Post-Eruptive History and Bathymetry, Northern CA Geological Society, 2003

Post-Eruptive History and Bathymetry of Crater Lake Highlights September Meeting cover-thumb

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As Crater Lake celebrates its 100th year as a National Park, the NCGS (Northern California Geological Society) was fortunate enough to have a nationally recognized volcanologist discuss its caldera development and filling after the explosive eruption of Mount Mazama 7700 years ago. Dr. Charles R. Bacon of the USGS, Menlo Park, spoke to NCGS members September 25th on “Volcanism in Nature’s Bathtubs: The Caldera Lakes at Aniakchak and crater Lake.”

Dr. Bacon, a member of the USGS Volcano Hazards Team, has spent much of his research time studying the evolution of Crater Lake and contrasting its development to that of other recent andesitic volcanoes in the Circum-Pacific arc. This talk specifically focused on the post-caldera Holocene history of Aniakchak volcano on the Alaskan Peninsula (Aleutian volcanic chain) as a guide for evaluating the post- caldera processes at Crater Lake. And why Aniakchak crater? Because this volcano lies about 400 miles southwest of Anchorage along major commercial air routes that could be affected by volcanic ash ejected into the atmosphere during an eruption.

Aniakchak erupted catastrophically about 3500 years ago and has exhibited much post-caldera activity. It is considered active, and consists of a 10 km-wide by 0.5 to 1.0 km deep caldera that was formed during this major eruptive event. Since then, at least a dozen vents in the caldera have erupted, spewing out tephra (ash) and lava. Aniakchak last erupted in 1931, a violent event, and although there are no current signs of impending volcanic activity, it will erupt again in the future. Soon after the 3500 year old eruption, a lake formed in the caldera depression. One side of the caldera wall failed suddenly, and the lake drained rapidly down the Aniakchak River to the ocean nearby. Various geomorphic features associated with the drainage event, including conspicuous floodplain sediments deposited by the raging torrent and large 20 meter-sized boulders scattered along its path, attest to the sheer force and quantity of water released when the lake drained. The exposed caldera floor had been intruded by shallow dacite domes and phreatic (sputtering) eruptive events occurred periodically as magma invaded groundwater-enriched layers. This well preserved Alaskan eruptive event was used as a template by Dr. Bacon and his colleagues to interpret bathymetric features discovered on the bottom of Crater Lake, Oregon.

By comparison, the Crater Lake caldera measures 10 km. by 8 km. The 7700 year B.P. (before present) eruption scattered ash over eight states and three Canadian provinces, covering 5000 square miles with ejecta 6 inches deep! This massive series of eruptions were 42 times greater than 1980’s Mount St. Helens event. The eruption demolished 12,000 foot-high Mount Mazama, a volcano that had erupted intermittently for 500,000 years. The 3000-foot deep crater left after Mazama exploded was initially too hot to hold water but eventually filled over an estimated 700 year interval with snow melt and rain-water to a constant level controlled by evaporation and seepage. Annual fluctuations in lake level are no more than 3 feet. At a depth of 1932 feet, Crater Lake is the deepest lake in the United States and the seventh deepest in the world (Lake Baikal, which fills a 400 mile long continental rift valley in southern Siberia is the deepest at slightly over 1 mile).