Geology of Mount Mazama

Quick Facts about Mount Mazama

Type of Volcano: Stratavolcano also known as a composite volcano
Age: From about 400,000 years ago; presently dormant but potential to erupt again
Igneous Rock Types: Basaltic andesite, andesite, dacite, and rhyodacite
Estimated height before collapse: 12,000 ft (3,700 m)
Highest point today: Mt. Scott 8,929 ft (2,722 m)
Size of caldera: 5 by 6 miles (8 by 10 kilometers) at rim
Last active: about 4,800 years ago
Comments: the largest explosive volcanic eruption in the past 400,000 years in the Cascades.
Highlights: World-renowned classic caldera; spectacular compositionally zoned caldera-forming eruption; eruptive history well displayed in caldera walls

Significance and Wonderment

The boundary of Crater Lake National Park protects most of what remains of Mount Mazama, an ancient volcano that once loomed around 12,000 ft (3,700 m) skyward, amongst the tallest in the Cascade Mountain Range. A rapid series of explosive eruptions around 7,700 years ago caused the cataclysmic emptying of Mount Mazama and the inward collapse of its structure.

Two highly visible results of these events include one of the most spectacular calderas in the world—a 6 by 5-mile (8 by 10 kilometer) basin more than .6 mile (1 km) deep. The other is Crater Lake which at its deepest place in this renowned caldera reaches 1943 feet. The lake and the caldera are incredibly prevalent and may seem to represent what is most important about Mount Mazama. But much more has been discovered about its eruptive past and present dormancy that provides insight into its potential future impacts and those of other volcanos.


Inside and Outside a Big Volcano

The view inside this partially lake-filled caldera is an unprecedented view inside a volcano. It presents geologic evidence of the rise and fall of Mount Mazama, one the biggest Cascade volcanoes. It exposes the layered story of lava flows and pyroclastic deposits. The caldera walls reveal broken and eroded evidence of Mount Mazama’s composite volcanic growth over several hundred thousand years, and the rim reveals the former presence of glaciers.

Outside the caldera, the explosive eruption laid waste to every living thing and dramatically changed the landscape, clogging rivers, filling valleys, and burying the ground with pumice, and ash as far away as southern British Columbia, and Alberta, Canada.

Marerial from inside the volcano and the magma reservoir from which it developed are visible throughout the park. The most notable result of the eruptions are the numerous pumus fields, which can be over 200ft (60.96m) deep with porous pumice, limiting the nutrition and soil needed for plant growth. Pinnacles, solidified in the aftermath of the fallen ash and later exposed through erosion, are visible spires within some river valleys in the park. Boulders along the trails represent different compositions and ages of igneous rock.

Active and Potentially Dangerous

The USGS Volcano Hazards Program monitors and studies active and potentially active volcanoes, assesses their hazards and conducts research on how volcanoes work to issue timely warnings of potential volcanic hazards to emergency-management professionals and the public.

A total of 161 volcanoes in the United States are judged to be potential threats. Eleven of the 18 very high threat volcanoes are in Washington, Oregon or California. These are where explosive and often snow-and-ice-covered volcanoes can project hazards long distances to densely populated and highly developed areas.

Mount Mazama, although its presence across the landscape lacks an exposed summit, is ranked 17th among the 161 potentially most dangerous volcanoes. About 4,800 years ago, a viscous rhyodacite dome nearly breached the lake surface. This is the youngest eruption product that has yet been recognized. Although presently dormant, Mount Mazama’s subterranean magma sources are unlikely to be dead and there is potential for future eruptions.

A Natural Laboratory

“The geology of the area was first described in detail by Diller and Patton (1902) and later by Williams (1942), whose vivid account led to international recognition of Crater Lake as the classic collapse caldera. Because of excellent preservation and access, Mount Mazama, Crater Lake caldera, and the deposits formed by the climactic eruption constitute a natural laboratory for study of volcanic and magmatic processes.” [from Geologic field trip guide to Mount Mazama and Crater Lake Caldera, Oregon (]

Three areas of knowledge enhanced by studies around Mount Mazama include:
  1. Ash from Mount Mazama is buried across the Pacific Northwest and therefore its presence is an identifiable stratigraphic marker of time.
  2. A detailed bathymetric survey [insert link] of the floor of Crater Lake in 2000 (Bacon and others, 2002) provides a unique record of post-caldera eruptions (including Wizard Island and three underwater volcanoes), and the interplay between volcanism and filling of the lake.
  3. Features exposured along the caldera wall give exceptional insight into how large volcanoes of magmatic arcs grow and evolve.

Last updated: December 6, 2022

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