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Lava Beds National Monument Geology


~ Though not as tall as most other Cascade volcanoes, the Medicine Lake Volcano is, by volume and in surface area, the largest volcano in the entire Cascade Range, covering over 700 square miles with an estimated volume of around 130 cubic miles.

~ At approximately 73 square miles, Lava Beds National Monument encompasses only 10% of the Medicine Lake Volcano's surface.

~ The Medicine Lake Volcano and its lava flows are independent of other Cascade volcanoes. That is, neither Mount Shasta nor any other nearby volcano produced any of the lava flows in this area.

~ Geologically speaking, the Tule Lake Basin is an "extensional environment". In this area, tectonic forces are slowly stretching the earth's crust, a process which continues today. This is evidenced by the dropping of the basin floor between north-south-trending faults along the west and east sides of the basin. More evidence of this stretching is seen in deep ground cracks running north-south through the Lava Beds backcountry. Some of the more recent volcanic eruptions at Lava Beds took place along these cracks, using these crustal weak points as easy routes for magma to reach the surface.

~ Prehistoric Tule Lake extended much farther south than its current southern limit. The lava flows from the north and east sides of the Medicine Lake Volcano filled in much of the former basin, leaving only the northern end as a lake.


~ The Medicine Lake Volcano, as a whole, is roughly 500,000 years old. Just a kid, geologically speaking!

~ The oldest geologic feature at Lava Beds is Gillem Bluff. A layer of tuff (a type of volcanic rock) found atop Gillem Bluff has been dated at around 2,000,000 years old. The rock layers seen in the face of Gillem Bluff (underlying the tuff) are therefore older than 2,000,000 years old. The southern extent of Gillem Bluff has been buried by the more recent formation of the Medicine Lake Volcano.

~ The oldest lava flow at Lava Beds that can be attributed to the Medicine Lake Volcano is the basalt of Hovey Point, located at the northern boundary west of Captain Jacks Stronghold. The Hovey Point flow is around 450,000 years old.

~ The youngest lava flow within the boundaries of Lava Beds is the Callahan Flow in the southwestern corner of the monument. This flow originated at Cinder Butte (just outside the boundary), and is 1,110,000 years old.

~ The youngest lava flow anywhere on the Medicine Lake Volcano is Glass Mountain (just south of Lava Beds). This large flow, containing large amounts of obsidian and pumice, is 885,000 years old. The light-colored pumice gravel covering most of the surface at Lava Beds fell from the sky during Glass Mountain's eruption.

~ The Medicine Lake Volcano has erupted 8 times during the last 3,000 years. Four of those eruptions directly affected the surface now within Lava Beds National Monument.

~ There are over 30 separate lava flows exposed at Lava Beds which can be attributed to eruptions of the Medicine Lake Volcano.

~ The basalt of Mammoth Crater erupted from Mammoth Crater, Modoc Crater, and other smaller vents to cover not only 70% of what is now Lava Beds National Monument, but also most of the basin to the east of the monument. This single eruptive event produced nearly a cubic mile of lava. Many of the lava tube caves in the monument are within this lava flow. This flow is approximately 30,000-40,000 years old.

~ Estimated ages of some of the eruptions and lava flows at Lava Beds and vicinity:

Gillem Bluff lava: >2,000,000 years old

Medicine Lake Volcano: ~500,000 years old

Hovey Point: 450,000 years old

Petroglyph Point: 275,000 years old

Eagle Nest Butte: 114,000 years old

Schonchin Butte: 62,000 years old

Mammoth Crater: 30,000-40,000 years old

Valentine Cave: 10,850 years old

Fleener Chimneys: 10,500 years old

Black Crater: 3,025 years old

Cinder Butte: 1,110 years old

Little Glass Mountain: 1,065 years old

Glass Mountain: 885 years old


~ The consistency and viscosity of lava are primarily determined by three main factors: its temperature, the amount of gas dissolved within the lava, and its chemical composition. The amount of silica (SiO2) within the lava is the primary factor in determining the lava's viscosity and category.


~ Lava Beds National Monument encompasses the largest known concentration of lava tube caves in the continental United States.

~ The longest known single cave at Lava Beds is Catacombs Cave, in the Cave Loop area, with a surveyed length of 6,903 feet.

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