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Geology Field Trip to Mt. Lassen

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Stop 1: Along Highway 32 east of Chico

We can see the Sutter Buttes in the distance—volcanics of 1.7 to 1.5 million years ago. They are not related to the Cascade volcanoes, which are older and of a different chemistry. Sutter Buttes are unique in the flat sediments of the Great Valley Sequence. Scientists don't know why this volcano is in this odd location, but probably is associated with the subducting North American plate. It erupted thick, gooey lava.

We examine the rocks along the road and see hard, sharp corners and edges, not the well rounded shapes of rocks deposited by rivers. The deposit here is a lahar. The angular boulders and rocks are cemented together by volcanic ash. Volcanic ash was sprayed out as mist, then it solidified, mixed with water created in the eruption to create a mud. This flows and picks up chunks of rock and goes down slopes. The deposit here is known as the Tuscan Formation, which is composed of lahars and is about 500 feet thick. It is 5 to 10 million years old and is a result of the earliest volcanics in the Cascades.

In the fields we can see the boulders that are left behind as the softer muds are eroded away. There is no agriculture here because of the boulders. Farther up the road we can see the lahar flow sequence into the valley. This went for 30 miles. The lower ends of the muds are covered by the sediments of the great valley. Lahars always come from explosive volcanoes, spraying magma. When you see a lahar you know there was a distinct, explosive volcano. This is like Mt. St. Helens which has not yet turned to stone. Most of the Mt. Lassen volcanics were violent. Another eruption of Mt. Lassen will even affect us in Rocklin.

Stop 2: Child's Meadow in the Lassen Peak region.

This area represents the competition between the building forces of volcanism and the wearing down forces of glaciation. Glaciers developed on the flanks of the volcanoes between eruptions. This happened over and over again. We are now in an interglacial period. The next stage will be volcanic. Child's Meadow was an ice age lake. As the glacier melted farther down it left end moraines—walls of loose rock and debris—as it melted back. These were big loops of moraines on the valley floor. The melt waters were dammed behind these moraines. There were many glacial lakes at the end of the ice age. Rivers washed in sediments and leveled out the lakes until the water was gone and meadows left behind. At higher elevations there are still lakes.

In the distance we can see Brokeoff Mtn. on the left. It is one edge of a semi-circle that was the old volcano, Mt. Tehama, 600,000 years ago. It was 11,000 feet high 30,000 to 40,000 years ago. It was like Mt. Shasta is today. Mt. Tehama was starting 600,000 years ago and developed by layers—a composite volcano. There were glacial conditions here 100,000 years ago. Mt. Tehama started as the climate started to warm so there were eruptions and glaciers simultaneously. The glaciers won the battle. The summit was collapsed quietly by the glaciers.

Mt. Lassen is a dome on the side of Mt. Tehama. Lassen was unaffected by glaciation so it formed after the glaciers. It is similar to Shastina on the side of Mt. Shasta. The core of Tehama which is sulfur has weathered out as very fine particles.

Stop 3 The Sulfur Works in Lassen Volcanic Park

This bright yellow patch is the core of Mt. Tehama which was 11,000 feet high. We can see the old arc of the volcano as represented by Brokeoff Mtn. The Sulfur Works is where there is still heat. Geologically this is a young volcano so the heat is subsurface—either hot rock or magma. Water seeps down and gets heated and the fluids and vapors come up here. The pH is around 2 or 3. There is sulfur gas and sulfur is associated with magma. When mixed with water it forms sulfuric acid. There is a dacite boulder which has been stewed, boiled, etc. and reacted with vapors. It is clay that has been produced in a hurry. The vapors here contain metals, copper, zinc and when they combine with sulfur they form pyrites. There are grains of sulfur minerals around the fumeroles which create a yellow crust. The bluish mineral is malachite.

Stop 4: Bumpass Hell

We can see the arc of the old Mt. Tehama. Mt. Lassen is outside this arc. It last erupted between 1914-1917. Lassen had large chunks of gooey lava which plugged the vent making a plug dome volcano. This makes an explosive situation so Lassen is a dangerous volcano. Any time you have magma and a plug dome you have an explosive possibility.

We stand on glacial polish and striations from the 600-700 foot thick glacier that was here. This equates to a pressure of 30 tons/sq ft. This amount of ice can move a lot of material. This glaciation was from 100,000 to 12,000 years ago. The glacier came out of where Emerald Lake is today. There is a boulder here which might be an erratic—a boulder left behind by the glacier. It is dacite and is the same material as the rock it sits on so it might not be an erratic.

Bumpass Hell is on the edge of Mt Tehama. The sulfide minerals and clay make the water here gray. The vents change and shift around over the years. A sign says there is cooling magma 3 miles deep here.

Stop 5: Cinder Cone trail

Lassen Volcanic Park was created because all types of volcanic activity is represented here. The eastern part of the park is a different volcanic composition. Bumpass is dacite which is similar to rhyolite. Both are magma which is 65-75% silica dioxide. Silicon influences the thickness (viscosity) of the lava. The thicker magma clogs vents and builds up back pressure leading to explosive volcanic events. The eastern part of the park is almost totally basalts which are 50% silicon. Being low is silica it is more "runny" and creates broad sheets of lava. This is not an explosive magma. There is some gas associated with basalts. Lava erupted which is larger than 64mm are volcanic bombs, between 64 and 4mm is lapilli, and under 4mm is ash. Basaltic flows can go for miles. Near the vent bombs and lapilli build up a cinder cone. Cinder cones erode easily so if you see a nice cinder cone it is recent. In the throat of the cinder cone is a neck of lava. This cinder cone is 800 feet above the lava flows below. This cone erupted less than 300 years ago, possibly in 1851 when there were immigrant reports of fires in this region. Down below we can see where Butte Lake was impounded by the lava flows.

Stop 6: Subway lava tube

2000 years ago the lava flows started. An eruption to the south came from a fissure. This basaltic flow oozed down the valley toward Burney Falls. The flow went over the irregularities of the valley floor and then there were flows on top of flows. Lava tubes form around the hot cylinder of lava which forms a “skin” while the lava still flows within. Rock is an excellent insulator so the lava flows until it all drains out and the lava tube is left. There ara lots of lava tubes here as can be seen by the many roof collapses in the valley. The Subway is a large tube with lavasickles—small drips of lava from the roof—on the roof inside.

Stop 7: Burney Falls

Basalts here overlay old impervious lakebeds. The lava flowed down Hat Creek Valley and when cooled cracked allowing water to flow down through it. Here at the lower end water flows over the top of the basalts and out through the sides which is the top of the old lakebeds. This is unique by having both surface water and ground water flowing out at the same place making this an unique waterfall in the world. Hat creek and Burney creek flow into Lake Britton and then into the Pit River.

Mike Price, Fall 1995

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