Crater Lake
Crater Lake partially fills a type of volcanic depression called a caldera that formed by the collapse of a 12,000 ft volcano known as Mount Mazama during an enormous eruption approximately 7,700 years ago. The climactic (caldera-forming) eruption of Mount Mazama changed the landscape all around the volcano. Pyroclastic flows of pumice and ash devastated the surrounding area, including all of the river valleys that drained Mount Mazama to as far as 40 miles away, and a blanket of pumice and ash fell to the northeast of the volcano at least as far as central Canada. Erosion removed much of this material, feeding rivers that carried it far from its source, ultimately into the Pacific Ocean. Prior to the climactic event, Mount Mazama had a 400,000 year history of activity similar to other Cascade volcanic centers such as Mount Shasta. Since the climactic eruption, there have been several less violent, smaller postcaldera eruptions within the caldera itself.
Mount Mazama was a large composite volcano constructed by episodic growth of many overlapping shield and composite volcanoes, each of which probably was active for a comparatively brief period. Mount Mazama rose to an approximate height of 12,000 ft above sea level. The erupted magma was mainly andesite. The caldera wall displays the geologic layering of lava flows through time. As the volcanic complex evolved, so did its eruptive style. The eruptions of the last ~70,000 years were highly explosive; the eruptive magma were silica-rich (dacite and rhyodacite). The eruptions of the last ~30,000 years were less explosive; the eruptive magma were low in silica. The only activity in the 30,000 years record, prior to the caldera-forming climactic eruption of ~7,700 years ago, was limited to a small number of preclimactic pyroclastic eruptions and ensuing lava flows of rhyodacite.
Mount Mazama began its climactic eruption about 7,700 years ago, blowing out about 12 cubic miles of magma as pyroclastic materials (mostly rhyodacite pumice and fine ash) in at most a few days (compared with 7/10 of a cubic mile for Mt. St. Helens). The volcanic ash covered parts of the northwestern states to as far as central Canada. Rare particles of Mazama ash have even been found in ancient ice from Greenland. The airfall pumice and ash covered a total surface area of more than 1,000,000 square miles at least 1 mm (fraction of an inch) thick, and no less than 5,000 square miles more than 6 in thick. A volume of 10-13 cubic miles of the mountaintop had disappeared.
The climactic eruption was fed by magma that had accumulated in a chamber about 3 miles below the surface. As the enormous volume of magma (13-15 cubic miles) was rapidly removed to feed the eruption, the roof of the magma chamber collapsed, forming the bowl-shape depression known as a caldera. The caldera has a diameter of approximately 5 miles north to south and 6 miles east to west, with an approximate depth of 4,000 feet from the caldera rim to the lake floor. Pyroclastic flows deposited pumice and ash on all flanks of Mount Mazama and in valleys below.
The landscape after the eruption was a scene of devastation. Geologists have determined that the collapse was a relatively quick event during the eruption, occupying perhaps a few hours or days. All the valleys surrounding the volcano were partially filled with hot pyroclastic flows. The caldera was partly filled with pyroclastic materials and rock debris from its unstable walls. Subsequent to the climactic eruption, all volcanic activity has occurred within the caldera itself. Renewed volcanism built the postcaldera volcanoes of the central platform, Merriam Cone, and Wizard Island.
Following the climactic eruption, it took perhaps 250 years of rain and snow accumulation for the caldera to fill to its present-day lake level. The lake level is maintained by a balance between precipitation and evaporation plus seepage. By that time, the postcaldera volcanoes, nearly all of which are hidden beneath the surface of the lake, had finished their eruptions of andesite lava. Only a small rhyodacite dome was erupted later, about 5,000 years ago.
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