1. What is sulfur volcanism?
On Earth, the mantle is heated mostly by the decay of radioactive material. On Io, it is tidal heating. Because of its slightly elliptical orbit and its resonance with Europa and Ganymede, the tidal bulge on Io (its rock) can reach 100 meters. Since this tide changes over time, it acts like stretching and releasing a rubber band—it heats up. Therefore, about 50 km below the surface, some of the rock and other materials are molten. If there is a path to the surface, this material will flow up to and over the surface—a lava flow. However, because Io’s interior contains sulfur compounds, something else can happen—sulfur plumes (see the image of Io below). Think of steam escaping from a pressure cooker or tea kettle. The liquid and vapor (water in the kettle and sulfur in Io) are hot and under pressure and you get a plume of steam. When this liquid and vapor reach the surface, they can be traveling at about 1 km/s, fast enough to create plumes that are hundreds of kilometers high!
2. Is the reason we dont see stratovolcanoes on other planets because you need to have plate convergence and we have not seen evidence of this?
As an example, the volcanoes on Mars are shield volcanoes. Magmas that make shield volcanoes are mafic (high in magnesium and iron, low in silica) in composition. In order to form stratovolcanoes, the magma must be intermediate in composition (more silica). On Earth, these intermediate composition magmas are usually formed near converging plates where mafic magmas (from the ocean plates) and felsic (high silica) magmas (from the continental plates) mix. The absence of stratovolcanoes on Mars is one line of evidence that the planet never had plate tectonics like the Earth.
3. What about Saturn, Neptune, Uranus and Pluto? Any volcanoes there?
Saturn, Neptune and Uranus are gas planets and have no solid surface. Therefore, they do not have volcanoes. Some of their moons may have ice volcanoes. Pluto is an icy planet and although we do not have sufficient images of its surface to say for sure, it is unlikely that it has volcanoes because it is too cold.
4. Why can’t you see lava inside some calderas in Earth imagery?
If you cannot see lava in the calderas (summit craters) of volcanoes it is probably because the volcanoes are not currently active.
5. How can you tell a volcanic crater from an impact crater?
Volcanic craters may have a cone or flanks associated with the crater. There may also be flows present. Impact craters may have central peaks, ejecta, raised rims and floors that are lower in elevation than the surrounding terrain that can distinguish them from volcanic craters. During an impact event, the rocks that are impacted are shocked.
6. How can volcanoes work if there is no atmosphere?
Without an atmosphere, lava can still be forced out through a volcano and flow onto the surface – controlled by gravity. However, without an atmosphere to insulate the lava, it would cool very quickly on the surface.
7. Are volcanoes only extrusive or can they be intrusive?
The definition of a volcano is an opening in the earth through which magma, ash and gas can escape from the interior. By that definition, volcanoes are only extrusive. However, sometimes magma associated with a volcano can cool inside the volcano and create a volcanic neck, a dike or a sill. These features are intrusive.
8. What gases accumulate to cause pressurization in magma?
On Earth, the common gases in magmas erupting onto the surface are mostly water and carbon dioxide (minor components include sulfur dioxide and hydrogen sulfide). The magmas with large amounts of dissolved gases come from rocks on the ocean floors that are thrust down into the mantle and are melted along with gases dissolved in the melt. As the magma approaches the surface and pressures go down, the gases come out of solution like the bubbles in a bottle of shaken soda water and drive the magma onto the surface in a large explosion – like the Mount St. Helens eruption in 1982. Volcanoes like Kilauea in Hawaii have much smaller amounts of dissolved gases-- derived from great depths or rocks from the ocean crust picked up by the rising magma. The composition of the gases is the same: water, etc.
9. Are gases in the Moon's magma the same as Earth's magma?
Geologic evidence for volcanism, from the study of lunar photos, indicate that the amount to gas dissolved in lunar magmas was very small, even less than Hawaiian lavas. The gases are long gone – escaped into space billions of years ago - likely water and carbon dioxide. Both of these compounds are abundant in the materials out of which the terrestrial planets – including the Moon – were made, and so they are the most likely candidates.