The Jovian moon Io is geologically active like Earth, yet it experiences heat pipe tectonics instead of plate tectonics.

Heat-pipe tectonics is a cooling mode of terrestrial planets in which the main heat transport mechanism on the planet is volcanism through the outer hard shell, also called the lithosphere. Heat-pipe tectonics initiates when volcanism becomes the dominant surface heat transfer process. Melted rocks and other more volatile planetary materials are transferred from the mantle to the surface via localized vents. Melts cool down and solidify forming layers of cool volcanic materials. Newly erupted materials deposit on top of and bury older layers. The accumulation of volcanic layers on the shell and the corresponding evacuation of materials at depth causes the downward transfer of superficial materials such that the shell materials continuously descend toward the planet's interior. - From Wikipedia

[From Wikipedia](https://en.wikipedia.org/wiki/Heat-pipe_tectonics)

Imagine an earthlike planet with a mass of about 1.5 earth-masses, that experiences similar heating and tectonics to Io. What would the crust be made of? Basalt, Andesite or a mixture?

Going by Io as an example, one would expect basalt. Basalt is what Earth's oceanic crust is made of and flood vulcanism, which will most likely create a lot of the surface of this world, creates basalt as well.

On the other hand andesites are created if volatile-rich material sinks into the mantle and returns through vulcanism. On earth andesite is created near subduction zones and accumulates there over the eons. However on this world, subduction would happen everywhere and so andesite might form everywhere.

Some might wonder why this is relevant to my worldbuilding, however this will determine how the world will look at large.

Silicate-rich magmas are typically formed at destructive plate boundaries, by partial melting and/or assimilation of crustal rocks (which are richer in silica than the rock of the mantle). Such magmas erupt as andesites and rhyolites or are intruded as granite masses. The more extensive silicate chain molecules render these magmas highly viscous, so when eruption occurs it is usually explosive (e.g. Mt St Helens).

Low-silica magmas are typically formed by partial melting of mantle rocks beneath mid-ccean ridges or at “hot spots” like Hawaii. These magmas erupt as basalts or intrude as gabbro, and are far less viscous. Eruptions are generally effusive. - From the Geological Society

So basaltic magma would mean that the world would have a shallow profile and would look a lot like Io and andesitic would mean that the world's profile would be sharper, as the more viscous lava can build larger mountains. If a mixture of say, regular andesitic vulcanism and periodic basaltic flood vulcanism is the most plausabile answer, that would be interesting as well.

So how will the geology of this world look like? Andesite or basalt? Or can I do whatever I deem plausable since noone can prove me wrong?


I'm basing this answer on the paper Heat-pipe planets by Moore, Simon, and Webb (2017, paywalled). The arguments of the paper are speculative, but I think the ideas they base their arguments on answer your question. I recommend reading through this paper if you have or can get access. It seems that the most likely answer to your main question is:


Moore et. al. argue that heat-pipe volcanism may be a phase which is common to terrestrial (rocky) planets during their early formation. They make this argument based on the presence of certain observed common features of solar terrestrial planets. One of these is the presence of a crust composed largely of mafic or ultra-mafic rock (that is, basalts and similar rocks). To quote the paper:

Heat-pipe operation leads to: 1) Thick, cold, and strong lithospheres even though heat flow is high, 2) Dominance of compressive stresses as buried layers are forced to smaller radii, 3) Continuous replacement of lithospheric material, 4) High melt-fraction (mafic to ultra-mafic), low-viscosity eruptions and efficient degassing of the interior, and 5) A rapid transition to stagnant-lid or plate tectonic behavior.

(Emphasis mine)

So, due to the efficient degassing of the interior, volatile-assisted remelting is unlikely, and Al and Si enriched rocks are less likely to form. Based on my reading of this paper I would expect a planet with a fairly smooth surface composed of mainly basaltic rocks. The main geographic features you might see would likely be compressive (e.g. faults and folds).

Note, though, that basalt may not be guaranteed. The authors propose a geological history for the Moon which includes a heat-pipe phase, during which the presence of volatiles in the interior of the Moon allows for buoyant plagioclase rock to rise toward the surface before being mixed into the lithosphere. Their proposed mechanism goes a bit over my head, but to me it seems that if you want that Si and Al rich crust, heat-pipe convection needs to stop relatively soon, or the plagioclase rock would be replaced with mafic rock over time. This might be what happened to form the lunar mare (which are basaltic).

Finally, one other thing which might be interesting to you: the authors mention in the conclusion that external heating (as in the case of Io) is not the only way to get a planet which stays in the heat-pipe volcanism phase in the long term. Here's the relevant bit:

Since the equillibrium heat flux of a planet scales as mass/area (for most plausible heat sources), terrestrial planets more massive than the Earth should experience longer heat-pipe episodes prior to the initiation of plate tectonics ... For the large "super-Earths" over five Earth masses, the lifetime of the heat-pipe phase may exceed the lifetime of Sun-like parent stars and thus any subsequent plate-tectonic phase may bever be observed. Such planets might better be called "super-Ios" ...

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Andesite, because in a planet with life subduction will not happen everywhere.

You have a system like Io. But you want life, so you need refuges that are not covered with lava every few years. You can confine your vulcanism to long term permanent pipes and lava pools, like Loki Patera.

As Io orbits Jupiter, gravitational tides yank at its interior, depositing massive amounts of energy that must then rise to the surface. But the moon has no tectonic plates that allow magma to easily slip through. Instead, the magma rises through pipes, not unlike the volcanism that built Hawaii. Across Io’s surface, periodic eruptions flare up and then die down. But elsewhere – most notably at Loki – the channel seems to stay open continuously...


There is a constant turnover of magma rising up, cooling and sinking back down at the giant lava pool that is Loki Patera. And it makes sense - if there is constant heat and constant pressure and an open channel to relieve it, that channel will stay open.

In your life-hospitable world, these active vents might be the relics of globe-spanning cracks, themselves the relics of a time when the surface was cooling from uniformly molten. As on Earth during that molten time lighter minerals floated to the top and as on Earth those areas became crust. Heavier basalts are confined to the regions around the active vents, and they will probably fall in soon.

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  • $\begingroup$ Thanks for the answer. Yet, I'm not so sure about needing refuges for life. Even if the entire surface is renewed within tenthousands of years, life will have time to migrate. New vulcanic Islands get settled quickly and can develop rich ecosystems in thousands of years. Especially considering that the ability to migrate and resettle new regions quickly will be championed by natural selection on this world. $\endgroup$ – TheDyingOfLight Jun 2 at 20:57

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