<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> 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>
the extinct martian volcano olympus mons is about the size of
the extinct martian volcano olympus mons is about the size of
Italy
Italy