The Geology of Crater Lake National Park

Like most of our national parks, the spectacular beauty of Oregon’s only national park is a result of the geologic processes that created it. But whereas most of our parks were created by gradual processes over long periods of time—for example, erosion by the Colorado River to create the Grand Canyon—Crater Lake was created in a few days during a cataclysmic eruption 7,700 years ago that destroyed almost the entire volcanic edifice. To imagine what Crater Lake looked like before the eruption, just picture other Cascade Range volcanoes such as Mount Shasta or Mount Hood. These volcanoes are composite or stratovolcanoes, because they are built up by layers (strata) of both lava and pyroclastic flows. They have steep sides and the classic cone shape.

Paul Rockwood’s 1930s artistic rendering of Mount Mazama 7,700 years ago, when the cataclysmic eruption began with a towering column of pumice and ash. The volcano’s collapse followed after this. Note Rockwood’s inclusion of glaciers on Mount Mazama. They are discussed later in the blog. Image from USGS publication:
Crater Lake today, after the eruption 7,700 years ago had removed most of the volcano known as Mount Mazama, and reduced the volcano’s height from around 3700 m (12,000 ft) to the current maximum rim height of 2,484 m (8,150 ft). The lake formed over a longer time, as rain and snow melt filled the depression formed by the volcano’s collapse. With a maximum depth of 594 m (1949 ft), Crater Lake is the deepest lake in the United States and the 9th deepest in the world. Its color is a result of its depth—longer wavelengths of light (red colors) are absorbed and shorter wavelengths of light (blue colors) are reflected—and because no rivers enter the lake to bring mud that would cloud the water column. Also, most of the water is below the photic zone, which prevents algal blooms.

Crater Lake should really be named Caldera Lake. Craters are small depressions at the summit of active volcanoes where lava and other volcanic products are emitted. The depression occupied by the lake is not a crater; rather, it is a caldera that was produced by collapse of nearly the entire volcano.

The eruption 7,700 years ago started with a blast that produced a column of tephra that reached a height of 50 km (30 mi). The airborne particles were carried by winds for long distances, producing a widespread ash-fall deposit. So much magma erupted that the vent widened and the tephra column collapsed. Circular cracks opened around the peak and produced pyroclastic flows that sped down the flanks of Mount Mazama and partially filled the valleys with up to 100 m (300 ft) of pumice and ash. As more magma was erupted and the magma reservoir emptied, the central park of the volcano collapsed, producing a caldera 8–10 km (5–6 mi) in diameter and 1.2 km (0.7 mi) deep. Small eruptions continued on the caldera’s floor, while rain and snow melt eventually filled the caldera with water to form the lake. Diagram from:
Diagram to illustrate the types of products emitted from volcanoes. Tephra is a general term for loose fragments of magma and pre-existing rock that can range in size from very fine ash to large pieces referred to as blocks or bombs. Tephra is typically emitted in eruptive columns and can be carried by wind over large distances. Pyroclastic flows also consist of magma and rock particles, but the particles are in a fluidized mixture of hot, expanding gases that flows rapidly down the flanks of volcanoes. They are the most dangerous hazard associated with volcanic eruptions. Lava is fluid rock (called magma while still underground) that also flows down the flanks of volcanoes, but typically at lower speeds and more predictable trajectories than turbulent, thick, gaseous pyroclastic flows. Consolidated layers of pyroclasts are referred to as layers of tuff (air fall) or ash-flow tuff (pyroclastic flow). Diagram from
Map showing the extent of the ash deposit that was produced during the eruption of Mount Mazama 7,700 years ago. Also shown is the extent of other large eruptions during the past million years, including the 1980 eruption of Mount St. Helens, which was very small in comparison to events like the 1912 Novarupta eruption near Kodiak, Alaska, that was the largest global eruption in the 20th century. Map from USGS web site:
Photograph of ash-flow tuff on the slope of Crater Lake. Most of the rock is ash-sized particles. The larger fragments of hot magma are lens shaped because they were flattened when the gaseous flow came to a halt and the material was compressed. Because most of the fragments were hot glass—there was insufficient time for mineral crystals to form—the fragments welded together to form a solid rock.
This outcrop is an ash-flow deposit from the eruption located on Highway 230 north of Crater Lake. It consists of pumice (the larger pieces that are made of frothy lava) and ash (the small-sized particles between the pumice fragments). Imagine a pyroclastic flow speeding downslope at 100 km/hr (60 miles/hour) and depositing this thickness of hot clasts (pyroclasts)—it would have been terrifying indeed. This deposit is only compressed enough to hold the clasts together—for the most part, the clasts are not flattened. These types of Mount Mazama deposits are up to 100 m (300 feet) thick; they are thinner farther away from the volcano and can be seen, for example, along Highway 97 on the way north toward Bend.

It is important to realize that Native Americans were living in the region at the time of Mount Mazama’s cataclysmic eruption, which was no doubt highly destructive to nearby communities. Accounts of the eruption are in stories still told by the Klamath and Umpqua peoples. You can learn more about these stories in a National Park Service publication:

Geologic history of the volcano—how it grew, blew, fell, and filled. Mount Mazama was formed as a succession of overlapping volcanoes that were erupting from about 420–40 thousand years ago, starting with Mt. Scott and then building up more toward the west. Layers of lava flows from these early volcanoes are visible in the caldera walls and in landmarks along the south rim of Crater Lake, including Applegate and Garfield Peaks (see map below).

By about 30,000 years ago, Mount Mazama began to generate increasingly explosive eruptions and thick flows of silica-rich lava (visible at Grouse Hill and Redcloud Cliff), indicating a large volume of magma had accumulated beneath the volcano. Because the magma had evolved to a more silica-rich composition, it was more gas rich and highly explosive. An eruption about 7,900 years ago formed a white layer of pumice and ash and the thick lava flow of Llao Rock and later at Cleetwood Cove, culminating 7,700-year ago in the largest explosive eruption in the Cascades during the past 1 million years.

Soon after the volcano exploded and the caldera was formed, eruptions from new vents built the base of Wizard Island, a mound of lava flows near the middle of the caldera called the Central platform, and Merriam Cone (see map below). Rain and snow melt continued to fill the lake until only Wizard Island remained above lake level. The last known eruption at Crater Lake occurred at the base of Wizard Island about 4,800 years ago. Since then, the volcano has been quiet, and up to 30 m (100 ft) of sediment has accumulated on the lake bottom.

During the growth of Mount Mazama, glaciers repeatedly carved out classic U-shaped valleys, most recently during the Last Glacial Maximum 18,000 years ago. Some of the valleys were later filled with lava, as seen at Llao Rock (see map and photo below). Other valleys, such as Kerr Notch and Sun Notch on the south rim, were not filled (see Google Earth image below).

Crater Lake digital elevation and bathymetric map showing surface features around and within the caldera. Map from USGS publication:
This photograph shows Wizard Island, which erupted soon after the caldera-forming eruption, and, on the cliff wall, Llao Rock, which consists of flows from an eruption about 7,900 years ago, just 200 years before the climactic eruption. Llao Rock has a lenticular shape because it filled in a valley formed by glacial erosion. The red head with me is my sister!
This Google Earth image of Crater Lake’s south rim shows Mt. Scott, the oldest (now extinct) volcano in the area, and Kerr and Sun Knotches, two U-shaped glacial valleys that were not subsequently filled by lava and/or ash flows.
This map shows eruptions of Cascade Range volcanoes during the past 4,000 years. Note that Crater Lake has not been active during this time period. Diagram from the Pacific Network Seismic Center:, and the USGS Cascades Volcanic Observatory:

What has created the Cascade Range—this line of active volcanoes that extends from northern California to northern Washington? The reason is the plate tectonic setting next to the Cascadia Subduction Zone (CSZ). Check out my May 10, 2020 blog post to see how subduction of the Juan de Fuca (oceanic) plate beneath the North American (continental) plate causes rock to melt beneath the surface and rise to the surface to create volcanoes. You can also read about other Cascade volcanoes in these older blog posts: Mount McLoughlin, Mount St. Helens, Mount Lassen, and South Sister.

Important reference

U.S. Geologic Survey Cascades Volcanic Observatory’s web site about Crater Lake:

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