Mars has a volcano so wide you couldn't see its summit from its slopes.
Olympus Mons on Mars rises more than 21 kilometres, roughly two and a half times the height of Everest, and covers an area about the size of Italy. Its slopes are so gentle and broad that, standing on it, you would not realise you were on a mountain at all.
Olympus Mons is a shield volcano, the same broad, gently sloping type as the volcanoes of Hawaii, built by many thousands of runny basalt lava flows. Laser altimeter measurements put it at about 21.3 kilometres high, making it the tallest mountain on Mars and roughly tied with Rheasilvia, a peak on the asteroid Vesta, as the tallest known in the Solar System. It is about 600 kilometres across and covers some 300,000 square kilometres.
The scale plays tricks on the eye. Its average slope is only about five per cent, so a person on its flanks would see the ground stretching gently beyond a horizon just three kilometres away, and would not be aware of standing on a giant. Even from a distance, the curve of the planet would hide its full profile. At the summit sit six overlapping collapsed craters, or calderas, forming a depression up to 80 kilometres across and more than three kilometres deep. Around its edge runs a cliff up to eight kilometres high.
Why is it so big? Mars has weaker gravity than Earth and gentler erosion, but the key is that it lacks moving tectonic plates. On Earth, a plate drifts over a volcanic hotspot, so the lava builds a chain of islands like Hawaii. On Mars the crust stays put above the hotspot, so a single volcano keeps growing.
Astronomers knew it long before spacecraft arrived, as a bright patch called Nix Olympica, the 'snow of Olympus'. It is also surprisingly young. It is thought to have last erupted about 25 million years ago, although crater counts suggest some lava flows could be as young as 2 million years, raising the possibility that it is not entirely extinct. The air at its peak is about 12 per cent of the already thin Martian surface pressure, yet clouds still drift over it.
Source: Wikipedia — Olympus Mons · Text summarised from Wikipedia (CC BY-SA 4.0)
