Explore Geology
geology fieldnotes title

Crater Lake

National Park


cover of park brochure

park geology subheading
Photo of Crater Lake
Crater Lake National Park, Oregon

Crater Lake is the deepest lake in the United States, the second deepest in the Western Hemisphere and the seventh deepest in the world.

A Brief Chronology

  • On the earth clock, natural forces only recently constructed this landscape.
  • Lava flows first formed a high plateau base on which explosive eruptions then built the Cascade volcanoes.
  • Humans probably witnessed the cataclysmic eruption of Mount Mazama about 7,700 years ago.
  • Shamans in historic time forbade most Indians to view the lake, and Indians said nothing about it to trappers and pioneers, who for 50 years did not find it.
  • In 1853, while searching for the Lost Cabin Gold Mine, some prospectors, including John Wesley Hillman, happened onto Crater Lake.
  • Soundings with piano wire by a U.S. Geological Survey party in 1886 set the lake's depth at 1,996 feet, close to sonar findings of 1,932 feet officially recorded in 1959.
  • The clean, clear, cold lakewater contained no fish until they were introduced by humans from 1888 to 1941.

A Vast Volcanic Region
The plateau base of the Cascade Range was built as the Earth's crust folded and uplifted, pushing seas westward. Molten rock pushed toward the surface, creating both violent eruptions and the welling up of lava through enormous cracks. In recent geologic time (the past 750,000 years) explosive eruptions built a string of volcanoes on this plateau base. This Cascade Range of volcanoes extends from Canada's Mount Garibaldi to Lassen Peak in northern California. One of these great volcanoes (Mount Mazama) now holds Crater Lake.

The Caldera
Today, the calm beauty of Crater Lake belies the violent earth forces that formed the lake. Crater Lake lies inside the top of an ancient volcano known as Mount Mazama. This dormant volcano is just one in a group of huge cones that extends along the crest of the Cascade Range. This range extends from Lassen Peak in California to Mount Garibaldi near Vancouver, British Columbia.

The volcanoes of the Cascade Range are the visible evidence of what geologists call "plate tectonics". In this process, the continents move over the Earth's surface as large land plates. As they move, the continents and adjacent sea floor either move apart or push into one another. Continental plates are lighter, and tend to "float" on heavier oceanic plates.

When an oceanic plate pushed into what is now the northwestern United States, it was forced under the less dense continental plate. Tremendous pressures were exerted on the oceanic plate, causing it to deform and even melt.

Mt. Mazama was built by successive flows of both andesite and dacite lavas. These "pasty" fluids piled up on each other resulting in a mountain much like today's Mt. Hood.

Mount Mazama
For half a million years this mighty volcano produced massive eruptions interrupting long periods of quiet. Ash, cinders, and pumice exploded upward, building the mountain to a height of about 12,000 feet. Parasitic cones on Mazama's flanks created Mount Scott and Hillman Peak. Glaciers periodically covered Mount Mazama's flanks and carved out the U-shaped valleys such as Munson Valley and Kerr Notch. About 7,700 years ago the climatic eruptions of Mount Mazama occurred. Ash from these eruptions lies scattered over eight states and three Canadian provinces, some 5,000 square miles were covered with 6 inches of Mazama's ash. In the park's Pumice Desert (see map) ash lies 50 feet deep. The eruptions were 42 times greater than those of Mount St. Helens in 1980. The Mazama magma chamber was emptied and the volcano collapsed, leaving a huge bowl-shaped caldera. The high mountain was gone. At first the caldera's floor was too hot to hold water. Renewed volcanism sealed the caldera and built the Wizard Island and Merriam cones, volcanoes within a volcano.

Volcanism stages leading to Mount Mazama's collapse

volcano 1
1) About 420,000 years ago, Mt. Mazama began to grow with the creation of Mt. Scott. As time passed, overlapping lavas flowed from several vents. This built the irregularly shaped Mt. Mazama volcano, which may have been as high as 12,000 ft. above sea level.

volcano 2
2) Mt. Mazama's most violent eruption occurred about 7,000 years ago. A column of hot gas and magma was ejected high into the air. This ejected material fell to the surface as fragments of frothy white pumice and volcanic ash.

volcano 3
3) Explosions from around the summit of Mt. Mazama produced fast moving flows of hot ash. Finally, as the magma chamber was emptying and the underlying support for the mountain was lost, the walls of the volcano began to collapse. The top of a mountain that was built over hundreds of thousands of years "disappeared" in just a few days!

volcano 4
4) The collapse formed a caldera (which comes from the Spanish word for "kettle" and is used by geologists to describe large basin-shaped volcanic depressions). Water from rain and snow filled the caldera, creating the nation's deepest lake at 1,932 feet.

park maps subheading

The General park map handed out at the visitor center is available on the park's map webpage.

For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.

photo album subheading

A geology photo album for this park can be found here.

For information on other photo collections featuring National Park geology, please see the Image Sources page.

books, videos, cds subheading
Discovering Crater Lake
Field, Nancy, and Sally Machlis., 1992.
In cooperation with the Crater Lake Natural History Association.
Dog-Eared Publications.
ISBN 0-941042-08-1, 8.5" x 11", paperback, 32 pages, black and white illustrations.

Discovering Crater Lake explores the formation of a crater, pinnacles and other geological features. Children learn about scientific research in deep water lakes, native plants and wildlife and winter survival! . They enjoy the many Indian legends and the history of modern people. Includes a sheet of 40 realistic forest stickers.

  • Explore the formation of Crater Lake
  • Delve into the secret depths of the lake
  • Discover why the lake is so blue
  • Play the "Adventure Trail Game"
  • Investigate people important to the history of Crater Lake

Please visit the Geology Books and Media webpage for additional sources such as text books, theme books, CD ROMs, and technical reports.

Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
ISBN 0-393-92407-6
9" x 10.75", paperback, 550 pages, full color throughout

The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!

Ordering from your National Park Cooperative Associations' bookstores helps to support programs in the parks. Please visit the bookstore locator for park books and much more.

geologic research subheading

Information about the park's research program is available on the park's research webpage.

For information about permits that are required for conducting geologic research activities in National Parks, see the Permits Information page.

The NPS maintains a searchable data base of research needs that have been identified by parks.

A bibliography of geologic references is being prepared for each park through the Geologic Resources Evaluation Program (GRE). Please see the GRE website for more information and contacts.

selected links subheading

NPS Geology and Soils Partners

NRCS logoAssociation of American State Geologists
NRCS logoGeological Society of America
NRCS logoNatural Resource Conservation Service - Soils
USGS logo U.S. Geological Survey

teacher feature subheading

Currently, we do not have a listing for any park-specific geology education programs or activities.

For resources and information on teaching geology using National Park examples, see the Students & Teachers pages.
updated on 01/04/2005  I   http://www.nature.nps.gov/geology/parks/crla/index.cfm   I  Email: Webmaster
This site is best viewed in Internet Explorer 6.0 or Netscape 7.0