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The central ediface stands 27 kilometres (88,600 feet) high over its base (over three times the height of Mount Everest); it reaches 25 kilometres above the mean surface level of Mars, since it stands in a two-kilometre-deep depression. It is 540 km (335 miles) in width, flanked by steep cliffs, and has a caldera that is 85 km (53 miles) long, 60 km (37 miles) wide, and up to 3 km (1.8 miles) deep with six overlapping pit craters. Its outer edge is defined by an escarpment up to 6 km (3.7 miles) tall unique among the shield volcanoes of Mars.
Olympus Mons is an apparently extinct shield volcano, the result of highly fluid lava flowing out of volcanic vents over a long period of time, and is much wider than it is tall; the average slope of Olympus Mons' flanks is very gradual.
The volcano is surrounded by a region known as the Olympus Mons Aureole ( Latin, "Circle of Light") with gigantic ridges and blocks extending 1000km (621 miles) from the summit that show evidence of development and resurfacing connected with glacial activity. Both the escarpment and the Aureole are poorly understood. In one theory this basal cliff was formed by landslides and the Aureole consists of material piled up at the bottom of these landslides.
The Hawaiian islands are examples of similar shield volcanoes on a smaller scale (see Mauna Loa). The extraordinary size of Olympus Mons is likely due to the fact that Mars does not have tectonic plates. Thus, the crust remained fixed over a hot spotIn geology, a hotspot is a location on the Earth's surface that has expreienced active vulcanism for a long period of time. Tuzo Wilson came up with the idea in 1963 that volcanic chains like the Hawaiian Islands result from the slow movement of a tectoni and continued to discharge lava, bringing the volcano to such a height.
Olympus Mons is located in the Tharsis bulge, a huge swelling in the Martian surface that bears numerous other large volcanic features. Among them are a chain of lesser shield volcanoes including Arsia MonsArsia Mons is the southernmost of three volcanos (collectively known as Tharsis Montes) on the Tharsis bulge near the equator of the planet Mars. To its north is Pavonis Mons, and north of that is Ascraeus Mons. The largest volcano in the solar system, Ol, Pavonis MonsPavonis Mons is the middle of three volcanos (collectively known as Tharsis Montes) on the Tharsis bulge near the equator of the planet Mars. To its north is Ascraeus Mons, and to its south is Arsia Mons. The largest volcano in the solar system, Olympus M and Ascraeus MonsAscraeus Mons is the northernmost of three volcanos (collectively known as Tharsis Montes) on the Tharsis bulge near the equator of the planet Mars. To its south is Pavonis Mons, and south of that is Arsia Mons. The largest volcano in the solar system, Ol, which are small only in comparison to Olympus Mons itself. The land immediately surrounding Olympus Mons is a depression in the bulge 2km deep.
The size of Olympus Mons is so great (roughly the size of the American state of MissouriMissouri named after the Missouri Siouan Indian tribe meaning "canoe", is a Midwestern state of the United States with Jefferson City as its capital. The state's nickname is the State the U. Post Office abbreviation for Missouri is MO and the state public) that a person standing on the surface of Mars would be unable view the profile of the volcano even from a distance as the curvature of the planet would obscure such detail. The only way to view the mountain properly is from orbit.
In the days before space probes revealed its identity as a mountain, Olympus Mons was known to astronomers as the albedo feature, Nix Olympica.