Geologic History and Formation

United States Geological Service Website

Crater Lake partially fills a type of volcanic depression called a caldera that formed by the collapse of a 3,700 m (12,000 ft) volcano known as Mount Mazama during an enormous eruption approximately 7,700 years ago. The climactic (caldera-forming) eruption of Mount Mazama changed the landscape all around the volcano. Pyroclastic flows of pumice and ash devastated the surrounding area, including all of the river valleys that drained Mount Mazama to as far as 64 km (40 mi) away, and a blanket of pumice and ash fell to the northeast of the volcano at least as far as central Canada. Erosion removed much of this material, feeding rivers that carried it far from its source, ultimately into the Pacific Ocean. Prior to the climactic event, Mount Mazama had a 400,000 year history of activity similar to other Cascade volcanic centers such as Mount Shasta. Since the climactic eruption, there have been several less violent, smaller postcaldera eruptions within the caldera itself.

Mount Mazama was a large composite volcano constructed by episodic growth of many overlapping shield and composite volcanoes, each of which probably was active for a comparatively brief period. Mount Mazama rose to an approximate height of 3,700 m (12,000 ft) above sea level. The erupted magma was mainly andesite. The caldera wall displays the geologic layering of lava flows through time. As the volcanic complex evolved, so did its eruptive style. 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. The only activity in the 30,000 years record, prior to the caldera-forming climactic eruption of ~7,700 years ago, was limited to a small number of preclimactic pyroclastic eruptions and ensuing lava flows of rhyodacite.

Mount Mazama began its climactic eruption about 7,700 years ago, blowing out about 50 km³ (12 mi³) of magma as pyroclastic materials (mostly rhyodacite pumice and fine ash) in at most a few days. The volcanic ash covered parts of the northwestern states to as far as central Canada. Rare particles of Mazama ash have even been found in ancient ice from Greenland. The airfall pumice and ash covered a total surface area of more than 2,600,000 km² (1,000,000 mi²) at least 1 mm (fraction of an inch) thick, and no less than 13,000 km² (5,000 mi²) more than 15 cm (6 in) thick. A volume of 42-54 km³ (10-13 mi³) of the mountaintop had disappeared.

Distribution of pumice deposits. Image acquired from the 1988 edition of the 1:62,500 scale topographic map entitled "Crater Lake National Park and Vicinity, Oregon."

The climactic eruption was fed by magma that had accumulated in a chamber about 5 km (3 mi) below the surface. As the enormous volume of magma (54-63 km³, or 13-15 mi³) was rapidly removed to feed the eruption, the roof of the magma chamber collapsed, forming the bowl-shape depression known as a caldera. The caldera has a diameter of approximately 8 km (5 mi) north to south and 10 km (6 mi) east to west, with an approximate depth of 1,200 m (4,000 ft) from the caldera rim to the lake floor. Pyroclastic flows deposited pumice and ash on all flanks of Mount Mazama and in valleys below.

The landscape after the eruption was a scene of devastation. Geologists have determined that the collapse was a relatively quick event during the eruption, occupying perhaps a few hours or days. All the valleys surrounding the volcano were partially filled with hot pyroclastic flows. The caldera was partly filled with pyroclastic materials and rock debris from its unstable walls. Subsequent to the climactic eruption, all volcanic activity has occurred within the caldera itself. Renewed volcanism built the postcaldera volcanoes of the central platform, Merriam Cone, and Wizard Island.

Following the climactic eruption, it took perhaps 250 years of rain and snow accumulation for the caldera to fill to its present-day lake level. The lake level is maintained by a balance between precipitation and evaporation plus seepage. By that time, the postcaldera volcanoes, nearly all of which are hidden beneath the surface of the lake, had finished their eruptions of andesite lava. Only a small rhyodacite dome was erupted later, about 5,000 years ago.

Studies show that hydrothermal activities are present at the lake floor. Chemical analysis of the lake water and hydrothermal studies indicate that warm water enters the lake from the lake bottom. The water is heated by hot rock beneath the fractured caldera floor. Scientists are not certain whether any magma still remains underground but it is likely that Mount Mazama will erupt again someday.

Formation of Crater Lake

Crater Lake occupies a basin in Mount Mazama, one of the volcanoes that make up the Cascades Mountain Range along the west coast of the United States. The major volcanoes of the Cascades include the following (listed from south to north):

• California: Lassen Peak and Mount Shasta.
• Oregon: Mount Mazama, Three Sisters, Mount Jefferson, and Mount Hood.
• Washington: Mount Saint Helens, Mount Adams, Mount Rainier, Glacier Peak, and Mount Baker.

This generalized diagram of the formation of Crater Lake is animated from the 1959 edition of the 1:62,500 scale topographic map entitled "Crater Lake National Park and Vicinity, Oreg."

Crater Lake is a beauty born from violent eruptions of spitting fires and rocks. The phrase "GREW, BLEW, FELL, and FILL" describes the process that created Crater Lake. Mount Mazama grew, erupted, then collapsed to form the caldera, and finally precipitation filled the caldera. Although Crater Lake has been dormant for about 5,000 to 6,000 years, there could be another eruption someday in the future and Mount Mazama might grow back. The long history of volcanism at Mount Mazama strongly suggests that this volcanic center will be active in the future.

GREW

Mount Mazama was a large composite volcano that was built during the past 400,000 years by hundreds of smaller eruptions of lava flows. Each layer is a different lava flow or, in some instances, pyroclastic deposit. These eruptions occurred from less than one year to thousands of years apart. Mount Mazama rose to an approximate height of 3,700 m (12,000 ft) above sea level. The caldera wall displays the geologic layering of lava flows through time.

BLEW

About 7,700 years ago, Mount Mazama erupted catastrophically, blowing out about 50 km³ (12 mi³) of magma as pyroclastic materials (mostly rhyodacite pumice and fine ash) in at most a few days. The volcanic ash covered parts of the northwestern states, spreading as far as central Canada. Rare particles of Mazama ash have even been found in ancient ice from Greenland. The airfall pumice and ash covered a total surface area of more than 2,600,000 km² (1,000,000 mi²) at least 1 mm (fraction of an inch) thick. A volume of 42-54 km³ (10-13 mi³) of the mountaintop had disappeared.

Where had all this mass gone? Did Mount Mazama blow its top off?

FELL

Mount Mazama did not blow its top off; it collapsed in on itself. As this enormous volume of magma (54-63 km³, or 13-15 mi³) was rapidly removed from the chamber (about 5 km, or 3 mi below Earth's surface) to feed the climactic eruption, it created a huge void underneath the mountain. Leaving no support for this massive dome, the roof of the magma chamber collapsed, forming the bowl-shape depression known as a caldera.

FILL

About 5,000 to 6,000 years ago, the accumulation of rain and snow filled the caldera. It took perhaps 250 years for the caldera to fill to its present-day lake level, which is maintained by a balance between precipitation and evaporation plus seepage.