Geology of Mars of the dichotomy places a significant constraint on theories of its origin. Straddling the dichotomy boundary in Mars's western hemisphere is a massive volcano-tectonic province known as the Tharsis region or the Tharsis bulge. This immense, elevated structure is thousands of kilometers in diameter and covers up to 25% of the planet’s surface. Averaging 7–10 km above datum (Martian "sea" level), Tharsis contains the highest elevations on the planet and the largest known volcanoes in the Solar System. Three enormous volcanoes, Ascraeus Mons, Pavonis Mons, and Arsia Mons (collectively known as the Tharsis Montes), sit aligned NE-SW along the crest
Arsia Mons the three Tharsis Montes volcanoes, it is the only major Tharsis volcano south of the equator. The volcano is in diameter, almost high (more than higher than the surrounding plains), and the summit caldera is 110 km (72 miles) wide. It experiences atmospheric pressure lower than 107 pascals at the summit. Excluding Olympus Mons, it is the largest known volcano in terms of volume. Arsia Mons has 30 times the volume of Mauna Loa in Hawaii, the largest volcano on the Earth. The caldera of Arsia Mons was formed when the mountain collapsed in on itself after its reservoir of