Ash flows ultimately stop because the gas finally escapes from between the particles. Glowing avalanches usually leave thin deposits of loose fragments like sand mixed with gravel. The ash fragments are hot, however, and deposits of ash have been found that are so thick that the hot particles actually fused together to form solid masses of rock after they stopped flowing. Geologists have never actually watched the formation of such a deposit, but the amount of ash in these deposits is so large that it requires ash flows hundreds of times larger than the glowing avalanches that have been seen. The destructive power in ash flows is a result of both the high flow speed and the high temperatures of the gas and fragmented rock. Even relatively small ash flows can be incredibly destructive: A glowing avalanche destroyed the town of St. Pierre in less than 30 seconds. The force of the flow literally ripped buildings apart all over town; only the sturdiest of stone walls were left standing. All but two of the 30,000 inhabitants of St. Pierre were asphyxiated or burned to death by the dense, superhot cloud. Ash flows occur in explosive eruptions and have been a significant problem with eruptions in Japan and the American Cascades. Photo: Courtesy of NGDC/NOAA

Ash falls are less devastating than ash flows but can be very disruptive to modern life. Ash falls are the blanketing deposits dropped downwind by the clouds of ash thrown into the atmosphere by explosive volcanoes. These deposits may be a thin dusting or a thick layer of grit. They appear grayish to whitish and look like fallen snow, but, unlike snow, ash deposits do not melt and must be physically removed. The ash is heavy and can collapse roofs, break branches, and coat the leaves of plants. Unless the plants are cleaned (manually or by rain), they can die. This is a significant problem for crops. The ash is particularly hard on machines. Some ash particles are so small that they can pass through the engine's air filters and ruin the engine. This image shows the ash layer on cars near Mount Pinatubo. Photo: USGS EROS Data Center

Rock/Debris avalanches are masses of cold, dry rock that have broken free from the sides of a volcano. They are driven downslope by gravity, smashing everything in their path by the force of their motion and then burying what remains under a mass of rock and debris. A good example of a rock avalanche triggered by an eruption is the mass of rock that broke off the north face of Mount St. Helens and buried the valleys to the north. However, an eruption is not needed to trigger a rock avalanche. Simple undermining of a mass of rock or an earthquake unrelated to an eruption can start a rock avalanche. Of course, such rock avalanches can occur on any steep mountain slope, but volcanoes are particularly prone to rock avalanches because their sides consist of outward-sloping layers of solid and fragmental rock (it is easier to slide down a sloping tabletop than across the ends of a bunch of stacked tables), and volcanic rock is easily changed by water into slippery clay, which promotes failure and sliding.

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