Lahars

Several hazards will likely accompany a future eruption of Lassen volcano.

When pyroclastic flows mix with large quantities of water, they can produce highly mobile flows of mud and debris, called lahars, that may rush down valleys leading away from a volcano.

Over the last 3 million years, regional volcanism in the Lassen segment of the Cascade arc constructed a broad platform of volcanic rocks. In periods of about 50,000 - 200,000 years, concentrated volcanism occurred in localized areas. These "volcanic centers" are defined as large, long-lived edifices erupting the full range of lava compositions from basalt to rhyolite.

On May 22, 1915, an explosive eruption at Lassen Peak, California, the southernmost active volcano in the Cascade Range, devastated nearby areas and rained volcanic ash as far away as 300 km (about 200 mi) to the east. This explosion was the most powerful in a 1914-17 series of eruptions that were the last to occur in the Cascades before the 1980 eruption of Mount St. Helens.