Approximately 600,000 to 400,000 years ago, a large, composite volcano built up in the southwest area of what is now Lassen Volcanic National Park. This ancestor of more recent eruptions, Brokeoff Volcano (also referred to as Mount Tehama) reached its full elevation of about 11,500 feet mid-way in the Pleistocene period. Towering 1,000 feet above the height of Lassen Peak, and with a diameter of 12 - 15 miles, it was similar in size to Mount St. Helens (pre-1980 eruptive activity). At the southernmost end of the Cascade Range, Brokeoff Volcano dominated the landscape much as Mt. Shasta (to the northwest) does today.
Today, all that can be seen of this once formidable volcano are remnants. Brokeoff Mountain, Mount Conard, Mount Diller and Pilot Pinnacle form a very visible semicircular pattern open to the southeast. From these fragments, we can gain a sense of the size and grandeur of this long gone peak. Sulphur Works in Lassen Volcanic National Park is considered to be the site of Brokeoff Volcano's central vent and continues to display active hydrothermal features. But what happpened to Brokeoff Volcano? Why is it no longer standing tall, marking the southern end of the magnificent Cascade Range? Two geologic processes are considered responsible for the demise of Brokeoff Volcano: glaciation and hydrothermal alteration.
Brokeoff Volcano's building phase was followed by a period of glacial erosion. Volcanoes are particularly susceptible to the erosional powers of glacial action. Much of a composite volcano is built of fragmented material that presents little resistance to glaciers and is easily eroded and carried away. Repeated glaciation throughout the upper Pleistocene period deeply eroded the cone of Brokeoff Volcano, aided by the erosive effects of spring snow melt. Some parts of a composite, or stratovolcano, are more resistant to the effects of glacial action - the interiors of lava flows and solid rock forming dikes for example - but even these have disappeared from the site of Brokeoff.
Glaciation, therefore, isn't the whole story. Geochemistry, in the form of hydrothermal alteration, also played a significant role in the demise of this mountain as it does for other volcanic features throughout the Cascade Range today. Hydrothermal alteration occurs when hydrothermal fluids react with surrounding rock. During this reaction, minerals are replaced in the rock and the rock is changed. In the case of Brokeoff Volcano, the steady emission of sulfurous gases, acidic fluids moving through cracks in the rock, heat and steam, chemically transformed the rock into a soft crumbly mass. In fact, at Sulphur Works and other places within the caldera of Brokeoff Volcano, this process is still at work as evidenced by colorful areas of rock that easily crumbles.
References:
Wood and Kienle: Volcanoes of North America (as cited in usgs.gov/Volcanoes/Lassen
Yant, Marcella: Hydrothermal Alteration (part of the document Volcanoes of the Eastern Sierra Nevada: Geology and Natural Heritage of the Long Valley Caldera)
Harris, Stephen L: Fire Mountains of the West: the Cascade and Mono Lake Volcanoes. 2005
Clynne, Michael A & Hopson, R Forrest: Geology of Lassen Country. 2019
U.S.Geological Survey and U.S. Department of the Interior: "Hot Water" in Lassen Volcanic National Park - Fumaroles, Steaming Ground, and Boiling Mudpots. USGS Fact Sheet 101-02 2003.
In order to claim this cache, please submit answers to the following questions by e-mail. Do not include answers in your cache log. The information needed is found on the interpretive signs at the cache site.
1. State the cache name please.
2. What do we call the heat loving organisms that live in these bubbling mudpots?
3. What role do these organisms play in the alteration of rock into clay?
4. The "rotten egg" smell indicates the presence of what gas?
5. What did Matthew Supan mine at this site in the late 1800s?
6. What is a magma system?
7. Following the sidewalk almost to the end (on the same side of the road as the parking lot), describe the very active hydrothermal feature seen alongside the road.