Olympus Mons, a shield volcano, is the tallest volcano in the solar system. It rises 27 kilometers (17 miles) high, spreads over 600 km (370 mi) at the base, and is surrounded by a well-defined scarp that is up to 6 km (4 mi) high. Lava flows drape over the scarp in places. Much of the plains around the volcano are covered by ridged and grooved terrain called the aureole. The origin of the aureole is controversial, but may be related to material sliding off of the flanks of an ancestral volcano. The summit