This mountain is the largest of the four Tharsis volcanoes. Spanning approximately 500 km (300 mi) horizontally and rising over 24 km (14.4 mi) above the surrounding plains, Olympus Mons is one of the largest volcanic constructs in the solar system. The volcano is so large that an accurate representation of the shape of the mountain must include the curvature of the planet’s surface. The average slope of the flanks of the volcano is about 5º (from horizontal), similar to terrestrial shield volcanoes. The summit of the volcano consists of a group of nested, quasicircular depressions; these depressions are very similar to calderas present at the summits of terrestrial shield volcanoes. Calderas on shield volcanoes are primarily the result of collapse, presumably related to the withdrawal of magma from the interior of the construct, rather than the direct result of explosive removal of summit material. Olympus Mons is surrounded by a scarp 3 to 6 km (1.8 to 3.6 mi) high that is unique among the martian volcanoes.
18 N, 133 W; Viking Orbiter frame 646A28; shading corrected version.