Lassen Volcanic National Park—the southernmost Cascade Range volcano
July 13, 2020
The Cascadia Subduction zone extends along the entire coast of Washington and Oregon, but also along the northern coast of California. South of Mendocino, California, the plates change from converging with each other (the ocean plate loses and slides beneath the continental plate in the subduction zone) to sliding past each other along the San Andreas fault (a transform-type plate boundary). Check out my last post, on 10 May 2020, to see maps of the entire Cascadia Subduction zone and an explanation of why volcanoes occur there.
Volcanism in the Lassen region has been ongoing for about 3.5 million years. The center of volcanism has shifted over time, as shown in the map and diagram below.
Mt Lassen erupted most recently in 1914–17. It was the youngest volcano eruption in the Cascades until Mt St Helens erupted in 1980. The results of this eruption are most visible in the Devastated Zone, which can easily be viewed from the main road through the park. But our focus in a July backpacking trip to Lassen was the eastern side of the park, where Cinder Cone was built in 1666—it must have been a big event for the native people who were living in the area at that time.
A favorite part of Lassen Volcanic National Park is Bumpass Hell, a hydrothermal feature located in the southern park of the park. The area includes roaring fumaroles (steam and volcanic-gas vents), thumping mud pots, boiling pools, and steaming ground. Water from rain and snow that falls on the highlands of the park feed the hydrothermal system. Once deep underground, the water is heated by a body of hot or molten (i.e., liquid) rock beneath Lassen Peak. Rising hot water boils to form pools and mud pots. Super-heated steam reaches the surface through fractures in the earth to form fumaroles at the surface.
Another process that formed Lassen Park is the action of glaciers during the Last Glacial Maximum about 20,000 years ago.
Lassen Volcanic National Park is one of the under-appreciated parks. It has a large variety of geologic features that make it well worth a visit, and it is possible to do short backpacking trips that enable visitors to enjoy solitary experiences.