The Geology of Crater Lake National Park

The Geological History of an Oregon Volcano in Northwestern USA

Sep 8, 2009

Crater Lake is one of Oregon's natural wonders. With a unique geological history, and active hydrothermal vents, could this volcano still be a force in the future?

Located in the southern portion of the Cascade Mountains of Oregon, Crater Lake National Park has been a popular tourist destination for well over 100 years with thousands of visitors from around the world coming to gaze upon its crystal clear blue waters. Established as a national park in 1902 by United States President Theodore Roosevelt, it is one of America’s oldest parks and natural scenic wonders. Its distinctive rock formations offer the visitor a comprehensive view of the structure and composition of volcanoes.

The Origin of Mount Mazama

Mount Mazama was the strato-volcano from which the Crater Lake caldera was formed. The mountain initially began about 500,000 years ago as part of an overlapping chain of basaltic shield volcanoes, which had erupted atop the remnant of the Western Cascade range. Over time the magma under the Cascades became more silica enriched, changing its composition from basalt to andesite. Andesitic magma tends to be a little more viscous than basalt, flowing slower and cooling quicker when it erupts as lava. It builds up tall steep sided cones that are typical of composite volcanoes. Mount Mazama grew from a succession of such eruptions, which lasted for over 400,000 years.

Interim Volcanic Activity

During its formation, the area around Mazama was covered with calc-alkaline basalt and andesite containing high concentrations of olivine and augite. Around 70,000 BCE, violent pyroclastic flows began to cover the east and southeastern portions with dacite lava. Andesite and dacite covered much of the remaining sides. Small intrusions of rhyodacite, in the form of pumice and lava erupted around the northern slope areas, including Llao Rock, Grouse Hill the Cleetwood Flow and Redcloud Cliff. Much of the rock in these areas are packed with small crystals of hornblende. By the year 30,000 BCE, the magma underneath the mountain had become a highly felsic rhyodacite mixture. Glacier activity during Pleistocene ice ages scored deep groves into the southeastern flanks creating Sun Notch, Kerr Notch and the Kerr Valley. At the time of its climactic eruption, Mount Mazama was approximately 10,000 to 12,000 feet, making it one of the largest volcanoes in the High Cascade range.

Creation of the Caldera

The caldera collapse began around 6,850 years ago when a sequence of explosive eruptions blasted steam and ash high into the atmosphere. The prevailing winds carried the ash fall north-northwest as far away as Canada. Ash deposits 16 inches in depth covered the area northeast of the mountain for up to 124 miles. Following the initial eruption, pressure began to rebuild within the magma chamber creating a ring of vents and dikes around the flanks of the mountain. A second sequence of eruptions began when pyroclastic flows of rhyodacite, andesite and mafic scoria exploded out of these ring-vents. According to David Alt and Donald Hyndman, in their book "Roadside Geology of Oregon", Montana, Mountain Press 1978, it is estimated that 10-12 cubic miles of rock was erupted, and that 15-17 cubic miles of mountainous rock collapsed in upon itself to create the caldera. After the main volcanic eruptions ended, smaller eruptions continued in the caldera for the next 6,000 years, creating the large raised area of the Central Platform, the Merriam Cone and the Wizard Island Cone.

Filling of the Caldera

During the volcanic activity within the caldera, water began to collect within the basin. The water inside the caldera comes mostly from rain and snowmelt and the level is maintained through seepage and evaporation. It is estimated that it took 300 to 250 years to fill the caldera to its present level. The clarity of the water is a result of its extreme purity. A low concentration of organic material (< 1ppm) and dissolved minerals (< 100ppm) creates a clarity that can penetrate to a depth of 40 to 60 feet on a sunny day. Its deepest area is 1,943 feet, making it the deepest fresh water lake in the United States. The water’s dark blue color is derived from its clarity and depth. As sunlight penetrates the surface, water molecules absorb the higher spectrum colors (red, orange, yellow and green). The blue spectrum is scattered at a molecular level and is reflected back to the surface. The lake is alive with both microorganisms and fish. Phytoplankton, zooplankton and microscopic water mites live throughout the lake and rainbow trout and Kokanee salmon were stocked from 1886 through 1941.

Current Volcanic Activity

While it is commonly believed that Mazama expended its volcanic energy in its climactic eruption, evidence of geothermal activity underneath the caldera may prove otherwise. Hydrothermal vents on the lake bottom were discovered in 1988-89 by Robert Collier and Jack Dymond, using a one-man submersible. One large vent named Llao’s Bath, situated on the Chaski Bay Landslide, supports a large colony of Gallionella and Leptothrix bacteria. The water in the vent was found to be very high in saline, manganese, radon and helium-3, which may indicate a magmatic source. A chain of additional vents was located off Palisade Point, and old hydrothermal spires were found on the east wall below Skell Head. The possibility of a future eruption at Mazama appears to be low at this time. Recent seismic activity at the neighboring peak of South Sister (90 miles to the north) make this volcano the more likely candidate.

Sources:

Collier, R. and Dymond, J. "Study of Hydrothermal Processes in Crater Lake: a report of field studies conducted in 1988 for the National Park Service", 1989 Ref no 145 OCR.pdf

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