Lassen Volcanic National Park—the southernmost Cascade Range volcano

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.

Here is a photograph of Mount Lassen, the southernmost volcano in the Cascade Range. This photo was taken in early July (2020), when it was snow-free enough to be climbed. This view is from the eastern side of the park, in the vicinity of Cinder Cone.

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.

This map shows the distribution of various volcanic units. Note how volcanism has shifted through time; the youngest volcanism is currently centered around Mt Lassen. The transform boundary is slowly migrating northward and shutting down the subduction zone, so it’s not surprising that volcanism has been shifting toward the north. Ma=millions of years ago.

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.

Cinder cones are the simplest type of volcano—they are built from particles and blobs of congealed lava ejected from a single vent to form a steep-sided cone. The 1966 eruption in Lassen included lava flows, two nested cinder cones, and an ash fall—because ash is small volcanic particles, it can be ejected far into the air and fall farther away from the volcano. The reddish material is some of the lava flow that was emitted from Cinder Cone. Photo taken by Jay Ach.
This view from the top of Cinder Cone, looking southwest, shows lava emitted from Cinder Cone that flowed around the Painted Dunes of volcanic ash. The red color is from oxidation of iron when hot water and/or steam penetrated the ash. The lake is Snag Lake, which was created when the lava formed a dam at the eastern end.
This view looking over the crater at the top of Cinder Cone shows how the younger cone is nested inside of the outer cone. There are good trails to the top of Cinder Cone, around the edge of each cone (note people on the edge of the inner cone) and even to the bottom of the crater. Mt Lassen is peaking over Cinder Cone’s top in this view to the west.
A “breadcrust bomb” on the slope of Cinder Cone. You wouldn’t have wanted to be standing there when this large blob of hot lava came out of the crater. The breadcrust-like texture on the outside forms because the exterior of the blob cools quickly and then cracks as the inside of the blob cools and shrinks.

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.

A view of Bumpass Hell’s hot water and steam features. The main trail to the area was closed because of snow, so we went in the back way that goes past Cold Boiling Lake. Incredibly, we were the only people in this area that is usually quite crowded. There are other hydrothermal features in the park, which are all connected to the same underground “furnace”.
Bubbling mud pots are a great people pleaser! Before reaching Bumpass Hell, we heard thumps that sounded like thunder or rocks falling in a landslide. Incredibly, it was just the thump of mud being shot into the air and descending again into its pot. The steam obscures the mud, but you may be able to see some mud being jetting upward in the photo.
Not all bubbling water is hot. This is Cold Boiling Lake, which is not really boiling of course (it’s cold!). Bubbles are from volcanic gases that are emitted from the groundwater system. Lassen has many beautiful lakes that are formed by a variety of processes.

Another process that formed Lassen Park is the action of glaciers during the Last Glacial Maximum about 20,000 years ago.

This lava rock has linear grooves from where glaciers flowed through the park and scraped the underlying rock surface. The high peaks were also eroded by glaciers, but on Mt Lassen (visible in the background of this photo) the subsequent eruptions have obscured much of the glacial evidence. The trees in this area were burned during a forest fire around 15 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.

The park is far from major cities and the night sky provides a spectacular show. Here the Milky Way is visible through the trees. Photo by Jay Ach, who is willing to get out of the tent in the middle of the night to capture these sights.

Important references

U.S. Geological Survey’s California Volcano Observatory’s web site about the Lassen Volcanic Center:

U.S. Geological Survey Field Guide to Lassen Park: Scientific Investigations Report 2015-5067 (

U.S. Geologic Survey Geologic Map of Lassen Volcanic National Park and Vicinity, California: Scientific Investigations Map 2899. (

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