Olympus Mons
Photo Mosaic

Olympus Mons is the largest volcano in the solar system.  It stands 17 kilometers high, which is three times as high as Mount Everest.  The caldera, the depression at the center of the volcano, spans 80 kilometers across.  Olympus Mons is a shield volcano, similar to those found in Hawaii, but considerably larger.   One reason for this is that Mars does not have plate tectonics, the movement of a planet's crust.  Consequently, the volcano is situated above a hot spot for millions of years, allowing for repeated eruptions that build up the surrounding structure.  There is some evidence that Olympus Mons may be the remains of an even larger volcano.

The lava that flowed from Olympus Mons created snakelike channels that stretch several hundred kilometers long and 200 meters across.  These flows are much larger than their Earth counterparts, which rarely extend more than 30 kilometers, and are not much wider than 10 to 30 meters.

Olympus Mons has been extinct for several hundred million years, which is relatively young in geological terms and makes it the youngest volcano on Mars.