The monument lies at the south end of the Klamath graben, 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 m to about 1,600 m at the southern monument boundary. Continuing south, the terrain rises gradually to the summit of the Medicine Lake shield volcano, which last erupted about a thousand years ago. Some of the most recent eruptions of the volcano were explosive, leaving a deposit of white, silica-rich pumice fragments on all units in Lava Beds National Monument. In the north, the pumice occurs as a thin dusting; near the south boundary it is locally as much as 0.3 m thick.
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 nearly 300 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 km3. It covers an area of about 250 km2, extending as far as 25 km from Mammoth Crater. 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.
Open ground cracks, some more than 10 m wide and deep, are common in Lava Beds National Monument, and to the west and southeast, as well as high on Medicine Lake volcano where they formed during Holocene, mostly silicic eruptions. Crack trends range from north-south to about N. 30=B0 E. =46ink and Pollard (1983) showed that east-west extension combined with dike emplacement was responsible for the ground cracks that accompanied the eruption of Little Glass Mountain and related Holocene rhyolites high on the northwest side of Medicine Lake volcano. Dominantly north-south vent alignments and major north-trending normal faults such as the Gillem fault (Donnelly-Nolan, 1983b) also indicate east-west extension. In this extensional environment, it is not surprising that very primitive basalts have erupted.
With increasing silica content, the morphology of flows in Lava Beds National Monument becomes more rugged. Basaltic andesites with silica contents higher than 54 percent do not have pahoehoe surfaces or lava tubes; instead they form rough-surfaced aa or have very rugged, blocky surfaces commonly described as block lava flows. Andesites with 57 percent silica or more are typically block lava flows.
Variable climate can also affect flow appearance. In Lava Beds National Monument, elevation and rainfall increase from north to south and the vegetation changes noticeably. At the north end of the monument, vegetation consists of scattered low brush and grass with occasional juniper trees. Farther south, larger brush in the form of mountain mahogany appears, and juniper trees are more abundant. At the southern boundary of the monument, ponderosa pine is common. Continuing up the north side of Medicine Lake volcano, vegetation zones continue to change and forest cover increases. This means that lava flows of identical age and surface morphology have greater vegetation cover and appear older at high elevation where there is greater precipitation. On Medicine Lake Volcano, the tephra cover also increases toward the higher parts of the volcano. Also, pre-Holocene units high on the volcano have been stripped and smoothed by glaciers. Thus, an important observation in this area is that appearance of a lava flow may not directly correlate with its relative age.
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