Mt. Lassen is one of numerous volcanic peaks dotting the Pacific Northwest.
It experienced the last major eruption in the continental U.S. prior to the
more well known Mt. St. Helens which erupted spectacularly in 1980.
Lassen Peak was last active from 1914 to 1921 with its major eruptions occuring
in 1915. In the spring of that year it expelled a flow of molten lava which melted
a heavy winter snowpack, creating a devasting mudflow. This was an unusual occurrence
because Lassen, like many Pacific Northwest volcanos, is a "plug" volcano which
generally erupts ash and solid debris rather than liquid material.
While the peak itself is not active there were (at the time of our last visit) numerous
active steam vents and fumaroles (boiling mud pots) in the park where the crust is
broken or cracked. This indicates the area is still geologically active
The photo at right shows several characteristics of an active thermal area.
The foreground has been scoured of vegetation by colorful mineralization
which has been deposited by the nearby steam vent. Note the wisp of steam in the
The peak in the background is the remains of a huge cone from a volcano
which collapsed long ago. (See below)
This photo shows more clearly the remnants of the ancient cone. The two peaks
are actually just one side of the huge cone which had a diameter of fifteen miles
and had an original elevation of 11,000 feet.
Two examples of steam vents within the park. Note in the photo right the
buildup of mineralization below the vent. In large vents such as the
Mammoth Hot Springs in Yellowstone National Park the mineralization can
take the fantastic look of fine marbled sculpture.
Surrealistic scenery on the trail to the top of Mt. Lassen. Although the
last eruptions took place decades ago very little vegetation has taken hold
in the rocky debris.
The trail is steep but can be easily walked in a few hours.
A view from the peak of Mt. Lassen. The remnants of the cone frame another
volcanic peak, Mt. Shasta, which can be seem in the distance, perhaps fifty
Lassen Peak is 10,457 feet high while Mt. Shasta is over 14,000 feet.