|MadSci Network: Astronomy|
Let's take the Earth as our example...it has the most data available. Heat in the interior of the planet arises from two causes: a. The potential energy released during initial formation. b. The decay of radioactive elements This heat is sufficient to create what are known as convection currents in the body of the planet. In the Earth, there is a solid nickel-iron core surrounded by a liquid nickel-iron core, then the largely magnesium silicate mantle which is solid. Even though solid (due to the enormous pressure), the mantle is quite hot and thus rather plastic for solid rock. This allows, especially over long time periods, for a convection current system to exist within the mantle. That is, the rock heats up down by the core, and then rises toward the top of the mantle; once at the top, it moves sideways under the crust (to which it loses heat) and eventually descends back down. Where the mantle rises, it pulls apart the crust of the earth. This process is almost entirely hidden deep under the oceans at what are called mid- ocean trenches. At certain locations (Iceland is by far the largest) it does come into view at the surface. Because the mantle plume pulls apart the crust, new rock erupts to fill the crack. This is volcanism, by definition. Once the new crust is exposed to the ocean or air, it rapidly cools, heating the water or air, and solidifies. Eventually, the crust slides down again, in what are called subduction zones. As it does so, the relatively water-laden crustal rock, which also drags down lighter sediments eroded from the continents, is reheated. It melts fairly readily, and because of its relatively low density (for rock, anyway), rises back toward the surface and again we get volcanism, this time forming island arcs (Indonesia, the Aleutians, and the Japanese islands are three examples). Once the descending old crust has been stripped of its volatiles, it sinks further down into the mantle and melts away back into it, thus completing the cycle. As to the other planets, I must admit I know less. Mercury, which is pretty small, may not have had convective volcanism of this type. Or, evidence of it may have been obliterated by later impact events. Venus seems to have some evidence, but it's far from clear that it behaves like the Earth does. Mars has certainly had volcanism (the planet has the largest volcano in the solar system) but again, the small size of the planet may not have allowed convection currents. Or, perhaps it did long ago but not for some billions of years.
Try the links in the MadSci Library for more information on Astronomy.