How Volcanoes Work

 Image Described in Caption
 Plinian pumice-fall deposit -- The white layer in the middle of the outcrop is a two-meter-think deposit composed of pumice lapilli (1-2 cm in diameter) in Arequipa, Peru. This deposit mantles an area of many hundreds of square miles. This large areal extent is typical of Plinian airfall which is derived from high eruptive columns, often reaching altitudes in excess of 30 km. The pumice-fall deposit is overlain by other airfall deposits of variable compositions, the darker layers being more mafic than the lighter layers. The age of the Plinian deposit is unknown. Courtesy of Peter Francis.