The following information was copied from Post-Caldera Volcanism and Crater Lake | U.S. Geological Survey (usgs.gov).
Since the climactic eruption of Mount Mazama, “postcaldera volcanism,” has been confined within the caldera. Most of the volcanic products are hidden from view beneath Crater Lake, but submersible and sonar studies gave scientists an eye beneath the water to the surface of lake floor.
Soon after the caldera formed, eruptions from new vents built the base of Wizard Island and a mound of lava flows near the middle of the caldera called 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.
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 submerged the central platform volcano as well. Only Wizard Island managed to grow high enough to stay above the waterline, and only 2% of it is above the lake level today. The last eruptions at Wizard Island took place when the lake was about 80 m (260 feet) 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.
The last known eruption at Crater Lake occurred when a small lava dome erupted underwater 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 30 m (100 ft) of sediment to accumulate on the lake bottom.
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Short video excerpted from the visitors center film "Crater Lake Into the deep".
Last updated: November 2, 2022