Which planet has the largest volcano in the solar system?

Olympus Mons on Mars is the largest volcano known in the solar system. Its summit stands about 22 km high, nearly three times the height of Mount Everest above sea level, and its base is roughly 600 km wide. The volcano is a shield volcano, built by many fluid lava flows over a very long time. Scientists think it could have grown so large because Mars has lower gravity and no moving tectonic plates like Earth.