Olympus Mons is a shield volcano 624 km (374 mi) in diameter (approximately the same size as the state of Arizona), 25 km (16 mi) high, and is rimmed by a 6 km (4 mi) high scarp.
Volcanoes erupt because of density and pressure. The lower density of the magma relative to the surrounding rocks causes it to rise and erupt.
Volcanoes stop erupting because all the trapped volatile gasses have degassed and there is no longer sufficient pressure to drive the magma out of the Earth.