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I have an exomoon that orbits close enough to its planet that it has notable geological activity. I would like to limit active volcanoes/geysers/lava lakes/etc. to at most, a couple dozen areas on the surface rather than having them be spread all over the moon like Io's volcanos or exist all over tectonic boundaries.

How could I achieve this?

A couple of theories off the top of my head:

  • The crust hasn't fully differentiated yet, so there are different concentrations of elements and minerals in different areas, creating weak spots in the crust through which the pressure can be released.
  • The moon has large tectonic plates that grind against each other, but large portions of boundaries are buffered by ice, limiting volcanic activity to areas not buffered by ice.
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    $\begingroup$ Your ice theory probably wouldn't work as Iceland has a lot of ice and a lot of volcanos. $\endgroup$
    – Bellerophon
    May 7, 2016 at 16:46
  • $\begingroup$ Yeah, that one was pretty off-the-wall. I have no idea how you'd get ice on top of a hot mantle. Not sure if Iceland is an apt comparison though, since it has ice on top of a rock crust instead of ice crust sections. $\endgroup$
    – emo bob
    May 9, 2016 at 2:34

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There are two reasonably simple solutions here.

They both start from the moon having a molten core but it has cooled more than earth has. The planet's crush has locked and there is no longer tectonic activity.

Then to create your hot spots you either have a hot plume coming up from the core that still reached the surface. Look for information on Mantle Plumes.

The second and more dramatic option is to have a huge meteor impact onto the moon, sufficient to shatter and weaken the crust enough for magma to come out. The shockwaves would then travel around the moon and meet at the far side, causing weakening there. This would explain two "hot spots" at opposite sides of the planet or moon.

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