Large volcano on Mars , the largest known volcano in the solar system.
It consists of a central structure that ascends about 13 mi (21 km) above Mars's mean radius and is 335 mi (540 km) wide at the base; it is surrounded by an outward-facing cliff rising as much 6 mi (10 km) above the surrounding area. At the summit is a crater 53 mi (85 km) in diameter. For comparison, Earth's largest volcano, Mauna Loa , is 75 mi (120 km) wide at the base and rises 5.6 mi (9 km) above the ocean floor.