Medicine Lake highlands: Self Guided Roadside Geology Tour

Shasta-Trinity National Forest

A self guide geology tour of the Medicine Lake Volcano highlands. The tour is primarily on the southern flake of the volcano.... OVERVIEW One of the most diverse and least visited volcanoes in the state of California is Medicine Lake Volcano. It lies east of the main volcanic axis of the Cascades , and is much different than the more typical Cascade Mountains stratovolcanos, such as Mt. Shasta and Mt. Lassen which stand nearby. Because this subrange of the Cascades is somewhat remote, the fascinating nature of this area is largely unappreciated. Medicine Lake Highlands is the largest identified volcano (in total area) within California. The large Hawaiian type shield volcano’s broad, gently sloping profile (stretching some 15 miles from east to west and nearly 25 miles north and south) belies the fact that it is actually larger in mass than nearby Mt. Shasta. Approximately 100,000 years ago, the great volcano underwent a series of eruptions which undermined the center of the mountain and the crest subsided to create a huge crater or caldera. Around the margins of this subsidence, new, smaller volcanoes arose; they are called rampart volcanoes. Medicine Lake now partially fills the crater. Geologists speculate over what events took place that caused the unusual shapes and features to form and how a series of eruptions changed the face of the countryside in the area. One thing is clear, however, more than a million years of volcanic activity have produced a landscape that is perhaps California’s most diverse volcanic field. Volcanic formations such as glass flows, pumice deposits, lava tubes and cinder cones are found throughout the Medicine Lake Highlands. Following are some descriptions and examples of these features…