Exactly what controls that seems hard to pin down even in formal sources, but conceivably it could depend on the mineral composition of the crust and mantle or internal temperature gradients. If there are more stagnant worlds at a similar rate of overall activity to Venus, they might tend to have a flatter global terrain without the high plateaus and stretched lowlands (on the other hand Mars has likely always been stagnant-lid and features large topographic features, but this might conceivably have been "locked in" by a rapidly cooling and thickening crust). It may also lack coronae, but it's not clear that these all form in the same way. There could conceivably be intermediate cases though, with only locally squishy terrain but no global crust motion, and indeed this may include Venus itself, as the indications of motion⁴² seem to be only regional and indicate fairly slow drift.

Regardless, squishy-lid worlds could recycle carbon in much the same way as described for drip-and-plume worlds, and may be better at it because even though individual plate boundaries are short-lived, globally the squishy-lid regime seems to imply a fairly consistent rate of overall motion and drip formation, and so might allow for a more stable climate.

Deformable/Plume Lid

A possible variant regime that's received less attention than squishy lid but has appeared in a few models. First, one recent study⁴³ investigating potential models for Venus's tectonic past found that the behavior of a semi-mobile crust may vary depending on its mineral composition: With an olivine-rich crust, as usually assumed for Venus, a squishy-lid regime can form when volcanic intrusions form weak boundaries in the crust between rigid (if short-lived) mobile plates, and volcanic activity is concentrated at these boundaries or long-lived hotspots. But with a more plagioclase-rich crust, the crust may still be similarly mobile but is substantially weaker even outside of intruded regions, and so rather than forming bounded plates, the entire crust may be subject to more fluid deformation, constantly squeezing and stretching over the entire surface. Individual volcanic regions would be more short-lived as rising magma shifts between opening and closing channels through the crust. Within this study this behavior alternates every few hundred million years with brief episodes of global subduction and flood volcanism replacing most of the crust, like the episodic mobile lids we'll discuss shortly, so they refer to this regime as a "deformable episodic lid", but this study only looks at a limited range of parameters (and I haven't found any other work looking at this particular tectonic mode), so it's not clear if a more persistent pure deformable lid tectonic regime is possible, or if these model cases can be taken to be representative more broadly of mafic (rich in magnesium and iron like olivine) and felsic (richer in silica and alkali metals, like plagioclase) crusts.

However, this behavior does have some resemblance to a regime of "plume lid" tectonics proposed in some earlier modelling⁴⁴ of young Earth, which included continental crust, ocean crust, and surface water; given a sufficiently hot mantle, it was found that converging sections of crust would tend to internally buckle and deform rather than form plate boundaries with subduction.

Details on surface features in these cases are sparse, but compared to squishy-lid tectonics, we'd probably expect the terrain to be more globally even, without low rifting zones or compressed highlands, but perhaps rougher in detail, with no undisturbed plains or flat plateaus and many ridges from folding and faulting of the crust. Conceivably, though, a planet (especially a wet one) might form different regions of crust with different compositions, potentially allowing for both deformable and squishy (or perhaps stagnant) regions. There's still significant lithospheric dripping in this mode, so that may allow for carbon-silicate cycling, but if periodic episodes of crust replacement are necessary, that may come with significant disturbances to the global climate.

This depiction is particularly speculative, based largely on the idea that the widespread deformation would manifest as folding and corona-like features.

Note that many models may not clearly distinguish squishy and deformable/plume lid regimes, so references to squishy lid behavior in later sections could perhaps apply to this regime as well. For that matter, many statements about stagnant lids also apply to some extent to squishy or deformable lids, as the distinction has only really been made recently.

Eggshell Tectonics

This is more of an interesting footnote than a distinct tectonic regime: Most of the worlds we're considering would have thick, cool brittle crusts, 10s of km thick, overlaying the more ductile lower lithosphere and relatively fluid mantle. Even in our more deformable cases, the brittle crust will tend to form by forming thin faults between rigid blocks of rock, which helps create local terrain features like ridges, cliffs, and so on between flatter terrain. But one recent paper⁴⁵ noted that either very high surface temperature or higher surface gravity reduces the depth of the brittle crust; in the most extreme cases they test, a surface temperature of about 1000 K and roughly 4 times Earth's surface gravity could reduce it to under 1 km thick. If we limit ourselves to more habitable temperatures, a planet with high gravity could have a brittle crust about 5 km thick.

They refer to planets with thin brittle layers as "eggshell planets", predicting that the thin crust will be too weak to support high mountains, forming very flat topography, and likely couldn't sustain subduction and plate tectonics either, as the thin crust would tear too easily. In the paper, they suggest that such planets might have squishy lid tectonics, but given the description I'd imagine a deformable/plate lid would be more likely. Though, if some of these cases could be just strong enough to maintain subduction, something like the distributed deformation regime we'll discuss later may be worth considering as well.

Mobile Lid

Speculative breakdown of major tectonic types, though lacking a squishy-lid mode. Lenardic 2018

The other major category of tectonic regimes, where all or at least large portions of the crust move horizontally over the surface, with boundaries separating mobile plates. Whether or not you choose to include the squishy- and deformable-lid regimes is somewhat semantic; on the one hand in at least the more active cases the whole of the crust may be mobile, but on the other without consistent plates and boundaries the scale of displacement is limited and so the expected global geography is quite different; a mobile-lid regime can perhaps be broadly defined as one that allows for two surface features to be on opposite sides of the planet at one point in time and directly adjacent at another.

Diagnostic parameters for representative models of several tectonic modes (each with constant internal heating): "plateness" is a measure of how concentrated deformation is around thin plate boundaries, "surface mobility" compares horizontal lid motion to mantle motion just below the lid, temperature is an average for the mantle, and the stagnant lid here is a heat pipes case. Lyu et al. 2025

This of course includes plate tectonics on Earth, and it's worth looking at that in a bit more depth. Plate motion on Earth, as I've said many times, is ultimately driven by subduction: old ocean crust becomes cold and dense enough to sink into the mantle, and this drags the rest of the attached plate with it. This encourages convection currents in the mantle that helps things along—the cooled mantle around the subducting slab of crust sinks along with it, and the the thinned and torn crust at the rift on the other side of the plate encourages upward convection of the mantle—but the majority of the force moving the plate still comes directly from the pull of the sinking slab. But for subduction like this to operate requires several conditions to be met: The temperature gradient and mineralogy of the upper mantle and crust has to allow young crust to remain buoyant but then allow old crust to sink with enough force to pull whole plates behind them against the immense friction with the underlying mantle. The crust then has to be weak and brittle enough to fracture so that one part of the crust can sink under another and begin subducting, but still strong enough to remain intact under these immense forces rather than promptly tearing and disintegrating as the subducting slab descends into the hot mantle and pliable enough to keep bending as new crust enters the subduction zone.

Typical surface features and mantle structure of various tectonic modes. Rolf et al. 2022

This requires a specific balance of conditions for plate tectonics to start and persist, and to be honest we aren't entirely sure⁴⁶ what those exact conditions are. It's clear enough, though, that plate tectonics is likely to be a transitional stage in the evolution of a planet, between an early stage where hot, vigorous mantle convection punches magma straight through the crust or the crust is too hot and squishy to form subducting slabs; and a late stage where the cold lithosphere thickens and any subduction locks up. Many planets may never achieve the right conditions at any age, and so miss plate tectonics entirely, but we can't be sure exactly how common that is. A number of different factors relating to Earth's overall composition, surface chemistry, and formation have been proposed to be in some way necessary for plate tectonics to develop:

One particularly frequent proposal⁴⁷ is that water oceans on the surface are critical, as water helps to lubricate the sliding of plates in subduction zones, but it's not clear if it's entirely impossible for subduction to succeed without water (whether with other liquids helping or just through the right combination of mineralogy and heat)—or, if water is critical, exactly how much would be needed.
Even if water specifically isn't necessarily, a cool surface with an Earthlike climate may be necessary⁴⁸ to maintain the proper thermal gradient subducting crust to sink into the mantle and help the crust⁴⁹ remain brittle and prone to fracturing.
Earth's particular composition may be important, as a planet richer in silicon or sodium may have form a crust too buoyant to subduct⁵⁰ (at a guess, it might instead be deformable/plume lid or ridge-only sluggish lid).
Even with something closer to Earth's balance of silicon and magnesium, a too even ratio of elements may form too pure a mineral composition that may not form⁵¹ the proper faults and weaknesses to accommodate plate motion.
A planet with a larger iron core and thinner mantle may struggle⁵² to form the proper convection patterns for plate tectonics.
The moon-forming impact may have influenced the structure of the mantle⁵³, encouraging the formation of larger mantle plumes that fracture the crust and start subduction.
Weathering of granite-rich continental crust causes radiogenic uranium and potassium to deposit on the seafloor, providing more heat⁵⁴ to encourage local melting when they're pulled into subduction zones, so formation and exposure of such crust above sea level may help sustain and accelerate more subduction.

Even just on Earth, it's still not clear exactly when and how plate tectonics began. Estimates for the start of plate tectonics range from 800 million⁵⁵ to over 4 billion⁵⁶ years ago, but most sources⁵⁷ seem to agree on 2.5-3-2 billion years as the most likely period when some kind of subduction must have started; about 2.5 billion years after Earth formed. Subduction itself could have started in a number of different ways:

Sufficiently old and dense crust may simply sink into the mantle⁵⁸, pulling in surrounding crust and creating a converging motion that eventually forces on section of crust over another. This is somewhat straightforward in that it requires no external trigger, but old crust also tends to be thicker and more rigid and so there's some skepticism it could undergo this degree of bending and tearing unprompted.
A plume of magma erupting through the surface could create a new layer of light surface rock and pull down on the existing crust⁵⁹ at its edges, much as coronae may form on Venus.
Large impact events could fracture the crust⁶⁰ and thin regions of it to create the density contrast necessary for subduction.

Lucy Reading-Ikkanda, Quanta Magazine

As mentioned earlier, shortly after the crust first forms, buildup of heat in the mantle may cause the entire surface to expand⁵, fracturing the crust with number of rifts which could then later reverse course and begin subducting. But this may only work if subduction begins very early after the planet forms.
As an early heat pipes stage ends, the sudden decline in volcanic activity and buildup of heat in the mantle may help the crust⁶¹ thin and begin to sag and fracture, leading to subduction.

However subduction first begins, it may then encourage the formation of continental crust, as well as create numerous rifts and transform faults which create the conditions for later subduction. However, the first continents may not have been formed by subduction zone volcanism like modern continental crust. There's some indication of lighter, more silica-rich crust minerals forming as much as 4 billion years ago, and then over the next billion years the first of the cratons formed; sections of crust of similar composition to modern continental crust but notably thicker, with lithospheric roots over 200 km deep, and so also very resistant to deformation or rifting. Their cores seem to have formed in large part by direct volcanic activity from the mantle rather than growth and collision of island arcs. Perhaps this was triggered by the rapid end of heat pipe tectonics⁶², or as mentioned it happened later³⁸ as a period of squishy-lid tectonics mixed material in the upper mantle, or it may just be a consequence⁴¹ of early patterns of mantle flow and mineral separation.

The formation of these first continents, significantly less dense than the surrounding ocean crust, may have helped⁶³ encourage subduction zones to form more frequently and last for longer, and may have had subtler influences as well such as the aforementioned weathering of exposed granite depositing radioisotopes on the seafloor. Thus the formation of the continents could have perhaps encouraged their own growth, so it's possible that something about Earth's particular composition or rate of cooling might have encouraged the formation of these early cratons in a way that shaped the development of plate tectonics thereafter.

There's clearly a lot of open questions and potential speculation to be had here, but little that can be said for sure. At any rate, it's probably time to look at variations in mobile lid tectonics in detail.

Episodic and Regional Mobile Lid

Even once subduction starts on a planet, it may not persist. As the mantle and crust cool and the lithosphere becomes more rigid and brittle, there's likely to be a stage where subduction can start, but the still-weak lithosphere⁶⁴ can't maintain it and the subducting slab soon tears away from the surface crust. Even once subduction can be maintained, it may not immediately lead to a globally mobile crust; every subduction zone eventually destroys itself, as it pulls together pieces of continental crust (which are either a precondition for subduction or formed by the subduction zone) which cannot subduct. To maintain plate tectonics thus requires new subduction zones to form as quickly as old ones close, which may not be the case with a younger, more buoyant and less brittle crust.

There may then be a transitional period of partial or transient mobility. At first, an otherwise stagnant or squishy planet may have small subduction zones forming due to mantle plumes or impact events but then quickly failing, lasting as little as⁶⁵ under 10 million years, forming volcanic arcs and stretching the crust but not breaking off proper plates (with a squishy lid planet in particular there may be a hazy gradient between large lithospheric drips and short-lived subduction zones, and some of the larger coronae on Venus have even been interpreted as transient subduction features⁶⁶). Later on, subduction zones may be more persistent and form proper plates with complementary rift zones, but only temporarily, pulling together pieces of early continental crust and closing themselves off before new subduction zones can form. This may manifest as episodes of global plate motion, potentially resulting in the replacement of most ocean crust and formation of a large supercontinent, and then long periods of stasis until a new subduction zone forms, with volcanic activity limited to hotspots.

Artemis Corona on Venus, sometimes interpreted as a stalled subduction zone forming an arcing trench with an associated ridge. Ghail et al. 2024

There could also be stages with regional mobility, where part of the planet has large plates bounded at least partially be rifts and subduction zones, but the rest of the surface remains stagnant or squishy; generally this probably isn't stable in the long term, representing either a transient local episode of mobility, or the beginning or end of a global episode, though the Lid and Plate regime we'll discuss a bit later imagines a more stable form of partial mobility.

Something like this sequence has been proposed as a model for Earth's tectonic behavior through the Proterozoic eon: several lines of evidence⁶⁷ indicate that some amount of subduction and plate motion likely began by around 2.5 billion years ago, culminating in the assembly and then prompt breakup of the early supercontinent Kenorland. But tectonic motion may have proceeded haltingly for a long time, with long pauses with less mobility and more hotspot volcanism: First⁶⁸ a perhaps complete halt around 2.4 to 2.2 billion years ago; then subduction and motion resumes, pulling together the next supercontinent Nuna (or Columbia, geologists seem to have trouble settling on a single name for it); then a longer period⁶⁹ of perhaps not total immobility but generally reduced activity and change around 1.6-1.0 billion years ago, covering much of the infamous "boring billion" period of static geology and climate and slow evolutionary change, though there was a gradual resumption of motion towards the end of this period pulling apart the continents and reassembled them into Rodinia. Something like modern plate tectonics and global motion finally begins around 800 million years ago as Rodinia rifts apart.

It's worth noting, though, that even though episodic mobile lids are often framed as transitional phases, that doesn't mean that they must inevitably end in full plate tectonics; a planet may have a long episodic phase and just never quite reach the conditions for sustained global plate motion, or it may be interrupted by a change in the surface conditions or interior cooling, and eventually slip back into a more stagnant regime. Episodic mobility is yet another model⁷⁰ proposed for Venus, with a global episode of mobility and crustal replacement 500 million years ago perhaps explaining the seemingly uniform age of much of the surface.

On Earth these mobile episodes are generally suggested to have alternated with a stagnant lid, but as mentioned some models⁷¹ suggest that alternation between mobile and squishy- or deformable- lid stages may also be possible.

Probable tectonic regimes (within one model and set of parameters) depending on internal heating and the resistance of the lithosphere against sinking and forming subduction zones; pie charts show relative time in different phases for episodic modes, and stagnant lid here includes heat pipe planets, with cold stagnant lids below the bottom of the chart. Lyu et al. 2025

Exactly what the mobile episodes look like may vary as well. Many of these models are quite vague on the details, but some seem to imply not so much large global plates with volcanic arcs and continents but more catastrophic events where the entire preexisting crust is consumed by numerous local subduction events and then replaced by widespread flood volcanism, with complete replacement of the crust in as little as⁷² a few tens of millions of years; one can imagine this might be somewhat more hazardous for life and climate stability than longer episodes of more gradual subduction and mobility.

With longer mobile episodes of more steady subduction, much of what we'll discuss later about the potential features of early plate tectonics (such as wider, lower orogenies) probably applies here as well, or perhaps exclusively to episode mobile lids if true plate tectonics only began within the last billion years as suggested by some models. Even early, one recent model⁷³ has suggested a pattern where a planet remains mostly stagnant (or squishy) with short episodes of mobility where there are concentrated subduction zones but no mid-ocean ridges, with instead broader regions of extension and rifting. These could perhaps resemble some back-arc regions on Earth, where local hotspots in the thinned crust produce new oceanic crust, but never form a single ridge of spreading.

Snapshots of tectonic motion at different points in time in several different models typifying different tectonic modes, with arrows showing direction and speed of surface motion, red showing areas of divergence, and blue showing areas of convergence. Names are from the paper, but these roughly correspond (left to right) to a young, infrequent episodic motion; more mature, sustained episodic motion; and plate tectonics. Modified from Xiang et al. 2024

We could perhaps expect that as a planet ages and cools, it would tend to transition from these brief catastrophic episodes of mobility towards more sustained global mobility with distinct plates, until eventually plate tectonics sets in fully. Some models⁷⁴ do suggest that certain circumstances (such as the loss of surface oceans) could prompt a plate tectonics world to return to episodic catastrophic resurfacing, though also with the intriguing possibility for regional catastrophic resurfacing that never spreads over the whole surface.

Even outside an obvious episodic mobile lid regime, a more consistently squishy-lid planet often seems to have periodic variation between more globally mobile or stagnant periods, and on the other hand modern Earth still has some stagnant-like behavior, with hotspot and mantle plumes still forming and occasional lithospheric dripping⁷⁵ from the continents. This seems to be particularly the case within large supercontinents, with Pangea forming a thick crust and several large mantle plumes late in its tenure. So we could say that even modern plate tectonics, through its supercontinent cycle, has a degree of periodic variation between more mobile and more stagnant or squishy behavior, at least on land.

Exactly what the surface of such a world looks like in detail will depend on the exact sort of cycle it's undergoing and what stage it's in. One can perhaps imagine a mix of stagnant or squishy lid features and those of early plate tectonics, either as different tectonics are ongoing on different parts of the planet or as remnants from one mode persist into another. During a mobile stage, there may be identifiable plate boundaries and subduction zones with volcanic arcs, but also scattered volcanic highlands and squishily deformed continents with numerous internal rifts and ridges; during a more stagnant phase, there may be broader volcanism and deformation, but still continents with mountain ranges across their interior, and perhaps stillborn island arcs over failed subduction zones, or large coronae on the cusp of sustained subduction that never quite reach it. But if the mobile episodes are more catastrophic, with total replacement of the crust, this might erase most old surface features and leave a single vast terrain of volcanoes and lava flows—though perhaps some fragments of more buoyant or just lucky crust can survive across the replacement period.

Sluggish Lid

Another tectonic mode that resembles plate tectonics in many ways but with important differences. There's an occasional misconception that Earth's plates are passively carried along by convection in the mantle, but really it's more the reverse: the crust and lithosphere are an integral part of the convection process and plates are primarily pulled by subducting slabs, such that they move faster than the underlying mantle and are slowed by their drag against it. In some cases⁷⁶, though, where the pull of subduction is weaker or even absent, convection currents may be more concentrated in the upper mantle such that it moves faster and surface plates are indeed carried along with it.

Conceptual cross-sections through the crust and mantle of a plate tectonics world (left) and sluggish lid tectonics on early Earth (right); some of these traits like smaller plates may not apply for other types of sluggish lid on older worlds. Asad and Lau 2024

This is similar to the mantle-driven movement we discussed for squishy-lid tectonics, but in this case a cooler mantle or more rigid lithosphere allows for large, persistent plates and long-lived subduction zones. Plate motion may still tend to be slower than in typical plate tectonics—perhaps still faster than squishy lid, but there could be significant overlap in the potential drift speeds of all 3 modes. Because plates are pushed into convergent boundaries rather than pulled, there's likely to be wider zones of deformation and ridge-building around them, and potential for double-sided subduction, where rather than one slab simply sliding under another, material is pulled from both plates on either side of the subduction zone and descends together, which could possibly induce⁷⁷ volcanic activity on both sides. But asymmetric subduction is still possible, and perhaps more likely for older cases closer to proper plate tectonics, especially if continental crust appears.

A sluggish lid mode is another proposed stage⁷⁸ of Earth's early evolution, providing a more gradual transition out of an early stagnant or squishy lid, with local rifts and subduction in a squishy lid spreading to form a global network of sluggish plates that gradually become more subduction driven (though much as with episodic mobility, conceivably⁷⁹ a planet could become sluggish but for one reason or another never quite achieve full plate tectonics). But a sluggish lid is also proposed⁸⁰ as a possible regime for older planets, as the cooling of the interior creates a thicker, slower-moving lithosphere.

Ridge-Only

This is a variant on sluggish lid tectonics seen in many models⁸¹, typically as an intermediate between stagnant lids and full sluggish lid motion with subduction zones. In this case, there are global plates that diverge to form mid-ocean ridges, but there is no subduction; instead crust piles up in vast compressed highlands around convergence zones, with material dripping off the bottom into the mantle.

Models of sections of the crust and upper mantle of planets with (top to bottom) distributed deformation, plate tectonics, and ridge-only tectonics, with lower viscosity indicating greater deformation, lowest near plate boundaries. Tackley 2000

Under constant compression and without any sharp plate boundaries, these convergent highlands probably wouldn't form just steep-walled plateaus but moreso fold-and-thrust belts like the Appalachians or Zagros, but on a larger scale. They'd also lack much volcanism or generation of continental crust, but a ridge-only mode is often predicted as a late stage in a planet's development as it cools too much to sustain other types of mobility, so there may be old continents still present that then would drift towards convergence zones and perhaps form larger, continuously compressed supercontinents.

Whether such a planet could maintain a carbon-silicate cycle is hard to say; ridges would have steady volcanism and outgassing, but quite a lot of carbon could be locked in the thick crust at convergent zones (though there would be some rate of material eroding off the bottom), and there may not be much generation of calcium-rich rock on top of the crust that would help sequester atmospheric CO₂, so how sustainable the climate is in the long term may depend on how large the existing reservoirs of these elements are and how the exact rates of outgassing (presumably declining for an old, cooling planet) balance with shifting patterns of weathering.

Lid and Plate

Yet another variant on sluggish lid proposed in a recent paper⁸², though also with elements of stagnant/squishy and regional mobility. In this case, it's imagined that early Earth may have started (perhaps after a brief heat pipes phase) in something like a squishy lid state, with small-scale deformation and dripping from local mantle convection but no large-scale mobility. But as churning and cooling alters the composition of the upper mantle, some plumes might start forming more rigid crust that better resists this frequent rifting and shifting of deformation boundaries, forming "proto-plates" with large undeformed interiors and persistant boundaries. Mantle convection will then cause sluggish motion of these proto-plates, and where they converge one will be forced to subduct under the other, which will cause subduction zone volcanism that forms the first continental crust and cratons.

Speculative evolutionary stages of Earth including a lid and plate transition, shown as cross-sections through the crust and upper mantle. Capitanio et al. 2019

The key point here, though, is that the conditions for formation of proto-plates may develop slowly, limited for a while to certain regions of the planet, and because the subduction zones aren't as self-perpetuating (with crust pushed into them rather than pulled by the subducting slab) they may not tend to spread or consume ocean crust as quickly, so there could be a long period (~500 million years in this model) where the world has regions of sluggish lid plate motion but other areas that remain squishy or stagnant lid.

Distributed Deformation

This is a somewhat enigmatic tectonic regime suggested by some models⁷⁹ but never described in much detail. On modern Earth, we have large plates with vast interior regions experiencing fairly little deformation and all moving in the same direction, and then concentrated regions of deformation—volcanoes, mountain ranges, concentrated uplift—in thin strips between them. But some models seem to indicate a different possible state with smaller stable regions divided by numerous broad deformation regions.

Surface velocity across a section of the surface (top) and regions of rigid crust on the surface of the globe (bottom) in models of different tectonic modes. Lenardic 2018

Some sources⁸³ classify this as essentially a variant of squishy or sluggish lid tectonics, which indeed tend to have broader deformation regions like this, but there does seem to be a meaningful distinction to be made between these modes and some model cases with fast plate motion and strong subduction (there's also a somewhat confusing clash of terminology with deformable lid tectonics, which is also similar but lacks any distinct undeformed regions or plate boundaries). To be honest it's not clear that all these models are really describing the same behavior, or how well these somewhat abstracted model cases could correspond to real planets—and trying to infer specific surface features from abstract global models is also challenging. But something like this tectonic mode has appeared enough times to be worth discussing.

Generally this regime appears⁸⁴ in models that are otherwise similar to plate tectonics worlds but lack a distinct asthenosphere, a thin layer of weaker and more fluid rock between the more rigid lithosphere above and lower mantle below, which helps allow surface plates to slide easily over the mantle; without it, the stresses at plate boundaries seem to be more broadly distributed across the nearby crust. Lack of an asthenosphere may also allow mantle currents⁸⁵ to erode away the deep roots of the cratons, making them more prone to deformation and rifting and so perhaps hampering the growth of large persistent continents. On Earth, subduction of water into the upper crust appears to have played a major role in the development of an asthenosphere, so perhaps one wouldn't appear on a drier world or one with some other factor preventing transport of surface water into the upper mantle.

A few⁸⁶ recent models⁸⁷ also seem to have hit on something like this mode by increasing the pliability of crust and lithosphere, making it easier to bend and subduct into the mantle. This results in far more subduction zones and small plates, and more broad deformation within plates, with a particular tendency for broad extension and stretching around divergent zones rather than thin ridges. It is worth noting, though, that these models generally lack distinct continent and ocean crust, so may not reflect how continents might control plate formation and subduction.

Snapshots of tectonic models with varying lithospheric strength. Without spending too much time describing the parameters here, the left of each figure shows the surface, with areas of lower viscosity showing plate boundaries or deformation, while the right is a cross-section through the interior, with low temperatures indicating subducting slabs. Mallard et al. 2016

How this might apply to real planets is hard to say. Earth's lithosphere has generally become weaker⁷¹ to bending over time as the ocean crust has thinned and become more brittle (this may seem at odds with the squishiness we've been proposing for early Earth, but here we're talking about how easy it is to bend and subduct whole sections of lithosphere rather than just causing local deformation; squishy behavior may actually help resist this sort of bending by distributing stress over a wider area). But eventually the cooling of the interior will likely cause the lithosphere to thicken again and become more rigid, as stresses from mantle convection also decrease; there should perhaps then be a point in Earth's history when the surface is weakest and most prone to subduction, and it happens to have fallen short of this distributed deformation mode (presuming it doesn't occur in the future, which no model seems to predict). Possibly tradeoffs between crust rigidity and brittleness just make it hard to ever reach this point of easy bending and subduction, but perhaps a somewhat different mineral composition or internal structure might make it easier, and as we discussed earlier a very high gravity or hot surface may make⁴⁵ for a thin, weak crust. But perhaps all these studies are reflecting the same tendency for more distributed deformation to appear when the lithosphere is weak relative to the stresses from the mantle below, so we could perhaps expect these same results for a planet without an asthenosphere.

In terms of specific features, we largely just have to guess from broad principles. Thin subduction zones seem to still be predicted at convergent regions, but there may perhaps be more distant compression features like fold-and-thrust belts and multiple parallel ridgelines, or perhaps even some areas of extension as different parts of the crust jostle at different speeds and pull apart (like the Basin and Range areas in the Rockies), with faults and thinning crust. At divergent regions, we might see more parallel rifting zones or more distributed spreading like backarc regions. In between may be more local faults, rifts, and hotspots than we generally see in the continent interiors on Earth.

It's worth noting that even on Earth, there's a fair bit of variation in the "width" of plate boundaries, with some plates having a fairly narrow region of faulting or spreading between them (like many mid-ocean ridges and ocean subduction and transform zones) but others having broader areas of deformation and "squishy" motion of regions of rock not cleanly divided into plates, like the Alpide belt across much of Eurasia; we can perhaps imagine a world where there aren't any thin rifts or collisional mountain ranges, and instead the broad interweaving mountains of Central Asia or multiple parallel rifts of East Africa are more the norm.

Hemispheric Tectonics

This tectonic mode has been explored in a recent string of papers mostly concerned with tidal-locked planets with surfaces far too hot for habitability, but there are some potential implications for more Earthlike worlds as well.

Close-orbiting tidal-locked worlds may have extreme contrasts in temperature between their day and night side, possibly hundreds of Kelvin, which may be enough to influence patterns of convection in the mantle. In particular there may be a tendency⁸⁸ for the convection to be biased from one side of the planet to the other, such that convergent zones with subduction form only on one side of the planet with divergent spreading zones appear mostly on the other side.

Stevenson 2019

Beyond that, the details can substantially vary⁸⁹ depending on the strength of the crust against bending and subducting, the dayside surface temperature, and its contrast with the nighttime surface temperature:

With weak crusts, as with our previous discussion, there's a tendency for there to be more frequent subduction and numerous convergent zones, but for planets with a very hot dayside and cooler nightside, the convergent zones will tend to be concentrated on the nightside, where the cooler crust can more easily sink into the mantle, while the dayside will be dominated by divergent zones.

With stronger crusts, there will be fewer subduction zones, to the point that in many cases there may be a single dominant convergent zone (though often with at least occasional brief episodes of additional nearby convergence) with most spreading occurring in the other hemisphere. For cooler planets, this convergence zone can be essentially anywhere and may shift around over time; For warmer planets with more of a temperature contrast, the convergent zone will tend to appear in the middle of the dayside or nightside (or in a few cases the terminator), but with no particular preference for which, essentially just picking one by chance, with the potential to later switch around. For worlds with very hot daysides and substantial contrasts, there is a more consistent preference: with a very cold nightside, convergence usually happens on the dayside, as the cold nightside crust is too rigid to subduct⁹⁰; but with a somewhat warmer nightside, the convergence is again often on the nightside.

Representative examples of modeled tectonic patterns: each chart shows the longitudinal position of upwelling and surface divergence (in red) and downwelling and convergence (in blue) over time as the simulation ran over time (with constant internal heat), with the substellar point at 0 longitude and the shading showing day and nightside, and the diagram above shows a cross-section through the mantle at the end of the simulation. The "M" labels for each shows the surface temperatures in K (dayside over nightside) and lithosphere strength in terms of the ductile yield stress in MPa. From Meier et al. 2024, rearranged and with added headers.

There's also a tendency in many of these models for a cyclical pattern of converging divergent zones: smaller regions of upwelling tend to join into larger convection cells, so divergent zones converge to a single rift, but then buildup of heat elsewhere causes the rapid appearance of new divergent zones.

But it is worth noting that this is all based on fairly simple modelling, with a slice through the planet rather than a full 3D model, uniform crust, and no particular accounting for the potentially important role of water in aiding subduction, even though most of these worlds would be too hot for surface water (these models are also all run for Super-Earths, so I couldn't say how much planet size and gravity figures into this). In reality we might expect somewhat more complex patterns of convergence and divergence, and perhaps more of a tendency towards episodic motion for stronger crust, but there could still be an overall preference in plate motion between hemispheres. Similarly, hot, dry planets may not have true subduction, but might still have squishy or sluggish lid motion with the same overall tendency for divergence in one hemisphere and convergence in the other.

Such a sustained pattern over billions of years will undoubtedly create strong contrasts in terrain, with perhaps rifts, hotspots, and generally low terrain across the divergent hemisphere, while crust piles up into high, folded plateaus in the convergent hemisphere. Such high dayside temperatures may cause the crust to be fairly thin and ductile, and likely fairly flat and prone to tearing and deformation (and note that some of these planets are potentially in the range of the "eggshell planets" we discussed earlier).

For cooler worlds with habitable temperatures, such consistent motion patterns appear unlikely even for tidal-locked worlds, but at least temporary hemispheric tectonics appears likely, and to an extent certain parts of the supercontinent cycle resemble hemispheric tectonics when all continents are diverging from their former center and moving towards the other side of the world; with a stronger crust, there may perhaps be a stronger bias for subduction to be concentrated in one region and rifting in another. There's some suggestion⁹¹ that Mars and perhaps even⁹² our moon may have had brief periods of hemispheric tectonics in some form, explaining the strong dichotomies of their surfaces between highland and lowland hemispheres.

Conceptual (not to scale) diagrams of potential early hemispheric tectonics on the Moon (left) and Mars (right). Santosh et al. 2017

For tidal-locked planets, even if the orientation of this hemispheric motion isn't initially aligned to the dayside or nights, the slight deformation of the planet's surface may cause⁹³ tidal forces to reorient the planet, placing the divergent area at the center of either the dayside or nightside (without any particular preference for either). Even absent a consistent global pattern of hemispheric tectonics, supercontinents tend to cause upwelling of the mantle below them, so after a supercontinent forms, it may tend to reorient towards either side of the planet, making the dayside either predominantly land- or ocean-dominated, which could have dramatic consequences for the global climate. Notably, if our moon did have a period of hemispheric tectonics, it has concentrated much of its younger lowlands on the Earth-facing side, with more highlands on the far side. It's unclear whether some aspect of the Earth's influence or the moon's early development favored this orientation, or whether it was essentially a coin flip and we could have just as easily ended up with highlands on the near side and a lower far side, but tidal forces would've likely forced the moon to orient one way or the other regardless of how the asymmetry originally formed.

Young, Old, and Different Plate Tectonics

Even if a world enters a proper plate tectonic regime, with large, rigid plates divided by thin boundaries, they might not look quite the same as Earth today.

For one, an early plate tectonics planet (or an episodic world during a mobile phase) may still have a hotter, weaker crust. This will limit the height⁹⁴ of mountain plateaus formed by tectonic collisions, as the crust will more readily slump and sink under their weight, such that the high mountains on early earth may have only reached a few kilometers high.

Mountain growth rates and eventual limits at different points in Earth's past; these are predicted averages, not necessarily strict maximums on individual high peaks. Rey and Coltice 2008

The more buoyant crust may also be less prone to the steep, rapid subduction typical on modern Earth. Instead early earth would have been dominated⁹⁵ by shallower flat-slab subduction, making Laramide-type orogenies extending far into the interior of continents the norm. But over time these subduction zones may also have been⁹⁶ prone to slab rollback, peeling away from the underside of the continent and pulling at its margin, forming a broad back-arc region interior to the mountain range with thinned crust and widespread volcanism (though possibly these two mechanisms could have varied in timing somewhat, with flat-slab subduction being more prominant in the earliest stages of subduction ~3 billion years ago and then rollback becoming more common after ~2 billion years ago).

Comparison between the potential structure of mature coastal subduction zones with rollback today (left) and during early plate tectonics (right). Roberts et al. 2023

Conversely, subducting slabs would still tear easier, so long continent-continent collisions and the resulting Himalayan-type orogenies may have been rarer. On the other hand, after collisions lithospheric dripping or peeling may have pulled away⁸² the denser lower portions of the crust under collisional orogenies, making the remaining surface crust more buoyant such that it rises to form a broader if less steep highland region.

Thus the picture we have of early Earth—or another world early in its plate tectonics regime—is one with broad coastal plateaus and volcanic regions, perhaps thin or at least less steep interior mountains, shorter mountains generally, and perhaps slower ocean subduction.

Potential development of Earth's tectonic mode, with a gradual transition from squishy/sluggish motion to modern plate tectonics. Cawood et al. 2022

As plate tectonics continues, subduction will form more continental crust, though exactly how fast and whether Earth today is still gaining more continental area is highly debatable⁹⁷; continental crust can't subduct, but it can be compressed in collisions, with material then removed by erosion both on the surface and through friction with the mantle below, and as continents cover more of Earth's surface they will more frequently collide with each other, so possibly Earth might tend towards a stable state of partial continent cover, though other models⁹⁸ suggest more potential variability in rates of continent growth and eventual land area.

Maximum plateau height may continue to get higher and flat-slab subduction may become rarer, and the nature of the supercontinent cycle may also change⁹⁹: as the mantle cools, the ocean crust formed at mid-ocean ridges becomes thinner, making it weaker and prone to quicker subduction that promotes more vigorous mantle convection. This may pull continents apart quicker early in a supercontinent cycle and encourage them to converge on the far side of the planet, causing an extroversion cycle—whereas a younger planet with thicker ocean crust will have slower disassembly of supercontinents and more time for new subduction zones in the new interior ocean to pull the continents back together, causing an introversion cycle. Supercontinents may also rift sooner¹⁰⁰ after assembly as a planet ages, as the cooler mantle and less intrusive volcanism means that colliding continents will form weaker sutures holding them together.

This also seems to imply that plate motion will generally become quicker as the planet ages and cools (which perhaps also leads to more high-speed collisions and high mountain plateaus), but I hesitate to guess how consistent this might be across different evolutionary stages and circumstances. Formation and subduction of ocean crust cools the interior, so we would expect that as the interior cools, plate tectonics would have to dissipate less heat overall (because either undercooling or overcooling the interior would eventually interrupt the thermal gradients and convection currents driving plate tectonics, and subtler feedbacks are likely to be in play as well). Perhaps the thinner ocean crust we expect in the future would carry less heat despite moving faster, or hotspots and occasional flood volcanism can help make up the difference, but at least at the extremes we would expect that an early, hot planet couldn't have too slow plate motion (at least as its dominant tectonic activity cooling the interior) or late, cool planets couldn't have too fast motion. This is in line with expectations that early episodic subduction may have been quite rapid or old plate tectonics worlds may eventually transition to sluggish lid motion, so perhaps the current quickening trend is just a temporary quirk that will eventually reverse.

Thinner ocean crust may also imply deeper ocean basins, and thus a lower sea level relative to the continents. But on the other hand, if subduction is faster then ocean crust will be younger and so float higher in the mantle, and if sea levels do decline then we would expect more erosion on the higher continents, cutting them down while filling the sea with more sediment. Still, in combination with the potential for more continental crust and for more water to be sequestered¹⁰¹ into the mantle, this may pan out to more land area overall, and with higher mountain plateaus and more frequent mountain formation due to faster plate motion, this may also lead to a generally greater range of land elevations.

But not every world with plate tectonics has to follow the same path as Earth. Faster or slower cooling of the interior could imply different rates of tectonic evolution and so different relationships of subduction speed and topography to land area and supercontinent cycles or different initial conditions at the start of plate motion. As mentioned, rates of continental crust accumulation could vary, and at least one recent study⁹⁸ has suggested that most planets may tend to very high or low continental areas due to positive feedbacks. Remember, for example, that the formation of the first cratons may have encouraged subduction⁶³ and the growth of continents, and some studies suggest¹⁰² that there may have been a relatively fast accumulation of continental crust in this early period of volcanic craton formation and episodes of rapid subduction, which ended as more modern plate tectonics settled in. Perhaps if cratons had started forming a bit earlier or Earth evolved a bit slower, there would have been a longer period of rapid growth that filled much of Earth's surface with continental crust; if they had started later, perhaps we'd have few large continents and a world dominated by long island arcs. More balanced cases like Earth may then require a happenstance combination of timing and development.

Given all the uncertainties with the start and development of plate tectonics, and potential variation in composition, age, etc., or possible negative feedbacks¹⁰³ stabilizing continental area, I doubt we can put too much confidence in that particular result. But it speaks to the amount of variation that may conceivably be possible with plate tectonics worlds without requiring radically different starting conditions.

Planet size and surface gravity could also influence tectonics, with a higher gravity likely implying a thinner crust (because temperature and pressure would tend to increase quicker with depth), and thus perhaps faster plate¹⁰⁴ motion for at least moderately higher gravity; though at much higher gravity, the upper mantle may become too viscous under high pressure or the crust too weak to allow for sustained subduction.

After Plate Tectonics

The balance of forces that maintains plate tectonics on Earth can't last forever. If nothing else, the thermal energy that drives tectonic motion is not infinite, but even aside from that a number of other shifts in the environment and makeup of Earth may interrupt the process sooner or later.

First off, a number of researchers have proposed that plate tectonics may depend on water⁴⁷ for lubricating the process of subduction. It's again unclear how much water may be necessary, whether there are any alternative conditions that may allow for subduction (if, for example, oceans of other liquids could fill the same role), or if subduction would stop immediately without water (water is delivered into subduction zones in the form of hydrated minerals deposited on the seafloor that then melt as they sink into the mantle, so if the ocean disappear the existing ocean crust could conceivably continue subducting for some while before these sediments are all consumed). Regardless, we expect that in around 1-2 billion years, increasing light from the sun will force Earth into a moist or runaway greenhouse state and rapidly remove all its oceans, which may then sooner or later cause subduction to fail as the friction of the sliding rock becomes too great.

Once this causes continuous subduction to stop, the planet could potentially transition back into⁷⁴ an episodic mobile lid regime. Tectonic plate motion helps transport heat out of the interior, so once it stops the crust may cool and thicken, but the upper mantle will warm up, which may eventually cause large mantle plumes to erupt through and fracture the crust, triggering a wave of subduction and volcanism that will spread across much of the planet but not necessarily become global, as the mantle soon cools and subduction locks up. There may be many such episodes, spread over hundreds of millions or billions of years, but eventually these will stop, and the planet will settle into some form of stagnant or squishy lid tectonics. Venus is sometimes thought to have gone through this transition in the past after its own runaway greenhouse event, with the apparent replacement of the whole crust half a billion years ago representing the last mobile episode, but this depends in part on Venus's uncertain climate history.

But if an Earthlike planet with plate tectonics was a bit farther out in the habitable zone or orbited a slower-evolving star, it could have tens or hundreds of billions of years longer in the habitable zone—what happens then? One issue could come from the growth of continents; as mentioned, continental area has increased over time and while it's not clear⁹⁷ if this trend will continue, if it does then that would imply less crust that can subduct and less area for continents to move before colliding back into each other. Continents also insulate the mantle more than ocean crust, so as they grow this will change the temperature and convection patterns of the interior¹⁰³. At first this may encourage more vigorous convection under the remaining oceans that leads faster subduction, but a critical juncture may come when about half the planet's area is covered in continental crust (though this may vary substantially depending on the planet's internal heating and just uncertainties in our model, it could potentially be over 60% of the surface area or under 40%), at which point global convection patterns no longer place enough stress on the remaining ocean crust to cause it to fracture and subduct, and so subduction peters out. This may also result in an episodic lid period as the cessation of subduction causes more heat buildup, but it's also been suggested⁷⁹ that a ridge-only sluggish lid mode may occur on an old world covered in continental crust too buoyant too subduct.

Likely evolutionary pathways depending on whether a planet's lithosphere tends to become weaker and more brittle (top) or stronger (bottom) as the interior ages and cools, with the red path showing the proposed evolution of Earth in one model. Lyu et al. 2025

Alternatively, the planet's interior could simply cool down to the point where either the crust is too rigid to break and subduct or mantle convection to weak. As we've discussed previously, a planet richer in radioisotopes may sustain heating¹⁰⁵ for substantially longer, but probably also implies that plate tectonics would have to start later, and so doesn't necessarily extend the total period of plate tectonics by much. Tidal heating from a star or by a moon from a planet may provide a steadier source of heat for longer, but that's not an indefinite source either. This likely also results⁸⁰ in a sluggish lid (perhaps ridge-only) phase transitioning to a stagnant lid, likely eventually resembling Mars, with some ongoing activity and accumulating impact craters but the remnants of its continents and ocean basins still marking to global topography (Mars is occasionally proposed¹⁰⁶ to have had some degree of plate motion in its past, but never on the scale of modern Earth).

(Curiously, I did find one source¹⁰⁷ suggesting that in a future sluggish lid mode, continued subduction may pull water into the mantle while weaker arc volcanism would fail to return it to the surface, such that eventually the oceans would disappear as almost all water was sequestered in the mantle, which might incidentally save Earth from a runaway greenhouse by converting it into a marginally habitable desert world. Alternatively, a more sudden end to plate motion could leave the oceans trapped on the surface while continued erosion washes much of the land surfaces into the sea. This is a somewhat older source, and I haven't seen the suggestion elsewhere, but we'll have to dig a bit more into the subject of Earth's long-term climate future another time.)

Exactly how plate motion stops in any of these cases isn't clear from the few models done on the subject. Ocean subduction rates appear to be generally increasing for now, but I'm not certain if we should expect that trend to eventually reverse as convection becomes weaker and resisting friction greater, or if instead existing subduction zones would continue at that rate but eventually close while new subduction zones fail to form. It may vary, with the former perhaps making more sense for a cooling planet gradually transitioning to sluggish lid and the latter better matching the start of a final phase of episodic mobile lid.

Tectonics and Life

Major events in the development of life (left) compared with possible origins of plate tectonics (or transitions between tectonic modes) (right). Stern 2016

There seems to be fairly good reason to expect that plate tectonics is not the only regime that could support climate-stabilizing geological feedbacks and Earthlike surface life. Indeed, life on Earth first appeared at a time when the planet was almost certainly operating under a different tectonic regime. But we also have to note that the first major diversification of animals and other complex multicellular life happened several hundred million years after the likely start of modern plate tectonics. So could there be a link between plate tectonics and complex or intelligent life, and why might that be? Most of the research on this subject focuses only on the distinction between plate tectonics and a stagnant lid, but of course we've now seen a more continuous spectrum of tectonic regimes with a mix of features, all of which have to be considered to some extent on their own.

Stern and Gerya 2023

First off, I've been emphasizing the importance of the carbon-silicate cycle, which maintains a stable climate through the feedback between surface temperatures and removal of CO₂ from the atmosphere by surface weathering. To maintain this balance, the planet's geology needs to provide a steady rate of CO₂ outgassing that can match the rate of removal; volcanic rock rich in calcium or other metals that can react with CO₂ during weathering; and ideally some way to recycle deposited carbon back into volcanic processes so it can be outgassed again, but this last point may not be critical if there's a sufficiently large reservoir of carbon in the interior that can be continuously pulled up by volcanic activity.

Plate tectonics achieves all this through subduction, which encourages steady volcanism¹⁰⁸ both at mid-ocean ridges and above subduction zones, and pulls deposited seafloor carbonate into the mantle where it can be fed directly into some of those volcanos, forming prominent mountain ranges that encourage steady weathering¹⁰⁹. The carbon-silicate cycle has not been without its hiccups¹ on Earth, but these generally occur due to more stagnant-lid-like behavior when mantle plumes produce flood volcanism. If Earth's plate tectonics were even more efficient at cooling the interior (perhaps in the distributed deformation mode, or if there was less tendency towards large, long-lived supercontinents) these plumes wouldn't form.

Excepting colder stagnant lids, all the tectonic regimes we've discussed today seem to have some prospect to provide all the necessary conditions, but the trouble is consistency: Several of these regimes (episodic mobile lid, perhaps squishy lid and drip-and-plume) seem to tend towards episodes of catastrophic volcanism and long periods of quiescence. Though a stable climate may be possible¹⁶ with very high rates of consistent volcanism, a sudden increase in volcanic outgassing likely forces rapid climate change and mass extinction, while long periods with little volcanic activity may cause CO₂ levels to drop to dangerously low levels, potentially causing the planet to freeze over.

Now, an occasional climate catastrophe isn't necessarily terrible for the development of complex life; occasional mass extinctions may give new groups chances to diversify and encourage overall more ecological diversity. For Earth in particular there's also a potential link between the rise of complex life and the snowball earth event, when much of the surface froze over around 700 million years ago. This climate instability may have been caused¹¹⁰ in part by the shift to modern plate tectonics, and could perhaps have given an opportunity for complex multicellular life to unseat simple unicellular life from global dominance (the whole episode is complex and impactful enough that I’ll definitely talk about it again in the future). But this requires a period of recovery after the mass extinction, with overall biodiversity generally taking something like 20 to 100 million years to recover from Earth's mass extinctions, depending on the severity. Climate cataclysms more frequent than that may simply not allow life time to recover and diversify. In more extreme cases, a sufficiently severe volcanic event could possibly even¹¹¹ permanently destabilize the climate and prompt a runaway greenhouse.

Still, this tendency towards inconsistent volcanism and rapid climate change may very from case to case; episodic lids may have a lot of variance in their tendencies towards cataclysmic change or gradual shifts in activity, and at least some models seem to indicate that a steadier rate of small-scale hotspot volcanism is possible for stagnant or squishy lids and stable temperate climates may be easily achievable¹¹². As with many factors, it may not be that a stagnant lid makes habitability impossible, just less likely.

For older planets, plate tectonics may also help¹¹³ maintain steady outgassing for longer, as the constant replacement of thin ocean crust helps make it easier for pockets of fluids to form and reach the surface, but a cooling interior may also cause plate tectonics to stop, so it's hard to judge how much of an advantage that might be, and in some cases near the inner edge of the habitable zone, lower outgassing may be an advantage¹¹⁴ as it delays a moist greenhouse.

It's also worth noting that while we've identified some potential for recycling of deposited carbon into the mantle for most of these regimes, in many cases (stagnant lid, squishy lid, ridge-only) this recycling may not be terribly efficient and doesn't necessarily deliver carbon directly back to surface volcanoes in the same way subduction does, so it's perhaps conceivable that for a world particularly poor in carbon, the upper mantle may become depleted and fail to outgas sufficient CO₂, though this is somewhat speculative on my part; Earth has a very large interior carbon reservoir compared to its surface content, and significant amounts of carbon in the crust may be released¹¹⁵ by volcanic activity that happens to pass through it, which could happen with hotspot volcanism in any of these regimes.

Recycling of water could be a similar issue. As Earth's interior has cooled and subduction started, water has been locked away in the mantle and there may be as little as¹⁰¹ half the total water on the surface as there used to be. Under different tectonic regimes, the amount of water pulled into the interior or returned to the surface by volcanic activity may vary, which implies scenarios¹⁰⁷ where the entire surface ocean is pulled into the interior, leaving a barren desert planet, or too much water remains on the surface, flooding all potential land surfaces. Some basic modelling¹¹⁶ indicates stagnant lid worlds may tend to outgas less water to the surface than similar worlds with plate tectonics, but they also tend to have flatter topography and so this may somewhat balance out in terms of resulting land area, and of course Earth may have been in some form of transitional mode when it produced most of its oceans. There may also be feedbacks¹¹⁷ tending to prevent extreme scenarios of completely dry or flooded surfaces, and we could always just imagine a planet happening to form with more or less water to offset different outgassing tendencies.

For planets that do have stable climates and oceans, another critical juncture may be the appearance of atmospheric oxygen. Oxygen first starts building up in the atmosphere after 2.5 billion years ago, at around the same time¹¹⁸ as one of the first potential major episodes of plate mobility on Earth, and then a second major rise¹¹⁹ in oxygen levels comes around 600 million years ago, shortly after snowball Earth and the likely beginning of the modern regime of steep-subduction plate tectonics, and is soon followed by the appearance of complex life.

Why might this be? For one, the felsic continental crust formed over subduction zones contains less iron and magnesium than the basaltic crust made by heat pipes and mantle plumes, and so absorbs less oxygen¹²⁰ from the air to make metal oxides. Uplift of mountains and resulting rapid erosion could also help increase the rate¹²¹ at which these oxygen-absorbing metals are buried in sediment. And long-lived continental crust could also store deposits of organic carbon¹²² formed from CO₂, leaving excess oxygen behind. High oxygen levels are vital for the energetic metabolisms of complex animals on Earth; if this is true everywhere, then mobile lid tectonics may be a necessity for complex animal-like life.

Of course, that initial rise in oxygen levels occurred when Earth was possibly in an episodic or sluggish lid regime, and we discussed earlier how Earth's first felsic continents may have formed in an even earlier stagnant or squishy lid, so how much any of this is tied to plate tectonics specifically is hard to say. We've also previously discussed a number of non-biological processes that may help produce atmospheric oxygen; in this case we wouldn't necessarily need them to even do the whole job themselves, just give the surface chemistry an extra boost towards oxygenation in the same way as continents and felsic volcanism did on Earth.

Finally, even aside from a stable climate and oxygen, the geography and change that plate tectonics produces could be more favorable to complex life and diversity. Broad, shallow continental shelves are ideal for the development of marine life, and rapid uplift of mountains creates a good source of nutrients¹²³ for these seas. Once complex life develops, breakup and assembly of continents and the relatively rapid shifting of geographic barriers in may encourage more diversification¹²⁴, and thus increase the chance of novel forms evolving and, eventually, intelligence. New forms of life will also have the opportunity to evolve in various climates and particular climate/geography combinations over the course of millions of years, rather than being locked into those accessible from whatever landmass they originate on in a stagnant-lid regime. Gradual climate change from drift and the supercontinent cycle may also help gently encourage more diversity and ecological innovation, unlike the rapid climate shifts of catastrophic volcanism.

Concept of how tectonic regimes could affect diversification of life ("P" for plants, "H" for herbivores, "C" for carnivores). Stern 2016

Episodic and sluggish lid regimes can of course reorganize landmasses in somewhat the same way, a squishy lid could shift and recombine nearby landmasses to some extent, and even on a stagnant lid world, local uplift and subsidence of the crust could produce¹²⁵ shifts in sea level that flood or expose land bridges between landmasses. But none may do so as frequently or dramatically as constant global plate motion. A distributed deformation regime, on the other hand, may cause even more shuffling of landmasses, though without knowing exactly what that looks like in practice, it's hard to say what consequences it might have for life.

While we’re here, the relationship between tectonics and life is not one-way. Though I have stressed several times on this blog that the carbon-silicate cycle probably doesn’t need life to function, life on Earth has certainly become involved in the process. Life on land breaks up surface material and so exposes more to weathering, and life in the oceans concentrates carbon and other materials in their bodies and deposit it on the ocean floor as they die. This means thicker sediment layers with more carbon and water entering the mantle at subduction zones, which could mean¹²⁶ faster production of felsic crust. All this to say that modern Earth with life could have larger continents than an Earth without life would have.

Ice Tectonics

Though I presume we're mostly concerned with habitable, rocky planets here, there is some indication that much colder bodies like Europa with ice crusts over water oceans could have similar behavior that's worth taking a quick look at.

Generally there are two broad models for how this happens. Ice does not get less buoyant as it cools like basaltic rock does, so isn't prone to sinking as it ages, making subduction difficult. Most models¹²⁷ therefore propose what we might call call ice-floe tectonics, which bears a certain resemblance to ridge-only sluggish lid tectonics: convection in the underlying water pushes two sections of ice together, and the ice along the boundary fragments, with the fragments pile onto a ridge above the boundary and a keel below it. Elsewhere, divergent sections of ice would cause water from below to rise up and freeze, much like mid-ocean ridges on Earth, though again without higher buoyancy it wouldn't necessarily be any higher than surrounding regions. Something like this plays out on Earth where sections of sea ice jostle and collide in the flow of ocean currents, but on icy bodies it might play out at global scale.

Stern et al. 2018

However, other models¹²⁸ suggest that variations in salt content between plates could allow a closer resemblance to plate tectonics, with one plate subducting under another, and even forming uplifted ridges and cryovolcanoes (outbursts of liquid water onto the surface) on the overlaying plate, just like over subduction zones on Earth. But there probably wouldn’t be any close analogue to our continental crust/ocean crust dichotomy, and so no continents.

Kattenhorn 2018

Both Europa and Enceladus show clear evidence¹²⁹ of widespread tectonic action, as tidal forces form their planets and other moons both deform their surfaces and produce heat in their interiors. Mostly these show indications of extension and faulting, but a number of still-enigmatic ridges might show convergence and even subduction¹³⁰. But a lot of this motion may be fairly small-scale and local¹³¹, so it's perhaps more comparable to squishy lid or distributed deformation than Earthlike plate tectonics (which makes some sense for crust that is weak to tearing but difficult to subduct). But much as with rocky planets, there could be a range of tectonics modes on icy worlds.

Several other icy bodies like Triton show indications¹²⁹ of at least local cryovolcanism, and ridge features on a number of bodies may indicate¹³² something like sluggish-lid tectonics at some point in their past. Titan has a number of long ridges in its equatorial regions that, similar to ridges on our moon, may be¹³³ the result of global contraction as the planet cooled and its spin may have slowed. Some have even suggested¹³⁴ this may have lead to local subduction and associated cryovolcanism, but the details remain unclear.

In Summary

Heat convecting out of planet interiors drives surface tectonics, which can manifest in many different ways depending on how the planet forms and evolves.
Tectonic modes can be largely divided into stagnant and mobile lids, though with some ambiguous in-between cases.

Stagnant lids have little or no lateral movement of the surface, with magma pushing up through the crust in scattered hotspots.

Young worlds may start molten but will cool quickly, potentially forming different terrain type as the initial surface is melted and reformed by frequent asteroid impacts.
High geothermal heating drives a heat pipe regime, where the surface is dominated by highly active volcanic hotspots.
Older stagnant lid worlds may enter a drip-and-plume regime, with more concentrated volcanic regions and slower replacement of the surface.
Aging stagnant lids will have declining volcanism and thick crusts potentially supporting high topography, and may have unstable climates.

Squishy lids have some lateral motion of the surface driven by mantle convection, but do not form large persistent plates or consistent plate boundaries.

Deformable/plume lids may lack even the small crustal blocks of squishy lids and so more fluidly deform their entire surface.

Mobile lids have large persistent plates that can move large distances across the surface, though they're not always fully bounded or rapidly moving.

Episodic mobile lids may alternate between mobile and stagnant periods, or have regional mobility of part of the surface.
Sluggish lids have slow plate motion driven by mantle convection.

Ridge-only sluggish lids lack subduction, such that crust piles up into high compacted plateaus around convergent zones.
Lid and Plate worlds may have regions of sluggish and stagnant lids coexisting for long periods.

Distributed Deformation tectonics has subduction and generally resembles plate tectonics, but with smaller plates and more widespread deformation within plates.
Early plate tectonics may have had lower, wider orogenies and wider backarcs.
As Earth has aged and cooled, thinner ocean crust may be causing faster plate motion and a tendency towards extroversion supercontinent cycles.

Plate tectonics may potentially end due to declining interior heat, loss of surface oceans, or buildup of continental crust, after with a planet might shift into episodic, sluggish, or stagnant regimes.
Though many tectonic modes might allow for a habitable climate, they may be less stable than plate tectonics or less suitable for the buildup of atmospheric oxygen and the development of diverse biospheres with intelligent life
Icy crusts can have mobile-lid tectonics, but with no distinct continents and little uplift.

In terms of our example system, there's not too much more to say for now: Teacup Ae has Earthlike plate tectonics, and presumably has a past of transitional states but that all falls largely before the period of tectonics and life we're most concerned with. Perhaps more interesting are the other worlds of the Teacup system:

Teacup Ab is a hot tidal-locked world with a dayside reaching over 800 K and a freezing nightside. This puts in well within the range where we might expect some sort of hemispheric tectonics, but Ab is a small world with a likely fairly cool interior, so it's not clear how well any of the reference models apply. Perhaps we might at least expect to see some ancient coronae and rift features, which a notable asymmetry across hemispheres; say, more divergence on the day side.
Teacup Ac is a dry world with declining but not fully dead tectonic activity. Given that it had some larger oceans in the past, we can perhaps say that it had an early period of mobility but slipped into an episodic regime and now a Mars-like stagnant lid, with remnant continental highland and ocean basins on its surface partially overlain by more recent cratering and flood volcanism.
Teacup Ad is a deep waterworld; though it would likely have some volcanic activity below its oceans and layers of high-pressure ice, we don't know much about how that might look and it has little influence on the surface appearance anyway.
Teacup Ad I is a small rocky moon that likely quickly cooled to a stagnant lid, but the competing tidal influence of the star and planet have perhaps helped maintain some internal heat, with sporadic volcanism perhaps concentrated in a couple persistent hotspots, and with perhaps some faulting and rifting of the surface as it has been stretched by tidal forces.
Teacup Ae II is another small rocky moon like Earth's, with no ongoing tectonics and heavy cratering across the surface, but perhaps a few surviving anorthosite highlands.
Teacup Af V is a Mars-like moon. Given that it's a bit larger than Mars and has a thicker atmosphere that perhaps implies more sustained volcanism—and let's say that it never had much surface water—we can perhaps imagine a more Venus-like squishy lid regime early in its history, though with activity now substantially declined and a thickening crust, so any remaining activity is concentrated in a few persistent hotspots.
Teacup Ah III is a blatant Io ripoff and so will have similar heat pipes tectonics.
Teacup Ah IV is similarly a Europa ripoff, and though Ah V has a substantial atmosphere it has a similar icy crust, so both might have some surface mobility driven by tidal heating.
Teacup Ai, a large icy body, doesn't particularly resemble any body in our solar system or one that has been modeled, but at a guess it might have a fairly vigorous style of ice tectonics, resembling something between a squishy lid and distributed deformation.
Finally, Teacup Aj III is another large icy moon with potential for some surface mobility.

In the next post we’ll discuss global climate patterns and the many factors influencing them, and then after that talk about the specific breakdown of climate zones on the surface.

Notes

Ah, what a pleasant time it must have been when you could write a paper with a section on “Flat Earth Hypothesis for the Pre-Neoarchean Earth” without that being wildly misinterpreted on the internet.

This is a pretty good (though fairly technical) talk on the relationship of different tectonic modes to climate and habitability.

Apparently at the same time they were pushing Intelligent Design on biologists, young-earth creationists made a play for legitimacy in geology as well. I guess they gave up when they realized mantle plumes and subduction get less press than crocoducks and monkeys.

Buy me a cup of tea (on Patreon)

Part VIa

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Comments

AbbydonOctober 27, 2020 at 8:41 PM
Very interesting. It's good to see another world building blog that includes links to scientific papers to ensure everything is scientifically plausible. In particular, I'm intrigued by the 5 Gyr estimate for habitability on a stagnant lid exoplanet. I'll have to read the paper in depth but a quick skim suggests that tidal heating can extend this and maintain CO2 outgassing. I assume that means a circumbinary planet with a relatively close orbit could potentially follow the stagnant lid model for a long period of time.
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AnonymousDecember 11, 2020 at 4:46 AM
Would a planet with heat pipe or drip and plume tectonics necessarily transition to an active lid scheme if flooded? I'm curious because I'm planning a project on a planet that switches modes from heat pipe or drip and plume to active lid, but with a fairly long time between it's flooding and the switch.
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AnonymousDecember 13, 2020 at 12:49 AM
What would the history of "continents" on a stagnant-lid world look like? Would they just build up in some areas from volcanic eruptions and slowly erode as time goes on? Also, what tips can you give in drawing maps for Drip and Plume worlds specifically?
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AnonymousJanuary 15, 2021 at 3:29 PM
How quick would the transition from active-lid to stagnant-lid be if a planet rapidly lost its oceans to a moist greenhouse in the span of less than 1 million years? Would its topogrophy before that influence future surface activity? (e.g.: a weak point in the crust near an old divergent boundary being the site of a major mantle plume)
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AnonymousFebruary 5, 2023 at 4:50 PM
I've been trying to find out more about distributed deformation as a variant of an active lid system, but I can't find anything about why it would occur instead of plate tectonics or if it would experience a supercontinent cycle. Are there any sources that might help?
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AnonymousApril 7, 2023 at 9:10 AM
I was wondering if conditions other than temperature difference could produce hemispheric tectonics IE: ice sheets on the dark side as well as glacial meltwater lubricating the subduction zones (the planet in question is around mercury insolation with a thin atmosphere so crust on the day side is still hot but not quite what's needed). Also would divergent zones look like ocean ridges, coronae, rift valley maybe? and could I see a transition into more martian geography with volcanoes only around hotspots.
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Kaden VancielJune 29, 2023 at 7:10 AM
For me, it’s easier to think of plans for which continent(s) to start off with, how many it/they could split/merge into, and where they’d be drifting over millions of years. Perhaps that could lead to figuring out where fault lines are.
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Lena SynnerholmApril 27, 2024 at 8:12 PM
I think plate tectonics only being 800 million years old is implausible. By then I think there were already had granitic continents. Moreover, the cratons mostly formed between 2 and 3 billion years ago. Many of these are too large to have formed though vulcanism in a single spot. Instead, I think they formed from several volcanic archipelagos crashing into each other. A process which requires subduction.
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AnonymousSeptember 9, 2024 at 1:36 AM
How would lakes, rivers and seas of liquid methane affect Ice Tectonics?
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AnonymousOctober 13, 2024 at 4:25 AM
This is really well put together. I was made aware of pre plate tectonics last year been doing reading on it too and I just found this just last week. I imagine the transition from one regime to another is gradual since episodic subduction or another mechanism of orogeny had to take place fairly early on with eoarchean metamorphic rocks (regional) and the known 3.8-4,0 gya Napier orogeny and 3.5 gya Rayner orogeny. You are doing great work here
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AnonymousApril 10, 2025 at 10:31 PM
How underwater part (ocean floor) on your map of stagnant lid - drift and plume tectonics looks like? What it should be? While map of continents on such world is clearly seen, underwater parts of planet surface are not seen because they are hidden under water.
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