USGS

As familiar as it seems to us, it’s easy to forget that, as far as we can see so far, Earthlike plate tectonics is a rather unusual state of affairs. Every other solid surface in the solar system (with the As familiar as it seems to us, it’s easy to forget that, as far as we can see so far, Earthlike plate tectonics is a rather unusual state of affairs. Every other solid surface in the solar system (with the possible exceptions of Europa and Enceladus) is either broiling all over with volcanic activity or frozen solid—And we really have no idea what exoplanets might be like in this regard. When it comes to talking about habitable worlds, certainly there seems to be a lot of intersection between the requirements for life and plate tectonics, but that doesn’t mean they’re exclusive to each other. In fact, a planet with nearly identical properties to modern Earth could lack plate tectonics due a different geological history.





Stagnant-Lid





Heat Pipe Tectonics

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





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





But the heat still needs to get out, and conduction through the crust isn’t enough. Thus the planet transitions to heat pipe tectonics: Plumes of hot magma rise through the mantle and burst through the crust here and there across the surface. Magma will flow up through the punctures—“heat pipes”—for a time, flooding over the surface and forming volcanic highlands for a while, until the mantle is locally cooled and so the flow stops and the heat pipes are filled in with solid rock.





Io is the archetype of such tectonics in our solar system, and the effects on surface geography are obvious: the moon is dotted with high volcanoes that give it a “pimply” appearance, though they often cluster together on highlands formed by uplift of the crust by mantle plumes. Elsewhere, the surface has few craters despite the absence of an atmosphere or water erosion, because of constant resurfacing with fresh magma.





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





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





On a more Earthlike world with an atmosphere and oceans, this burial and melting of crust could carry down carbonates with it, allowing for the formation of a climate-stabilizing carbon-silicate cycle. Intermittent magma flows across the whole globe may not sound too hospitable, but they need not be quite as frequent as on Io; Earth may have had similar heat pipe tectonics for over a billion years

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





Drip-and-Plume

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





Venus is the archetype for this mode of tectonics, and though it shares some of the scattered volcanism as Io there is a clearer division of the surface into highlands and lowlands. There are also some features present that wouldn’t last long on Io—impact craters and ancient lava flow channels.





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

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





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





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





Were Venus’s surface partially submerged in water, there would be large landmasses we could call continents separated by large bodies of water we could call oceans, but they wouldn’t be directly analogous to those features on Earth. Rather than concentrated mountain ranges and volcanic island arcs, the landmasses would mostly have high interiors sloping gradually down to the coasts, and islands would be randomly distributed off the coasts.





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



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





Oddly enough those fractal-based terrain generators I dragged in the last post might do a decent job of simulating the appearance of a world with drip-and-plume tectonics—though it would still lack the interesting features like the coronae and uplifted plateaus.





Formation of coronae (and through other plume-related processes) can cause some surface material to be pulled down into the mantle, allowing for a carbon-silicate cycle—though not in Venus’s case, given the lack of water. Long-term stability may still be an issue, though; Some models predict that rather than continuously resurfacing, heat may build up in the mantle over several hundred million years until it bursts through in a catastrophic global resurfacing, after which volcanism dramatically reduces . Such swings in volcanism would be disastrous for climate stability and life. But presuming Earth must have experienced a stage of drip-and-plume tectonics in transitioning between heat pipe and plate tectonics modes, either this cycling is not ubiquitous or it’s survivable.

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





Presuming a similar size, composition, and orbit to Earth, a stagnant-lid world could remain habitable for up to 5 billion years ; shorter than Earth could last (with a slower-evolving sun), but still substantial. It could even maintain a magnetic field 2 content or less land area, it would be more prone than a similar mobile-lid world to reach a moist greenhouse and end up like, well, Venus. And without constant CO 2 outgassing at ridges and subduction zones, consistency of volcanism is an issue; a gap without volcanism of 100,000 years would send the planet spiraling into a snowball state. But however intermittent its major volcanic events are, however important that may or may not be. But if it had a higher CO­content or less land area, it would be more prone than a similar mobile-lid world to reach a moist greenhouse and end up like, well, Venus. And without constant COoutgassing at ridges and subduction zones, consistency of volcanism is an issue; a gap without volcanism of 100,000 years would send the planet spiraling into a snowball state. But however intermittent its major volcanic events are, recent evidence for ongoing volcanism on Venus is cause for optimism.









Upwelling and Delamination

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



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



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





Cold Stagnant-Lid

moon Whatever stages it passes through as it ages, every planet eventually runs out of heat. The crust will thicken, the mantle become ever more viscous, and volcanism will cease entirely (exactly when is hard to say; some volcanic features on the may be under 100 million years old but it’s still under debate ). There can still be some tectonic motion: As a planet cools, it will shrink slightly, and so the surface is compressed. This may be responsible for some ridges on Earth’s moon and Mercury.



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



But such features will become increasingly drowned out by impact craters. Even the erosion that might obscure these features will decrease: carbon-silicate cycling is impossible, so sooner or later all surface water will either freeze or escape to space. The atmosphere too will no longer be replenished by volcanic outgassing, but it may take much longer to escape (and some small amount of gasses can continue diffusing out of the crust).





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

Mobile-Lid

Notes

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





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



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

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