Lahars

The greatest potential volcanic hazard at Mount Rainier is a lahar, also known as a volcanic mudflow.

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.

he primary hazard to people from lavaflows is low, but a more serious hazard arises when such flows come into contact with snow and ice.

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.

Lahars as the greatest of threats that Mount Rainier poses to people and property downstream.

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.

Lahars are common at Mount Rainier, because its mantle of snow and ice provides water when melted, and parts of the upper flanks of the volcano contain abundant loose, weak, hydrothermally altered rock.

The combination of weak, hydrothermally altered rocks, large quantities of snow and ice, and a seismically active volcano make landslides and rockfalls potential hazards at Mount Rainier.