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Lava Beds National Monument Geologic Formations

Roughly ninety percent of the lava in the monument is basaltic. There are primarily two kinds of basaltic lava flows: pahoehoe and aa. Pahoehoe is smooth and ropy and is the type most common in Lava Beds. Aa is formed when pahoehoe cools and loses some of its gases. Aa is rough, sharp, and jagged; an excellent example is the Devils Homestead lava flow, which originated at Fleener Chimneys. Most of the rest of the lava in the monument is andesitic. Pumice, a type of rhyolitic lava, also is found covering the monument; this rained down around 900 years ago during the eruption of Glass Mountain.

The flows from Mammoth and Modoc Craters comprise about 2/3 of the lava in the monument. These flows have been dated to about 30,000-40,000 years ago; most of the caves in the monument were formed from these flows. As the hot basaltic lava flowed downhill, the top cooled and crusted over, insulating the rest of the lava and forming lava tubes. Lavacicles were formed when lava splashed up on the ceiling of the tube and dripped back down as it cooled. Dripstone was created when lava splashed on the inside walls of the tubes.

Cinder cones are formed when magma is under great pressure. It is released in a fountain of lava, blown into the air from a central vent. The lava cools as it falls, forming cinders that pile up around the vent. When the pressure has been relieved, the rest of the lava flows from the base of the cone. Cinder cones only erupt once.

The cinder cones of Hippo Butte, Three Sisters, Juniper Butte, and Crescent Butte are all older than the Mammoth and Modoc Crater flows (that is, more than 30,000-40,000 years old). Eagle Nest Butte and Bearpaw Butte are 114,000 years old. Schonchin Butte and the andesitic flow from its base were formed around 62,000 years ago. The flow that formed Valentine Cave erupted 10,850 years ago. An eruption that formed The Castles is younger than the Mammoth Crater flows. Even younger were eruptions from Fleener Chimneys (the Devils Homestead flow, 10,500 years ago) and Black Crater (3,025 years ago). About 1,110 years ago, plus or minus 60 years, the Callahan flow was produced by an eruption from Cinder Butte. Though Cinder Butte is just outside the boundary of the monument, the Callahan flow is in Lava Beds and is the youngest flow in the monument.

Spatter cones are built out of thicker lava. The lava is thrown out of the vent and builds, layer by layer, a chimney surrounding the vent. Fleener Chimneys and Black Crater are examples of spatter cones.

Gillem Bluff, a fault scarp, was created as the region stretched and a block of earth dropped down along this fault. The tuff layer on top of Gillem Bluff is 2,000,000 years old, inferring the rock layers beneath are even older. The oldest lava flow from the Medicine Lake Volcano within the monument is the Basalt of Hovey Point, near Captain Jack's Stronghold, which is 450,000 years old. Petroglyph Point was created about 275,000 years ago when cinders erupted through the shallow water of Tule Lake; violent explosions of ash and steam formed layers upon layers of tuff.

Geologic forces at work today:

Seismic/Tectonic activity: A series of small earthquakes in late 1988 has been attributed to subsidence in the caldera.

N-NE trending ground cracks, as well as N-NE trending vent series show relationships between tectonism and volcanism. One very prevalent ground crack exists along the northeastern boundary of the monument- "The Big Crack."

Mineral deposition: The leaching of minerals from pumice gravel, soils, and overlying rock provides for deposition of secondary speleothems in lava tubes.

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