What is the tallest known volcano in the Solar System?

Olympus Mons is the tallest known volcano in the Solar System.

Located on Mars, Olympus Mons is a gigantic shield volcano measuring about 21.9 kilometres from its base on the surrounding plains to its summit. Its summit stands roughly 18 to 22 kilometres above the Martian datum, depending on the measurement used. The volcano’s enormous size is partly possible because Mars has lower gravity than Earth and lacks active plate tectonics that would move a crustal plate away from a volcanic hotspot.

Olympus Mons has very gentle outer slopes, typical of a shield volcano built by repeated flows of fluid basaltic lava. Near the summit is a complex caldera about 80 kilometres wide, formed when magma withdrawal caused the summit area to collapse in stages.

Although often called the Solar System’s highest mountain, comparisons vary because planetary elevations use different reference surfaces. Olympus Mons is unquestionably the tallest known volcanic edifice and is far larger than any volcano on Earth.