Answer

Olympus Mons, the tallest volcano in the solar system at roughly 21 km above the Martian datum, sits on the Tharsis Plateau in the western hemisphere of Mars. The Tharsis region is a massive volcanic highland that also hosts three other large shield volcanoes known as the Tharsis Montes. The low gravity and lack of tectonic plate movement on Mars allowed magma to keep erupting in one spot for billions of years, building Olympus Mons to its extraordinary height.