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Geology

Introduction

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Photo of Mount Shasta by Bill Hirt, Fall 1998

Mount Shasta is one of the twenty or so large volcanic peaks that dominate the High Cascade Range of the Pacific Northwest. These isolated peaks and the hundreds of smaller vents that are scattered between them lie about 200 kilometers east of the coast and trend southward from Mount Garibaldi in British Columbia to Mount Lassen in northern California (Figure 1). Mount Shasta stands near the southern end of the Cascades, about 65 kilometers south of the Oregon border. It is a prominent landmark not only because its summit stands at an elevation of 4,317 meters (14,162 feet), but also because its volume of nearly 500 cubic kilometers makes it the largest of the Cascade STRATOVOLCANOES (Christiansen and Miller, 1989).

Mount Shasta's prominence and obvious volcanic character reflect the recency of its activity. Although the present stratocone has been active intermittently during the past 200,000 years, two of its four major eruptive episodes have occurred since large glaciers retreated from its slopes at the end of the PLEISTOCENE EPOCH, only 10,000 to 12,000 years ago (Christiansen, 1985). Mount Shasta's most recent eruption occurred about 200 years ago (Miller, 1980), and low-levels of geothermal and seismic activity still occur on and around the mountain today. Because of the potential hazards that Mount Shasta's future eruptions and debris flow events may pose to the surrounding communities and the thousands of visitors who pass through them each year, it is important for everyone who spends time around the mountain to know how to respond safely in the event of renewed activity.

This chapter was written to provide a general introduction to the mountain's geology, and has been adapted from part of a National Association of Geoscience Teachers conference guidebook (Hirt, 1999). Individual sections describe Mount Shasta's geologic setting, the processes active in its development, its geologic history, and its potential hazards. More detailed information on each of these topics is available from the sources cited in the references section, and from the many other geologic papers listed in the accompanying annotated bibliography compiled by Miesse. I want to emphasize that the research presented in this chapter is not my own. My contribution has been to weave together material from a variety of sources and add explanations that, I hope, will clarify geological ideas for general readers. Any errors in fact or interpretation are my responsibility, however, and should not be attributed to the original authors. Also, please note that terms written in SMALL CAPS are linked to definitions in a glossary at the end of the chapter.

Bill Hirt
Weed, California
June 2001

 

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