4: Crater Lake

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

 Crater Lake

Soon after the cataclysmic eruption, eruptions from new vents within the caldera built the base of Wizard Island and the central platform. As eruptions continued, rain and snowmelt also began to fill the caldera. For the next few hundred years, eruptions from these new vents kept pace above the rising water level. Lava flowed into the deepening lake, creating benches on the flanks of the growing cones that tell scientists how deep the lake was during these eruptions. One set of eruptions from a crater on the west edge of the central platform formed lava tubes or channels that sent lava far out onto the caldera floor.

Later eruptions from a vent in the northern part of the caldera, just south of present-day Cleetwood Cove, built Merriam Cone. The erupting Merriam Cone probably never reached the lake surface. The ever-deepening lake eventually drowned the central platform volcano as well. Only Wizard Island managed to grow high enough to stay above the waterline. The last eruptions at Wizard Island took place when the lake was about 260 feet (80 m) lower than today. All of this activity occurred within 750 years after the cataclysmic eruption. The water level continued to rise until reaching near present-day levels, where it encountered a thick layer of porous deposits in the northeast caldera wall. These deposits stabilize lake levels like an overflow drain in a bathtub.

Sometime after these eruptions, the Chaski Bay landslide, the top of which is still visible above the southern shore of Crater Lake, slumped into the caldera and ran up onto the edge of the central platform. Other landslides within the caldera were deflected by the base of Wizard Island and the central platform, burying explosion craters and other features on the lake floor with even more debris.

The last known eruption at Crater Lake occurred when a small lava dome erupted under water on the east flank of the base of Wizard Island about 4,800 years ago. Since that time, the volcano has remained quiet, allowing as much as 100 feet (30 m) of sediment to accumulate on the lake bottom.

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