Olympus Mons, a large shield volcano on the planet Mars, is the highest and largest known volcano in the solar system. The volcano is located in Mars’s western hemisphere at approximately 19° N, 133° W. It stands 14 miles high and 435 miles across and is almost three times as tall as Mount Everest’s height above sea level. A caldera 50 miles wide is located at it’s summit.
The extraordinary size of Olympus Mons is likely because Mars does not have tectonic plates. Thus, the crust remained fixed over a hotspot and the volcano continued to discharge lava, reaching ever expanding heights.
On Earth the Hawaiian islands resulted from the northwesterly movement of the Pacific plate over a stationary hotspot producing lava. As the plate moves over the hotspot, new volcanoes formed and the existing ones disappeared. This distributes the total volume of lava among many volcanoes rather than one large volcano.
On Mars, the crust remains stationary and the lava piles up in one, very large volcano. Crater counts indicate that lava flows on the northwestern flank of Olympus Mons range in age from 115 million years to only 2 million years old, suggesting that the mountain may still be volcanically active, though in a very quiescent and episodic fashion.