Glowing Avalanches Pyroclastic flows are mixtures of very hot gas and tephra that cascade down a volcano's sides at high speeds. A pyroclastic flow covered the city of Herculaneum in A.D. 79, killing many residents. Because pyroclastic flows can spread destruction over large areas and move at very high speeds, they are extremely hazardous. Most people are not aware that this danger exists. Mud and Debris Flows Debris flows, fragments of mud and other debris that flow down the sides of a volcano, are another serious and little-known hazard.
Debris flows often form when part of the volcano collapses, breaking up and flowing downhill.
If the collapse is a major one, the large flow that results can travel great distances, often burying everything in its path. Other Dangers Lava, ash, and debris flows are the most common and serious volcanic hazards, but others do exist. Severe eruptions can disrupt the climate for long periods or cause atmospheric shock waves. Eruptions can also cause tsunamis, volcanic earthquakes, or the release of suffocating gases. These hazards pose serious threats to both life and property.