|MadSci Network: Astronomy|
Hi Rebecca, On Earth, we think of a volcano as a kind of vent along which molten rock (of basalt-like or granite-like composition, or at some point in between) is released at the surface. It is powered by the fact that the molten material is less dense and less viscous than the cooler country rock it travels through, so bouyancy forces drive it to the surface. As the magma body reaches the surface depths, pressure exerted drops rapidly and, especially if it has a more granitic composition, gasses trapped in the magma are released rapidly, giving an explosive "kick" to the eruption. At different temperatures, pressures, and crustal composition, solids other than what we generally think of as rock can be involved in volcanism, including ices. An ice volcano is therefore only a volcano that emits liquids that freeze into ices. In fact, the distinction between Earth volcanoes and ice volcanoes is somewhat blurred when it is considered that ices are technically minerals, since they are non- biological, naturally occuring, crystalline solids and some textbooks list water ice as one. Considering that, liquid water, in the right geological context, qualifies as a magma or lava every bit as much as more traditional lavas we see on Earth. On Triton there seem to have been three kinds of ice volcanism: (1) The type associated with "Cantaloupe terrains", where the surface is cracked by faults and slushy fluids (thought to be water or mixes of water and ammonia or methane) that flowed viscously across the surface, creating a texture that resembles the skin of a cantaloupe. (2) The type associated with the volcanic plains, in which a less viscous molten material than that in (1) (probably water at a higher temperature with or lesser amounts of ammonia and/or methane) formed lava lakes and depressions interpreted as volcanic calderas. (3) The active volcanoes or geysers generating dark streaks in Triton's atmosphere during the Voyager 2 flyby. The explosive agent here is believed to be methane or liquid nitrogen that forms at very shallow depths. The liquid ascends to the surface because it is less dense than the ice, but vaporizes as pressure drops. Because this is a fairly shallow phenomenon that is mostly powered by vaporization, it may be more accurate to call these geysers. Only the phenonmenon in (3) is known to be happening today. Triton may have long-since cooled past the point where it could support the other two types of volcanism, leaving "fossil" landforms behind as evidence. (Reference: "Exploring the Planets" by Christiansen and Hamblin. Prentice- Hall, 1995.) --Gene Marlin
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