NSS Convention ~ July 25-30, 2021

Crater Lake NP

Cascade Volcanic Arc

  • Excerpted from USGS and Wikipedia Publications

The Cascade Arc includes nearly 20 major volcanoes, among a total of over 4,000 separate volcanic vents including numerous stratovolcanoes, shield volcanoes, lava domes, and cinder cones, along with a few isolated examples of rarer volcanic forms such as tuyas. Volcanism in the arc began about 37 million years ago; however, most of the present-day Cascade volcanoes are less than 2,000,000 years old, and the highest peaks are less than 100,000 years old. Twelve volcanoes in the arc are over 10,000 feet in elevation, and the two highest, Mount Rainier and Mount Shasta, exceed 14,000 feet. By volume, the two largest Cascade volcanoes are the broad shields of Medicine Lake Volcano and Newberry Volcano, which are about 145 cubic miles and 108 cubic miles respectively.

Crater Lake

Crater Lake inspires awe. Native Americans witnessed its formation 7,700 years ago, when a violent eruption triggered the collapse of a tall peak. Scientists marvel at its purity: fed by rain and snow, it’s the deepest lake in the USA and one of the most pristine on earth. NPS photo.

Over the last 37 million years, the Cascade Arc has been erupting a chain of volcanoes along the Pacific Northwest. Several of the volcanoes in the arc are frequently active. The volcanoes of the Cascade Arc share some general characteristics, but each has its own unique geological traits and history. Lassen Peak in California, which last erupted in 1917, is the southernmost historically active volcano in the arc, while the Mount Meager massif in British Columbia, which erupted about 2,350 years ago, is generally considered the northernmost member of the arc.

Mount Mazama is one of the major volcanoes of the Cascades Arc. Crater Lake is located within the collapsed caldera of Mount Mazama on the crest of the Cascade Range in southern Oregon about 100 miles north of Weed, California and about 60 miles northeast of Medford, Oregon. This volcano formed at the intersection of the Cascade chain of volcanoes with the Klamath graben, a north-northwest low-lying basin that is surrounded by tectonic faults, and is bounded to the east by the Basin and Range province. 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.

Phantom Ship Island - Crater Lake

Phantom Ship Island is mainly formed from andesite rock that dates from about 400,000 years ago, partly altered by hydrothermal activity. The island is situated on the south east end of Crater Lake and projects more than 656 feet out from the wall of the caldera. NPS photo.

Click here to download a geologic field-trip guide to Mount Mazama and Crater Lake Caldera.

Mount Mazama began erupting relatively continuously 420,000 years ago as a complex of overlapping shields and stratovolcanoes, each of which probably was active for up to 70,000 years. The massive volcano erupted violently 7,700 years ago, accompanied by collapse of the entire upper half of the edifice. Prior to its climactic eruption Mount Mazama had a summit elevation of about 12,000 feet. The present high point is Mount Scott at 8,929 feet, about 2 miles east of the caldera rim.

Today, the caldera rim ranges in elevation from approximately 6,700 feet to 8,150 feet, with maximum relief of about 2,000 feet above the surface of Crater Lake, virtually equal to maximum water depth. Surviving flanks of Mount Mazama consist of lava that slopes gently away from the caldera rim, incised by deep glacial valleys that are partially filled with pyroclastic-flow deposits of the climactic eruption. All but the steepest slopes are covered with deposits of the climactic eruption.

In the area surrounding Mount Mazama, regional volcanism has been active throughout at least the last 700,000 years, but continuity of regional activity prior to approximately 200 ka is uncertain. Several regional volcanoes, some of them large shields, were active during the interval from about 200 to 100 ka. There was comparatively little regional volcanism between about 100 to 40 ka. However, after 40,000 years, during the growth of the climactic magma chamber, large amounts of basalt to basaltic andesite lavas were erupted from several vents west of Mazama; this suggests that abundant magma from deep within the earth was moving into the crust around Mount Mazama at this time.