Abstract

The discovery of numerous extra-terrestrial volcanoes, including active ones, has stretched our traditional definition of what is a volcano. We now know that the nature of volcanism is highly variable over the Solar System, and the traditional definition of a volcano as defined for Earth needs to be modified and expanded to include processes such as cryovolcanism, in which aqueous mixtures are erupted from the interior to the surface. Plate tectonics, which largely controls the location and types of volcanoes on Earth, has not been identified on any other planetary body. Volcanic and tectonic structures on other bodies may have different origins from their terrestrial morphological counterparts. For example, calderas on Io are associated with effusive rather than explosive eruptions. Lava lakes on Io may be more similar to the Earth’s East Pacific Rise eruptions that give rise to temporary lava lakes rather than to terrestrial lava lakes where there is a shallow magma chamber. Volcanic features on Io, and also Titan, appear to be randomly distributed on the surface and not associated with a global tectonic control or with the locations of mountains. Cryovolcanism on Titan, which may still be active, is likely made possible by bottom crevasses opening in the icy crust and formation of ammonia-water pockets in the ice shell. Large scale tectonic stress (tides, global volume changes and/or topography) may promote resurfacing. Mountains on Titan may be the result of long-term cooling of the interior causing global volume contraction (Mitri et al. 2009, JGR submitted). This paper will focus on active volcanic features on Io, cryovolcanism on Titan, and how these phenomena have led us to suggest the following definition that encompasses the different forms of volcanic activity seen in other worlds: A volcano is an opening on a planet or moon’s surface from which magma, as defined for that body, and/or magmatic gas is erupted.