This massive rounded boulder on the SW side of Cotopaxi volcano was carried in a lahar, or volcanic mudflow, possibly during a major eruption in 1877. Scale is provided by volcanologists Minard Hall and Patty Mothes of Ecuador's national university, who have spent many years assessing hazards from Ecuador's volcanoes. More than 130,000 persons live in areas subject to lahar risk from Cotopaxi. The 1877 eruption produced lahars that covered this valley, swept into the Amazon basin, and reached the Pacific Ocean along valleys to the NW.
The glacier-capped summit of Ecuador's Cotopaxi volcano is truncated by two nested craters. The outer crater, seen here from the SE, is 800 x 550 m wide. A cone that grew inside this crater is cut by a smaller crater that is 250 m wide and 120 m deep. Frequent explosive eruptions during historical time have modified the shape of the summit crater. In 1903, prior to growth of a broad central cone, it was 450 m deep.