It’s been said that the reason dinosaurs are fascinating is because they’re ‘big, fierce and extinct.’ Olympus Mons, or Mount Olympus, on Mars almost fulfills those criteria – it’s the solar system’s biggest volcano, it’s a volcano, and seeing it’s on another planet it’s as inaccessible as dinosaurs.
Firstly the stats –
Olympus Mons is a shield volcano, like the Hawaiian volcanoes on earth. But it is 25km high and 624km in diameter, which makes it about 3 times taller than Mt Everest and bigger than Victoria. If you were standing on the top of it you would not be able to see the flat plain around you because it would be below the horizon.
It has a complex caldera 80km wide with six craters that have been formed by collapses. The two youngest can be seen as circles, overlying older craters. Lava flows have been discovered that are over 200 million years old, ranging down to others less than 30 million years old. We can tell it is geologically young because it has few impact craters compared with the moon.
There is a scarp around the base that is up to 6km high. It’s not certain how it formed, there are many different theories including earthquakes and faulting, which you would expect near an active volcano, or even erosion. There is some erosion on Mars because of wind and dust storms, however with the thin atmosphere they aren’t as strong as on earth. Ice has been found beneath the surface of Mars as well as large clay deposits, indicating there was possible water erosion in the past.
Why is it so big?
We have a reasonable understanding of how shield volcanoes form, which suggests why this one grew so big. The Hawaii volcanoes are on top of a ‘hot spot’ which gives them a constant build-up of magma underneath, causing frequent eruptions. However the Pacific plate is slowly moving over the top while the hot spot stays still, leading to a line of extinct volcanoes that get older as you move away from the active ones.
Mars does not have tectonic plates, but it still had molten magma under the mantle. Olympus Mons formed above a hot spot, with multiple eruptions over millions of years, but because the crust isn’t moving all the eruptions happened in one place. It is so big because it is all of the Hawaiian volcanoes piled on top of each other.
Like Earth, Mars does have a core, mantle and crust. However Mars is much smaller than Earth, which causes two problems as far as tectonic plates are concerned. Firstly, the heat to drive movements is absolutely much smaller. It needs enormous amounts of energy to move parts of a planet around – they are incredibly heavy. The Martian core is too small and too far down to provide this type of energy.
Secondly, heat is produced in the volume of the core, but it is lost through the surface area of the planet’s crust. Because of the way these are related, small objects have relatively much larger surfaces. Think about mice – they get cold much easier than an elephant. So poor Mars had a smaller core engine to start with, and it lost its heat to space more easily. This led to a thicker cold crust forming, which is too thick to split and too heavy to move.
Like fossil dinosaurs, technology is bringing Mars tantalisingly within reach, showing us a mix of the familiar and the bizarre. It gives us a sense of ‘could have been.’ I, for one, hope we make it there during my lifetime.
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