Olympus Mons is a large shield volcano on Mars. It is over 21.9 km high as measured by the Mars Orbiter Laser Altimeter (MOLA): about two and a half times taller than Mount Everest above sea level. It is Mars's tallest volcano, its tallest planetary mountain, and is approximately tied with Rheasilvia on Vesta as the tallest mountain currently discovered in the Solar System. It is associated with the volcanic region of Tharsis Montes. It last erupted 25 million years ago.
Mars Global Surveyor image showing lava flows of different ages at the base of Olympus Mons. The flat plain is the younger flow. The older flow has lava channels with levees along the edges. Levees are quite common to lava flows on Mars.
Lava flows on Olympus Mons with older and younger flows labeled, as viewed by HiRISE during the HiWish program
Calderas on the summit of Olympus Mons. The youngest calderas form circular collapse craters. Older calderas appear as semicircular segments because they are transected by the younger calderas.
Oblique view of Olympus Mons, from a Viking image mosaic overlain on MOLA altimetry data, showing the volcano's asymmetry. The view is from the NNE; vertical exaggeration is 10×. The wider, gently sloping northern flank is to the right. The more narrow and steeply sloping southern flank (left) has low, rounded terraces, features interpreted as thrust faults. The volcano's basal escarpment is prominent.
A shield volcano is a type of volcano named for its low profile, resembling a shield lying on the ground. It is formed by the eruption of highly fluid lava, which travels farther and forms thinner flows than the more viscous lava erupted from a stratovolcano. Repeated eruptions result in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form.
Mauna Loa, a shield volcano in Hawaii
An Ancient Greek warrior's shield—its circular shape and gently sloping surface, with a central raised area, is a shape shared by many shield volcanoes.
ʻAʻa advances over solidified pāhoehoe on Kīlauea, Hawaiʻi
A pāhoehoe lava fountain on Kīlauea erupts