Like most of our national parks, the spectacular beauty of Oregon’s only national park is a result of the geologic processes that created it. But whereas most of our parks were created by gradual processes over long periods of time—for example, erosion by the Colorado River to create the Grand Canyon—Crater Lake was created in a few days during a cataclysmic eruption 7,700 years ago that destroyed almost the entire volcanic edifice. To imagine what Crater Lake looked like before the eruption, just picture other Cascade Range volcanoes such as Mount Shasta or Mount Hood. These volcanoes are composite or stratovolcanoes, because they are built up by layers (strata) of both lava and pyroclastic flows. They have steep sides and the classic cone shape.
Crater Lake should really be named Caldera Lake. Craters are small depressions at the summit of active volcanoes where lava and other volcanic products are emitted. The depression occupied by the lake is not a crater; rather, it is a caldera that was produced by collapse of nearly the entire volcano.
It is important to realize that Native Americans were living in the region at the time of Mount Mazama’s cataclysmic eruption, which was no doubt highly destructive to nearby communities. Accounts of the eruption are in stories still told by the Klamath and Umpqua peoples. You can learn more about these stories in a National Park Service publication: https://home.nps.gov/crla/planyourvisit/upload/History-508.pdf.
Geologic history of the volcano—how it grew, blew, fell, and filled. Mount Mazama was formed as a succession of overlapping volcanoes that were erupting from about 420–40 thousand years ago, starting with Mt. Scott and then building up more toward the west. Layers of lava flows from these early volcanoes are visible in the caldera walls and in landmarks along the south rim of Crater Lake, including Applegate and Garfield Peaks (see map below).
By about 30,000 years ago, Mount Mazama began to generate increasingly explosive eruptions and thick flows of silica-rich lava (visible at Grouse Hill and Redcloud Cliff), indicating a large volume of magma had accumulated beneath the volcano. Because the magma had evolved to a more silica-rich composition, it was more gas rich and highly explosive. An eruption about 7,900 years ago formed a white layer of pumice and ash and the thick lava flow of Llao Rock and later at Cleetwood Cove, culminating 7,700-year ago in the largest explosive eruption in the Cascades during the past 1 million years.
Soon after the volcano exploded and the caldera was formed, eruptions from new vents built the base of Wizard Island, a mound of lava flows near the middle of the caldera called the Central platform, and Merriam Cone (see map below). Rain and snow melt continued to fill the lake until only Wizard Island remained above lake level. The last known eruption at Crater Lake occurred at the base of Wizard Island about 4,800 years ago. Since then, the volcano has been quiet, and up to 30 m (100 ft) of sediment has accumulated on the lake bottom.
During the growth of Mount Mazama, glaciers repeatedly carved out classic U-shaped valleys, most recently during the Last Glacial Maximum 18,000 years ago. Some of the valleys were later filled with lava, as seen at Llao Rock (see map and photo below). Other valleys, such as Kerr Notch and Sun Notch on the south rim, were not filled (see Google Earth image below).
What has created the Cascade Range—this line of active volcanoes that extends from northern California to northern Washington? The reason is the plate tectonic setting next to the Cascadia Subduction Zone (CSZ). Check out my May 10, 2020 blog post to see how subduction of the Juan de Fuca (oceanic) plate beneath the North American (continental) plate causes rock to melt beneath the surface and rise to the surface to create volcanoes. You can also read about other Cascade volcanoes in these older blog posts: Mount McLoughlin, Mount St. Helens, Mount Lassen, and South Sister.