Flat-topped, steep-walled depression, shaped like a horseshoe and resembling an ancient Greek theatre. It may be the result of glacial erosion, forming a cirque, or the collapse of a volcano. In the case of Mount St Helens, the catastrophic collapse of the summit and northern slopes of the cone and the subsequent pyroclastic flow eruption on 18 May 1980 created a north-facing amphitheatre enclosing a small active vent, later occupied by a small dacite dome.