The martian volcano Olympus Mons (18°N, 133°W) is one of the largest volcanos in the solar system, measuring over 600 kilometers across and rising more than 27 kilometers above the surrounding plain. In this view, Olympus Mons is compared to the Hawaiian Islands, and demonstrates the value of comparative planetary volcanology. Notice that the Island of Oahu would easily fit inside the summit caldera of the martian volcano. Volcanologists on Earth have a good understanding of Hawaiian volcanism, but the eruptive style and duration of planetary examples such as Olympus Mons remain poorly understood.
Viking Orbiter 646A28.