Contrary to popular belief, Haleakalā crater is not volcanic in origin, nor can it accurately be called a caldera (which is formed when the summit of a volcano collapses to form a depression). Scientists believe that Haleakalā's crater was formed when the headwalls of two large erosional valleys merged at the summit of the volcano. These valleys formed the two large gaps — Koʻolau on the north side and Kaupō on the south — on either side of the depression.