Crater Lake

Geological Summary
Crater Lake partially fills a caldera that formed by the collapse of a volcano known as Mount Mazama during an enormous eruption approximately 7000 years ago. Prior to the climactic event, Mount Mazama had a 400'000 year history of activity. The erupted magma was mainly andesite. The caldera wall displays the geologic layering of lava flows through time. The eruptions of the last ~70'000 years were highly explosive; the eruptive magma were silica-rich (dacite and rhyodacite). The eruptions of the last 30'000 years were less explosive; the eruptive magma were low in silica.

Mount Mazama, a large composite volcano, rose to an approximate height of between 3300 and 3700 meters (view from South; Mount Scott is below the two small clouds). It began its climactic eruption about 7000 years ago, blowing out about 50 km3 of magma as pyroclastic materials (mostly rhyodacite pumice and fine ash) in at most a few days.	Pyroclastic flows of pumice and ash devastated the surrounding area to as far as 64 km away. The volcanic ash covered parts of the northwestern United States and even parts of central Canada. Rare particles of ash have even been found in ice from Greenland. Pyroclasic deposits can best be seen alonc canyon walls as here at «The Pinnacles».	The climactic eruption was fed by magma from a chamber about 5 km below the surface. As the enormous volume of magma was rapidly removed to feed the eruption, the roof of the magma chamber collapsed, forming the caldera. Wizard Island is composed of a cinder cone and lava flows, erupted soon after the caldera formed.	Several more postcaldera volcanoes are hidden by the lake. Studies of the lake bottom were conducted with the Deep Rover Submersible. Hydrothermal activity is present at the lake floor. Warm water enters the lake from the bottom. 3D-view courtesy USGS.
Crater Lake is filled with rain and melted snow (average annual precipitation 168 cm). There is no inlet to or surface outlet from the lake. The lake maintains its level because the amount of precipitation is equals to the evaporation and seepage. Lake level has varied only 5 m in the past 100 years. Crater Lake is the seventh deepest in the world. The maximum depth is 594 m. The lake water is exceptionally clear due to its isolation from streams and rivers.
maximum depth (July 2000)		594 m (seventh deepest lake in the world)
maximum diameter of caldera at the rim		9.7 km (east-west)
minimum diameter of caldera at the rim		7.3 km (north-south)
suface area		54.4 square kilometres
highest peak in the park (Mount Scott)		2721.6 m
highest peak on the rim (Hillman Peak)		2484.4 m
record clarity depth (August 1994)		40.8 m (measured by secchi disk, black and white disk lowered into the water with a cable)
average clarity depth		27 to - 31 m
Information adapted from Crater Lake Data Clearinghouse.		Copyright: J. Alean