Icy or Extinct (or Maybe Not)
Elsewhere in the solar system, you can find active volcanoes spewing not molten rock but water, methane or ammonia liquids and vapors. These cyrovolcanoes, as they're called, are found on icy moons that feel the powerful tidal pulls of the gas giants they circle. Cryovolcanoes have been observed on Neptune's moon Triton and Saturn's moon Enceladus, and could also exist on Saturn's moon Titan and Jupiter's moon Europa.
The Kuiper Belt Object Quaoar could also have cryovolcanic activity, driven in this case not by tidal pull but by heating caused by the decay of radioactive materials.
Other planets and moons in our solar system show signs of volcanic activity in the recent (geologically speaking) past. The number of impact craters visible on Venus's surface, for example, indicates the planet might have had major makeover thanks to lava flows and other activity some 500 million years ago. Pointing to fast and wild variations in sulfur dioxide levels in Venus' upper atmosphere, some scientists speculate the planet might still have active volcanoes.
And then there's Mars. With a surface made up mostly of basalt, the red planet clearly was volcanically active at some point in the past. The peak of that activity came some 3.5 billion years ago, but some volcanism might have continued long after that. Instruments on the European Space Agency's Mars Express found signs of lava flows as recently as 20 million to 200 million years ago, and discoveries of small amounts of both methane and ammonia in the atmosphere suggest the planet could have life of either the geological or biological kind somewhere.
Above right: A false-color image of Triton, with streaks that might be emissions from small volcanic vents. (Image credit: NASA/JPL, http://photojournal.jpl.nasa.gov/jpegMod/PIA02214_modest.jpg)
Left: This photo taken by Mars Express shows fields of volcanic cones up to 600 meters hight at Mars' north pole. (Image credit: ESA/DLR/FU Berlin (G. Neukum), http://esamultimedia.esa.int/images/marsexpress/011_Mars_northPole_H_volcanoes.jpe)