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The Spring 2011 issue of The Earth Scientist is focused on modernizing seismology education. Thanks to IRIS, you can download this issue for free as a pdf. Print copies are available in our online store.
This is an image of Crater Lake in Oregon, USA.
Click on image for full size
Image from: U.S. Geological Survey

Crater Lake

Mt. Mazama was once among a cluster of Pleistocene stratovolcanoes stretching along the Washington and Oregon coast. This cluster of volcanoes includes Mt. Hood, Mt. Rainier, and Mt. St. Helens. The magma chamber formed over a period of 15,000-40,000 yrs. During that period, eruptions of basalt, andesite, dacite, and rhyolite occurred in the Mt. Mazama area.

During the few centuries preceding the last, climactic eruption, at least two large eruptions of rhyolite occurred in the area underlain by the magma chamber. Tephra from one of these eruptions extended into eastern Washington and western Nevada.

The climactic eruption of Mt. Mazama occured 6850 yrs ago, and took place over a number of months. The eruptions produced tephra-fall and two episodes of pyroclastic-flow deposits. Tephra deposits from this series of eruptions have been found in 8 western states and 3 Canadian provinces. The major episode of pyroclastic-flow in this series of eruptions extended to 60 km (40 miles). Following this series of eruptions, the caldera finally collapsed into the magma chamber, and Mt Mazama became dormant except for activity which formed a small cinder cone in the center. The collapsed caldera filled with water and became what we now know as Crater Lake.

The total volume of magma erupted during the climactic eruption was about 10 times larger than that produced during any other explosive eruption in the Cascade Range during postglacial times.

The Yakima People who lived near Mt. Mazama when it fell have a different story of what caused the mountain to collapse.


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