Titan

I wonder how much ice is in Titan's makeup. Imagine a "mantle" made of ice. Earth's mantle has pressures of order 1E11 Pascals at the bottom, which would get you well into the ice VII/VIII/X phases. Ice has lots of phases. The density change between the ice VII/VIII and ice X looks especially interesting. If you had any migration of material within the mantle, and some of the ice X rose to a level where the pressure dropped below the phase transition boundary, it could undergo a "sudden" change in volume, rather like some of the transitions in mantle rock on Earth. I'd think this would make any convection go faster, and maybe drive some of the crust around. That would mean earthquakes, subduction, etc. The energy would have to come from somewhere--formation energy, tidal compression a la Io, radioactives in a metal core. I gather that the first two aren't very significant at this point, so if Titan doesn't have a metal core it would be pretty inert. Still, if it did...

And, of course, near the surface you should get some of the methane in the ice. If you subduct that, you could get some cool methane/water volcanoes.