Lassen Peak - Geology

Geology

Lassen Peak is the southernmost in the chain of eighteen large volcanic peaks that stretch from southwestern British Columbia to northern California. These peaks were formed during the past 35 million years while the large Juan de Fuca tectonic plate and the much smaller Gorda plate to its south have been pushed underneath the overriding North American plate. While the oceanic crust in the Juan de Fuca plate melts under the subterranean pressure, this creates pools of molten lava that have uplifted the Cascade Range and created volcanic eruptions from time to time.

Roughly 27,000 years ago, Lassen Peak started to form as a mound-shaped dacite lava dome pushed its way through Tehama's destroyed north-eastern flank. As the lava dome grew it shattered overlaying rock, which formed a blanket of angular talus around the emerging steep-sided volcano. It probably looked much like the nearby 1,100-year-old Chaos Crags. Lassen Peak reached its present height in a relatively short time, probably in just a few years.

From 25,000 to 18,000 years ago, during the last glacial period of the current Ice Age, the shape of Lassen Peak was significantly modified by glacial erosion. For example, the bowl-shaped depression on the volcano’s northeastern flank, called a cirque, was eroded by a glacier that extended out 7 mi (11 km) from the dome.

The most recent eruptive period of Lassen Peak began in 1914, and lasted for about seven years (see below). The most powerful of these eruptions was an explosion in 1915 that sent ash and steam in a 6 mi (10 km) tall mushroom cloud, making it the largest recent eruption in the contiguous United States, until the explosion of Mount St. Helens in 1980. The region around Lassen Peak remains a mildly-volcanic one, with mud pots, active fumaroles, and boiling water features demonstrating their activity. The area around Mount Lassen and the nearby Mount Shasta are considered to be the most likely volcanoes in the Cascade Range to erupt during the coming decades and centuries.