3: Mount Mazama’s Final Eruption

Mount Mazama and Crater Lake: Growth and Destruction of a Cascade Volcano

U.S. Geological Survey and the National Park Service

USGS Fact Sheet 092-02, August 2002


Mount Mazama’s Final Eruption

The cataclysmic eruption of Mount Mazama 7,700 years ago started from a single vent on the northeast side of the volcano as a towering column of pumice and ash that reached some 30 miles (50 km) high. Winds carried the ash across much of the Pacific Northwest and parts of southern Canada. So much magma erupted that the volcano began to collapse in on itself. As the summit collapsed, circular cracks opened up around the peak. More magma erupted through these cracks to race down the slopes as pyroclastic flows. Deposits from these flows partially filled the valleys around Mount Mazama with up to 300 feet (100 m) of pumice and ash. As more magma was erupted, the collapse progressed until the dust settled to reveal a volcanic depression, called a caldera, 5 miles (8 km) in diameter and one mile (1.6 km) deep.





Cataclysmic eruption to present. Eruptions of ash and pumice: The cataclysmic eruption started from a vent on the northeast side of the volcano as a towering column of ash, with pyroclastic flows spreading to the northeast. Caldera collapse: As more magma was erupted, cracks opened up around the summit, which began to collapse. Fountains of pumice and ash surrounded the collapsing summit, and pyroclastic flows raced down all sides of the volcano. Steam explosions: When the dust had settled, the new caldera was 5 miles(8km)indiameterand1mile(1.6km)deep. Ground water interacted with hot deposits causing explosions of steam and ash. Today: In the first few hundred years after the cataclysmic eruption, renewed eruptions built Wizard Island, Merriam Cone, and the central platform. Water filled the new caldera to form the deepest lake in the United States. Figure modified from diagrams on back of 1988 USGS map “Crater Lake National Park and Vicinity, Oregon.”

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