Olympus Mons is Mars’s enormous shield volcano and the largest volcano in the Solar System.
Scientists estimate that Olympus Mons rises about 22 kilometres above the surrounding plains, while its summit stands roughly 21.9 kilometres above Mars’s global reference level. Its broad base spans approximately 600 kilometres, making it far wider than most terrestrial volcanoes. The volcano formed through repeated eruptions of fluid basaltic lava.
Olympus Mons could grow so large partly because Mars has lower gravity than Earth and lacks Earth’s moving tectonic plates. A volcanic hot spot could therefore remain beneath one region for a very long time, allowing lava flows to build the same structure repeatedly. Its summit contains a complex group of overlapping calderas formed when underground magma chambers emptied and collapsed.
A common mix-up is calling Olympus Mons the tallest mountain by every possible measurement. Earth’s Mount Everest has the greatest elevation above sea level, but Olympus Mons is generally described as the Solar System’s tallest volcano and highest planetary mountain above its surrounding terrain.