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What might you expect of volcanoes erupting on other planets similar to Earth, such as Mars or Venus?
Venus has a thick atmosphere of carbon dioxide, but water vapor is apparently absent from these planets.
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This is a great question! While no volcanic activity is currently being observed on any of the terrestrial plants except for Earth, there are lots of signs that it has happened extensively in the past on our neighboring planets.(However several "hotspots" on Venus were recently identified, check out the second link below for more info on that.)
One big difference appears to be that the other terrestrial planets lack plate tectonics movements. Here on Earth these movements tend to create chains of volcanoes, as the plate is dragged across an underlying hot spot in the mantle. Without mantle movement, the same area of plate will remain above the hot spot for millennia, resulting in a single volcano of truly massive size, as opposed to a chain of smaller ones. Another result of this lack of motion is that volcanoes on other terrestrial planets tend to be shield type volcanoes.
The lower gravity of our slightly smaller neighboring planets also allows for the formation of immense lava plains. There is a lava plain on Mars that is the size of the state of Oregon, and it is believed to have been formed in less than a year; some scientists believe it may have formed in just a few weeks!
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In terms of mass and radius, Venus is the most similar to Earth. Venus is about 0.82 Earth masses and 95% the radius of the Earth. Near the surface of Venus, there is very little wind. The atmosphere on Venus consists of carbon dioxide (96%) and nitrogen (3%). The atmosphere is so thick that the pressure at the surface of Venus is 90 times the atmospheric pressure on the surface of the Earth. Due to the high atmospheric pressure, the winds on Venus are also relatively slow. Thus, neither wind nor rain can really affect the surface on Venus. As a result, volcanic features will look freshly formed for a long time. Venus shows no evidence for plate tectonics. There are no long, linear volcano chains. There are no clear subduction zones. Although rifts are common, none look like the mid-ocean ridges on Earth. Also, continent-like regions are rare, and show none of the jigsaw fits seen on Earth. Thus, where volcanism on Earth mostly marks plate boundaries and plate movements, volcanism on Venus is much more regional and much less organized. In addition, volcanism on Venus shows fewer eruptive styles than on the Earth. Almost all volcanism on Venus seems to involve fluid lava flows. There is no sign of explosive, ash-forming eruptions on Venus, and little evidence for the eruption of sludgy, viscous lavas. This may reflect a combination of several effects. First, due to the high air pressure, venusian lavas need much higher gas contents than Earth lavas to erupt explosively. Second, the main gas driving lava explosions on Earth is water, which is in very short supply on Venus. Lastly, many viscous lavas and explosive eruptions on Earth occur near plate subduction zones. Thus, the lack of subduction zones should also reduce the likelihood of such eruptions on Venus.
Mars, on the other hand, is the most distant terrestrial planet from the Sun. The mass of Mars is about one-tenth the mass of the Earth and Mars is about half as large in diameter. Like the Earth, Mars has polar caps; unlike the Earth, one of these is mostly CO2 ice. Temperatures on Mars range from 133 K to 293 K, and the atmospheric pressure at the surface is very low, about 1/100 the pressure at the surface of the Earth. The atmosphere on Mars is 95% CO2, similar to the atmosphere of Venus. Because the atmosphere is so much thinner, however, there has been no runaway greenhouse eﬀect on Mars. The atmosphere is thick enough to support tenuous water ice clouds, and enormous dust storms, some of which are over a mile high. Although Mars has no plate tectonic activity, the surface of Mars has been shaped by geologic activity. The Tharsis Plateau contains a string of volcanoes, the largest of which is Olympus Mons. Olympus Mons grew to such enormous size because the hole in the crust that brought magma to the surface stayed stationary over time. Also spectacular is Valles Marineris, whose feature is not the result of plate tectonics, as it would be on the Earth. Instead, Valles Marineris is the result of the cooling and shrinking crust splitting open at the equator, much like the surface of drying clay.
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