Olympus Mons is a volcano that originally erupted under a Martian glacier. It was a shield volcano, a type that doesn’t really explode. It just kinda dribbled out lava like a runny nose. It erupted for a long time, producing enough rock to make the mountain. Olympus Mons has about the same footprint as Poland.
Volcanic heat made a melty spot in the middle of that Martian glacier. The lava could only flow so far before it reached the limit of the melty spot, which defined the boundary of the glacier and the volcano. The sides look so steep because the lava couldn’t spread out, it just piled up higher and higher, confined by the walls of the hole it melted in the glacier. It basically created a steep-sided mold for itself, then filled it.
The glacier later melted, leaving behind what we see today. Volcanos like this also exist on Earth, notably in Greenland.
Both mountains are measured from the datum used for their planet. On Earth, we measure things above sea level. On Mars, which has no liquid surface water, the datum is the elevation at which the atmospheric pressure is 610.5 Pa (lowest pressure for which liquid water is stable). It’s kinda like sea level for Mars. If there was liquid surface water, it couldn’t exist above that elevation.
Notably, Olympus Mons is only about 22 km tall when measured from its base. It sits in an area that’s already slightly elevated. The same holds true for Mt. Everest, which is about 3.5 km tall when measured from surrounding valleys. This is a measurement known as prominence, how far something sticks out of its surroundings.
There are all sorts of ways to define the height of a mountain depending on where you measure from. My favorite is distance from the center of the planet, which puts Chimborazo as the most extreme terrestrial point on Earth.
Someone correct me if I'm wrong, but it did evaporate. When the interior of the planet solidified, it's magnetic field was lost, which allowed the sun's radiation to strip the atmosphere and seas away.
There used to be, though some of it is still around at the poles. To my understanding, it evaporated and was stripped away in the solar wind (like the atmosphere) when Mars lost its magnetic field.
It is a shield volcano, meaning the volcano spews out lava that very slowly spreads outwards from a centre point, causing this type of geological feature
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u/[deleted] Oct 07 '22
Can someone please explain to me why the outside edges are so eroded? Or at least appear to be eroded?