Areas as far as 15 km (10 mi) from an explosive eruption could be swept by hot, fast-moving pyroclastic flows and surges.
A pyroclastic flow is a ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 100 km/hour (over 62 mi/hr) or more. The temperature within a pyroclastic flow may be greater than 500° C (932° F), sufficient to burn and carbonize wood. Once deposited, the ash, pumice, and rock fragments may deform (flatten) and weld together because of the intense heat and the weight of the overlying material.
Scientists examine the distances that pyroclastic flows are known to have traveled in the past few thousand years to determine areas where they might occur in the future. The actual areas covered by pyroclastic flows or surges during a future explosive eruption depends primarily on the location of the vent(s), the surrounding topography, and volume of magma erupted.
Pyroclastic flows generated from vents on Mammoth Mountain could travel farther than 15 km (10 mi) because of the extra momentum that would be gained as the flows descend hundreds of meters down Mammoth's steep flanks and move across low-lying areas.
This hazard zone is based on explosive eruptions from vents located along the chain in the past 10,000 years that are known to have ejected <1 km3 (0.25 mi3) of magma and generated pyroclastic flowsor surges. The zone is centered along the south moat of the caldera, which is the location of epicenters of many swarms of earthquakes since 1980 and the area of most intense ground movement (deformation). Thus, scientists suggest that future eruptions might occur from this restless zone, and pyroclastic flows and surges could travel as far as 15 km (10 mi) from a new vent. Future pyroclastic flows and surges from a single eruption in the south moat would affect only a part of the total hazard zone shown on the map.
Potential pyroclastic flow and surges hazards for small to moderate-sized eruptions in the greater Long Valley Caldera region, California
This hazard zone is based on explosive eruptions from vents located along the chain in the past 10,000 years that are known to have that ejected <1 km3 (0.25 mi3) of magma and generated pyroclastic flows or surges. Future pyroclastic flows and surges from a single eruption from along the Mono-Inyo chain or in the south moat would affect only a part of the total hazard zones shown on the map.