Lava Beds National Monument is the site of the largest concentration of lava tube caves in the United States. The monument lies on the northeast flank of the Medicine Lake shield volcano, the largest volcano (total area covered) in the Cascade Range. The region in and around the monument is unique because it lies on the junction of the Sierra-Klamath, Cascade, and Great Basin physiographic provinces. In addition, the monument is geologically outstanding because of its great variety of "textbook" volcanic formations; i.e., lava tube caves, fumaroles, cinder cones, spatter cones, maar volcanoes, and lava flows. 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: Callahan Flow, Schonchin Flow, Mammoth Crater Flow, Modoc Crater Flow, and Devils Homestead Flow.
The high elevation, semi-arid desert environment of Lava Beds receives an average of 15 inches (381mm) of precipitation annually. The climate is characterized by warm, dry summers and cold winters. The average annual high temperature is 60 F (16 C) and average annual low temperature is 35 F (2 C). Temperature extremes range from -18 F ( -28 C) to 102 F (39 C). Average annual snowfall is 44 inches (1118 mm). The lava tube collapse systems and lava outcrops support a great diversity of plant life, from an impressive variety of lichens and mosses to plants such as desert sweet (Chamaebatiaria millefolium) and the aromatic desert (purple) sage (Salvia dorrii carnosa). An impressive variety of fern species are present in cave entrances including the spreading wood fern (Dryopteris expansa) and the western swordfern (Polystichum munitum). These species are well outside of their normal range 90 - 125 miles away on the northern California coastline.