<P> The volcano is located in Mars's western hemisphere at approximately 18 ° 39 ′ N 226 ° 12 ′ E ﻿ / ﻿ 18.65 ° N 226.2 ° E ﻿ / 18.65; 226.2, just off the northwestern edge of the Tharsis bulge . The western portion of the volcano lies in the Amazonis quadrangle (MC - 8) and the central and eastern portions in the adjoining Tharsis quadrangle (MC - 9). </P> <P> Two impact craters on Olympus Mons have been assigned provisional names by the International Astronomical Union . They are the 15.6 km (9.7 mi) - diameter Karzok crater (18 ° 25 ′ N 131 ° 55 ′ W ﻿ / ﻿ 18.417 ° N 131.917 ° W ﻿ / 18.417; - 131.917) and the 10.4 km (6.5 mi) - diameter Pangboche crater (17 ° 10 ′ N 133 ° 35 ′ W ﻿ / ﻿ 17.167 ° N 133.583 ° W ﻿ / 17.167; - 133.583). The craters are notable for being two of several suspected source areas for shergottites, the most abundant class of Martian meteorites . </P> <P> As a shield volcano, Olympus Mons resembles the shape of the large volcanoes making up the Hawaiian Islands . The edifice is about 600 km (370 mi) wide . Because the mountain is so large, with complex structure at its edges, allocating a height to it is difficult . Olympus Mons stands 21 km (13 mi) above the Mars global datum, and its local relief, from the foot of the cliffs which form its northwest margin to its peak, is nearly 22 km (14 mi) (a little over twice the height of Mauna Kea as measured from its base on the ocean floor). The total elevation change from the plains of Amazonis Planitia, over 1,000 km (620 mi) to the northwest, to the summit approaches 26 km (16 mi). The summit of the mountain has six nested calderas (collapsed craters) forming an irregular depression 60 km (37 mi) × 80 km (50 mi) across and up to 3.2 km (2.0 mi) deep . The volcano's outer edge consists of an escarpment, or cliff, up to 8 km (5.0 mi) tall (although obscured by lava flows in places), a feature unique among the shield volcanoes of Mars . Olympus Mons covers an area of about 300,000 km (120,000 sq mi), which is approximately the size of Italy, and it is supported by a 70 km (43 mi) thick lithosphere . The extraordinary size of Olympus Mons is likely because Mars lacks mobile tectonic plates . Unlike on Earth, the crust of Mars remains fixed over a stationary hotspot, and a volcano can continue to discharge lava until it reaches an enormous height . </P> <P> Being a shield volcano, Olympus Mons has a very gently sloping profile . The average slope on the volcano's flanks is only 5 ° . Slopes are steepest near the middle part of the flanks and grow shallower toward the base, giving the flanks a concave upward profile . The shape of Olympus Mons is distinctly asymmetrical - its flanks are shallower and extend farther from the summit in the northwestern direction than they do to the southeast . The volcano's shape and profile have been likened to a "circus tent" held up by a single pole that is shifted off center . </P>

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