Pyroclastic Surges

The most serious hazard posed by a hydromagmatic eruption is a pyroclastic surge. Surges are mixtures of air, volcanic gas, steam, and magma or rock fragments that move along the ground surface at high velocities (Waters and Fisher, 1971). Surges differ from pyroclastic flows in that they contain less solid debris, and are therefore less dense and more capable of flowing over topographic barriers. Surges may transport debris away from vents at velocities up to hundreds of meters per second (many hundreds of miles per hour). With temperatures that range from the boiling point of water to the temperature of magma, they can destroy or incinerate most structures and living things in their path.

The distance that a surge travels from its source is greatly dependent on the type of eruption. Discrete explosions at well-observed volcanoes typically send surges only a kilometer or so from the vent, though some larger explosions can produce surges that travel several kilometers. The most mobile surges are generated by the most violent hydromagmatic eruptions that combine an influx of water and high rate of magma discharge over sustained periods of time. Such eruptions generate columns of ash and debris extending several kilometers or more into the atmosphere. If all or part of the gas/magma mixture in those columns is heavier than air, it falls back to earth, in some cases from plumes that have drifted kilometers from the vent. Gravity-driven descent of particle-laden clouds can accelerate them to high velocities. Surges from such eruptions have extended more than 30 kilometers from their source vents.