How many tectonic plates should my moon have?

Question

I have been wondering, what factors influence the number of tectonic plates a planet would have? Are there criteria to choose the number?

Case in point, for my habitable moon project, here is the information:

• Moon mass: 0.7 earths
• Moon radius: 0.885 earth diameters
• Moon density: 5.56 g/cm³
• Gravity: 0.9 G
• Primary mass: 954 earths=3 Jupiters
• Average distance: 1003917 km from the giant (about 157 Earth radii)
• Eccentricity: 0.025
• Rotation: 36 hours
• Revolution: 90 hours
• Spin-Orbit resonance: 5:2
• 60% Ocean coverage, with a maximum depth of 2km aside from the trenches.

As you can probably imagine, the planet would have some serious geological activity, comparable with that of Io but slightly offset by the distance from the planet and the presence of oceans and a molten core. How many tectonic plates would make sense?

Here is some additional information:

• The moon has a composition similar to earth's, however it is slightly denser as it has 10% more iron in it. Also due to the frequent meteoric impact there is also a larger presence of heavy metals.
• Age: circa 3 Billion years
• There are 4 other inner moons which would be analogues to the galilean satellites (especially ganymede) except for one that is a mars analogue. I have also been toying with the idea of the moon having two stable lagragian companions, akin to earth's moon, though only if they make sense.
• The orbital ranges are as follows from the primary: 70 Earth Radii, 80 Earth radii, 100 Earth Radii (this is the Mars analogue), 120 Earth Radii.

Answer 1

With the additional information now available we can estimate that this world will probably be extremely geologically active:

• It will have similar retained heat of formation (because of it's smaller size and mass it is cooling faster than the Earth but it's only two thirds the age), the greater proportion of heavy elements will mean it has greater proportional radiogenic heat than Earth, and it experiences quite a lot of tidal heating due to its crowded orbital neighbourhood. So there will be more energy driving geothermal/tectonic processes.

• It will be cooler than Hadean Earth though and have definite tectonic plates but they're likely to be relatively fragile compared to those we see today.

• More heavy metals is going to mean that the igneous rock chemistry is a bit strange because the lithophillic elements are going to be relatively scarce. This is going to make the mantle a little more viscous than ours would be at the same temperature.

• The mantle is relatively hot though so it will be less viscous over all leading to tectonic motion that is quite rapid by Earth standards.

• Magmas are going to tend to be more mafic compared to Earth leading to less violent volcanism. The volcanoes of this world are going to be predominantly shields with a few andesitic cinder cones along subduction zones but are far less likely to be rhyolitic and highly explosive. An exception exists for a Yellowstone-like situation where a deep mantle plume burns a volcanic hotspot up through continental crust.

• The volcanoes and mountain ranges of this moon will be able to grow higher under the slightly reduced surface gravity before they start collapsing under their own weight.

• The higher iron and heavy metals will also mean that there is a lower total percentage volume of continental crust due to the relatively lower levels of light elements.

Now to the question of overall tectonic plate configuration: I can make arguments for having many small tectonic plates and for having only 3, and for both scenarios, and many in between, being stable over geological time. Pick the number of separate landmasses you want for story purposes, each one gets a separate plate, or possibly two (if two you need to decide on the whether the boundary is convergent (Himalaya/Alps), divergent (Rift Valley), or transform) (San Andreas/Southern Alps). All land masses will have active or passive margins, normally, but not always, a mix of the two, these show you where mid-ocean spreading and eventual plate subduction occur. You can play around with minor, and even micro, plates but they're not needed in basic world building.

Important Note: as things stand the slightly lower volume of continental crust coupled with it's tendency to be a little thicker in the lower gravity means the oceans are going to dominate this world even more than on Earth covering 75-80% of the surface and probably be deeper than you would like too.

By request some notes on reasonable ways to reduce ocean cover: if all the active continents are in the temperature storm bands i.e. the roaring forties and have one active margin on their leeward side and a passive margin to windward then you'll get a situation similar to the Amazon basin with a high mountain range like the Andes being rapidly eroded to create a wide plain of sediment that is close to flat and near sea level. If you also have an inactive continent or two which are basically large peneplains with very little relief, similar to Australia but closer to sea level, that will make the most of the small volume of continental crust you do have. The tropics should also be dotted with many atolls formed around monogenetic volcanic islands.

As always hit me up in comments if there's something I missed or you want more details on.

Answer 2

Even if the details in your question were sufficient to guesstimate an answer for a general level of geological activity, and they're not, we don't really have a concrete understanding of how and why the modern tectonic plates of Earth formed. We're yet to understand why certain parts of the crust have such longevity, how they formed or why they continue to survive. We also don't understand why they've stuck together in their current formations i.e. the modern continental plates for the last 500 million years and change either.

We do understand how they behave both now and for quite some time into the past though. The better approach may be to ask yourself what you want and whether you can justify those features scientifically. Particularly look at triple point formation and stability and the boundary types, convergent, divergent and transform and their attendant landforms to get an idea of where to go.

Answer 3

Treat your planetary core like an atmosphere. Hot(ter) zones, Cold(er) zones, and jetstreams. There are a couple of good videos on YouTube on how to design an atmosphere. Wherever there's a hot spot, put a volcanic zone. Wherever there's a cold spot, put a subduction zone. Wherever there's a barrier between two magma flows going in opposite directions, put a fault. If your planetary body doesn't have a liquid core, then you won't have any techtonics.