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. Every other solid surface in the solar system (with the possible exceptions of Europa and Enceladus) is either broiling all over with volcanic activity or frozen solid—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 tectonics due 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.

• Stagnant-Lid
• Heat Pipe Tectonics
• Drip-and-Plume
• Upwelling and Delamination
• Cold Stagnant-Lid
• Mobile-Lid
• Episodic Subduction
• Sluggish-Lid
• Shallow Subduction
• Distributed Deformation
• After Plate Tectonics
• Tectonics and Life
• Exotic Tectonic Regimes
• Ice Tectonics
• Hemispheric Tectonics
• In Summary
• Notes 

Back to Part Va 

Stagnant-Lid 

Stern et al. 2018

All the tectonics modes we’ll discuss can be divided into two broad categories: Mobile-lid, where distinct sections of the crust (tectonic plates) are moving across the surface; and stagnant-lid, where—aside from local deformation—the crust is a single static piece. Stagnant-lid is the default, occurring for planets with hotter interiors than Earth (e.g. Io), colder interiors (Mars), or similarly warm interiors but with surfaces unsuitable for plate tectonics (Venus). Earth itself began and eventually will return to a stagnant-lid regime. But just because the surfaces of these worlds aren’t broken into moving plates, that doesn’t mean they don’t move or change at all, and indeed there’s a lot of variation.

Heat Pipe Tectonics

Every planet necessarily begins in a hot state, due to impacts during formation. 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.

At first, the result is a molten surface, with easy internal convection and lots of heat radiating into space. But lacking an outside source of heat this won’t last long. Within a few million years the crust cools and solidifies and so forms an insulating cover over the still-warm interior.

But the heat still needs to get out, and conduction through the crust isn’t enough. Thus the planet transitions to heat pipe tectonics: Plumes of hot magma 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 for a while, until the mantle is locally cooled and so the flow stops and the heat pipes are filled in with solid rock.

Io is the archetype of such tectonics in our solar system, and the effects on surface geography are obvious: the moon is dotted with high 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 of constant resurfacing with fresh magma.

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 magma flows pile up, older ones are pushed down until they melt again at the base of the crust. This constant inward flow of material compresses the interior, forcing blocks of crust upwards to form ragged ridglines.

 

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 magma flows across the whole globe may not sound too hospitable, but they need not be quite as frequent as on Io; Earth may have had similar heat pipe tectonics for over a billion years, possibly lasting until after the origin of life.

Typical geography and notable features of a heat pipe world with oceans.

Drip-and-Plume

As a planet continues to cool, the crust thickens and strengthens and internal convection becomes less vigorous. Plumes of magma continue to rise from to mantle to form volcanic highlands, but at any one time they’re more concentrated rather than bursting through all across the crust, and resurfacing by magma flows is slower than for heat pipe tectonics.

Venus is the archetype for this mode of tectonics, and though it shares some of the scattered volcanism as Io 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.

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

 

Then there are the coronae, circles of ridges and trenches between 60 and 1,000 km in diameter. These are likely formed by magma plumes that push into the crust and then form local convection cells, lifting up the center and then pulling down crust at the sides. This regime is sometimes also called "squishy-lid tectonics" because of how the crust can be squeezed and torn by action of the mantle.

And though the surface isn’t broken into plates, there may still be some lateral movement driven by the flow of material in the mantle, rifting apart some areas and compressing others into high plateaus.

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.

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 appear under such conditions, though; the action of mantle plumes could pull water into the mantle and cause the formation of andesitic lava that would rise to the surface. Our cratons on Earth may have first formed 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.

Formation of coronae (and through other plume-related processes) can cause some surface material to be pulled down into the mantle, allowing for a carbon-silicate cycle—though not in Venus’s case, given the lack of water. Long-term stability may still be an issue, though; Some models 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 would be disastrous for climate stability and life. But presuming Earth must have experienced a stage of drip-and-plume tectonics in transitioning between heat pipe and plate tectonics modes, 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 habitable for up to 5 billion years; shorter than Earth could last (with a slower-evolving sun), but still substantial. It could even 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 evidence for ongoing volcanism on Venus is cause for optimism.

Upwelling and Delamination

If a planet continues in this mode without transitioning to mobile-lid tectonics, it will eventually come to resemble Mars; largely similar to drip-and-plume in terms of the processes involved, but generally slower and less mobile. A mantle plume may rise in one region of the planet and then continue a slow rate of volcanism there for billions of years, as is the case with Mars’s Tharsis bulge. This plume can also produce huge deformation features like the Valles Marineris.

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

Most of Mars’s current large-scale features are billions of years old, and most of the surface is now dominated with impact craters. Some occasional volcanism can occur, but even on a larger planet it will be separated by long dormant periods that prevent a stable carbon-silicate cycle. Thus, complex surface life is unlikely to survive.

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 (exactly when is hard to say; some volcanic features on the moon may be under 100 million years old but it’s still under debate). There can still be some tectonic motion: As a planet cools, it will shrink slightly, and so the surface is compressed. This may be responsible 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

But such features will become increasingly drowned out by impact craters. Even the erosion that might obscure these features will decrease: carbon-silicate cycling is impossible, so sooner or later all surface water will either freeze or escape to space. 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.

Mobile-Lid

Speculative breakdown of major tectonic types. Lenardic 2018

Plate tectonics is a transitional stage in the evolution of a planet. Somewhere between an Io-like heat pipe stage and a Moon-like dead stagnant stage, the interior isn’t producing enough heat to constantly punch plumes through the crust, but it is still producing enough heat to drive large convection cells in the mantle. Many planets will probably enter a drip-and-plume regime like Venus, but under the right conditions—crust composition, internal heat gradient, presence of water—the occasional extension and compression of the crust transitions into the motion of whole plates.

But exactly how abrupt is this transition? What causes it? And what does the world look like as it’s happening, and directly after?

There are currently 3 dominant models for transition to plate tectonics from a drip-and-plume regime. All concern the beginnings of subduction, which should then lead to rifting, plate motion, and the formation of continental crust:

• A section of the crust becomes denser as it ages, and it sinks into the mantle; similar to the “drip” part of drip-and-plume tectonics, e.g. at the edges of coronae. The surrounding crust is pulled in ever tighter around this descending section, until one side tears and so the crust can sink further and drag down the still-attached crust on the other side.
•  A plume of magma erupts through the surface and lava spreads over a wide area. The surrounding crust is pushed down, and eventually some of it peels off from the light, young crust above and sinks into the mantle, pulling nearby crust with it.
• Large impact events in the past could have both fractured the crust and thinned regions of it to create the density contrast necessary for subduction.

Lucy Reading-Ikkanda, Quanta Magazine

Exactly when any of this may have happened is not clear, with estimates ranging from 800 million to over 4 billion years ago—though most sources seem to agree on 2.5-3-2 billion years as the most likely period; about 2.5 billion years after Earth formed. But it may not even have been one event; there could be multiple stages of mobile-lid regimes as the planet cools and the properties of the interior change.

Episodic Subduction

Even the earliest stage of mobile-lid tectonics may not have started in one event. Once subduction starts, it has to keep pulling an entire plate along with it to continue. A hotter young mantle could mean a weaker lithosphere, meaning subduction could only continue for so long before the descending slab of crust tore away and the subduction stopped. Thus, early subduction would have occurred in intense bursts, beginning near 3 billion years ago but with gaps of hundreds of millions of years (very brief events—under 10 million years—may have occurred even earlier due to impacts).

The weaker lower crust may also have limited mountain plateaus formed by orogenies to under 2,000 meters elevation (on average; there could still be higher individual peaks, though lower than today).

Mountain growth rates and eventual limits at different points in Earth's past. Rey and Coltice 2008

So the picture we have of an early mobile-lid Earth (or another planet in a similar tectonic stage) is a world with a mix of Venus-like and modern Earth-like tectonic features: No permanent plate boundaries, and topography dominated by volcanic highlands and shallow slopes, but with some true continental crust (including the early cores of the cratons). During episodes of subduction some familiar features like volcanic island arcs or coastal ranges would have formed, though they would start at lower elevation than on modern Earth and have long periods to erode down between episodes.

Sluggish-Lid

As an alternative to episodes of rapid plate motion (or perhaps as another stage in its evolution) early Earth may have had a gradual transition to mobile-lid tectonics. In modern plate tectonics it is the low buoyancy of subducting slabs that directs plate motion and drags the mantle along with it, but in the past the mantle may have dragged the plates along instead (much as mantle convection may do so locally on Venus).

Profiles through the crust in different modes; black lines show movement in different layers over equal time, color is temperature (red=hot). Lenardic 2018

In this sluggish-lid regime plates would have started moving far slower than today and with less distinct boundaries and only localized subduction. Compared to today, there would have been fewer volcanic arcs, and mantle plumes would still be responsible for much of the topography, but formation, collision, and compression of continents would still have been possible—if slow. As the mantle cooled, plate speed increased and subduction became more common.

But in some cases a sluggish-lid world may have no subduction at all; a ridge-only mode may develop where divergent plate boundaries form rifts and mid-ocean ridges as today, but at convergent boundaries the crust simply compresses, forming a broad mountain plateau. It would also extend deep into mantle, so some material can fall off the bottom and cycle back into the mantle; so far as I’m aware, whether or not this is sufficient for carbon-silicate cycling hasn’t been modelled, but mantle plumes driving drip-and-plume processes may still appear in this mode.

Shallow Subduction

Even once plate tectonics was well underway and uninterrupted (somewhere after 2 billion years ago)—or at any rate during periods of plate motion in an episodic regime—it did not necessarily resemble that of modern Earth. Higher mantle temperatures and more buoyant crust may have made steep subduction, with the crust descending almost straight down into the mantle, more difficult—so the world would have been dominated by flat-slab subduction, making Laramide-type orogenies extending far into the interior of continents the norm.

On the other hand, subducting slabs would still tear easier, so long continent-continent collisions and the resulting Himalayan-type orogenies may have been rarer. Thus: thin interior mountain ranges and thick coastal mountain ranges.

Exactly when subduction ceased for the last time (if it ever did) is not clear, but after around 1 billion years ago the mantle had cooled enough for steep subduction to become dominant and Earth entered the regime of modern plate tectonics.

 

Distributed Deformation

On the 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 at least some modelling has suggested that this may not always be the case. Different internal properties could still lead to widespread motion of the crust, but rather than distinct plates there may by large regions of deformation with many small sections of crust, and few undisturbed interiors.

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

Exactly what kind of geography this would create hasn’t been modelled in detail. My best guess is either large numbers of small plates between all the large ones (so, many parallel ridges, volcanic arcs, or rifts at boundaries), or perhaps there would still be identifiable large plates but they would be surrounded by broad belts of “plastic” crust that would compress or extend large distances as plate motion shifted (so, broad irregular plateaus or scattered volcanoes at boundaries).

After Plate Tectonics

Given how many times the nature of tectonics has changed in Earth’s past, could it happen again? Well, plate tectonics appears to depend on water for lubrication, so we can expect that the loss of Earth’s oceans in about 1 billion years due to the brightening sun will bring it to the end and return the planet to a drip-and-plume regime.

But suppose that didn’t happen, or suppose that an Earthlike exoplanet with plate tectonics remained in the habitable zone far longer than Earth will. What then?

One persistent trend over Earth’s history in various mobile-lid regimes is the growth of the continents, given that they can’t subduct—though they can compress lose some material off their bases into the mantle. Whether or not the continents are still growing or will continue to do so is debatable, but even if they don’t we can wonder what might happen if continent growth continued for longer on another planet, or one with far faster continental growth rates.

Of course, subduction is necessary for plate tectonics, and continental drift requires some space between continents for them to move around. So if the continents keep growing, would they eventually “jam up” and stop further plate motion?

Perhaps, but other factors may come into play first: Continental crust is a better insulator than ocean crust, and so greater continent area can heat the upper mantle even as the planet as a whole cools down. At first this might enhance convection and speed up plate motion (this, combined with smaller oceans, could lead to a shorter supercontinent cycle) but once the continents surpass 50% of the planet’s surface area the heat will weaken the crust and could return it to a sluggish, episodic, or stagnant-lid regime.

We don't really have any good model for what a cold formerly mobile-lid world would look like. Venus may once have had plate tectonics—we don't really know one way or the other—but it transitioned because of changing surface conditions, not declining internal heat, so it has been completely resurface since then. Mars might actually be a better model, in a broad sense: the continents will remain as heavily deformed highlands, while the younger ocean floors will be flat lowlands, and there may be some uplifted bulges from the latest stages of stagnant-lid volcanism. There shouldn't be as many craters, though: Earth is unlikely to experience as many large impacts in the future as Mars has in the solar system's tumultuous past.

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

I think I can confidently say 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?

 

For one thing, the geography that plate tectonics produces could be more hospitable to complex life than stagnant-lid or slower mobile-lid alternatives. Broad, shallow continental shelves are ideal for the development of marine life, and rapid uplift of mountains creates a good source of nutrients for these seas. Once complex life develops, breakup and assembly of continents and the relatively rapid shifting of geographic barriers in may encourage more diversification, 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.

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

But perhaps the most compelling case for the link between plate tectonics and complex life is plate tectonics’ possible role in the buildup of atmospheric oxygen. Oxygen-producing life first appears in the fossil record over 200 million years before oxygen begins building up in the atmosphere. The period when oxygen does begin to accumulate is also one of the likely periods of rapid tectonic motion identified in episodic mobile-lid models of Earth’s development. And then the second major rise in oxygen levels, just before the appearance of complex life, comes near to the likely beginning of the modern regime of steep-subduction plate tectonics.

Why might this be? For one, the andesitic 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 oxygen from the air to make metal oxides. Uplift of mountains and resulting rapid erosion could also help increase the rate at which these oxidizing ions are buried in sediment. And long-lived continental crust could also store deposits of organic carbon 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.

Another interesting role for plate tectonics may be its relationship to Snowball Earth events, where all or most of the planet is covered in glaciers. Several have occurred throughout Earth’s history, but the most recent—around 800-600 million years ago—again coincides with the likely period of transition between earlier tectonic modes and modern steep-subduction plate tectonics. This transition may have caused wild shifts in climate that caused the snowball event, and then this in turn gave 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).

And then once fully underway, plate tectonics tends to provide a steady, high supply of CO₂ for the carbon-silicate cycle, which may have helped to prevent further snowball events after the last one. Contrast this with the irregular rates of volcanism in a regime where mantle plumes are more dominant; indeed, the rare cases where large mantle plumes have appeared since plate tectonics began often seem to be associated with dramatic shifts in the climate and mass extinctions; a few such events might help “clear the table” and encourage the evolution of novel forms, but too many will discourage the development of complex ecosystems with specialized species.

But at the end of the day we have to remember that there may not be strict divisions between tectonic modes. Even on the modern Earth dominated by plate tectonics mantle plumes continue to appear and deformation of continent interiors can happen by sluggish-lid-like processes, especially during supercontinent breakup. Earlier in Earth’s history (and perhaps again in the future) episodic or sluggish-lid regimes may have included a mix of stagnant-lid and mobile-lid features. And on the flip side, stagnant-lid Venus has elements of sluggish-lid motion.

So, excepting perhaps the latest stages of cold stagnant-lid, it’s hard to say with confidence that any tectonic mode is totally incompatible with complex life.

But 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 mean faster production of andesitic crust. All this to say that modern Earth with life could have larger continents than an Earth without life would have.

Exotic Tectonic Regimes 

That about does it for the tectonics of Earthlike planets with continents and liquid water oceans, but there are couple of tectonic modes that might operate on more exotic planets that are worth mentioning—either as settings for exotic life or space travelers, or just as points of interest:

Ice Tectonics

First off, I’ve mentioned a couple times that something resembling plate tectonics could occur on cold bodies with thin icy crusts underlain by water oceans. But ice does not get less buoyant as it cools like basaltic rock does, so most models suggest something more similar to ridge-only sluggish-lid tectonics, what we might call ice-floe tectonics: convection in the underlying water pushes two sections of ice together, the ice along the boundary fragments, and the fragments pile onto a ridge above the boundary and a keel below it.

Stern et al. 2018

But other models suggest that variations in salt content between plates could allow a closer resemblance to plate tectonics: sinking slabs connected to the subducting plates and uplifted ridges and cryovolcanoes on the overlaying plate. But there probably wouldn’t be any close analogue to our continental crust/ocean crust dichotomy, and so no continents.

Kattenhorn 2018

Hemispheric Tectonics

Exactly what effect tidal-locking might have on tectonics hasn’t been modelled in detail. One suggestion is that large, long-lived mantle plumes might tend to be drawn towards the substellar point, and as a consequence there will be more LIPs and hotspots there but continents will tend to be driven towards the twilight zone, though this isn’t likely to be more than a weak trend.

But for close-orbiting, tidal-locked worlds with significant heating by the sun—hot enough to lack atmospheres, but not so hot as to melt—the extreme temperature difference between the day and night sides may affect tectonic motion on planets with warm interiors. At 800 K temperature difference, the day side is expected to exhibit distributed deformation mobile-lid tectonics with widespread volcanism, transitioning to something similar to plate tectonics on the night side with plates trending towards subduction zones near the point furthest from sunlight. This could eventually lead to buildup of thick crust on the nightside with subduction zones around its perimeter.

Thus: a high compacted plateau on the nightside and volcanic lowlands on the dayside.

Stevenson 2019

In Summary

• Large planets evolve through multiple tectonic regimes, starting and ending as stagnant-lid planets and possible transitioning through mobile-lid regimes.
• Stagnant-lid worlds are dominated by volcanic features, which become more concentrated and less active as they age and cool.
• Stagnant-lid worlds can still have lateral motion of the crust and non-volcanic uplift.
• Mobile-lid worlds may pass through episodic or sluggish-lid regimes before plate tectonics, with a mix of stagnant-lid and mobile-lid features.
• Subduction on young mobile-lid worlds will produce broader, lower mountain ranges.
• Plate tectonics is unlikely to operate when oceans cover less than half a planet’s surface.
• Stagnant-lid worlds can support life, but mobile-lid worlds are more hospitable to complex life.
• Icy crusts can have mobile-lid tectonics, but with no distinct continents and little uplift.

There’s little more to say about the Teacup system for now: Teacup Ae has Earthlike plate tectonics, and aside from perhaps one or two icy moons every other solid body in the system is in some type of stagnant-lid regime—heat pipe for Ah III drip-and-plume for Ad and Af V.

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.

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Part VIa