Overview of the Geology of Mount Shasta

Overview of the Geology of Mount Shasta Geology 60 Fall 2007 William Hirt College of the Siskiyous 800 College Avenue Weed, California Introduction 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 Lassen Peak 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). Figure 1: Locations of the major High Cascade volcanoes and their lavas shown in relation to plate boundaries in the Pacific Northwest. Full arrows indicate spreading directions on divergent boundaries, and half arrows indicate directions of relative motion on shear boundaries. The outcrop pattern of High Cascade volcanic rocks is taken from McBirney and White (1982), and plate boundary locations are from Guffanti and Weaver (1988). 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 quarter of a million 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 paper 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. A road log for the field trip we will take on Saturday is included as Appendix 1. More detailed information on each of the topics discussed here is available from the sources cited in the references section, and from the many other geologic papers listed in the comprehensive bibliography on Mount Shasta compiled by Miesse (1993). I want to emphasize that the research presented in this paper 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 throughout this paper that definitions of terms written in SMALL CAPS are given in the glossary that follows the references. Geologic Setting of Mount Shasta The High Cascades is the younger of two volcanic mountain ranges that have risen parallel to the Pacific Northwest coast during the past 35 to 40 million years. The lofty stratovolcanoes that dominate the range are less than 2 million years old, but they stand atop a massive platform of BASALTS that has been built by eruptions from scores of vents during the past 12 million years. This entire suite of High Cascade rocks, in turn, overlies the eroded remnants of an older volcanic chain called the Western Cascades that was active between about 35 and 17 million years ago. In order to understand why lavas have risen to build these volcanic mountains over tens of millions of years, we need to review a bit about the concepts of plate tectonics and, in particular, the process of SUBDUCTION. Plate Tectonics Geologists have long recognized that earthquakes and volcanic activity are not spread uniformly across Earth's surface. Instead, they are largely confined to narrow zones, like the circum-Pacific "Ring of Fire", that are the boundaries between the great lithospheric plates that cover the planet's surface (Figure 2). These rigid slabs of rock include the crust − thin seafloor basalts and the thicker continental granitic rocks − as well as the cold dense PERIDOTITE of the uppermost mantle. The plates are 100 to 150 kilometers thick, and move slowly across the hotter, softer ASTHENOSPHERE beneath them in response to the tug of sinking ocean lithosphere and thermal circulation in the deeper mantle. The plates interact with one another along three types of boundaries: divergent boundaries, where they are moving apart; convergent boundaries, where they are coming together; and shear boundaries, where they are sliding horizontally past one another. Here in the Pacific Northwest the Juan de Fuca ridge system, the Cascadia subduction zone, and the Mendocino fault, respectively, exemplify these three types of plate boundaries (Figure 1). Figure 2: Map of the boundaries between Earth's lithospheric plates (Simkin and others, 1994). Divergent boundaries are shown as gray lines; convergent boundaries as gray lines with sawteeth pointing in the direction of the downgoing plate; shear boundaries as dotted black lines; and broad plate boundary zones where individual boundaries are not well defined as gray ruled areas. Dots mark the positions of hotspots, where plumes of hot mantle rock rise to the surface from Earth's interior. The Juan de Fuca ridge system is a chain of seafloor volcanoes that marks the rift along which the Gorda, Juan de Fuca, and Explorer plates are pulling away from the Pacific plate. Beneath the ridge, hot asthenospheric rock flows slowly towards the surface and partially melts due to a decrease in confining pressure. Basaltic MAGMAS rise from the zone of partial melting, filling fractures between the plates and solidifying to form new oceanic crust. In this way the seafloor lithosphere on each side of the ridge grows wider by about 3 centimeters per year. The Cascadia subduction zone is a shallowly dipping fault that separates the Gorda, Juan de Fuca, and Explorer plates from the overriding North American plate (Figure 3). The subduction zone dips eastward at an angle of 10 to 15° from its surface trace 50 to 100 kilometers offshore. The boundary lies at a depth of about 100 kilometers below the High Cascades, and continues deep into the mantle beneath North America. Only the upper part of the zone however, where the down going oceanic plates are rigid and water- rich, is marked by seismic and volcanic activity. Like most faults, the Cascadia subduction zone is often "locked" so that plate motion creates strain in rocks of the adjoining lithosphere. When these rocks break, part of the stored strain is released suddenly as an earthquake. The Cascadia subduction zone has produced an average of one large quake every 500 years. The last of these, which occurred in 1700, had an estimated MAGNITUDE of 9 (Satake and others, 1996). In addition to producing earthquakes, the subduction zone is also the source of the magmas that sustain volcanism in the Cascades as described below. Figure 3: Schematic cross section of a subduction zone similar to the Cascadia subduction zone beneath the Pacific Northwest. The angle of subduction in Cascadia is shallower than in this illustration, but the basic process of triggering partial melting of the mantle by dewatering of the downgoing plate is the same. From Chernicoff and Fox (1997). The Mendocino fault is a steep boundary that separates the Gorda plate, which is moving eastward relative to the underlying mantle, from the Pacific plate, which is moving westward. As in the Cascadia subduction zone, the sudden release of strain accumulated along this fault can produce large earthquakes (Dengler and others, 1995). Because the fault offsets relatively thin oceanic lithosphere and accommodates shear rather than convergent motion, however, its quakes are likely to be smaller than those generated by the subduction zone. Nonetheless, because the Mendocino fault lies entirely offshore its quakes also have the potential to create large TSUNAMIS if the faulting offsets the seafloor vertically or triggers an undersea landslide. Cascadia Subduction Zone As the Gorda, Juan de Fuca, and Explorer plates descend along the subduction zone, they are warmed by heat that flows into them from the surrounding mantle. The upper parts of the plates carry water in fractures, seafloor sediments, and the altered minerals of the oceanic lithosphere itself. As the plates heat up, this water is expelled and rises into the "wedge" of asthenosphere that lies above them (Figure 3). The presence of the water lowers the melting temperature of the asthenospheric rock, and enables it to partially melt and produce a variety of basalt and BASALTIC ANDESITE magmas. At depths of about 100 kilometers, the asthenosphere is so hot that enough melt forms that it is able to separate from the partially molten peridotite and rise buoyantly towards Earth's surface. Some of subduction zone magmas rise through parts of the lithosphere that are being stretched. Here, faults and fractures channel the magmas rapidly to the surface so that they have little opportunity to cool or interact with crustal rocks. Elsewhere, subduction zone magmas rise more slowly, and many become "trapped" in parts of the crust that are less dense than they are.

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