I left George and Wilma's ranch and headed for Lava Beds Volcanic National Monument, on the Modoc Plateau. This involved curving around Mt. Shasta from the south and the east, giving me more views of the majestic mountain. The geologic structure of the Modoc Plateau consists almost entirely of flood basalt flows and cinder cones. The first major flood basalt flows began about 17 million years ago and are believed to have been triggered by an asteroid or comet falling somewhere in Southeast Oregon. The asteroid or comet created a crater that penetrated several kilometers down to magma in the Earth's mantle. The reduced pressure on the magna caused it to expand and heat up, causing the crater to collapse and forming a much vaster caldera. This magma lake periodically emitted flows of basalt lava that over the next two million years covered much of Eastern Washington, Eastern Oregon, parts of Idaho and Nevada, and the Modoc Plateau in the Northeastern corner of California. Since then, there has been additional volcanic activity that has covered much of the great flood basalt with additional volcanic material.
Lava Beds National Monument is located in part of a huge shield volcano known as the Medicine Lake Volcano. It is part of the High Cascade Range, and is the largest volcanic by volume in the range. A shield volcano generally has a large size and a low profile, and is usually built from fluid lava flows. It gets its name resembling a warrior's shield. The Medicine Lake Volcano does not have a since vent at its center. Rather, it has had numerous vents and has produced many lava flows over the past two million years, and it is still considered to be active. Contained within the shield are quite a few cinder cones. The major attration of Lava Beds are caves consisting of basalt tunnels through which the fluid lava flowed. As the lava flows from the vent, it cools as it comes in contact with the ground and the are. The surface of the flow hardens, forming the tunnel. The hotter lava in the center remains liquid and continues to flow, extending the tunnel until the lava has cooled enough to stop its flow. The tunnels can be miles long, though most are measured at a few hundred yards from the vent.
Mileage: 175. Cumulative mileage: 1,717.
