Geology of Lassen Volcanic National Park, California

Part of the NPS Geodiversity Atlas, which describes each park's significant geologic features and processes. Geodiversity is the full variety of a park's natural geologic resources — rocks, minerals, sediments, fossils, landforms and physical processes — and its soils. The atlas comes from the NPS Geologic Resources Inventory.

The park's Geologic Resources Inventory report. Image from the NPS's page.

The park

Lassen Volcanic National Park lies in the southern Cascade Range of northern California, across Lassen, Plumas, Shasta and Tehama Counties. It began as two monuments — Lassen Peak and Cinder Cone National Monuments, proclaimed on May 6, 1907 — joined into one national park on August 9, 1916. Its 43,135 hectares (106,589 acres) protect a wide range of volcanic landforms and hydrothermal features:

• composite volcanoes, shield volcanoes, cinder cones and lava domes — all four main kinds of volcano;
• lava flows, pyroclastic flows (hot, dense currents of volcanic gas, ash and rock) and tephra (fragments thrown out in an eruption);
• steaming fumaroles, mudpots and boiling springs.

An active volcano

Lassen Peak is the southernmost active volcano in the Cascades. After a series of eruptions beginning in 1914, it erupted explosively on May 22, 1915: pyroclastic flows, debris avalanches, debris flows and floods devastated the area around it, and ash fell hundreds of miles to the east. Activity continued intermittently until 1917. Until Mount St. Helens erupted in 1980, Lassen Peak was the most recent volcanic outburst in the contiguous 48 states, and the only Cascade volcano to have erupted in the 20th century.

Why volcanoes here? Lassen and the other Cascade volcanoes belong to the Cascade volcanic arc, which runs from southern British Columbia to northern California. The arc exists because the Juan de Fuca plate — with its Explorer and Gorda subplates — is sliding obliquely beneath North America. Inland of the sinking plate, magma has risen to build a chain of striking volcanic landscapes, from Miocene to Holocene in age. The Cascades form part of the Pacific's "Ring of Fire", the circle of volcanoes and earthquakes around the ocean named for this kind of subduction.

The Lassen volcanic center, of which Lassen Peak is part, lies within the park, on top of a platform built by older regional volcanoes. Over its 825,000-year life it has held a collapsed caldera complex, filled by the now heavily eroded Brokeoff Volcano, and the Lassen domefield. It has erupted three times in the last 1,050 years, most recently in 1914–1917. The magma chamber that powers it heats the park's roaring fumaroles, mudpots, boiling pools and thermal ground — signs that future eruptions remain possible.

Ice on the volcanoes

Lassen is known for volcanoes, but ice shaped it too. There are no glaciers in the park today, yet ice caps covered its high ground several times in the Ice Age.

• Glaciers advanced at least five times in the Pleistocene. Valley glaciers spread out from the base and northeastern flank of Lassen Peak, and an ice cap sat on the park's central plateau. Two small moraines near the base of Lassen Peak record glaciers of the early Holocene, about 8,000 to 12,000 years ago.
• Erosion was the main effect. Glaciers deepened valleys, stripped bedrock from large areas, created or enlarged hundreds of lake basins, polished and scratched bedrock, and carved U-shaped valleys, cirques and arêtes high up and roches moutonnées in the valleys. Rock softened by hydrothermal alteration made the glaciers' work easier.
• Deposits matter too: glaciers spread till (a mix of clay, silt, sand, gravel and boulders), built moraines, dropped erratics, and their meltwater laid down outwash sand and gravel beyond the ice. Moraines and outwash overprint much of the volcanic landscape.

Geologic map of the park, from the Geologic Resources Inventory. NPS.

Maps and reports

Lassen belongs to the Cascade-Sierra Mountains province, sharing its geologic history with a region well beyond the park. The park's Geologic Resources Inventory maps, reports and posters are in the NPS DataStore, and a Soil Resources Inventory project has been completed. More: the park's pages on volcanoes and lava flows and seismic activity, and the USGS California Volcano Observatory.

Crumbaugh Lake. Image from the NPS's page.

Sources

Based on "NPS Geodiversity Atlas—Lassen Volcanic National Park, California," National Park Service; a work of the United States government in the public domain.