Molossian Institute of Volcanology

Geologic Background of Lassen Volcanic National Park

The Lassen volcanic center consists of the ancient Brokeoff (or Mount Tehama) stratovolcano, a 6-km-wide caldera largely filled by a dacitic lava dome field, and peripheral small andesitic shield volcanoes and pyroclastic cones. Activity spanning 600,000 years began with construction of Brokeoff stratovolcano. Beginning 400,000 years ago activity shifted to the north flank of Brokeoff, where episodic, more silicic eruptions produced a small caldera and field of a dozen dacitic lava domes along the inferred margin of the caldera, including Bumpass Mountain, Mount Helen, Ski Heil Peak, and Reading Peak. At least twelve eruptive episodes took place during the past 100,000 years, with Lassen Peak being constructed about 28,000 years ago. The Chaos Crags dome complex was constructed about 1100-1000 years ago north of Lassen Peak. The Cinder Cone complex NE of Lassen Peak was erupted in a single episode several hundred years before present and is considered part of the Lassen volcanic center. Wide range of volcanic products including a andesitic stratovolcano, dacite domes, small andesitic shield volcanoes, and cinder cones and flows.

Glacial erosion, enhanced by hydrothermal alteration of permeable cone rocks, has resulted in deep erosion of Brokeoff volcano. The major remnants, Brokeoff Mountain, Mount Diller, Mount Conard, and Diamond Peak, enclose a central depression that marks the position of Brokeoff volcano, which was approximately 3,350 meters (10,921 feet) high, had a basal diameter of approximately 12 kilometers (7.5 miles). The current area of the Bumpass Hell geothermal area is believed to be the main vent for the ancient volcano.

Lassen Peak Lave Dome

Lassen Peak began as a volcanic vent on Mount Tehama's northern flank. Considered the world's largest plug dome volcano, it rises 600 meters (2,000 feet) to an elevation of 3,207 meters (10,457 feet). Lassen Peak is the largest of a group of more than 30 volcanic domes erupted over the past 300,000 years in Lassen Volcanic National Park in northern California. These mound-shaped accumulations of volcanic rock, called lava domes, were created by eruptions of lava too viscous to readily flow away from its source. Eruptions about 27,000 years ago formed Lassen Peak, probably within only a few years. When Lassen Peak formed, it looked much like the nearby 1,100-year-old Chaos Crags Domes, with steep sides covered with angular rock talus. However, from 25,000 to 18,000 years ago, during the last ice age, Lassen's shape was significantly altered by glacial erosion.

The most recent eruptive activity occurred at Lassen Peak in 1914 - 1917. This eruptive episode began on May 30, 1914, when a small phreatic eruption occurred at a new vent near the summit of the peak. More than 150 explosions of various sizes occurred during the following year. By mid-May 1915, the eruption changed in character; lava appeared in the summit crater and subsequently flowed about 100 meters (326 feet) over the west and probably over the east crater walls. Disruption of the sticky lava on the upper east side of Lassen Peak on May 19 resulted in an avalanche of hot rock onto a snowfield. A lahar was generated that reached more than 18 kilometers (11 miles) down Lost Creek. On May 22, an explosive eruption produced a pyroclastic flow that devastated an area as far as 6 kilometers northeast of the summit. The eruption also generated lahars that traveled more than 20 kilometers (12 miles) down Lost Creek and floods that went down Hat Creek. A vertical eruption column resulting from the pyroclastic eruption rose to an altitude of more than 9 kilometers (5.5 miles) above the vent and deposited a lobe of pumiceous tephra that can be traced as far as 30 kilometers (18.5 miles) to the east-northeast The fall of fine ash was reported as far away as Elko Nevada, more than 500 kilometers (300 miles) east of Lassen Peak. Intermittent eruptions of variable intensity continued until about the middle of 1917.

A trail leads to the top of Lassen Peak, reachable from a parking lot just above Lake Helen, and just off State Route 89. The trail allows complete access to the crater as well as outstanding views of all of Lassen Park and even Mount Shasta, some 121 kilometers (75 miles) away. This trail is a well-graded climb of 600 meters (2,000 feet), from 2600 meters (8,500 feet) to 3200 meters (10,457 feet) in 4000 meters (2.5 miles) of hiking. Allow four hours for the round trip.

Lassen Peak.

Lassen Peak from the trailhead to the summit.

The highest point atop Lassen Peak.

Looking over the crater atop Lassen Peak.

Looking down into the crater.

Looking northwest from atop Lassen Peak. Cinder Cone is in the distance.

Looking south from atop Lassen Peak, toward Lake Helen. The mountains at left and right are remnants of Mount Tehama.

Cinder Cone

Cinder Cone in Lassen Volcanic National Park is a 214 meter (700 feet) high volcanic cone, created around 1650. The creation of Cinder Cone occurred during one of three episodes of volcanism that have occurred at the Lassen volcanic center in the past 1,100 years. The Cinder Cone eruptive episode consists of 4 basaltic andesite lava flows, a complex vent cone, and an ash blanket covering 200-300 square kilometers. Two lava flows erupted before the ash blanket and two followed the ash blanket. All of these erupted over a short time interval. Cinder Cone is composed of loose scoria. Scoria forms when blobs of gas-charged lava are thrown into the air during an eruption and cool in flight, falling as dark volcanic rock containing cavities created by trapped gas bubbles. The summit of Cinder Cone has a crater with a double rim, probably created by fluctuating eruptive activity late in its formation. The cone also has several associated blocky lava flows, and a related, wide-spread ash deposit identifiable for 13 to 16 kilometers (8 to 10 miles) from the cone.

To reach Cinder Cone, drive 17 kilometers (10.5 miles) east on Highway 44 from Old Station, CA . Take the signed Butte Lake Campground turnoff 11 kilometers (7 miles) to the trailhead at Butte Lake. The trail to the base of the cone is 2.4 kilometers (1.5 miles) in length, and ends at the base of the cone. The trail up the cone is extremely steep, and made more difficult by the loose surface of the cone. It is about 1 kilometer (.7 miles) and you gain about 214 meters (700 feet) in elevation. From the top you can see the Fantastic Lava Beds and the Painted Dunes, all part of the volcanic activity that created Cinder Cone.

Cinder Cone.

Looking across the
top of Cinder Cone.

Looking down into
the crater of Cinder Cone.

Fantastic Lava Beds, at
the foot of Cinder Cone.

Painted Dunes, also at
the foot of Cinder Cone.

Bumpass Hell

Bumpass Hell is Lassen's most spectacular and diversified hydrothermal area, with 3 of the 4 main types of geothermal feature, fumaroles, mudpots and hot springs. A fairly easy 2.4 kilometer (1.5 mile) trail leads to the hot spring basin. The walk through the basin is another 1.2 kilometer (0.8 mile) on raised wooden walkways traverse the unstable ground and lead towards points of interest. Most noticeable are several large hot pools, grey-blue in color and up to 10 meters (32 feet)across, often with simmering, bubbling water. There are also many mudpots of varying viscosity; some are quite thin and effervesce vigorously whereas others are thicker and plop every few seconds. Other features include small water fountains, steaming soil, noisy fumaroles (sulphurous steam vents) and a stream with warmish acidic water that flows out of the basin.

Bumpass Hell was named after Kendall Vanhook Bumpass who discovered it and lost his leg as a result of burns suffered when stepping into a thermal pool.

A trail leads to Bumpass Hell, reachable from a parking lot below Lake Helen, and just off State Route 89.

Bumpass Hell.

Steaming stream
through Bumpass Hell.

A boiling pool.

Mud Pots.

Boiling pool.

Boiling pool.

Sulphur Works

The Sulphur Works hydrothermal area is the most accessible hot springs area in Lassen Volcanic National Park. It is thought to be part of the central vent system of ancient Mount Tehama. A short boardwalk trail leads to sputtering hot springs, steaming fumaroles and hot bubbling mudpots. Slippery clay and thin crusty coverings could lead to a dunking in 76 degrees celsius (195 degrees F) water and mud. Most water in the thermal areas of the park contains sulphurous or sulphuric acid. The odor is mainly that of hydrogen sulphide. Much of the white clay is tinted yellow, tan, or pink by minerals, chiefly iron oxides.

The name Sulphur Works was first used in 1865, when efforts were made by T. M. Boarman and Dr. Milton Supan to develop the sulphur and clay potential of the area. This land was filed upon originally as a mining claim, but was actively used only for the tourist trade beginning about 1940. Some of the acidic thermal water was used for hot baths. The property was acquired by the National Park Service in 1951.

Sulphur Works boardwalk is reachable from the parking lot just off State Route 89, about 3.2 kilometers (2 miles) north of the Southwest Entrance Station.

Fumarole at Sulphur Works.

Another fumarole at Sulphur Works.

Subway Cave

Subway Cave is a lava tube, 500 meters (1,300 feet) long and between 2 and 6 meters (6 and 17 feet) high. It was formed about 20,000 years ago when a river of molten lava flowed across the floor of Hat Creek Valley. As it flowed, it cooled and hardened on the outside, but the interior lava remained insulated, stayed fluid, and continued to move. When the lava flow stopped at the source, the inner molten material soon drained out of its shell, leaving behind a dark, hidden tunnel. Lava tubes are common in volcanic areas but ordinarily become visible only when the roofs fall in. The entrance and exit to Subway Cave was formed by partial collapses of the cave roof many years ago.

Subway Cave is located just .4 kilometers (1/4 mile) north of the Highway 44/89 intersection near Old Station. The cave is cold, 8 degrees celsius (46 degrees F.) and dark. Dress warmly and take a good flashlight with fresh batteries. The are self-guiding trail signs inside. The floor is rough, so sturdy shoes are a must. Do not tour the cave alone.

Entrance to Subway Cave
(Devil's Doorway).

Looking inside.

Wind Tunnel.

Molossia's Flag
inside the cave.

Collapsed ceiling
and the way out.

Lava bubble
inside the cave.

Spatter Cone Trail

This loop trail will take you to the origin of the recent Hat Creek Lava Flow, an area with many spattercones and associated volcanic features, including three of the four kinds of volcanoes. About 20,000 years ago fluid lava poured out of a series of fissures in this area; the many spatter cones on this trail mark the origin of this lava. As lava poured out of these fissures, built up gasses caused some of the lava to be hurled out in fountains. As the large air-born clots of lava fell back to earth, they flattened and plastered themselves together, forming spatter cones around the lava fountains. When the fountains died, they left craters behind, the spatter cones that dot this area.

Spatter Cone Trail is located about 1.6 kilometers (1 mile) south of the Highway 44/89 intersection near Old Station. The trailhead is located just across Highway 44 opposite Hat Creek Campground. It is about 2.4 kilometers (1.5 miles) in length. While the trail begins in trees, most of it is in the open, among manzanita and sagebrush. Water and sturdy hiking shoes are strongly recommended.

Mysterious lava
tube entrance.

Peering into
a lava tube.

Rock formations caused
by rapidly cooling lava.

Sugarloaf Peak,
old volcano.

Lava Tube.

Inspecting another
lava tube.

Another lava

Beaut Cone,
Spatter Cone.

Steep Cone,
the largest crater.

Deep volcanic

Lava tube,
with lavacicles
and sulfur stains.

Spatter Tube

Return to the Molossian Institute of Volcanology Main Page.