Ash fall on village in Indonesia from eruption of Galungung volcano, in 1982. Photograph by Maurice and Katia Krafft.

Ash fall on hut 3 km from rim of Pinatubo volcano, Philippines, after the major 1991 eruption. Photograph by R. P. Hobblitt, U.S. Geological Survey.

What is Pyroclastic Fallout?

Eruption of Anak Krakatau, Indonesia, September, 1979, sending ash into the atmosphere. Photograph by Maurice and Katia Krafft.

Pyroclastic fallout consists of particles that have been ejected from vents and have travelled through the atmosphere before falling to earth or into water. Fallout ash can also be derived from elutriation of ash the boils up from pyroclastic flows as they travel across the land. Pyroclastic material that falls on land is "subaerial fallout," and that falling into water is subaqueous fallout.

Transport of pyroclastic fallout material is by ballistic trajectory and by turbulent suspension. Energy is supplied initially to fragments by the eruption and later by wind. Tephra falling from above characteristically drapes over the landscape.

Pre-historic ash fall layers that blanket earlier topography. Oshima Island, Japan. Photograph by Fisher.

Troubles with Long-Distant Pyroclastic Fallout

Lessons learned at Mount St. Helens, Mt. Pinatubo and many other eruptions are that it is difficult to clean up after the fall of just a few centimeters of ash, and that ash in the atmosphere severely damages jet engines, thereby endangering the lives of passengers.

Sakurajima volcano, Kyushu, Japan. A volcano in frequent small-magnitude eruption.

Living with an erupting volcano. Sakurajima city, located on the flanks of Sakurajima volcano, is commonly dusty from frequent eruptions. The photograph shows ash being stirred by traffic after fallout of a small eruption. Children often go to school wearing hardhats as protection against occasionally larger ballistic lapilli.

Copyright (C) 1997, by Richard V. Fisher. All rights reserved.