24 September 2008

Low-Gravity Volcanoes

Posted by Ryan Anderson

During my series of posts about volcanoes last month, a reader emailed me and asked what the effect of lower gravity would be on martian volcanoes, and I thought it was such a good question that I decided to answer it here! Most of my answer is based on a (rather large) review paper by Wilson and Head that is available here.

One of the first effects of low gravity on a volcano is when the magma is rising from deep in the planet. If gravity is lower, things like the buoyant rise of magma beneath a volcano, the settling of crystals in a partially melted lava, and convection will all happen slower. Wilson and Head show that this would mean that dikes (cracks through which lava travels) would be several times larger, and that therefore the amount of lava erupted (the effusion rate) could be 5 times greater than on Earth. Also, on Mars, magma chambers are expected to be deeper than on the earth.

The effect on lava flows themselves are a little counter-intuitive. According to Wilson and Head, lava flows on Mars should be longer than the ones on Earth, due to the decreased gravity! I did a double-take when I read that because I assumed that with less gravity pulling lava downhill, the flows would not go as far. But the key is that lava stops when it cools down and stops acting like a liquid. On Mars, since gravity isn’t as strong, lava flows can be thicker, and that means that they don’t cool as effectively. Combined with wider dikes and therefore more lava, Wilson and Head predict that lava flows on Mars could b 6 times longer than those on Earth! That could certainly help to explain the giant volcanoes seen there!

The lower atmospheric pressure on Mars would mean that eruptions with any gas in the lava are much more likely to be explosive: the gas will expand much more violently in the near-vacuum of mars than it would under Earth atmospheric pressure. This means that gas-rich eruptions will bust rocks up into finer particles, and that the eruptions will go much faster. Fire-fountains on Mars would be twice as high as they are on earth. Also, the ash and debris produced in explosive eruptions would be spread out much farther. Small cone volcanoes on Mars would tend to be broader and shorter than on Earth. Pyroclastic flows, which can occur when a column of erupting gas collapses back on itself and then flows down the side of the mountain, would be more common on Mars and would travel three times as far.

One of the things that I love about planetary science is taking processes that we (think we) understand on Earth, and seeing how they work on other planets. Mars still has some atmosphere and some substantial gravity, so we still see things like shield volcanoes and cinder cones that we see on Earth. But you can extrapolate to places like the Moon or Io, where gravity and atmospheric pressure are even lower, and eruptions would be even more explosive. But, without any atmosphere, you can’t get things like pyroclastic flows, and there is no wind to carry the volcanic ash. On Venus, the gravity is about the same as the earth’s but the atmospheric pressure is a hundred times higher, and the surface is almost 900 degrees fahrenheit (~480 C). Pretty much all the volcanoes on Venus ooze rather than “pop”, and they ooze a long way because the lava cools so slowly. Finally, a really bizarre example is Titan. It has low gravity but high atmospheric pressure. It’s way below freezing at the surface and any lava will be made of molten ice instead of molten rock! It’s hard to image how volcanoes would work on such a world, but by analogy with the volcanoes that we do understand, we can make hypotheses. I don’t do that sort of work, but there are people who do. They may soon be able to test their hypotheses too: the Cassini mission may have seen a volcano through Titan’s haze.