Lava Beds National Monument is geologically significant because of its wide variety of volcanic formations, including lava tubes,
fumaroles,
cinder cones,
spatter cones,
pit craters,
hornitos,
maars,
lava flows, and
volcanic fields. Volcanic eruptions on the
Medicine Lake shield volcano have created a rugged landscape punctuated by these many landforms of
volcanism.
Cones cinder cone 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 are typically
monogenetic. The cinder cones of Hippo Butte, Three Sisters, Juniper Butte, and Crescent Butte are all older than the Mammoth and Modoc Crater flows, more than 30,000–40,000 years old. Eagle Nest Butte and Bearpaw Butte are 114,000 years old. The
Schonchin Butte cinder cone 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, such as the Devil's 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.
Lava flows from
Captain Jack's Stronghold Roughly ninety percent of the
lava in the Lava Beds Monument is
basaltic. There are primarily two kinds of basaltic lava flows:
pahoehoe and
ʻaʻā. Pahoehoe is smooth, often ropy and is the most common type of lava in Lava Beds. ʻAʻā is formed when pahoehoe cools and loses some of its
gases. ʻAʻā is rough, sharp, and jagged; an excellent example is the Devil's 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 two-thirds of the lava in the monument. Over 30 separate lava flows located in the park range in age from 2,000,000 years BP to 1,110 years BP. Some of the major lava flows within Lava Beds National Monument include the Callahan Flow; Schonchin Flow; Mammoth Crater Flow; Modoc Crater Flow; and Devil's Homestead Flow.
Lava fields Gillem Bluff, a fault scarp, was created as the region stretched and a block of earth dropped down along this fault (see
Basin and Range Province). The
tuff layer on top of Gillem Bluff is 2,000,000 years old, indicating 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. The
caldera is thought to have formed by subsidence, during which basalt and
andesite were erupted up on the slopes.
Lava tubes Lava flows dated to about 30,000–40,000 years ago formed most of the lava tubes in the monument. As the hot basaltic lava flowed downhill, the top cooled and crusted over, insulating the rest of the lava and forming
lava tubes.
Lavacicles on the ceiling of a lava tube were produced as the level of lava in the tube retreated and the viscous lava on the ceiling dripped as it cooled. Dripstone was created when lava splashed on the inside walls of the tubes. The leaching of
minerals from pumice gravel,
soils, and overlying
rock provides for deposition of secondary speleothems in lava tubes. Lava Beds National Monument has the largest concentration of
lava tubes in North America. One has electrical lighting; the others are illuminated by ceiling collapse portals or require flashlights, available on loan. Notable examples in the park include
Catacombs Cave,
Merrill Cave,
Mushpot Cave, and
Valentine Cave.
Recent activity A series of small
earthquakes in late 1988 has been attributed to subsidence in the
caldera. North-northeast trending ground cracks, as well as north-northeast trending vent series show relationships between tectonism and volcanism. One notable ground crack, the Big Crack, extends along the northeastern boundary of the monument. ==Climate==