College of the Siskiyous Chautauqua Courses

Contrasting Volcanism at Mount Shasta and the Medicine Lake Volcano (Course COS-01)

Northern California is an outstanding natural laboratory for learning about the diverse styles of volcanic activity that characterize the High Cascades, a young continental volcanic arc. It’s also an ideal place to develop an understanding of the potential hazards that such activity may pose to people living throughout the western United States.

The focal point of this region is Mount Shasta, a 14,000-foot peak whose steep ice-clad flanks mark it as a typical High Cascade stratovolcano. Mount Shasta’s violent past has often been punctuated by explosive eruptions that filled valleys tens of kilometers from the peak with searing clouds of hot rock and gas and torrents of muddy debris. The mountain is also something of a phoenix, rising from the remnants of an older peak that collapsed catastrophically three to four hundred thousand years ago to produce one of Earth’s largest debris avalanches. Although the mountain is dormant today, dating suggests a person has about a one in ten chance of witnessing another eruption during his or her lifetime.

As imposing and potentially dangerous as Mount Shasta is, however, studies have shown that like other High Cascade stratovolcanoes it accounts for only a small fraction of the range’s activity. Most eruptions in the California and Oregon Cascades have built small shield volcanoes of fluid basaltic lavas. Throughout this region there are only two large shield volcanoes and one of them, the Medicine Lake volcano, stands just 50 kilometers east of Mount Shasta. It forms a broad highland whose surface is dotted with smaller cones and domes that have produced both fluid basalts and pasty rhyolites during the past several thousand years. This massive volcano probably poses fewer hazards than Mount Shasta, but explosive eruptions like those that accompanied the growth of the aptly named Glass Mountain about 900 years ago could still blanket large parts of the region with pumice. Controversy also simmers on the Medicine Lake volcano as community members seek to balance the potential environmental degradation that will accompany a proposed geothermal development with the desire for “green power”.

This four-day program will include day trips to each of these volcanoes so that participants can study the landforms and eruptive products unique to each. In addition, two days of classroom discussions and laboratory sessions at College of the Siskiyous will introduce the tectonic setting and geologic processes that are shaping the southern Cascades, review how volcanic hazards are being monitored and assessed in this region, and acquaint participants with materials and activities that they can use to facilitate student learning about volcanism and volcanic hazards in their own classrooms. Limit, 20 participants.

Contrasting Volcanism at Crater Lake and Lassen Volcanic National Parks (Course COS-02)

Northern California and southern Oregon are natural laboratories for studying volcanic activity and its potential hazards in the High Cascades, a modern continental arc. The region is home to two national parks that preserve landscapes shaped by recent volcanism. Surprisingly, however, neither Lassen Volcanic National Park nor Crater Lake National Park is home to the sort of massive snow-clad stratovolcano that so many people envision as a typical High Cascade peak. Instead, these parks preserve the remnants of two volcanoes, Mount Tehama and Mount Mazama, whose "lives" ended in very different ways. The histories of these mountains are a fascinating study in contrasts and offer insights into what may lie ahead for the towering peaks that dominate the range today.

The centerpieces of Lassen Volcanic National Park are the hot springs and fumaroles that belie an active magmatic system beneath the park’s western half, and the imposing dome of Lassen Peak which last erupted in 1917. All of these features lie within the footprint of Mount Tehama, an 11,000 foot stratovolcano that dominated the area about 610,000 years ago. This massive peak succumbed slowly to erosion during the past 600,000 years, however, as hot acidic water circulated by the present magmatic system “rotted” its interior and streams and glaciers subsequently carved away the altered rock. Today, lava flows that snaked down Mount Tehama’s flanks slope skyward towards its missing summit and remind us of the impermanence of even massive volcanic peaks in the face of alteration and gravity.

To the north, the centerpiece of Crater Lake National Park is the 10 km-wide caldera that marks where the summit of Mount Mazama stood until 7,700 years ago. Unlike Mount Tehama, this volcano was destroyed in a matter of days when massive eruptions from a shallow magma reservoir undermined its upper slopes and led to collapse. Although there is little hydrothermal activity within the caldera today, deposits on the volcano’s flanks tell of a climactic eruption that burned and buried forests tens of kilometers from the peak. Although large caldera-forming eruptions are rare in the Cascades, their violence makes them an awesome threat.

This four-day program will include day trips to Lassen and Crater Lake National Parks so that participants can study the landforms and eruptive products unique to each volcano. In addition, two days of classroom discussions and laboratory sessions at College of the Siskiyous will introduce the tectonic setting and geologic processes that are shaping the southern Cascades, review how volcanic hazards are being monitored and assessed in this region and acquaint participants with materials and activities that they can use to facilitate student learning about volcanism and volcanic hazards in their own classrooms. Limit, 20 participants.

Geology of the Eastern Klamath Mountains (Course COS-03)

During Mesozoic time, as the Farallon plate subducted beneath the western edge of North America, a chaotic mixture of marine sediment and oceanic crust was piled up along much of the continental margin. Today this mélange is exposed in California’s Coast Range. Farther north however, in northernmost California and southern Oregon, the older part of this mélange is missing. In its place are a series of elongate blocks of thickened crust—terranes—that collided with the continent one after another and were sutured onto its western margin. Many of these terranes were chains of offshore volcanic islands, like those of modern Japan, and today the diverse assemblage of igneous and sedimentary rocks that built them underlie the Klamath Mountains.

This course will explore the geology and geologic history of the oldest and most diverse of the Klamath terranes: the Eastern Klamath. From ancient mantle exposed high in a glacial cirque to a Late Paleozoic reef hollowed out to form a limestone cavern, the Eastern Klamath terrane preserves a record of more than 200 million years of Earth’s history. After this terrane was added to North America about 400 million years ago subduction that accompanied the accretion of terranes farther west produced magmas that rose into the Eastern Klamath. These magmas formed younger bodies of granite like Castle Crags and circulated groundwater to create the veins of gold-bearing quartz that drew European miners to the region in the mid-1800s.

Although the mines that took gold, chromium, and mercury from the Eastern Klamath terrane are now silent, the legacy of environmental damage they caused remains. Dredge spoils near Callahan, for example, are the result of placer mining that turned the Scott River’s channel “upside down” and still affect the stream’s ability to support populations of native salmon. Terrane accretion weakened the Klamath bedrock by creating faults and fractures. Combined with the steep slopes produced by the stream and glacial erosion that have developed as the modern Klamath Mountains have risen during the past 5 million years, this weakening makes landslides a constant threat. Similarly, deforestation caused by logging, road building, and wildfires worsened by climate change increase the dangers of both flooding and landslides and reflect humanity’s impact on the geologic processes that are shaping the Klamath Mountains today.

This four-day program will include day trips to the Scott Valley and Sacramento River Canyon so that participants can study the diverse rock types and landforms of the Eastern Klamath Mountains and see recent changes wrought by mining, road building, and fire. In addition, two days of classroom discussions and laboratory sessions at College of the Siskiyous will describe the tectonic setting and geologic processes that are shaping the Klamath Mountains, introduce the origins of its mineral wealth and the geologic hazards that confront the region’s residents today, and acquaint participants with materials and activities that they can use to facilitate student learning about terrane accretion and geologic hazards in their own classrooms. Limit, 20 participants.