Lava Beds National Monument sits on the northeast corner of the Medicine Lake Volcano, a little-known mountain in the Cascade Range. This volcano has been active for at least 500,000 years. There are two types of volcanoes in the Cascades. Most of the more famous Cascade volcanoes, such as Mt. Shasta, Mt. McLoughlin and Mt. St. Helens, are composite volcanoes. These are easily recognized because they form massive cones of layers of ash and thick lava that piled close to the vent. These volcanic mountains can reach over 14,000 ft. above sea level and may be covered by snow and ice most of the year.
Medicine Lake Volcano is a shield volcano that forms a gently-sloping mound with a surface area of over 700 square miles. The most active shield volcanoes are found in the Hawaiian Islands and are built of very fluid lava that flows a great distance from its source. The highest point of the Medicine Lake highlands is almost 8,000 feet above sea level.
The last great surface flow of lava emanated from Mammoth Crater, just around the corner, about 40,000 years ago. This crater, nearly 400 feet deep, was once filled with highly-fluid lava that spilled over its banks and flowed towards the northeast, forming many of the monument's lava tube caves. Today, over 435 caves have been documented within the monument.
Most caves can be explored safely without a guide but it is always better to go with a partner. Caving equipment should include at least two lights per person, a hard hat, gloves and kneepads if you plan to do much crawling. Cave temperatures average 55 and cave floor surfaces are rough. Wear warm clothes with long sleeves, long pants and sturdy shoes. Lights are available for free at the visitor center.
Generally, lava tube caves lie just beneath the surface and do not continue far before they reach a dead end. The deepest cave passages lie about 150 feet below the ground. The longest tube is Catacombs that winds for almost 7,000 feet, often through narrow tunnels. Most caves can be explored in less than 45 minutes.
Please remember that many cave features are fragile and irreplaceable. Caves are also homes for many kinds of animals, including fourteen different species of bats. Some caves are closed in the summer season in order to protect female bats and their babies from disturbance. Travel gently through these amazing underground passageways.
Prehistoric Tule Lake extended much farther south than its current limit. The lava flows from the north and east sides of the Medicine Lake Volcano filled in much of the former basin, leaving only the northern end as a lake.
Geologically speaking, the Tule Lake Basin (which includes the Lava Beds area) is an "extensional environment". In this area, tectonic forces are slowly stretching the earth's crust, a process which continues today. This is evidenced by the dropping of the basin floor between north-south-trending faults along the west and east sides of the basin. More evidence of this stretching is seen in deep ground cracks running north-south through the Lava Beds backcountry. Some of the more recent volcanic eruptions at Lava Beds took place along these cracks, using these crustal weak points as easy routes for magma to reach the surface.
Regarding "recent" volcanic events in the Lava Beds area, four new lava flows have covered part of the monument over the last 11,000 years. Also, pumice (a frothy, rhyolitic lava) is found covering the monument; this rained down around 900 years ago during the eruption of nearby Glass Mountain.