The Tharsis plateau, which sits in the highland-lowland boundary, is an elevated region that covers roughly one quarter of the planet. Tharsis is topped by the largest shield volcanoes known in the solar system. Olympus Mons stands 24 km tall and is nearly 600 km in diameter. The adjoining Tharsis Montes consists of Ascraeus, Pavonis, and Arsia. Alba Mons, at the northern end of the Tharsis plateau, is 1500 km in diameter, and stands 6 km above the surrounding plains. In comparison, Mauna Loa is a meager 120 km wide and stands 9 km above the sea floor.