Oblique view of Crater Lake caldera looking southeast. The colored area shows shaded-relief bathymetry of the lake floor. The gray area shows shaded-relief of the caldera walls and Wizard Island. The distance across the bottom of the image is about 6 kilometers (3.7 miles).

Crater Lake, the deepest lake in the United States, occupies a caldera in Mount Mazama, a Cascade Range volcano that once stood about 3,700 meters (12,000 feet) above sea level. About 7,700 years ago in a catastrophic eruption that lasted only a few days at most, Mount Mazama ejected about 50 cubic kilometers (12 cubic miles) of magma in the form of pumice and ash. Towards the end of the eruption, the mountain collapsed upon itself to form the caldera shown in this view. After this climactic event, volcanic activity resumed within the caldera, creating Wizard Island and other new landforms. All but the uppermost portion of the Wizard Island volcano is hidden from view below the surface of Crater Lake.

Within perhaps 200 to 300 years after the formation of the caldera, the lake filled to its present level. As a result, many of the volcanic landforms that rose from the caldera floor during this period display the effects of rising lake waters in the form of shorelines and other features. An exception is the rhyodacite dome, which formed underwater about 2,500 years later.

The Wizard Island volcano grew as the lake filled. Older submerged shorelines of the island (see S1 and S3 in Arrow 6) can be seen where lava flows shattered after entering the lake, creating steep underwater talus slopes. The gently sloping bench around Wizard Island consists of lava flows that were later drowned by the rising lake. Submerged near the center of Crater Lake is the central platform volcano, which also has breaks in its slopes that indicate the location of shorelines. Below its steep north and east flanks, however, are sinuous lava flows that apparently flowed underwater, down over the shattered older lava.

Near the north shore of the lake is Merriam Cone, another postcaldera andesite volcano which erupted under water. Merriam Cone is bordered by the flat-bottomed northwest basin, a depression that contains up to about 80 meters (262 feet) of sediment largely derived from the adjacent caldera walls (Nelson and others, 1988). To the east of Merriam Cone is the east basin, where up to 100 meters (328 feet) of sediment have been deposited. Merriam Cone and the lava flows north of the central platform partially dam the northwest basin. In the foreground of this view, at Llao Bay, sediment transport from the upper walls to the basins can be seen. Chutes in rock outcrops mark the beginning of talus cones that extend in the form of debris-flow lobes out onto the caldera floor. Extensive landslide and debris-flow deposits can be seen in Chaski Bay and Danger Bay.

Source: Gardner, James V., Peter Dartnell, Laurent Hellequin, Charles R. Bacon, Larry A. Mayer, and J. Christopher Stone. 2001. Bathymetry and selected perspective views of Crater Lake, Oregon. USGS Water Resources Investigations Report 01-4046.