Mars' volcanic features can be likened to Earth's geologic hotspots. Pavonis Mons is the middle of three volcanoes (collectively known as Tharsis Montes) on the Tharsis bulge near the equator of the planet Mars. The other Tharsis volcanoes are Ascraeus Mons and Arsia Mons. The three Tharsis Montes, together with some smaller volcanoes to the north, form a straight line. This arrangement suggests that they were formed by a crustal plate moving over a hot spot. Such an arrangement exists in the Earth's Pacific Ocean as the Hawaiian Islands. The Hawaiian Islands are in a straight line, with the youngest in the south and the oldest in the north. So geologists believe the plate is moving while a stationary plume of hot magma rises and punches through the crust to produce volcanic mountains. However, the largest volcano on the planet, Olympus Mons, is thought to have formed when the plates were not moving. Olympus Mons may have formed just after the plate motion stopped. The mare-like plains on Mars are roughly 3 to 3.5 billion years old. The giant shield volcanoes are younger, formed between 1 and 2 billion years ago. Olympus Mons may be as young as 200 million years.