Mars is the planet that has Olympus Mons, the tallest known volcano in the Solar System.
Olympus Mons is a giant shield volcano in Mars’s western hemisphere. It rises about 21.9 kilometres above the planet’s reference surface and spreads across a base roughly 600 kilometres wide. Its enormous size is possible partly because Mars has lower gravity than Earth and lacks Earth-like moving tectonic plates that would carry a volcanic region away from a persistent hotspot.
The volcano has gently sloping sides typical of shield volcanoes, formed by many eruptions of relatively fluid basaltic lava. Its summit contains a complex caldera made up of several overlapping collapse depressions. Olympus Mons is part of the Tharsis volcanic province, which also includes several other huge volcanoes.
Olympus Mons is often compared directly with Mount Everest, but the measurements use different reference surfaces and planetary conditions. Everest is taller above sea level on Earth, while Olympus Mons rises much farther above Mars’s reference level.