The Geology of Crater Lake National Park, Oregon

Crater Lake National Park, in the Cascade Range of Klamath County in southwestern Oregon, was established on May 22, 1902, and protects about 183,224 acres of forest and alpine terrain. Its centerpiece, Crater Lake, lies in the caldera of Mount Mazama, which erupted catastrophically about 7,700 years ago. The caldera is roughly 5 by 6 miles across and more than 1 kilometer deep, making Crater Lake the deepest lake in the United States. Visitors come to fish, bicycle, hike more than 90 miles of trails, and take in the lake’s clear blue water.

The NPS Geodiversity Atlas, a product of the Park Service’s Geologic Resources Inventory, describes the full variety of a park’s rocks, minerals, sediments, fossils, landforms, soils and the processes that shape them.

Mount Mazama

Before it destroyed itself, Mount Mazama — about 12,000 feet high — was one of the major volcanoes of the Cascade arc, the chain of tall composite cones (stratovolcanoes) and hundreds of smaller shield volcanoes and cinder cones stretching from northern California to southern British Columbia. The arc sits above the Cascadia subduction zone, where the Juan de Fuca plate and the smaller Gorda and Explorer plates slide beneath North America, and it has erupted within the past century.

Mazama was not a single cone but a succession of overlapping shield volcanoes and stratovolcanoes built on lava flows more than 400,000 years old. Each was probably active for a few thousand to perhaps 40,000 years, and activity migrated west-northwest as the mountain grew. Smaller shield volcanoes and cinder cones around it reflect the region’s wider volcanism.

The climactic eruption

The great eruption lasted only a few days but expelled about 50 cubic kilometers (12 cubic miles) of magma, in two phases:

1. A single-vent phase, a towering “Plinian” column of pumice and ash.
2. A ring-vent phase, as the volcano began to collapse into itself and circular cracks opened around the peak, sending pyroclastic flows — fast, chaotic mixtures of rock, gas and ash hotter than 800 °C (1,470 °F) — from vents ringing the upper mountain.

Ash from the eruption, mostly from the first phase, settled over about 656,000 square miles of the western United States and southwestern Canada. Because it fell so quickly and so widely, the Mazama ash is a valuable marker layer for dating deposits across many environments.

The caldera and the lake

The collapse left a caldera about 3,900 feet deep and 5–6 miles across at the rim. Since then it has been filling: first with rock fallen from its walls, then with lava and lake sediment, a process that continues today. Five vents have erupted lava onto the caldera floor since the collapse, all of it inside the caldera; only one post-caldera volcano, Wizard Island, rises above the water. These volcanoes were active while the lake was filling.

Crater Lake has no inlet or outlet. Its level is set by a balance between precipitation and inflow on one side and evaporation and seepage on the other. An average water supply of about 88 inches a year, and a thick layer of permeable glacial till in the caldera wall that acts as a “bathtub drain,” largely control the lake level.

Caves, hot water and ice

• Caves. About 40 caves are known in the park, most at old lake levels inside the caldera, carved by waves, plus at least five significant caves outside it. Some caldera caves and shelters have held sensitive archeological materials. The park has at least nine species of bats, though whether they use the caves is unclear. All park caves are protected under the Federal Cave Resources Protection Act of 1988.
• Hydrothermal features. The Geothermal Steam Act of 1970, as amended in 1988, names Crater Lake a significant thermal feature. On the lake floor there are pools of warm, mineral-rich water, bacterial mats where warm water vents, and tall silica spires built by underwater thermal springs.
• Fire and ice. No glaciers remain, but the park has long been known as a place where volcanoes and glaciers met. Lava flows chilled against ice into polygonal columns, and flat-topped tuyas formed beneath it. Classic glacial features abound — till and moraines, polished and scratched rock, U-shaped valleys and notches, cirques and horns. As many as six advances of glacial ice carved the park, including the notches at the heads of Sun and Kerr valleys, and a seventh is recorded in the caldera walls; researchers have matched them to global ice ages.

Maps and reports

The Geologic Resources Inventory has produced digital geologic maps, reports, posters and scoping summaries for the park, and a Soil Resources Inventory has been completed; both are available through the NPS DataStore.

Sources

Based on "NPS Geodiversity Atlas—Crater Lake National Park, Oregon," Geologic Resources Division, National Park Service; a work of the United States government in the public domain.