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Geology of Crater Lake National Park

Learn about the geology of Crater Lake National Park!

Crater Lake and bathymetric image of the lake subsurface and caldera floor
Photo of Crater Lake and bathymetric image of the lake subsurface and caldera floor.

Crater Lake, which fills one of the most beautiful calderas of the world, lies atop of Mount Mazama in Southern Oregon. This caldera, a large volcanic depression created by the collapse of the volcano’s structural support due to the underground magma reservoir emptying through eruptive processes, is located in the Cascade Range about 90 km (55 mi) north of the city of Klamath Falls and about 100 km (60 mi) northeast of Medford. Explosive eruptions about 7,700 years ago created a basin on Mount Mazama measuring 8-by-10-km (5-by-6-mi) across and more than 1 km (0.6 mi) deep. Crater Lake, which has since partially filled this basin, has a maximum depth of 594 m (1,949 ft), making it the deepest lake in the United States and the seventh deepest lake in the world.

Deep blue water at Crater Lake
Yes, the water really is this blue (no filter)! The great depth of the lake (nearly 2000 feet deep) helps to impart the vibrant, deep blue color. Photo by Annie Scott. 
A location map of Crater Lake, Oregon
A location map of Crater Lake, Oregon.

The Cascade Range, in which Mount Mazama and Crater Lake sits, is a perfect example of a fundamental concept in geology. It involves a common interaction between tectonic plates and the resulting chain of volcanoes that forms parallel to and inland from the plate boundary. This specific area is known as a subduction zone, or a collision zone where a slowly-sliding dense oceanic plate, the Juan de Fuca Plate, sinks below the less-dense continental North American Plate. As the Juan de Fuca plate dives deeper beneath the Earth’s crust, temperatures and pressures increase causing the plate to partially melt and release any less dense material within the rock (water, gases, etc.). The less dense material rises, melting and absorbing surrounding rock as it bubbles upwards to form magma. This magma eventually collects and creates magma chambers just below the crust of the Earth. These chambers behave similarity to a soda can, staying dormant most of the time unless a sudden disruption (such as an earthquake) occurs. Just as a sudden and violent shake of a soda can will cause the liquid to explode when opening, volcanoes will react to this quick change in motion and pressure by erupting onto Earth’s surface.

Cascade Range Map
Cascade Range
The subduction zone creating the Cascade Range volcanoes
The subduction zone creating the Cascade Range volcanoes.

The relatively continuous eruptive history of Mount Mazama dates back to 420,000 years ago, with geologic evidence of a complex system of overlapping shield and stratovolcanoes. These two types of volcanoes have very different eruptive behaviors and physical appearances. Shield volcanoes typically have a broad mound shape and an effusive eruption styles, or a relatively calm outpouring of lava onto the surrounding ground. Stratovolcanoes, also called composite volcanoes, tend to have a steep-sided conical form and highly explosive eruptions. Earlier eruptions built Mount Scott, which lies east of Crater Lake. As time continued, volcanoes began growing to the west building as layers of lava flows and pyroclastic deposits. The presence of glaciers played a role in growth of Mount Mazama, leaving traces of U-shaped valleys carved on the volcano flanks, some of which were filled in with lava flows. The summit of Mount Mazama grew to 3,700 m (12,000 ft) at its maximum elevation. Roughly 30,000 years ago, the composition, or chemical make-up, of the magma began to change, causing more explosive eruptions of low-density rock and ash. These smaller eruptions eventually led to the largest explosive eruption in the Cascade Range during the past one million years and one of Earth’s largest eruptions in the past 12,000 years.

The climactic eruption of Mount Mazama occurred 7,700 years ago in a two-phase process. The initial phase began as a Plinian eruption, shooting a column of tephra, or rock and ash, 50 km (30 mi) high from a single vent on the northeast side of the volcano. The massive amount of ejected magma allowed the vent of the volcano to widen, causing the eruption column to collapse on itself and send pyroclastic flows down the north and east flanks. The winds carried material great distances causing widespread ash-fall deposits throughout the Pacific Northwest. The second phase of the eruption introduced the opening of circular cracks, or ring-vents, around the peak. These cracks acted as a wider dispersion path for a larger volume of magma to escape the reservoir. This sequence of events led to the structure of the central portion of the volcano to collapse in on itself, resulting in an 8–10 km (5–6 mi) diameter and 1.2 km (0.7 mi) deep caldera.

The Pinnacles, remains of hot ash and pumice, form tall spires above Wheeler Creek
The Pinnacles, remains of hot ash and pumice, form tall spires above Wheeler Creek. Photo by Annie Scott.
Crater Lake Hoodoos
A close up view of the Crater Lake hoodoos shows a wide variety in the size of the volcanic debris. The larger fragments are pumice and the fine-grained material is known as ash. Photo by Annie Scott.

Volcanism has continued in the caldera after the climatic eruption of Mount Mazama. Shortly after the formation of the caldera, smaller eruptions formed Wizard Island and a mound of lava flow deposits. Rain and snowmelt began to fill the caldera, with eruptions ejecting enough material to keep pace with the rising water levels through the next 750 years. In the current conditions, the peak of Wizard Island (displaying only 2% of its actual size) is the only volcano that managed to remain above the waterline. The present-day water levels are stabilized by a thick layer of porous deposits in the  northeastern wall of the caldera, which acts as the “overflow drain” for Crater Lake. The last known eruption took place at the base of Wizard Island 4,800 years ago, and the volcano has remained fairly quiet, allowing roughly 30 m (100 ft) of sediment to accumulate on the lake bottom.

Volcanic Rock Statue at Crater Lake
An ominous statue of volcanic rock, covered with several varieties of lichen, is part of the Devils Backbone dike which cross-cuts four lava units. Photo by Annie Scott.

What lies beneath Crater Lake was not explored until the late 1800s and mid-1900s. Researchers from the USGS, the National Park Service, and the US Coast and Geodetic Survey have used submersible and sonar studies to evaluate beneath the water to the surface of lake floor. In 1979, a USGS marine geologist was able to use acoustic imaging techniques (like CAT scans) to map the accumulation of sediments after the caldera formed. Measurements of heat flow led to the discovery of thermal vents in the lake. Further exploration of these vents and the caldera floor took place in 1987- 1989 during a remote underwater surveying followed by a submersible. In 2000, scientists from the USGS, the National Park Service, the University of New Hampshire, and C & C Technologies surveyed the lake floor with modern techniques to provide a bathymetric (depth) map for interpreting the post-caldera geologic history, providing the clearest understanding of the history since the eruption taking place 7,700 years ago.

Crater Lake is an anomaly among most mountainous lakes because there is no streamflow running into or out of the caldera. The water level of the lake is controlled by precipitation, evaporation, and seepage through surrounding rocks. Precipitation is defined as water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that delivers atmospheric water to the Earth. Evaporation is the reverse process of precipitation. Evaporation is the process by which water changes from a liquid to a gas or vapor. It is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric water vapor. Seepage involves groundwater leaking into porous surrounding rock and creating an area of saturation. This process allows Crater Lake to maintain water level without overflowing. 

The water cycle
A visual aid of the Water Cycle. 

Currently, the volcano is considered active. Mount Mazama is expected to erupt in the future, most likely beneath the water’s surface. Water tends to play a big role in the explosivity of an eruption. The interaction between water and magma could produce highly explosive hydrothermal eruptions that send rock fragments and large blocks out of the caldera. Factors that determine how violently water and magma interact include the type of magma, its rate of extrusion, how much gas is in the magma, and the depth of the water in which it erupts. Shallow water eruptions may be highly explosive, while those in the deep lake are expected to be much less violent. More recent monitoring data shows that most of the post-caldera volcanic activity has been concentrated on the western half of the caldera, and indication that future activity will likely occur within that area. There is the possibility of the formation of a new vent on the flanks of Mount Mazama or the surrounding area. There is a north–south trending active fault zone traversing the west half of Crater Lake National Park, which could cause damaging earthquakes. 

Crater Lake hazards map
Crater Lake, Oregon simplified hazards map showing potential impact area for ground-based hazards during a volcanic event.