Haleakalā is a massive shield volcano that forms more than 75% of the Hawaiian Island of Maui. The western 25% of the island is formed by another volcano, Mauna Kahalawai, also referred to as the West Maui Mountains.
The tallest peak of Haleakalā ("house of the sun"), at 3055 m, is Puʻu ʻUlaʻula (Red Hill). From the summit one looks down into a massive depression some 11.25 km across, 3.2 km wide, and nearly 800 m deep. The surrounding walls are steep and the interior mostly barren-looking with a scattering of volcanic cones.