Located at the summit of most volcanoes is a somewhat funnel-shaped depression, called a crater (crater = a bowl). Volcanoes that are built primarily of pyroclastic materials typically have craters that form by gradual accumulation of volcanic debris on the surrounding rim. Other craters form during explosive eruption as the rapidly ejected particles erode the crater walls. Craters also form when the summit area of a volcano collapses following an eruption. Some volcanoes have very large circular depressions called calderas that have diameters greater than one kilometer and in rare cases can exceed 50 kilometers. We consider the formation of various types of calderas later in this chapter.