What is the largest volcano on Mars?

The largest volcano on Mars is Olympus Mons. It is a gigantic shield volcano in the Tharsis region and rises about 21.1 kilometers above the surrounding Martian datum, making it the tallest volcano and mountain on Mars.

Olympus Mons grew through repeated eruptions of fluid basaltic lava. Mars does not have Earth-like moving tectonic plates, so the crust could remain over a volcanic source for a very long time. The planet’s weaker gravity and limited erosion also helped the volcano build an enormous, gently sloping profile.

The volcano is about 600 kilometers wide, roughly the size of Italy or the Philippines. Its summit contains a complex caldera formed by the collapse of several magma chambers. Its outer escarpment can reach about 8 kilometers high, adding to its dramatic scale.

Valles Marineris is a vast canyon system, not a volcano. Mount Everest is Earth’s highest mountain above sea level, while Mauna Kea is a large volcano on Earth, so neither is the answer to this Mars question.