Pyroclastic Flows

Because of its elevation (4,392 m), relief, hydrothermal alteration, icecap, glacier-fed radial valleys, and proximity to encroaching suburbs of the Seattle-Tacoma metropolis, Mount Rainier is the most threatening volcano in the Cascades. Its next eruption could produce volcanic ash, lava flows, and avalanches of intensely hot rock and volcanic gases, called pyroclastic flows.

Volcanism occurs at Mount Rainier and other Cascades arc volcanoes because of the subduction of the Juan de Fuca Plate off the western coast of North America.

Timeline of eruptions at Mount Rainier.

Mount Rainier erupts explosively to produce small to moderate volumes of tephra, but volumetrically the volcano is primarily effusive.

We know more about the recent volcanism at Mount Rainier because deposits postdate extensive glaciation and therefore are well preserved.

Pyroclastic flow deposits are not as common at Mount Rainier than at some other Cascades volcanoes, such as Mount St Helens.

Mount Rainier is behaving about as it has over the last half-million years, so all evidence suggests that the volcano will continue to erupt, grow, and collapse.