volcano on the planet Mars, the highest point on the planet and the largest known volcano in the solar system. Centred at 19° N, 133° W, Olympus Mons consists of a central edifice 21 km (13 miles) high and 540 km (335 miles) across. Around its perimeter an outward-facing cliff ascends as high as 10 km (6 miles) above the surrounding area. At the summit is an 85-km- (53-mile-) diameter crater, or caldera, comprising several mutually intersecting craters. For comparison, the largest volcano on Earth, Mauna Loa, Hawaii, measures 120 km (75 miles) across at its widest extent and rises 9 km (5.6 miles) above the ocean floor. Broad, gradually sloping flanks and the presence of numerous long flows and lava channels identify Olympus Mons as a shield volcano and suggest that it was built up from eruptions largely of fairly fluid basaltic (basalt) lava. Its tremendous size has been attributed to the stability of the Martian crust and to a long accumulation time, possibly more than a billion years.