El Altar consists of a large stratovolcano of Pliocene-Pleistocene age with a caldera breached to the west. Inca legends report that the top of El 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 form a horseshoe-shaped ridge about across, surrounding a central basin that contains a crater lake at about , known as Laguna Collanes or Laguna Amarilla.