Lassen Peak, located at the southern terminus of the Cascade Range in Northern California, represents a significant chapter in U.S. volcanic history. It holds the distinction of being the site of the most recent explosive eruption in the lower 48 states prior to the 1980 awakening of Mount St. Helens. The mountain’s intense eruptive period in the early 20th century fundamentally altered its physical structure. Examining the mountain’s stature before it was violently reshaped helps appreciate the scale of this geological event.
Defining Lassen Peak
Lassen Peak is geologically classified as a plug dome volcano, a type of lava dome formed by highly viscous dacite magma. This magma is too stiff to flow easily, so it piles up directly over the volcanic vent, creating a steep-sided, dome-like structure. The mountain is one of the largest examples of this volcanic type on Earth, rising dramatically within what is now Lassen Volcanic National Park. Lassen Peak originated approximately 27,000 years ago on the northern flank of the much older, eroded Mount Tehama.
The Pre-Eruption Height
Before the eruptions began in 1914, Lassen Peak stood at an historically accepted elevation of 10,437 feet above sea level. This measurement was established by a reconnaissance survey conducted in the 1880s, serving as the official height for decades. The summit at the time featured a distinct, pre-existing crater that was approximately 1,000 feet across and 360 feet deep. Although early surveys of rugged, high-altitude features often contained minor variations due to technical limitations, this established elevation provides the benchmark for understanding the subsequent volcanic impact.
The 1915 Eruption and Summit Transformation
The long-dormant volcano began its destructive sequence on May 30, 1914, with a series of minor steam (phreatic) explosions that lasted nearly a year. The eruption changed dramatically in May 1915, when incandescent lava blocks appeared in the summit crater, signaling the extrusion of new dacite magma. On the evening of May 19, a large explosion shattered the newly formed lava dome plug, sending hot rock debris cascading down the northeast slope. This debris fell onto a deep snowpack, rapidly melting the snow and transforming the avalanche into a devastating, high-speed mudflow known as a lahar.
The most violent event occurred three days later on May 22, 1915, when a powerful explosion launched rock fragments and pumice into the air, with the resulting ash cloud reaching over 30,000 feet high. A partial collapse of this eruption column generated a scorching pyroclastic flow—a fast-moving current of hot gas and rock—that swept down the mountain’s northeast face, destroying a three-square-mile area of forest. The force of this blast significantly altered the summit’s topography, creating the larger and deeper crater visible today.
Lassen’s Current Status
Following the 1915 climactic event and subsequent steam explosions that continued until 1917, the mountain settled into its current form. Today, the official elevation of Lassen Peak is 10,457 feet. This measurement is slightly higher than the historically recorded pre-eruption figure of 10,437 feet, likely due to the extrusion of new lava dome material just before the main explosive phase. The overall effect of the eruption was a transformation of the summit from a single crater to a configuration featuring two distinct, overlapping craters.