16. Olympus Mons
The largest of the martian volcanos is visible in this photograph taken by Mars Global Surveyor. The base of the volcano is surrounded by a huge scarp with 3–6 kilometers (2–4 miles) of vertical relief, a feature unique to this particular volcano. The scarp encircles an area roughly 500 kilometers (300 miles) in diameter and is comparable in size to the state of Arizona. The summit of Olympus Mons is about 26 kilometers (16 miles) above the plains around the base. The relatively shallow slopes on the flanks of the volcano give the entire mountain a shape similar to a shield, leading to the term “shield volcano” for features such as this. Near the summit is a group of nested craters forming a caldera complex, formed by repeated lava flooding and collapse of the summit area. Olympus Mons and several other martian volcanos (see slide #15) demonstrate that volcanism played a major role in the geologic evolution of Mars.
Mars Global Surveyor Press Release P-50134.