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Most of the pyroclastic flows generated during the eruption Soufriere Hills volcano in Montserrat (above) were generated by partial collapses of a growing lava dome. The hot cascading lava fragments swept down the volcano's flanks as pyroclastic flows and sometimes as surges and spread across the countryside reaching distances as far as 2 to 4 km from the dome. Some pyroclastic flows were intiated by strong explosive events from the dome that ejected hot lava fragments onto the flanks of the volcano and then swept down slope (see effects of one of the largest pyroclastic flows from Soufriere Hills). Small- to moderate-sized pyroclastic flows typically move down hill away from an erupting vent and into low-lying areas, including gullies, canyons, and river valleys. In contrast, pyroclastic surges, being of relatively low density, are less confined to valleys. They tend to cover broad areas and can sweep up and over ridges adjacent to the vent or in their path. Since both pyroclastic flows and surges occurred during some eruptions along the Mono-Inyo chain, the hazard zones take into account the potential of surges to move some distance upslope as they move away from an erupting vent. |
