Mars has the Solar System’s largest known volcano, Olympus Mons.
Olympus Mons is a gigantic shield volcano in the Tharsis region of Mars. It rises about 22 kilometers above the surrounding plains, although the exact height depends on the reference surface used. Its broad base spans roughly 600 kilometers, and its summit contains a complex caldera formed by repeated collapse as magma was withdrawn.
The volcano grew to such a size partly because Mars has lower gravity than Earth and lacks Earth’s active plate-tectonic system. A stationary volcanic hotspot could therefore build one enormous structure over a long period. Olympus Mons is much wider and less steep-sided than a typical composite volcano. Spacecraft observations show that Mars was volcanically active in the geologically recent past, though Olympus Mons is not currently known to be erupting.