An Apple Pie From Scratch, Part Va: Tectonics: Construcing a Plate Tectonic History
USGS |
But in reality, a better metaphor for the evolution of a planet’s interior might be a stage play: There’s a lot going on behind the scenes, and for a complex production the stagehands may outnumber the actors. But what happens on stage drives everything else, and a perceptive audience member can infer the nature of the whole beast from what they are shown.
Crustal tectonics is not a sideshow. It is an engine converting heat from the interior into motion, and what happens on a planet’s surface can affect the behavior of the interior as deep as the core. As a planet ages and cools, this engine operates in different modes which can have a profound effect both on the landforms and conditions on the surface and on the hospitability of the planet to complex life.
- Earth Today
- Patterns of Plate Motion
- Convergent-Continent-Divergent
- Convergent-Continent-Convergent
- Convergent-Divergent-Convergent
- Convergent-Divergent-Continent
- Simulating Plate Tectonics
- In Summary
- Notes
Earth Today
To start off, a quick tour: On average, the Earth’s radius is 6370 kilometers, but only the top 35 km is the crust (varying locally between 5 km at young sections of ocean and 70 km at old sections of continent). Underneath, the mantle stretches for over 2800 km to the boundary with the core.Kelvinsong, Wikimedia |
Below that, the core is composed largely of a 9:1 mix of iron and nickel, with around 3% lighter elements (mostly oxygen and sulfur). The outer core, stretching to 5150 km depth, is liquid in spite of the high pressure thanks to a temperature of 4000 to 5400 K (hotter with depth). Though the inner core is even hotter—as high as 6000 K—it is under even higher pressure, around 3,500,000 atm, such that it is forced back into a solid.
As mentioned in the last post, most new heat is generated in the mantle and crust by decaying radioactive isotopes, but the core retains a good deal of primordial heat and is the best-insulated part of the planet, so has the highest temperature. As it cools, iron from the liquid outer core crystallizes onto the surface of the inner core. The liquid just outside the inner core thus becomes iron-depleted and so lighter and more buoyant than the overlaying liquid. It rises to the core-mantle boundary and there loses its heat to the cooler mantle, gains back its iron by mixing with surrounding liquid, and so becomes less buoyant and sinks back down towards the inner core, thus forming a convection cell. The liquid metal is conductive, and so this motion—along with the Earth’s rotation—forms the planet’s magnetic field.
"Advection" is transfer of heat by magma reaching the surface. Bkilli 1, Wikimedia. |
As this fresh rock is pushed further from the ridge, it cools and becomes denser, causing it to “float” lower on the underlying mantle, like a ship laden with cargo. This sinking helps to pull more material away from the ridge, and it’ll all move together as one tectonic plate. Eventually the rock will encounter another tectonic plate, and if that other plate is younger and less dense, then the rock will be pushed below it, forming a subduction zone. The rock sinks into the mantle, adding more pull to keep the rest of the plate behind it moving. It sinks lower, breaking up and mixing into the mantle as it goes, and eventually reaches the core-mantle boundary, closing the loop of mantle convection.
Surachit, Wikimedia/USGS |
I’ve been emphasizing the pushing force from the mantle plume here to
keep a narrative thread, but in reality the relative importance of “ridge-push”
from mid-ocean ridges and “slab-pull” from subduction zones is an ongoing
debate, and the current consensus leans towards the latter being more
important. I’ll explain more in a moment, but the key point is that subduction
causes ridge spreading, not the latter, and the mantle convection cell exists
only so long as surface conditions allow it and the plate keeps moving.
What might stop a tectonic plate moving? Well, if the original seam of lava bursting through the crust is underwater—as is usually the case—then the rock forming at the mid-ocean ridge will have some water mixed in with it. As it travels from mid-ocean ridge to subduction zone, it will also tend to accrue layers of sediment from material settling out of the water. These sediments will trap more water, and some—like carbonate minerals—will themselves be formed of other volatiles. As the plate subducts and warms up in the mantle, these volatiles will melt out of the rock and rise upwards, puncturing the overlaying plate and bursting through the surface as volcanoes. The volatiles themselves will mostly be released as gasses, but as they melt and rise, they bring some of the surrounding silicate rock with them. Heavy elements like iron will tend to sink out of the mix as it rises, and so once this magma reaches the surface, it will form iron-depleted andesitic rock that is less dense than the basaltic rock formed at mid-ocean ridges. Islands of this material will pile up on the overlaying plate along the subduction zone, forming an island arc.
Given enough time, the individual volcanoes can produce enough rock to form a plate of their own; a section of continental crust, as opposed to the oceanic crust that forms at mid-ocean ridges. This lighter and thicker crust floats higher on the mantle, causing much of its surface to rise above the oceans.
What might stop a tectonic plate moving? Well, if the original seam of lava bursting through the crust is underwater—as is usually the case—then the rock forming at the mid-ocean ridge will have some water mixed in with it. As it travels from mid-ocean ridge to subduction zone, it will also tend to accrue layers of sediment from material settling out of the water. These sediments will trap more water, and some—like carbonate minerals—will themselves be formed of other volatiles. As the plate subducts and warms up in the mantle, these volatiles will melt out of the rock and rise upwards, puncturing the overlaying plate and bursting through the surface as volcanoes. The volatiles themselves will mostly be released as gasses, but as they melt and rise, they bring some of the surrounding silicate rock with them. Heavy elements like iron will tend to sink out of the mix as it rises, and so once this magma reaches the surface, it will form iron-depleted andesitic rock that is less dense than the basaltic rock formed at mid-ocean ridges. Islands of this material will pile up on the overlaying plate along the subduction zone, forming an island arc.
Given enough time, the individual volcanoes can produce enough rock to form a plate of their own; a section of continental crust, as opposed to the oceanic crust that forms at mid-ocean ridges. This lighter and thicker crust floats higher on the mantle, causing much of its surface to rise above the oceans.
Note that continental crust extends further down into the mantle as well. USGS |
Once formed, continents grow by various mountain-forming events called orogenies.
When a continent encounters plates of oceanic crust, the oceanic crust
always subducts under the continental crust. As it does, more subduction
volcanism causes more rock to build up on the continent along the plate
boundary. If the oceanic plate already carries some volcanic islands formed of
andesitic rock on it, these will collide with the continent and fuse with it,
forcing some of the rock to fold up and form high mountain ranges.
Eventually subduction can pull two large continents together. They can continue to be pushed together for some time, forming a large plateau of folded rock, and one plate may even partially subduct under the other. But continental crust is too light to sink into the mantle, and so before long the plates will stop moving and fuse together. The thickened section of crust will be eroded from above and below, somewhat reducing the total amount of andesitic rock (some rock is also pulled from the overlaying plate at subduction zones), but this is a far slower loss process than subduction and cannot destroy a continent completely, so continents are largely permanent once formed. Sections of the continents are billions of years old, while the oldest oceanic crust—in the eastern Mediterranean—is no more than 280 million years old, and most of the oceans are far younger.
Eventually subduction can pull two large continents together. They can continue to be pushed together for some time, forming a large plateau of folded rock, and one plate may even partially subduct under the other. But continental crust is too light to sink into the mantle, and so before long the plates will stop moving and fuse together. The thickened section of crust will be eroded from above and below, somewhat reducing the total amount of andesitic rock (some rock is also pulled from the overlaying plate at subduction zones), but this is a far slower loss process than subduction and cannot destroy a continent completely, so continents are largely permanent once formed. Sections of the continents are billions of years old, while the oldest oceanic crust—in the eastern Mediterranean—is no more than 280 million years old, and most of the oceans are far younger.
NOAA |
Thus, our modern planet is a patchwork of old continents and young oceans. Most of the tectonic plates contain sections of both types of crust, but the sections of oceanic crust are ephemeral attachments to the more persistent continental cores.
domdomegg, Wikimedia |
Divergent boundaries, where plates are spreading apart; mostly the mid-ocean ridges, such as in the Atlantic. But mantle plumes can also burst through continental crust, forcing the continent to spread apart and forming a rift valley, with volcanic ridgelines to either side. Before long, the continent is split apart, and a new ocean basin forms with a mid-ocean ridge. East Africa is undergoing rifting of this sort, though it’s unclear if this will ultimately split the continent apart or stop before that happens.
Red dots indicate earthquake epicenters. Note that in the continent-continent case (right) the continental crust will not completely subduct |
Transverse boundaries, where plates are moving in different directions past each
other—or in the same direction but at different rates. Friction between these plates can cause earthquakes and land
deformation, such as near the San Andreas fault in western North America, but
no crust is being formed or subducted, and there is no significant volcanism.
In an idealized case such as the one I first described, a plate has a divergent boundary on one side and convergent on the other, such as is the case for the Nazca plate. But this need not always be a case. The African plate is surrounding mostly by divergent boundaries, meaning it is growing on all sides, while the Filipino plate is surrounded mostly by convergent subduction zones and will be completely consumed before long. The total area of the plates must, of course, remain constant, but the different motions of all the plates can allow area to effectively be traded between plates that are not in contact. Even where 2 plates are in contact, their complex shape or rotation of the plates can cause different types of boundaries to exist along the same border.
In an idealized case such as the one I first described, a plate has a divergent boundary on one side and convergent on the other, such as is the case for the Nazca plate. But this need not always be a case. The African plate is surrounding mostly by divergent boundaries, meaning it is growing on all sides, while the Filipino plate is surrounded mostly by convergent subduction zones and will be completely consumed before long. The total area of the plates must, of course, remain constant, but the different motions of all the plates can allow area to effectively be traded between plates that are not in contact. Even where 2 plates are in contact, their complex shape or rotation of the plates can cause different types of boundaries to exist along the same border.
Major plates and the relative motion at their boundaries. USGS |
No one plate can get too large, though; the older the oceans, the
greater the chance they’ll subduct, and the larger the continent, the greater
the chance that it will rift apart.
Patterns of Plate Motion
The exact origins of plate tectonics are a bit murky, and a subject I’ll dig into more in the next post, but most researchers agree it has been ongoing in some form for at least 2 billion years. However it starts and whatever drives it, most of what we know about how plate tectonics works comes from observing Earth’s history, as interpreted through various surviving geological clues—common rock layers and fossil species across now-distant regions, records of shifts in the magnetic field and the orientation of continents relative to it, the motion of plates across stationary hotspots, and so on. Altogether, this gives us a near-complete record of how plates have moved and changed over the last 250 million years, since the supercontinent Pangea began to rift apart, and a fairly solid picture of motions going back 600 million years to the breakup of the supercontinent Pannotia. Past that there are multiple plausible models for the appearance and breakup of the supercontinent Rodinia about 1 billion years ago, decent indications for the assembly of supercontinent Nuna in some form 1.8 billion years ago, and a few vague hints at another assemblage around 2.5 billion years ago—perhaps another supercontinent we’d call Kenorland, perhaps multiple large continents in sequence.
Looking through all this data, there are a couple patterns
that emerge. The most obvious is the supercontinent cycle. Over long
periods the world’s continents have joined into a single landmass, rifted
apart, and then joined together again several times, completing a cycle roughly
every 750 million years. Of course we only have detailed records of a few of these
cycles, and none are perfect: Rodinia started rifting before it was fully
assembled, Pannotia appears oddly out of sequence and never included all major
landmasses so you might just call it a stage in Rodinia’s breakup, and Pangea
was spalling off microcontinents all throughout its tenure. But there is a
definite trend, and it provides a convenient framework for constructing our own
continents: Every continental world must be in some stage of the breakup,
assembly, or tenure of a supercontinent.
The exact mechanisms of the cycle are debated—particularly the role of the mantle—but we can at least say that the mantle helps in the rifting process. During its tenure, a supercontinent forms a large insulating layer above the mantle, and the trapped heat causes plumes of magma to form that eventually burst through the surface in the form of hotspots—isolated volcanoes—and flood volcanism—vast regions of volcanic activity and lava flows (it’s also possible the plumes have less to do with insulation and more with how patterns of subduction affect mantle convection, but at any rate these plumes consistently form under supercontinents). These plumes do not themselves cause rifting, but they form weak points in the crust which allows for subductions zones outside the continent to pull it apart. They also push the continental crust up slightly, creating a slight slope between the supercontinent core and surrounding subduction zones that the continents can slide down.
The rifting begins at triple junction, where rifting spreads out from weak points in 3 different directions. Eventually the spreading rifts from nearby triple junctions join together into a single zig-zag shaped rift, which you can still see in the overall shape of the modern Atlantic coasts. The third rifts from the triple junctions that don’t join with others become failed rifts and cease spreading, though they remain as weak points in the crust and can be reactivated later.
The exact mechanisms of the cycle are debated—particularly the role of the mantle—but we can at least say that the mantle helps in the rifting process. During its tenure, a supercontinent forms a large insulating layer above the mantle, and the trapped heat causes plumes of magma to form that eventually burst through the surface in the form of hotspots—isolated volcanoes—and flood volcanism—vast regions of volcanic activity and lava flows (it’s also possible the plumes have less to do with insulation and more with how patterns of subduction affect mantle convection, but at any rate these plumes consistently form under supercontinents). These plumes do not themselves cause rifting, but they form weak points in the crust which allows for subductions zones outside the continent to pull it apart. They also push the continental crust up slightly, creating a slight slope between the supercontinent core and surrounding subduction zones that the continents can slide down.
Source |
The rifting begins at triple junction, where rifting spreads out from weak points in 3 different directions. Eventually the spreading rifts from nearby triple junctions join together into a single zig-zag shaped rift, which you can still see in the overall shape of the modern Atlantic coasts. The third rifts from the triple junctions that don’t join with others become failed rifts and cease spreading, though they remain as weak points in the crust and can be reactivated later.
Rifts formed in the Pangea breakup. Note how failed rifts often become the cores of river valleys later. Source. |
The rift zones will appear first as volcanically active rift valleys, but eventually split open into new ocean basins with mid-ocean ridges down their middles. Rifting often isn’t clean or simultaneous; Africa split from North America long before South America, and North America and Eurasia are still partially connected today over the Bering Strait.
USGS |
Once the continents have rifted apart, they can reassemble into a new supercontinent in two primary ways. The simpler method is extroversion, where the new interior ocean basin continues to spread, the exterior ocean basin is fully subducted, and the continents collide more-or-less on the opposite side of the world from where they originally rifted apart, such that the exterior coasts of the old supercontinent become the interior of the new supercontinent. The alternative is introversion, where the interior ocean grows for a while, but new subduction zones form that cause the continents to stop and reverse course, closing the new basin and colliding along the same coasts that had previously rifted apart (though not necessarily with the exact same geometry). We’re not entirely sure what causes one or the other to happen, and often a particular supercontinent cycle can contain versions of both in separate ocean basins.
Earth in cross-section through different types of supercontinent cycles. Bradley 2011 |
Part of the mystery is uncertainty about how new subduction
zones form. They’re unlikely to form directly between the interior ocean basin
and attached continental coasts; though oceanic crust sinks as it ages, it also
becomes stiffer and harder to break. More likely is that subduction begins
along transverse boundaries,
where the crust is already broken and two sections of oceanic crust of very
different ages can come into direct contact. The geometry of linear spreading
on a spherical surface requires that these boundaries must form somewhere
within the new ocean, and once subduction starts there it can spread outwards
along the continental coastlines and consume the entire interior ocean. Why
these new subduction zones should come to dominate over the older subduction
zones in the exterior ocean is an unresolved issue.
As the supercontinent cycle is ongoing, there will be many local cases of rifting, collision, subduction, ridge spreading, basins opening and closing, and so on. I want to go through a few common geometries to clarify how they play out, but the essential rule throughout is that subduction is active and rifting and ridge spreading is passive. Subduction zones dictate the movement of plates, though the relative importance of different subduction zones seems to depend on mantle processes that aren’t fully understood. The best interpretation right now is that mantle convection forces the supercontinent cycle to occur, but subduction determines all the details.
As the supercontinent cycle is ongoing, there will be many local cases of rifting, collision, subduction, ridge spreading, basins opening and closing, and so on. I want to go through a few common geometries to clarify how they play out, but the essential rule throughout is that subduction is active and rifting and ridge spreading is passive. Subduction zones dictate the movement of plates, though the relative importance of different subduction zones seems to depend on mantle processes that aren’t fully understood. The best interpretation right now is that mantle convection forces the supercontinent cycle to occur, but subduction determines all the details.
Convergent-Continent-Divergent
This is the typical
state of continents during an introversion cycle, with South America serving as
a good example. The coast facing the direction of motion forms an active
margin, with a subduction zone just offshore and a volcanic orogeny near
the shore, forming a steep coastline. The leeward coast forms a passive
margin, with well-eroded topography, vast coastal plains, and wide
continental shelves.
The orogeny by the active margin is usually relatively thin, with a sharp transition to the interior lowlands, but in some cases a continent may experience flat-slab subduction, where a subducting plate doesn’t immediately dive into the mantle but slides underneath it for some distance before finally turning downwards.
The orogeny by the active margin is usually relatively thin, with a sharp transition to the interior lowlands, but in some cases a continent may experience flat-slab subduction, where a subducting plate doesn’t immediately dive into the mantle but slides underneath it for some distance before finally turning downwards.
EOS.org, modified from Finzel et al. 2016. |
This causes uplift and deformation far into the interior of the continent, and either an end or motion inland of volcanism. This is how the Rockies in North America formed, though it's largely ceased now. It’s not totally clear why this happens, but it may be due to high speed of the continent relative to the subducted plate or subduction of a mid-ocean ridge.
Convergent-Continent-Convergent
This is the typical
state of continents in the later stages of an extroversion cycle. There are no
good current examples of this, but the eastern coast of Asia shows some of the
features. Though both sides are active margins, there is still a clear
direction of motion for the plate and different features on either side. The leading
edge is much like the active margin in the previous example: an offshore
subduction zone and coastal orogeny.
The trailing edge has a subduction zone and orogeny, but because of the continent’s motion the neighboring ocean is spreading faster than the subduction zone is consuming it. Thus the age of the subducting crust is increasing, meaning that it is becoming less buoyant. This leads to slab rollback; the ocean crust sinks further from the subduction zone, which has the effect of pulling the subduction zone away from the continent.
The trailing edge has a subduction zone and orogeny, but because of the continent’s motion the neighboring ocean is spreading faster than the subduction zone is consuming it. Thus the age of the subducting crust is increasing, meaning that it is becoming less buoyant. This leads to slab rollback; the ocean crust sinks further from the subduction zone, which has the effect of pulling the subduction zone away from the continent.
Niu 2014 |
The orogeny is pulled out with it, though because subduction is slower and the land is being stretched out it won’t form as high a mountain range as the leading edge, and can often be broken apart into an island arc. Between the volcanic arc and the continent interior the crust is thinned out, forming a back-arc basin. Fractures in the crust turn into hotspots, forming more volcanic islands. New oceanic crust can even form in the basin, but there is no mid-ocean ridge and the basin can never open up into a new ocean. The Sea of Japan is a typical back-arc basin. (Back-arc basins can form along the leading edge as well, but for different reasons and without significant thinning of the crust.)
Once the continent’s motion stops (e.g. if it collides with another continent) the trailing edge will be compressed and transition into a more typical active margin.
To be clear, the neighboring plates could be continents as well. |
Convergent-Divergent-Convergent
This is the typical state of the exterior ocean during supercontinent tenure and breakup, with the Pacific and Nazca plates together being a good example. Such an ocean can remain stable for a long time so long as the subduction rate is symmetrical on both sides. But this is unlikely to be true forever, and so eventually the ridge will be subducted, leading to closure of the ocean.
In some cases there may not be a single ridge down the middle of an ocean, but 3 ridges meeting at a triple junction. Sometimes—not often, but sometimes—a new plate is formed in the middle of the triple junction, and then a triangle of ridges spreads outwards, growing the new plate at the expense of the old ones. Because this tends to push the ridges towards the edges of the plate, this can hasten the closure of the ocean. The Pacific plate formed this way, though it’s not yet clear if this will lead to its closure.
As the oceans close, island arcs formed over subductions zones will collide with the advancing continents, forming collisional orogenies along the coast—similar to volcanic orogenies in many ways, but tending to form higher and wider mountain ranges (though still not as wide as flat-slab subduction) with more internal deformation. The most recent example of this is along the coasts of Alaska and British Columbia, though this orogeny is largely concluded by now.
This is the typical state of the exterior ocean during supercontinent tenure and breakup, with the Pacific and Nazca plates together being a good example. Such an ocean can remain stable for a long time so long as the subduction rate is symmetrical on both sides. But this is unlikely to be true forever, and so eventually the ridge will be subducted, leading to closure of the ocean.
In some cases there may not be a single ridge down the middle of an ocean, but 3 ridges meeting at a triple junction. Sometimes—not often, but sometimes—a new plate is formed in the middle of the triple junction, and then a triangle of ridges spreads outwards, growing the new plate at the expense of the old ones. Because this tends to push the ridges towards the edges of the plate, this can hasten the closure of the ocean. The Pacific plate formed this way, though it’s not yet clear if this will lead to its closure.
As the oceans close, island arcs formed over subductions zones will collide with the advancing continents, forming collisional orogenies along the coast—similar to volcanic orogenies in many ways, but tending to form higher and wider mountain ranges (though still not as wide as flat-slab subduction) with more internal deformation. The most recent example of this is along the coasts of Alaska and British Columbia, though this orogeny is largely concluded by now.
When the oceans finally close, the subduction zones will not
close all at once, but may remain in fragments along complex coastlines. The continents
may slow in some areas or even stop while trapped sections of ocean remain. The
surviving sections can undergo slab rollback, closing some sections of ocean
and widening others, leaving complex patterns of land and sea. The
Mediterranean was formed in this fashion.
Where the oceans do completely close, the continents form
one last collisional orogeny in between them. This has started in some sections of the Mediterranean (in the Alps and the Caucasus) and the Urals are a good example of the eventual result. These collision zones remain weak points in the crust, and can rift apart again later.
Convergent-Divergent-Continent
This is the typical state of an interior
ocean in the later stages of an extroversion cycle. The former Tethys ocean was
an ideal example, with the current collision between India, Arabia, and Africa
into Eurasia as the final result. Because ridge spreading is symmetrical and
the crust is only being subducted on one side of the ocean, the ridge must move
towards the subduction zone and eventually be subducted by it. This can lead to
closure of the ocean, much as in the previous case, but in some cases the
continents on either side of the subduction zone may be connected elsewhere,
such that they can’t move relative to each other.
The subduction zone keeps pulling on the subducting plate, and so it has
to break somewhere, and though it is thicker continental crust tends to break
easier than oceanic crust. If the continent across the ocean has no internal
rifts, then a section of that continent on the coast facing the subduction zone
will be torn off, forming a microcontinent (or multiple along the coast)
with a new mid-ocean ridge behind it.
The microcontinent will cross the ocean and collide with the continent or
island arc on the far side of the subduction zone, forming a collision orogeny,
but at that point the subduction zone can “jump” across the microcontinent—that
is, a new subduction zone will form on the trailing side of the microcontinent
after or even before the collision. That puts us right back where we started,
with a subduction zone, ridge, and continent passive margin.
Eventually the continent—or a large section of it—must close the ocean
basin and collide with the continent overlaying the subduction zone. Because
the continent is part of the subducting plate, it will collide with the
overlaying plate at high speed, and because the margin of the overlaying plate
is composed of many pieces of former microcontinents, it is fairly weak. Thus
the collision will produce a wide, high, intensely folded collision orogeny,
such as the Himalayan Plateau we have today. Only such a large
continent-continent collision can permanently stop a subduction zone.
Plate speed depends on a couple different factors. Subducting plates move fastest
due to slab pull, and subduction even helps pull overlaying plates through slab
suction. Ridge push has a secondary effect, which is stronger for plates
spreading away from a former subcontinent thanks to the slight slope created by
mantle plumes—but to be clear, a continent can only be moved by ridge push if
there is subduction elsewhere helping it along. Countering these forces is drag
from the mantle, which has a stronger effect on continents because they have
deeper lithospheric “roots”. As points of reference:
- India, on a mostly-oceanic subducting plate, topped out at 20 centimeters/year in the late Cretaceous, and continues to move north at around 6 cm/yr.
- The Nazca plate, a subducting oceanic plate, is moving east at 10 cm/yr, and the Pacific plate west at around 8 cm/yr.
- North and South America, large continents with large active margins, are both moving west at 3 cm/yr.
- When it initially starting rifting from Africa, North America only moved at 1 cm/yr for the first 25 million years, before abruptly accelerating to 3.5 cm/yr in a 6-million-year-window.
- Eurasia, a huge continent with a small active margin, is only moving about 1.5 cm/yr. Despite being a “convergent” boundary, the eastern coast is actually spreading at 1-2 cm/yr to accommodate the motion of the Americas (which is to say, oceanic crust is still subducting but the boundary is moving away from the mid-ocean ridge), which is causing slab rollback and back-arc extension.
- Africa and Australia, continents surrounded by divergent boundaries, both move at less than 1 cm/yr.
1 cm/yr is about 0.9° of rotation around the Earth every 10 million
years (i.e. 0.9° latitude or longitude on the equator). In other words:
Situation
|
Subducting
Ocean
|
Recent
Subduction Collision
|
Active
Margin Continent
|
Passive
Margin Continent
|
Plate
speed (°/10 myr)
|
7.2-18
|
5.4
|
2.7
|
<0.9
|
For planets of other sizes, divide these values by the radius in Earth
radii (presuming equivalent plate speed, which probably isn’t true but there’s
no way to get a better estimate right now).
There appears to have been a general increase in average plate speed
since the onset of plate tectonics, roughly doubling over the last 2 billion
years,
which may be associated with the period of the supercontinent cycle decreasing
by the same factor, but at any one time overall variation will be larger than this
general trend. Plates can accelerate very quickly when a new subduction zone is
formed or a mid-ocean ridge is subducted.
Note that the speed of a plate’s movement is not necessarily
the speed at which it’s boundaries are moving, save for in the case of a
continent’s leading-edge active margin; an ocean plate can be moving at high
speed but bounded by static convergent and divergent margins (though no
boundary is totally still today).
The area of the continents is also increasing, but only slightly. The Earth
gains an average of 0.6-0.9 cubic kilometers of continental crust per year—at an average
thickness of 35 km, that’s 0.017-0.025 km2/year, or 17-26 thousand
km2/million years, compared to a current continental crust area
(including submerged continental shelves) of ~200 million km2. That’s
around 9-15% growth over the last billion years; not a lot, but worth bearing
in mind (all of this assuming average continental crust thickness has remained
constant, which might not be the case). This may go faster on larger planets because of square-cube scaling between the area of the crust and the volume of the mantle, but it's not clear.
Finally, sea levels tend to rise and fall relative to the continents with
the supercontinent cycle. During a supercontinent’s tenure, the oceans are
mostly composed of old, low-lying oceanic crust and the continents are lifted
up by mantle plumes, combining to cause low sea levels. Once the continent
breaks up, the old oceanic crust is replaced with younger, more buoyant crust
and the continents move off the mantle plumes, so sea levels rise over the next
hundred million years or so, until the oceanic crust ages and continents begin
reassembling. Global climate and glaciations also affect sea level, but the
relationship of those to tectonic events is harder to parse; we’ll tackle that
in a later post.
Simulating Plate Tectonics
To get a map with a realistic tectonic history, there’s a few different methods; 3 that I'll share here today. I don’t suggest trying to retcon a tectonic history onto a map you’ve already made—sorry—or trying to just slap down plate boundaries at random. These boundaries are formed by the patterns of continental drift, not the reverse.
Let Someone Else Do It
Find a program to model the tectonic history for you. The best available one I know of right now—by a huge margin—is that in the alpha version of Songs of the Eons, a strategy game in development. The map generator simulates a tectonic and even glacial history for the planet, and the results are fairly impressive from a standpoint of geological detail and accuracy (though sadly it can’t quite seem to do fold-and-thrust belts properly, probably due to resolution limitations).
Find a program to model the tectonic history for you. The best available one I know of right now—by a huge margin—is that in the alpha version of Songs of the Eons, a strategy game in development. The map generator simulates a tectonic and even glacial history for the planet, and the results are fairly impressive from a standpoint of geological detail and accuracy (though sadly it can’t quite seem to do fold-and-thrust belts properly, probably due to resolution limitations).
Elevation map produced by Songs of the Eons--from their website. |
Do It Myself: Quick and Dirty
If you want more control over how the resulting continents look, then we can try to make our own tectonic history by hand. But if you don’t want to spend too long on it, then we can use supercontinent cycle as a shortcut by working forward from the last supercontinent to work out a reasonable modern distribution of continents and plate boundaries. Most people seem to want a world roughly resembling modern Earth—late in the breakup of the last supercontinent—so I’ll start with one of those.
If you want more control over how the resulting continents look, then we can try to make our own tectonic history by hand. But if you don’t want to spend too long on it, then we can use supercontinent cycle as a shortcut by working forward from the last supercontinent to work out a reasonable modern distribution of continents and plate boundaries. Most people seem to want a world roughly resembling modern Earth—late in the breakup of the last supercontinent—so I’ll start with one of those.
Basically, we just take a vaguely positioned supercontinent,
slash it down the middle with a zig-zagging rift, and move the resulting
fragments directly away.
My supercontinent. Red is the rift, arrows indicate motion of the continents. |
The arrows within the continents show the rotation of each fragment. |
We should end up with an interior ocean surrounded by
passive margin coasts—that are vaguely mirrored on opposite sides—and an
exterior ocean surrounded by active margins.
Blue lines are active margine, grey lines are passive margins. |
If we follow modern Earth’s example we might designate the
active margins on one side of the ocean as leading-edge, with smooth coastlines
and high coastal ranges, and those on the other side as trailing-edge, with
jagged coastlines and offshore island arcs. But we could also make them all
leading edge, and in either case place a couple more subduction zones between
the continents, and perhaps even breaking in between them into the interior
ocean as has happened in the Caribbean on Earth.
Blue lines are leading-edge active margins, orange lines are trailing-edge active margins, purple lines are new subduction zones. |
Finally in the continent interiors we can place high
mountain ranges on active margin coasts and continent collision boundaries and
low mountains randomly and along the interior coasts as remnants from older
collisions.
Dark brown is new high mountain ranges, beige is old lower ranges |
If we wanted a world during supercontinent assembly, we have
to decide whether to do an extroversion or introversion cycle. For
extroversion, we keep moving the continents out—rotating and crashing them into
each other as they go—until they start meeting on the opposite side of the
world—throwing in some trailing-edge active margins on the coasts of the
interior ocean as the crust there ages and more subduction zones break into it
from the sides.
Orange lines are new active margins, and brown areas new collisional mountain ranges. I haven't adjusted to coastlines to reflect this, just moved the landmasses. |
For introversion, we place subduction zones in the interior ocean, but turn them into leading-edge active margins and leave the exterior coasts as trailing-edge active margins (this needn’t happen on all sides of the ocean, just one will do). Carry the continents back towards each other, but with some rotation as they’re pulled about by the new subduction zones; the coasts shouldn’t meet back up neatly in the same places they split
Blue lines are new leading-edge active margins, orange lines are former leading-edge active margins that have become trailing-edge. Again, coastlines aren't adjusted. |
If we want a supercontinent, we just carry forward either case until the continents have all met back up and closed the oceans between them, leaving high mountain ranges along the sutures and active margins on most—or all—of the coasts. The supercontinent should still be moving somewhat relative to the oceans, so we’ll have a leading edge and trailing edge, and we can also have microcontinents moving around the edges if we don’t want all the land area in one piece.
New supercontinent, carried forward from the introversion model. Blue lines are leading-edge active margins and orange lines are trailing-edge, after the supercontinent has formed. |
Do It Myself: Obsessively Detailed
If we really want a complete, detailed, and realistic history for the whole planet, then we need to fully simulate the history back not just from the last supercontinent, but all the way back to the supercontinent before it—around 500 million to 1 billion years of history. This is far enough back to cover the formation of almost all major geological features on Earth today.
And while we're doing that, we'll be tracking not just the movement of the continents, but all the plates, and the boundaries between them.
If we really want a complete, detailed, and realistic history for the whole planet, then we need to fully simulate the history back not just from the last supercontinent, but all the way back to the supercontinent before it—around 500 million to 1 billion years of history. This is far enough back to cover the formation of almost all major geological features on Earth today.
And while we're doing that, we'll be tracking not just the movement of the continents, but all the plates, and the boundaries between them.
So we want to start with a supercontinent, break it apart,
let it reassemble into a new supercontinent, and then break that apart and
continue to the stage in the cycle we want to reach, all the time following the
rules we established in the last section. As we do this, we’ll be recording the
positions of a few key geological features:
Orogenies, distinguishing between Andean-type
orogenies, where a subduction zone forms along the coast of a continent; Laramide-type
orogenies, where a continent quickly subducts a mid-ocean ridge;
Ural-type orogenies, where a continent collides with an island arc,
microcontinent, or the active margin of another continent; and Himalayan-type
orogenies, where a continent on a subducting plate collides with a continent
on an overlaying plate. We shouldn’t have to worry much about Japan-type
trailing edge subduction zones, as the affected regions should experience larger
orogenies in short order—we just have to note the ones still active at the end.
For reference, here are the widths of modern examples of
these orogeny types:
Location
|
Orogeny Type
|
Width (km)
|
Ecuador
|
Andean
|
80
|
Chile/Argentina
|
Andean
|
120
|
Peru
|
Andean
|
200
|
Chile/Bolivia
|
Laramide
|
750
|
US southwest
|
Laramide
|
1300
|
Russian Urals
|
Ural
|
50
|
Switzerland/Italy
|
Ural
|
180
|
British Columbia
|
Ural/Andean
|
600
|
Nepal/China
|
Himalayan
|
1200
|
Failed Rifts, (formally called aulacogens) as these may rift again later, and even
if they don’t they tend to form low-laying regions that become the core of
major river valleys later on, like the modern Mississippi.
Hotspots, which form local volcanic features and
remain mostly (but not always) stationary relative to the mantle—Hawaii is a
prime example. There is some evidence that these are mostly associated with
sections of subducted crust sitting above the core-mantle boundary,
but they can appear elsewhere. As a guideline, we’ll expect them to mostly
appear either near the rifting center of a supercontinent, roughly on the
opposite side of the planet, or in areas of weakened crust like trailing edge
back-arc basins, but they’re also possible anywhere else. They’re small
features that tend to be erased by other processes in the long run, so we don’t
really have to worry about them until the last 100 million years or so of the
sequence. Definitions vary but there are around 60 hotspots active today,
though many are very small or appear at continent boundaries where we already
expect there to be volcanic activity—so really 10-20 should be fine.
Large Igneous Provinces, which are massive regions of
flood basalts created by giant mantle plumes. Iceland is the closest equivalent
we have today, but it’s nowhere near the same scale (I’ll describe them and
their effects in more detail when I discuss mass extinctions). They appear in roughly
the same areas as hotspots, though with no particular association with trailing
edges—LIPs are more consistently associated with large mantle plumes. The largest
known—the Central Atlantic Magmatic Province—formed along the North
America-Africa rift at the start of Pangea’s breakup. We’ll only concern
ourselves with the largest examples, and also only those on the
continents—though they appear frequently in the oceans as well, they don’t
typically create islands or any other particularly notable feature. Again, definitions
vary but there have been about a dozen large continental LIPs in the last
half-billion years, maybe 5 of which were associated with the breakup of
Pangea. Also note that LIPs often leave hotspots behind after they conclude.
To do all this, I’ll be using a program called GPlates,
a free program designed to allow paleogeographers to map the motions of Earth’s
plates in the past and does a pretty decent job with fictional plates as well.
In principle everything I’m going to do here could also be achieved with a
beach ball and some cut-out pieces of paper, but using software just seems
easier for me. I would not recommend attempting this on a flat map.
I've now added a supplemental tutorial on using GPlates in a separate post; I still recommend the tutorial written up here as a secondary source. Refer to those for the specifics of using the program; what I want to lay out here is a more conceptual outline of how to simulate tectonics in semi-realistic way; at least, well enough to produce realistic "modern" geography.
Remember that you can click each of these images for a bigger version. All are created by me in GPlates and exported to Mollweide projections. |
I’ll start out with an ideal supercontinent containing 10 major cratons (marked in grey).
These are particular old sections of continental crust at the core of all of
Earth’s continents that are thicker and tougher than the rest and so are rarely
broken or even much deformed—so we’ll keep them intact through the exercise,
and give them rather flat interiors at the end. Each craton has some accreted
continental crust around it, and for now there are no additional bodies outside
the supercontinent.
Overall this supercontinent occupies about 1/4 of the planet’s surface
area in order to about match Earth’s land area around a billion years ago, but
there’s no particular reason that should be the case for all worlds. We don’t
need to worry about the plates in the oceans or any features of the continents
aside from the placement of the cratons, because all of these will be
near-completely erased and overwritten with new plates and features over the
course of this exercise.
From here I'll move forward in 50-million-year steps, moving the continents, creating new oceans, and splitting and fusing plates as required by the rules laid out so far—though still with some space for artistic interpretation in the exact speed and direction of plate movement and placement of new rifts. Though we haven't marked out any boundaries in the oceans, we only need a few subduction zones to start out with and before long we'll have completely replaced the old oceans with new crust.
But first, a little bookkeeping: I’ll give each of the cratons a letter designation, and
refer to any continents they form by the combination of the cratons in them; so
this first landmass carries the admittedly awkward title of supercontinent
ABCDEFGHIJ. As new ocean plates appear I’ll designate them with lowercase Roman
numerals (i,ii, iii, iv, etc.) and microcontinents (sections of continental
crust without a craton) I’ll designate with numbers (1, 2, 3, 4, etc.).
I also want to break up the sequence of events into geological periods,
but with more memorable names. I’ll define the time starting with the breakup
of this supercontinent as the Cuvieric Period.
I’m going to be taking a very “let’s see how it goes” approach, but just
to keep everything organized I’m going to decide now that our first
supercontinent cycle should be an extroversion cycle. Using my powers of
prophecy, I can see that this means most of the action is going to occur on the
other side of the planet, awkwardly on the edges of the above map—so for the
sake of consistency I’m going to shift our view 105° west. I can also see that
the entire process is going to take 850 million years.
Let’s get started.
850 mya: Early Cuvieric
We start off much as we did before, with a major rift
bisecting the supercontinent (marked in red). But this time I’ve made sure to
avoid the cratons, and mark out the third-arm failed rifts. I’ve also added
subduction zones around the rim of the supercontinent to help pull it apart
(marked in blue) and noted the Andean-type orogenies caused by them (marked in
black).
Presumably there would be LIPs and hotspots associated with
this rifting, but the landforms created won’t last 850 million years so we
don’t have to worry about them this time around. It’s just something to bear in
mind when we work out the history of life on this world.
800 mya: Mid Cuvieric
The two supercontinent halves—ABCDEF and GHIJ—drift apart, leaving a mid-ocean ridge between them (marked in red, like the rifts) and transform boundaries on their edges (marked in green). New ocean crust forms between them (shaded in lighter blue than the Pre-Cuvieric ocean crust). They do not break apart evenly—on such a long border on a spherical planet that’s not really possible, but in this case especially the subduction zone on GHIJ’s north shore helps rotate the continents as they split.
Arrows indicate relative plate motion at boundaries, with length roughly correlating to speed. All these closeup maps are in orthographic projections; i.e., like looking at a globe. |
But already, GHI is rifting away from J, with associated
failed rifts—the main rift itself is an extension of a “failed” rift from the
initial rifting event. Such multi-stage rifting appears to be typical of
supercontinent breakups, with all the resulting continents moving away from a
specific rifting center.
As the continents move, the subduction zones on their flanks
churn away, and I’ve added volcanic island arcs along those in the oceans. I’ll
continue to do so as new subduction zones appear, though to be clear these
aren’t meant to be accurate depictions of the types of islands that would
appear—just abstract representations that there are islands there.
750 mya: Late Cuvieric
But already there are other forces causing rifting: The
subduction zone on GHI’s east coast has spread south, creating an enclosed
ocean basin with a subduction zone on one side and no mid-ocean ridge to feed
it. As such, a section of land—Microcontinent 1—is being torn away from shore
opposite the subduction zone.
By this point, new ocean crust and motion of the continents
away from the magma plume under the rifting center has caused a significant
rise in sea level, and we can expect inland seas on many of the
continents—though I won’t bother showing that here.
With the supercontinent rifting coming to an end and other
tectonic processes starting to play out, we’ll call this the end of the Cuvieric
and start another period, the Anningic.
700 mya: Early Anningic
The continents continue to rift apart, moving at increasing
speed. EF and J—which is now crossing over the north pole—are speeding towards
each other due to the subduction between them, and Microcontinent 1 is moving
particularly fast towards a collision with the other shore of GHI.
650 mya: Late Anningic
Microcontinent 1 collides with GHI at 660 mya, forming a
continent-continent convergent boundary (marked in blue) and a Ural-type orogeny
(marked in black). The subduction zone “jumps” across the thin landmass,
consuming the new oceanic crust created behind 1 as it travelled.
EF is speeding towards its own collision with J, and the
pull of J’s subduction zones is causing it to rotate clockwise. This compressed
the oceanic crust near its north shore, and transform boundaries there become a
new subduction zone that spreads down its east coast. The mid-ocean ridge to
its east continues to spread, but now will drift west due to the subduction.
This is a typical ocean introversion cycle, but at a local rather than global
scale.
As EF drifts away from ABCD, the subduction zone on its west
coast spreads as well. EF is still moving westward, even though it is
subducting crust on it east side.
600 mya: Early Owenian
At 620 mya, EF finally collides with J. We’ll take this
collision to conclude the Anningic and start another period, the Owenian.
As the continents collide and slow down, subduction pulls E
and F apart, and the subduction zone on E’s east coast has consumed the
mid-ocean ridge there and begins pulling a section of land, Microcontinent 3,
away from J. To the south, E’s subduction zone spreads and links up with F’s
subduction zone, which allows the whole assemblage—EFJ—to drift south.
Meanwhile, the subduction zone on GHI’s interior ocean has
consumed the mid-ocean ridge there, and begins pulling another
microcontinent—Microcontinent 2—away from the far shore.
550 mya: Mid Owenian
The complicated EF-J collision continues. E and F have now
both solidly collided with J, forming Ural-type orogenies, but a remaining
section of subduction zone pulls a small landmass, Microcontinent 4, from E’s
shore. In the east, 3 begins to collide with E.
In the south, Microcontinent 2 swiftly moves towards
collision with GHI, but GHI itself is moving towards collision with ABCD.
At sea, the mid-ocean ridge between EFJ on one side and ABCD
and GHI on the other has grown so broad that spreading along its whole length
is no longer possible—a result of the geometry of spherical planets. A
subduction zone forms along its eastern edge—north of GHI—which starts pulling
GHI and EFJ together. EFJ has also consumed the mid-ocean ridge to its south,
so we’ve solidly moved from supercontinent breakup to supercontinent assembly.
Though we said it would be an extroversion cycle, as is typical it’s actually
more of a mix: an extroversion between ABCD and GHI, and an introversion
between EF and J and between EFJ and ABCDGHI
With continents over both poles, we can expect climate to be
cooling—perhaps there’s even an ice age. This, along with aging of the new
oceans, will pull sea level down from its late-Cuvieric high.
That said, there might be a brief warm spell associated with
our first LIP in northeast GHI (marked in orange), as well as a possible
extinction event.
500 mya: Late Owenian
The EF-J collision event has now largely concluded; E and F
are solidly attached to J, and Microcontinents 3 and 4 have collided with E and
J, respectively. The trapped sea between E and J would probably be filled in
before long, but I’ll leave it in place as a weak point in the crust. I could also
have the subduction zone “jump” these microcontinents and continue pulling
chunks away—this is probably more realistic—but given that these regions will
soon be involved in the collision with the southern continents, I’ll leave them
for now.
As EFJ moves south and its island arcs collide with ABCD’s,
the islands accrete to the overlaying plate and form a section of continental
crust—eventually a whole new continent could be formed this way, but that won’t
be the case here.
In the south, Microcontinent 2 collides with GHI at 530 mya,
compressing the already-deformed lands of former Microcontinent 1. If this
process continued uninterrupted, H might eventually be torn away and finally
close the subduction zone—but again, that won’t be the case here.
Note here that we can infer the presence of an old mid-ocean ridge from before our simulation began due to relative plate motion around the pre-Cuvieric ocean. |
450 mya: Early Huxleyic
ABCD finally collides with GHI at about 460 mya, closing the
Owenian and starting the Huxleyic period. As with the EF-J collision,
it’s a messy process, and subduction zones pull G—and its accreted
microcontinents—away from HI.
Meanwhile, the oceanic crust formed at the start of the
Cuvieric is now pushing 400 million years old. A subduction zone spreads down
the west coast of GHI, eventually separating the continent from the oceanic
crust and forming our first ocean-only plate, Ocean i, which continues to move
east.
A subduction zone also spreads along EFJ’s eastern coast, so
the continent is pulled in 2 directions. E splits from FJ, taking a large part
of J’s former plate with it. FJ continues south towards the already-starting
collision with the assembling supercontinent, and E drifts east out to sea.
Just for fun, I’ve also dropped a small LIP in FJ.
400 mya: Mid Huxleyic
The ABCD-HI collisions continues, with widespread mountain
formation and land deformation, including a proper Himalayan-type orogeny
between ABCD and G, where 2 microcontinents are sandwiched.
FJ also collides with the supercontinent’s north, forming an
impressive orogeny of its own thanks to the trapped sections of former island
arcs.
E continues east, though it’s rotating clockwise thanks to
subduction between it and ABCD. It also experiences a massive LIP, potentially
causing a large mass extinction.
At sea, the triple junction of mid ocean ridges spreads
apart to create another plate of new oceanic crust, Ocean ii. Though it has no
subduction zones of its own, this plate will spread at the expense of
surrounding plates as they are subducted near the coasts. At this point, almost
all remaining oceanic crust from before the Cuvieric has been subducted (save
for a couple patches near the south pole and between the colliding continents),
so we have a complete image of the tectonic forces at play in our world.
One consequence of this is that to keep Ocean i moving in
the right direction to feed all the subduction zones on GHI’s west coast, it
has to rotate clockwise and so start subducting part of the ocean connected to
ABCD.
350 mya: Late Huxleyic
The ABCD-GHI collision concludes and FJ more solidly
attaches to the assemblage. We can definitely say that we’ve entered the tenure
of a new supercontinent, ABCDFGHIJ. We’ll mark the occasion by closing the
Huxleyic and starting the Marshian period.
We could let the situation continue to play out until E
joined the supercontinent, but just to make things a little more interesting
we’ll have a minor rifting event that separates D and Microcontinent 5 from the
supercontinent’s east coast.
As these new landmasses move east, the subduction zone on
E’s east coast pulls the distant ocean crust west. The result is a new
subduction zone splitting the 2 sections of crust apart, forming Ocean iii.
Meanwhile, subduction zones spread along the north coasts of E and FJ, splitting
them from Ocean iv. There is now a complete ring of subduction zones separating
the assembling supercontinent from the surrounding superocean—which is exactly
what should happen as the oceans age and the continents stop moving relative to
each other.
As Ocean I moves south, the mid-ocean ridge between it and
Ocean iv begins to subduct under the supercontinent’s west coast, so we’ll put
a Laramide-type Orogeny there.
300 mya: Early Marshian
Internal mountain building has largely ceased on ABCDGHI,
though it continues along subduction zones and a sliding collision in the south
with Microcontinent 5. The whole landmass drifts slowly north and rotates
clockwise. The vast interior will be fairly arid, with a large desert at the
center.
D drifts north and E drifts south, accreting island arc
crust along its south coast. A small section of the ocean crust attached to
Microcontinent 5 gets pinched out between E and Ocean I, forming a small ocean
plate, but it won’t last long enough to be worth giving a name.
Ocean ii continues to grow, and the very last of the pre-Cuvieric oceans have either been consumed by subduction or trapped within the supercontinent. Some small fragments will have been lifted onto the continents and survive there as ophiolites.
250 mya: Late Marshian
Microcontinent 5 impacts E, sandwiching some island arcs, and other island arcs from ocean i impact E’s south coast, creating a long, complex orogeny. The subduction zone “jumps” across plate 5, and E is pulled towards a collision with the supercontinent.
D continues north, and has a moderate LIP.
The mid-ocean ridge between Ocean iv and Oceans i, ii, and
iii is now so broad that, as has happened before, it can no long spread across
its entire length. Ocean iv begins subducting below Ocean i.
200 mya: Early Copian
E finally collides with ABCFGHIJ at about 220 mya, forming a
vast Himalayan-type orogeny, and D begins colliding with the same landmass, so
for short time we have a complete supercontinent ABCDEFGHIJ.
But not for long. A rift crosses the interior, and with it
comes a massive LIP, flooding the region with lava. ACG in the south begins
splitting from BFHIJ in the north. So ends the Marshian and begins the Copian
period.
The northward motion of BFHIJ encourages the formation of a
new subduction zone on its north coast to subduct the old crust inside the
now-separated Ocean v. Ocean i is also rapidly subducting, and the subduction
of its mid-ocean ridge with Ocean ii causes a Laramide-type orogeny along ACG’s
west coast.
Rifting also breaks apart E. The new landmass contains much
of the crust from former Microcontinent 3, back in the Owenian, so we’ll give
it the same name. It may look like an odd snaking continent here, but in reality it would probably appear as a long archipelago like Indonesia.
Now that E has stopped, a subduction zone spreads along its
north coast, separating it from Ocean vi. This subduction also pulls away a
section of D, Microcontinent 6.
150 mya: Late Copian
The supercontinent has now solidly split in two, but like
last time it doesn’t spread evenly; subduction zones in the east cause BFGHIJ
to rotate clockwise and ACEG to rotate counterclockwise.
Also like last time, the rifting doesn’t conclude in one
step. G begins rifting from ACE, with an associated LIP.
To BFHIJ’s north, Ocean i has subducted completely and Ocean
v will soon follow. Their associated island arcs attach to BFHIJ’s coast,
forming Ural-type orogenies. This is the typical fate of island arcs offshore
of continent active margins.
In the east, D finishes colliding with BFHIJ and
Microcontinent 6 speeds south towards ACE. In this process, another oceanic
plate is cut off, Ocean vii. This whole eastern ocean is now a maze of
subduction zones, island arcs, and mid-ocean ridges, but it’s all being
inexorably pulled towards collisions with the continents.
In the south, Microcontinent 3 collides with G to its west
and islands from Ocean vi to its east. Take special note here how odd this appears in the Mollweide projection, as opposed to the view over the south pole below; always keep a map's distortion in mind when working near the poles.
Finally, as we approach the end we should start adding
hotspots (marked with purple dots)—some near rifting centers, some randomly across the globe. For now, though, just one left over from the Early Copian LIP.
100 mya: Early Andrewsian
At this point I’m trying to construct more of a continuous
geological history rather than doing everything in 50-million year steps. As
such, shortly after G rifts from ACE, CE rifts from A at 110 mya—closing the
Copian and beginning the Andrewsian period.
But the supercontinent breakup isn’t over—an LIP within
BDFHIJ foretells another rifting event.
Microcontinent 6 also collides with CE around this time, and
the mid-ocean ridge behind it is quickly consumed. This pulls CE towards
BDFHIJ, starting an introversion cycle.
Oceans vi and vii are also being quickly subducted, and
Oceans iii and iv are following them. But Ocean ii’s subduction on the other
side of the world helps keep their mid-ocean ridges balanced in the center.
To the south, 3 continues to collide with G over the south
pole, and subduction of the ii-iii mid-ocean ridge causes a Laramide-type
orogeny on its north coast.
Again like last time, at this point we should expect high
global sea levels and inland seas—though perhaps that may be moderated by the
presence of a large, likely-glaciated mountain range over the south pole.
50 mya: Late Andrewsian
DJ rifts from BFHI at 80 mya, and both this rift and the
A-CE rift have several associated hotspots—as well as one final LIP in CE’s
south.
G is moving pretty swiftly at this point, and as it shifts
north from the south pole it creates something of a pocket between it and BFHI
that Ocean ii is pushing into—such that there is a small section where the
oceanic crust attached to BFHI and G is subducting under Ocean ii. The
collision with Microcontinent 3 also continues, and has now enclosed a section
of ocean, where slab rollback could lead to interesting patterns of land and
sea.
Ocean vi has almost completely subducted away, and there is
also a small section of Ocean vii remaining but it’s so small that I was too
lazy to bother labelling it.
I did bother to add the vi-vii rift in this one, and I've noted an early rift pulling microcontinent 4 towards CE. |
0 mya: Ostromian
At 30 mya, BF rifts from HI—the last rifting in the breakup
of the supercontinent. At the same time, CE barrels into DJ, forming a broad
Himalayan-type orogeny. We’ll use these events to mark the close of the
Andrewsian and opening of our final period, the Ostromian.
Just before the collision, Microcontinent 4—from back in the
Owenian—is pulled from DJ’s south coast and collides with the island arc to
CE’s north, then is carried back north into DJ. This, in combination with slab
rollback in the enclosed sea, should lead to an interesting, Mediterranean-like
coastline.
Microcontinent 3 continues to collide with G, but by 0 mya
there is still a substantial enclosed sea remaining.
Ocean ii continues to push into the pocket between G and HI.
It isn’t a separate plate yet, but it will be before long so we can go ahead
and call it Ocean viii. There’s also a small new section of subduction between
A and G due to G’s counterclockwise rotation.
And because this is the last section of tectonic history,
I’ve also pulled out some sections of the east coast of DJ, as it’s been moving
slowly and we can expect some back-arc extension.
And that finally brings us to the “present day”. Here's a chart I threw together to help us keep track of the periods I've established and drift of the continents throughout them:
Widths are not proportional to continental area because I only realized I should have done that after I finished. |
And indeed we can pretty well see a supercontinent cycle of breakup (Cuvieric, Anningic), assembly (Owenian, Huxleyic), Tenure (Marshian), and breakup once more (Copian, Andrewsian, Ostromian).
Now, we can’t really go on calling our continents “HI” and
“BF”, so I’ve come up with some interim names for our 6 continents (I’m still
counting CE and DJ as separate, given the high mountain range dividing them)
and 4 major oceans until we figure out what sort of names their eventual
inhabitants might use.
As a final touch for this post, we can use our recorded history to get a rough sense of the modern topography. As we’ve moved along, I’ve been marking out the areas affected by orogenies and LIPs, and now we have a record for all the major mountain-building events of the last 850 million years (You can use gplates to generate these age maps for you):
This map is in an equirectangular projection, which will be more useful for modifying it later on. |
Mountain range height doesn’t directly correlate to age, but
there is a relationship; Based on the heights and ages of many of Earth’s
current ranges, average mountain height tends to begin around 2500 meters and drop about 5 m every
million years, with peak height starting around 4000 m and dropping 8 m/myr. But there are plenty of exceptions
and local variation and initial height is particularly broad (and really an exponential decay might be a better model, but we're working in generalities here).
With this as a guide and a bit of artistic interpretation, I’ve put together a
rough pass of Teacup Ae’s global topography:
These are average elevations, mind; we'll add plenty of peaks and valleys later on. |
I’ll talk a bit more in a later post about the exact considerations regarding different mountain types, erosion, and uplift, but for now this gives us a general idea of the planet’s terrain and will help in working out the climate.
And that about does it. There’s still a lot of cleaning up
and fleshing out to be done: In addition to further work on internal
topography, the coastlines need to be refined, the island arcs expanded, and
other islands added. Bear in mind that the boundaries shown aren’t actually
coastlines, but boundaries between continental and oceanic crust (roughly).
Even when sea level is low, much of the margins of the continental crust
sections will be submerged continental shelves—more on passive margins. Though,
in this specific case, I’ve tallied up the areas of crust at the start and end
of the sequence and found that continental area didn’t increase as I was aiming
for (I should have added more land during island arc collisions). It actually
decreased, though only by about 0.2%. So
I might expand out these shelves a bit so I can retain more land area.
But for now, it’s a pretty good result.
Orogenies are black when active then fade to white as they age, LIPs are orange areas, hotspots are purple dots. Some of the borders can be a bit janky during collisions because I only bothered updating them every 50 million years.
An alternate view showing tectonic boundaries (red for divergent, blue for convergent, green for transverse) and distinct plates in different colors.
I’ll admit, this is a pretty tedious method to get a world
map; what you see here is the result of a solid week of long afternoons (though
it probably would have gone a lot faster if I was less obsessive about matching
up all the lines just right). But I’m pretty happy with the result: A world
like Earth, with a similar diversity of landforms, and a solid geological
history to work off of (we’ll see the utility of that over the next few
sections) but different in interesting ways:
- Except between Hutton and Lyell, there are no land connections between the continents as there are between most continents on Earth. Hutton and Steno might still have a land bridge in the depths of an ice age, but at such points it would be covered with ice sheets anyway. There are only thin seas separating Steno, Holmes, and Hutton, and (once I’ve expanded the islands) Lyell and Wegener, so there will be some exchange of plant and animal life and intelligent Teacupites. Still, natural and social divergence of the continents will be easier than on Earth—especially on Agassiz, which is completely isolated from the other continents.
- Hutton has a European-like west coast and Asian-like east coast, but a much thinner (though mountainous and probably arid) interior than Eurasia, which could bring these diverse regions into closer contact than in our world.
- Steno and Agassiz both have sections near the poles that we should probably expect to develop ice caps, but also extend into temperate climate zones. We don’t have that full gradient on any one landmass on Earth today.
- Holmes is an “island continent” somewhat smaller than Australia, but less arid and closer to other landmasses and likely centers of civilization.
Next post, we’ll take a quick detour into looking into
alternate modes of tectonics that we might see on more alien worlds, and then
we’ll move on to planetary climates and biomes.
In Summary
- Convection of heat in Earth’s interior drives plate tectonics, but the motion of particular plates is determined by patterns of subduction.
- Plates spread apart at divergent boundaries—rifts and mid-ocean ridges—come together at convergent boundaries—subduction zones and collisional mountain ranges—and slide past each other at transverse boundaries.
- Ocean crust subducts and is completely replaced on ~250 million year timescales
- Continental crust is formed at subduction zones and never subducts, though it can be deformed and compressed by collisions.
- Earth undergoes a cycle of supercontinent formation, tenure, breakup, and reformation in a new form roughly every 750 million years.
- Supercontinet cycles can be either extroversions, with continents spreading and consuming the exterior ocean, or introversions, with continents reversing direction and consuming the interior ocean—or a combination of both.
- The position of subduction zones is the primary determining factor in the motion of plates, though the influence of mantle plumes plays a role as well.
- Mountains form at orogenies due to subduction zone volcanism or continental collisions, and then gradually erode down afterwards.
- Regular leading-edge, slab-rollback trailing-edge, and flat-slab subduction zones produce different types of active margins.
- Broad, Himalayas-like plateaus are formed by repeated impacts of small continental fragments followed by a large, high-speed continent collision.
- Complex, Mediterranean-like seas are formed by trapping of oceanic crust between converging continents.
- LIPs, hotspots, and failed rifts are other tectonic features that can affect global geography.
Notes
So, I'm going to be opening a patreon for the blog. Though I enjoy doing all this, it’s a big time commitment and I could use a little support as my bills start looming. But I won’t be pushing it past adding some links to the front page and other posts. I don’t want anyone to think I’m relying on this, and for now I won’t be putting anything behind a paywall (and I don’t intend to ever put anything behind one permanently). I’m just testing the waters a bit and hoping for a little extra motivation.Scotese has assembled a list of plate tectonics “rules” that he uses in his paleogeography reconstructions—I tried to contextualize them and translate them all into more layman’s terms throughout this post, but the originals are worth a read.
If you want to see a model of Earth's development from the very beginning, this video is a pretty decent representation, but with the major caveats that any geographic or atmospheric data before about 2 billion years ago is very speculative, and the landmasses shown don't necessarily represent all the landmass that existed at the time; just what we have direct evidence for.
Awesome!
ReplyDeleteneat
DeleteI understood most of ther article, got to the Gplates part. Dd all the tutorial you linked. Came back here and now i'm completely lost. Like I can't move past your original supercontinent. I just don't know how to do what you have done. Is your supercontinent just a bunch of features that you arelady planned ? Do you draw each 50 Million years something new ? How do you keep track of failed rift ? I have so many questions. I'm so sad i was pumped and ready to simulate my world tectonic with my new found knowledge (very well explained even for a non native speaker), but now i'm stuck :(
ReplyDeleteI'm planning on adding a proper gplates tutorial of my own at some point, but haven't got around to it. But the short answers to your questions: I started with the supercontinent as one big piece, but gave each craton its own plate ID; whenever I split a piece of continent off, I had the old supercontinent end and created 2 new objects occupying the same space. I used 50 million year steps for moving objects, and sometimes redrew borders at these steps when 2 objects collided (have the old object end at that point, make a copy with adjusted borders that starts at the same time). Failed rifts and so on, I just drew as new objects with plate ids of whatever the nearest craton was. I know this still probably doesn't clear everything up so, again, I'll have to make a proper tutorial sometime soon.
DeleteThank you that is actually helpful. When I typed the comment i had been on this article and Gplates tutorial for something like a whole day so my brain was not functionning properly. With sleep and time to think outside of Gplate i figured that's what you did somehow. Except failed rift I was at loss of idea so you unlocked that for me hehe.
DeleteThank you for answering my comment. Looking forward to see a Gplate tutorial of your own. The one you linked was very good for the bases but I figure there's a lot to be said. I think one thing that helps is when you realize Gplates isn't a plate tectonic simulator in the sense that it will simulate for you what happens. When you see it like a "movie maker", things starts to make more sense. Thank you !
Apologies, I'm stuck again, how did you move cratons and subduction zone with your continent fragment ? Do you move all of them manually or did you have a trick that helped you move everything at the same time ?
DeleteYou give all these objects the same plate id, and then use the Euler rotation tools to move one and all the others should automatically follow (as well as any other plate ids you've set to follow them). The rotation tool is a bit awkward to use, but produces more realistic results anyway.
DeleteThe post after the next one should be my gplates tutorial.
Oh can't wait for that one.
DeleteYeah i tried to do that but then eventually your continent fragment divide and so it changes it's ID. It's hard to explain what i mean but like let's say in your case you have two continents because they split. It's okay ABCD will have the ID 1 and all cratons A, B, C and D will also have the ID 1. But then eventually the continent ABCD will become two continents called AB and CD. And at that point AB and CD have to have two distincts ID's right ? AB will be 2 and CD will be 3. You change cratons A and B to ID 2 but all the animation you made so far will be canceled because for Gplates, they never had the ID 1 ?
So my guess is that you "kill" the Cratons when you split and just redraw them, correct ? I'll try that but most of all i'll be waiting your Gplates tutorial :')
Thank you very much for taking the time to explain to me in comments.
Mate, you might wanna put a captcha on your comment section :S
DeleteYeah, I've put a moderation block on comments, which isn't ideal but will have to do for now.
DeleteAnyway, regarding your question: you can set objects to only exist over a certain time period, so in the the case you describe you have the original craton exist until the point of splitting, copy the geometry (there should be a button for this) and create a new object that starts to exist just as the other one ends.
But also, when I start I give all the big cratons unique IDs from the start just to save time in the future (it can also avoid some weird rotation issues that can crop up when you create a new ID halfway through the process). The tutorial I linked includes a section on linking objects of one ID to follow the motions of another ID.
Found the capcha setting (just testing it now)
DeleteI thought it was conjugate plate ID but it seems not. I'll look into the tutorial again then tofind the option. Very weird. But thanks to you it's progressing very fast now :D
DeleteMan those bots are annoying. Used to have a little blog, had to put a dumb 5+3= ?? thing to keep them at bay back then haha.
Is it possible to start at 850ma and then move back to 0ma in Gplates? I've messed around with it a bit and all the rotations/positions are based on 0ma. In yours you've started at 850ma, but I can't figure out how to make movements adjust from there, and not work "backwards" from a map of what the world will be.
ReplyDeleteI'm not sure if this is a limitation of the software.
I should have a more in-depth Gplates tutorial out soon, but in short: You should be able to draw from the start and move forward, so long as you properly set up a rotation.rot file like in the linked tutorial (I actually started at 1000ma in the program, but came to the "modern" position at 150 ma). One issue you might run into is that once you create features, you shouldn't alter their position at 0ma or it can cause weird issues; if you need to alter their position close to the end, change their position at 1ma.
DeleteRealized the issue - I needed to set the time on the initial rotation to 1 instead of 0. Thanks for the help!
DeleteHello !
ReplyDeleteI hope you are doing well. Wanted to know if you had a vague idea to when your Gplates tutorial would be ready. I kinda gave up on Gplating my world for now as I'm sure I'm making stupid mistakes and not doing it properly so I'd rather wait to see how you do it.
Thank you :)
It's essentially done, just gotta proofread and upload. Check back on Tuesday.
DeleteOh wow ! Super cool :) Thank you very much for all the quality content.
DeleteThis is quite a comprehensive lesson. Thank you for taking the time to educate us - I'm fascinated by this science and very appreciative for the worldbuilding advice.
ReplyDeleteYou mentioned that you don't recommend trying to import a tectonic history on an existing map, but do you have any advice for somebody who already has a map that they are attached to?
ReplyDeleteI don't want to start over from scratch (because I worry that this method doesn't leave me with enough control over the final shapes of the continents, which is rather important to me - see: attachment to existing map). But just drawing in tectonic plates and assuming that the tectonic history works out to get their present position and landforms is apparently not a reasonable solution.
Hey, sorry it took a bit to get back to you. I wouldn't say it's totally unreasonable to keep with a map you're attached to, but you won't get the same depth of detail (which will matter more in future, as-yet-unwritten steps regarding terrain and resources). Best I can say is you can look at your current continents and try to work out if they might reasonably have originated from a single supercontinent (though it's probably fine to have one or two landmasses left out) and then that gives you a sense of the current plate directions (generally away from the center of the former supercontinent, though plates can quickly change direction when new subduction happens) and from there you can predict current plate boundaries and ongoing orogenies. Then it's basically just an artistic choice where to place older mountains and features.
DeleteI'm pretty late on finding this blog admittedly. But these tutorials have been so fascinating and helpful to read! I recently started a new worldbuilding project, but I was wondering- Would you ever back track tectonic history? Say you have the contemporary stage of a world's plates and might speculate some history? How far back would you be able to go potentially for that?
ReplyDeleteI'm asking because I did make a very interesting landmass and plate set up for the contemporary stage of the world, but hadn't really gone into tectonic history to get a lot of geographic and resource based features better painted.
It's not what I recommend, but it may be possible depending on what your current map is and how much you're willing to tweak it. The key point is whether you can trace things back to the breakup of the last supercontinent, without having to account for anything too weird like random new landmasses popping up in the middle of the ocean far from any subduction zone, or the lack of any leading-edge active margins on your continents. Once you've done that, you've sort of "cleaned the slate" and can keep extrapolating back as far as you like.
DeleteThis won't give you all the benefits of the process in terms of the shape of continents, but if might be useful for later (as yet unwritten) parts where we'll use tectonic history to inform fine terrain details and natural resources.
If I'm modeling my planet's tectonics over an extremely long time with the extremely in-depth method that you did, do I necessarily need to start with a supercontinent? Based on your post on alternate tectonic schemes, wouldn't it be possible for a watery planet switching from drip and plume tectonics to plate tectonics to have multiple large plateaus that transition to continental interiors?
ReplyDeleteYeah there's no requirement to start out with a supercontinent, it's just easy to set one up without worrying too much about the previous tectonic history. You could go right to the start, but there are disagreements on what exactly the transition to plate tectonics looked like, and generally speaking stuff like rifts and orogenies are almost totally erased over 500 million-1 billion year timescales anyway (like, there may be traces in the geologic record, but they won't have a clear impact on the current terrain).
DeleteThanks! While I have you here, would you expect a larger or smaller planet to have more or less cratons and plates respectively than Earth? I'm currently making a world with only around 78% earth's surface area, starting around 3.2 billion years back in its 5.2 billion year age. Could there be fewer old interiors on such a world?
DeleteWe currently know very little about the relationship between planet size and plate tectonics. If I had to guess, I'd say maybe slightly less but random variation between individual planets is probably going to be a bigger factor than any such trend. (I've also somewhat overstated the extent to which cratons are clearly distinct and easily counted for simplicity; whether Africa has like 2 or 5 cratons is kinda a matter of opinion, though they haven't been divided into more than 2 independently moving groups for a good while now).
DeleteCan someone make a video on this? I find videos easier to follow.
ReplyDeleteIt's something I might explore when this series is finally concluded, but I'm trying to keep focused on just getting the research and writing done for now.
DeleteIn the Cuvieric era, why does the subduction zone continue north past the F orogeny when there's already a subduction zone at J? Why wouldn't the transform boundary continue all the way to the northern edge of the F orogeny, since that's the direction the continent's travelling in anyways? Is either scenario possible?
ReplyDeleteBecause that was the first timestep, I had a lot of latitude to decide arbitrarily how the initial plate boundaries appear, and those sort of parallel subduction zones off the coast aren't unusual (see for example the filipino plate). So I didn't have to do that, but adding a bit of extra complexity at the start helps breed complex geography later.
DeleteHow do "shattered" masses of microplates, such as (old) Europe, (old) central Asia, the Caribbean, and Indonesia, form?
ReplyDelete"Microplates" is something of a vague term, applying in some cases to essentially just small plates but in others to regions that aren't totally moving independently but jostling around a bit relative to the main body of the plate (there's basically a continuous spectrum there).
DeleteSo with island arcs like the Caribbean and Indonesia, you'll sometimes see maps marking out various microplates and boundaries in them, but these are towards the latter end of the scale; the stress of subduction creates lots of faulting, compression, and sometimes extension that can move around bits of crust relative to each other, but these aren't really breaking apart into distinct plates in the long term.
With Eurasia, there is more of a history of independent motion there; Asia was constructed from several rounds of collisions of island arcs and microcontinents, south europe is sort of still in the process of such a collision, and sections of west europe have been passed back and forth between the Baltica and Laurasia cratons. Now all these pieces are mostly sutured together, but they're still jostling around a bit as Africa and India push north into them.
What kind of speed are you moving things in the worked example? Because they seem to be moving a lot slower than the suggested speeds, especially with how long the collisions take to play out? And what does the 'compressed the oceanic crust' at 650 million years look like in practice? The mid ocean ridge overlapping EF's ocean crust? starting to get close to doing that?
ReplyDeleteI checked and the speeds are around 1-5 cm/year on average for major continents and 2-10 cm/year for ocean-only plates, but pretty variable overall, so it's about right, I don't really see how you're judging the speeds. The compression at 650 million years just means that there's some compressive stress on that area that should make formation of a subduction zone easier. Exactly what the earliest parts of subduction look like is a bit of an open question in geology, but presumably it involves a lot of faulting. Broadly speaking the goal here is to get a very broad picture of the major tectonic motions throughout the planet's history to inform it's modern geography rather than to capture every step in detail (which would probably require juggling something like a few hundred tectonic units--without even getting into thin-skinned motion and deformation--and learning how to deal with a lot more special cases).
Delete1.How can I decide the speed of plate? Is it random?
ReplyDelete2.If oceanic crust meets the border of plates, then what happens?
3.The time when the plates merge/divide is random?
1, I give some guidelines at the bottom of the patterns section, but it's not important to be super precise.
Delete2, I don't quite get what you mean, I suppose it depends on the context? You shouldn't really be having ocean crust meeting other plates without a preexisting subduction zone causing that to happen.
3, To a certain extent you can arbitrarily decide on rifting events and formation of new subduction zones arbitrarily, but it kinda depends on context and where you are in the supercontinent cycle.
For question #2: I mean, If oceanic crust move forward to other plate, then what happens?
DeleteI still don't really understand what you're describing and how it comes about.
DeleteSee: https://cdn.discordapp.com/attachments/826031364595384370/1063480500108410940/20230114_001805.gif
ReplyDeleteWhite: Border of Plates, Gold:Oceanic crust
Is it possible? If not, what happens?
Okay but the thing is that the ocean crust is itself part of the plate so I don't really see how it is moving towards the plate boundary (i.e. there should already be other crust between that piece and the boundary)? Like, what is this boundary? If this is a divergent boundary, that shouldn't be happening, you should ensure there is spreading along the entire boundary, and if that doesn't seem possible, open up more subduction zones. If this is a transform boundary, you can allow there to be a little overlap and figure in reality there's some small-scale faulting and deformation there as the plates slide past each other, but try to avoid a lot of overlap (transform boundaries can transform into subduction zones if that helps). If it's a convergent boundary, it would be a subduction zone, so the crust just subducts. Broadly speaking, pretty much all plate motion is driven by subduction, so you mostly shouldn't be getting crust moving towards a plate boundary that isn't a subduction zone.
DeleteThere is two plates, divided by divergent boundary and transform boundary.
ReplyDeleteIn this picture, oceanic crust(Yellow) is next to right plate.
In the case that oceanic crust is pulled in different directions as a result of subduction, would this open up a new ridge in the ocean? In gplates I've only seen continental rifting, in this circumstance would oceanic crust rift or create a new plate?
ReplyDeleteSorry if it's in the post I've read it several times in the past but cant find the information im looking for at 1am.
Thank you for these lovely tutorials. Between your blog and Artifexian's video's I've learned so much.
We do occasionally see new rifts appearing within oceans, as is happening today in the Indian ocean as India and Australia are pulling apart. But there are some feedbacks that tend to prevent this; See, in addition to pulling on the subducting plate, subduction zones also exert some pull on the overlaying plate towards the subducting plate, and this pull tends to get stronger the older (and so cooler and denser) the subducting crust is. If you have an ocean plate surrounded by subduction zones and with no ridges to feed it new crust, then its crust must be getting older and it will be pulling ever stronger on the surrounding plates, which may cause them to converge on it and eventually completely consume it and collide on each other.
DeleteOf course you may have cases where there are competing forces involved (or with australia and india, the geometry is more complicated) and so a rift eventually appears.
In the section of the speed of tectonic plates you write: "For planets of other sizes, divide these values by the radius in Earth radii". Does that mean that tectonic plates move faster on smaller planets and slower on larger planets? Say an active margin continent on a planet with a radius that is about 60% of Earth's radius. If I interpret you correctly then the movement should be 2.7/0.6=4.5 cm/year. Wouldn't that mean that the supercontinent reforms a lot faster on a smaller planet? That seems strange to me, but I'm a total novice when it comes to tectonic movement. Sorry if I misunderstood...
ReplyDeleteIn that section I was showing plate speeds in terms of degrees of latitude or longitude, which are scaled to the size of the planet. It is my guess (emphasis there) that plate speeds are governed mostly by local forces and so should be consistent in absolute terms (i.e. cm/year) across planets, meaning that they will move across a greater portion of a smaller planet's surface in a given amount of time.
DeleteIn retrospect "degrees longitude/latitude per 10 million years" is a weird unit and I'll probably switch it all back over to cm/year in a future edit.
Hm, for some reason I can't log in to comment (this is Kurodani again). Any way, thanks for the reply, that makes more sense! My bad!
DeleteI'm looking at the video of the worlds history showing the plates and boundaries, and I'm wondering how you are able to make them work so dynamically together. I've only just recently started to use the topology tool in Gplates creating line and boundary topologies & only manage to use half stage rotation for the dynamic mid-ocean ridges but cant figure out how to make dynamic collisions, like where your plates and boundary's look like their being absorbed by the overriding plate mine will just overlap. is their a trick to making dynamic Collisions or have I misread your Gplates tutorial?
ReplyDeleteCreate line topologies to link all your subduction zones or rifts together into boundaries, and then use those to create area topologies for each plate. For this to work out neatly involves a lot of fiddling to make sure the base features overlap properly to make sure the topologies have the shape you want, and then any time objects appear or disappear or move so that they overlap differently, you often need to remake topologies to make it all pan out right. It's about as tedious as it sounds.
DeleteAlso, if you have topologies starting and ending at the same time, they'll look weird, so slightly offset the export time; e.g. rather than starting at 850 ma and exporting a frame every 1 ma, start at 849.99 ma.
Okay so I fiddled around in Gplates and I've think I've figured it out. They both use the same method of making line and boundary topologies that have features with multiple plate id's in them lining up with the sides of the plates too make L and V shapes so that their movement/rotation make it look like one is being absorbed when its really just zipping up alone the sides overriding plate. Is this right?
DeleteIs it possible for a extroversion cycle to not fully "eat up" the old ocean before forming the next super continent ? My North pole simply has a ring of subduction zones around it for some reason, and a portion of ocean on the southern hemisphere is also still old ocean for some reason, or did i go wrong somewhere ?
ReplyDeletesupercontinent cycles need not always be as neat and complete as the ideal archetypes, so you might have patches of older crust surviving, but very old ocean crust next to younger crust is likely to encourage the formation of new subduction zones.
DeleteI am somewhat confused regarding the highlands on the coast of Brazil and southern Africa; I've always just assumed that big Atlantic ocean-style rifts made these big plateau/highland regions on continental coasts of divergent boundaries, but nothing seems to be supporting this assumption. Sure rift valleys and such form along divergent boundaries but they seem to be unable to form a large mountainous region like that? Other divergent boundaries between continental landmasses don't seem to have this kind of topography either, like Florida or most of Australia. They're also not orogenies, and mostly not LIPs, actually the only thing I can find that overlap with these features seem to be cratons? Aren't cratons supposed to be too old/stable for mountains, such that if they ever did have mountains they'd practically be completely eroded away by the modern day? What's going on here?
ReplyDeleteOn both sides it's a somewhat complicated combination of ridges and highlands formed in the original assembly of Gondwana and the pretty substantial uplift and volcanic activity associated with its breakup. South Africa was essentially right in the middle of this large landmass and Brazil not far off, so they experienced a fair bit of uplift from the mantle plume that developed under it and helped precipitate the breakup. The cratons in these areas have to some extent actually helped to preserve these highlands by preventing the formation of smaller rifts and faults that would have broken them up and allowed for quicker erosion.
DeleteDespite its limitations, do you suggest using Eons if you don't want to spend a lot of time going into detail? Do you have any suggestions for attempting to solve the limitations with eons with external editing?
ReplyDeleteFor its quirks, Eons still gets my recommendation as the most realistic global map generator by a wide margin. Taking its terrain and running it through gospl or something might be an interesting experiment, but is probably ultimately almost as much work as just making new terrain from scratch. Working out climate externally from eons might be a bit more rewarding, it doesn't have anything like the worst climate model for a generator but it's still not hugely reliable.
DeleteHello. I have a question about beginning a tectonic simulation (using GPlates). How do you go about shaping and placing cratons? Mostly placing them. Do you think about what you want the supercontinent to look like and then drop the cratons around or do you place cratons randomly and then draw the continental crust around it? I'm struggling to get a good shape that's around 25% of my planet's surface area. For realism does a supercontinent need to have any specific features? I assume it can't just be a massive blob.
ReplyDeleteI do generally tend to have a vague shape in mind, but not much more specific than like "triangle with one point over the north pole" or "east-facing crescent". I do sometimes think about where I want my first rift to be so I'm not blocking it, and where the initial subduction zones should be to work with that--with those then having somewhat straighter or gently curved coastlines. I have seen a lot of people draw their cratons oddly small lately, and also some people seem to want to give them really detailed shapes, when really actual cratons have pretty indistinct, fuzzy boundaries, especially over these long timescales. I don't see everyone doing that, though, so don't overthink it. Ultimately a blob is fine; the idea is to add detail over the course of the run, and if you go long enough than any work you put into trying to make detailed features at the start will largely be overwritten at some point anyway.
DeleteQuestion about mountain typs overlapping; so I have two continents that collided to form a Ural type orogeny, with part of an Andean type overlapping with part of the Ural type shortly after formation. Note the Outer Arc would form on the back slope side.
ReplyDeleteFurther down(southern part)the chain a purely Andean portion would be overlapped with a Ural(outer Arc on the Foreslope side).
How would these two portions work?
Would I just put an Ural type for the southren portion of the chain or would the outer Arc still exist? And for the northern part should I combine the two, for lack of better words, with each parts respective erosion?