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Geologic Setting

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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 (McBirney and White, 1982). 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 distributed uniformly around the Earth. Instead, they are mostly confined to narrow zones, like the circum-Pacific "Ring of Fire", that mark the boundaries between rigid plates of rock that cover the planet's surface (Figure 2). These lithospheric plates include the crust -- both thin seafloor basalts and the thicker continental granitic rocks -- as well as the cold dense PERIDOTITE of the uppermost mantle (Figure 3). 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.

In most areas 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 these three types of boundaries are exemplified, respectively, by the Juan de Fuca ridge system, the Cascadia subduction zone, and the Mendocino fault (Figure 1). 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. Basalt MAGMA rises from the zone of partial melting, filling fractures between the plates and solidifying to form new oceanic crust. In this way the seafloor lithosphere at 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 4). From its surface trace 50 to 100 kilometers offshore, this great fault dips eastward beneath the Pacific Northwest at an angle of 10 to 15°. The subducting oceanic plates reach depths of about 80 to 100 kilometers beneath the High Cascades, and sink to still greater depths farther east. Only the upper part of the subduction zone is marked by seismic and volcanic activity, however, because this is where the downgoing oceanic plates are rigid and water-rich. Like most faults, the Cascadia subduction zone is often "locked" so that plate motion creates strain in the rocks of the adjoining lithosphere. When these rocks break, part of the stored strain energy is released suddenly as an earthquake. Recent studies indicate that the Cascadia subduction zone has produced an average of one large quake every 500 years during the past 3,000 years. The last of these occurred in 1700 and is thought to have had a MAGNITUDE of 9 based on the height of a TSUNAMI that reached Japan (Satake and others, 1996). In addition to producing earthquakes and tsunamis, however, the subduction zone is also the source of the magmas that sustain volcanism in the Cascades.

The Mendocino fault is a steep shear 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 Mendocino fault offsets relatively thin oceanic lithosphere and accomodates shear rather than convergent motion, however, the earthquakes it produces are likely to be smaller than those generated by the Cascadia subduction zone. Because the Mendocino fault lies entirely offshore, however, its quakes also have the potential to create tsunamis if the faulting offsets the seafloor vertically or triggers undersea landslides.

Cascadia Subduction Zone

As the Gorda, Juan de Fuca, and Explorer plates descend along the subduction zone, they are warmed by heat flowing 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 the subduction zone (Figure 4). The presence of the water lowers the melting temperature of the asthenospheric rock and enables it to partially melt to produce a variety of basalt and BASALTIC ANDESITE magmas. At depths of about 80 to 100 kilometers the high temperature of the asthenosphere enables perhaps 20 to 30% of the peridotite there to melt in the presence of water. When this melt becomes sufficiently abundant it separates itself from the surrounding partially-molten peridotite and rises buoyantly towards Earth's surface, forming the magmas that sustain volcanism in the High Cascades.

Some of the melts formed above the subduction zone rise through parts of the lithosphere that are being stretched. In these areas faults and fractures channel them rapidly to the surface so that they have little opportunity to cool or interact with crustal rocks. Elsewhere, however, the melts rise more slowly and many become "trapped" in parts of the crust that are less dense than they are. These "stalled" melts cool, crystallize, and mix with the surrounding crustal rocks to form new magmas -- ANDESITES and DACITES -- that have compositions and properties quite different form those initially formed along the subduction zone. The wide range of volcanic rocks found on and around Mount Shasta indicates how complex the structure and composition of the crust are beneath the southern Cascades.


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