The mountain consists of a large stratovolcano of Pliocene-Pleistoncene age with a caldera breached to the west. Inca legends report that the top of Altar collapsed after seven years of activity in about 1460, but the caldera is considered to be much older than this by geologists. Nine major peaks over 5,000 metres (16,400 ft) form a horseshoe-shaped ridge about 3 kilometres (1.9 mi) across, surrounding a central basin that contains a crater lake at about 4,200 m (13,800 ft), known as Laguna Collanes or Laguna Amarilla.