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News of the long-lived and not unusual cloud on Mars associated with 21 km tall Olympus Mons has people commenting. This tweet by planetary scientist Dr. Tanya Harrison says

I don't understand the conspiracy theory contingent that's going off re: the clouds over Arsia Mons.

What possible reason would we have to hide volcanic eruptions on Mars? Heck, NASA has shared plenty of pics of volcanoes erupting on Io (shown here) & geysers on Enceladus!

(see the tweet for the video)

Why exactly is the feature on Io called a volcano, and the feature on Enceladus called a geyser? I understand they are substantially different, but I'm asking about how the geological classification works on bodies with different materials and temperatures than those on Earth.

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The difference between a volcano and a geyser is defined by what is being ejected.

Volcanoes eject lava, which is molten rock. Molten rock that stays underground is called magma, but when it reaches the surface, usually via a volcano, it is called lava.

A rock is composed of minerals, such as sulphides and silicates. The volcanoes on Io eject lava that is "composed of various forms of elemental sulfur. The colouration of the flows was found to be similar to its various allotropes".

Some volcanoes on Earth also eject elemental sulfur, such as Mount Ijen in Indonesia.

Geysers on Earth typically eject water or steam. But elsewhere in the solar system cryogeysers exist; they eject "volatiles, together with entrained dust or ice particles, without liquid".

On Enceladus, the geysers eject water, ice particles "and smaller amounts of other components (such as carbon dioxide, nitrogen, ammonia, hydrocarbons and silicates)". Unlike in lava from volcanoes, the silicates ejected from geysers on Enceladus are a minor component of the ejected material.

In addition to volcanoes and geysers, there are cryovolcanoes, also known as ice volcanoes, on some bodies in the solar system. Like geysers, they eject volatiles "such as water, ammonia or methane, instead of molten rock. Collectively referred to as cryomagma, cryolava or ice-volcanic melt, these substances are usually liquids and can form plumes, but can also be in vapour form. After eruption, cryomagma is expected to condense to a solid form when exposed to the very low surrounding temperature."

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  • $\begingroup$ Thanks for tackling this question. What then would be the difference between a water-spewing geyser and a water-spewing cryovolcano? I realize you are still in the process of writing, sorry if I pre-empted your next paragraph. $\endgroup$ – uhoh Oct 26 '18 at 2:37
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    $\begingroup$ @uhoh: I need to look into it more, but my suspicion is that material from a cryovolcano ends up on the surface of the celestial body near the ejection site, whereas with a geyser the material may end up spread out over a larger area, or ejected into the cosmos. $\endgroup$ – Fred Oct 26 '18 at 2:43
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    $\begingroup$ @uhoh: another thought is the difference may be due to the density/viscosity of the material being ejected $\endgroup$ – Fred Oct 26 '18 at 2:45
  • $\begingroup$ Looking for my questions Enceladus; why use the words “geysers”, “jets”, and “plumes” interchangeably? and also How can “Geysers” on Europa reach heights of 100km? in order to link here, I also found my very similar yet unanswered question from almost two years ago. Is there an established distinction between a geyser and a cryovolcano? which is too old to migrate. These may be helpful, or not. I could delete that last question there. $\endgroup$ – uhoh Oct 26 '18 at 2:48

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