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Lava Beds National Monument

Lava Beds National Monument is located in northeastern California about 50 km (31 mi) south of Klamath Falls, Oregon. The monument, established in 1925, includes the sites of many important battles of the Modoc Indian War of 1872-73. It is also known for scores of lava-tube caves and for well preserved young volcanic features.

The monument lies on the north flank of Medicine Lake volcano. The north edge of the monument coincides with the south edge of Tule Lake, much of which is now reclaimed for farmland. South of the lake the terrain rises from about 1,200 meters to about 1,600 meters at the southern monument boundary.

Most lava flows in the monument issued from cinder and spatter cones within the monument boundaries. These numerous flank vents of Medicine Lake volcano have fed mafic lava including basalt, basaltic andesite, and some andesite. The basalt of Mammoth Crater is the dominant unit in the monument in terms of area and volume and is host to most of the hundreds of lava-tube caves. This unit, erupted from several vents including Mammoth Crater and Modoc Crater, is volumetrically one of the largest of Medicine Lake volcano, amounting to about 5 km (1.2 mi3. It covers an area of about 250 km2 (96 mi2, extending as far as 25 km (15.5 mi) from Mammoth Crater and was fed via approximately a dozen lava tubes. The basalt of Mammoth Crater was erupted in a geologically brief period of time, probably less than 100 years, as judged from the consistent paleomagnetic direction obtained from different parts of the unit.

The basalt of Mammoth Crater interacted with the water of ancient Tule Lake, producing pillow lava between Captain Jacks Stronghold and the northeast corner of the monument. The pillows are visible in small quarries at the edge of the flow. The lake must have been very shallow, as it is today, because the upper surface of the flow shows no effect of interaction of water. At Hospital Rock, in the northeastern part of the monument, several littoral cones, similar in appearance to small cinder cones, apparently formed where a lava tube emptied directly into water. Much older pillowed flows are exposed in Gillem Bluff and apparently entered a much older (and probably deeper) version of Tule Lake.