Geology of the Area
Sisters are a part of the Cascade Range which extends from northern California
to northern Washington. In Oregon, the Cascade Range varies from 50 to 120
kilometers wide. This portion of the Cascades is primarily upper Eocene
to Quaternary in age and is composed of volcanic, volcaniclastic, sedimentary,
and igneous intrusive rocks. The average altitude is 1,500 to 2,000 meters
but some of the stratovolcanoes exceed 3,000 meters (USGS, 2002a).
Included in these high stratovolcanoes are the Three Sisters. Most of these stratovolcanoes are of rhyolitic to basaltic composition. Their structure is an interlayering of thin lava flows and pyroclastic deposits over cinder cone cores. A typical example of these volcanoes would have a broad shield of vesicular basaltic andesite with a cinder cone core that would have plugs and radial dikes (USGS, 2002a). Looking at the specific composition of each of the Three Sisters, it is found they somewhat fit the general description.
North Sister is
an example of a shield volcano (Jacobs, 1996). It has a shield base around 10 miles wide
and is a steep-sided cone. It is a large composite structure consisting of
basaltic andesite. North Sister is not believed to have had explosive
eruptions. Typically evidenced are lava flows and thick ash deposits
usually within a 10 mile radius (USGS, 2002d). North Sister has been inactive for
probably 100 millennia (Jacobs, 1996).
Middle Sister is an example of a major composite volcano. Its age is somewhere between the older North Sister and the younger South Sister. Middle Sister is compositionally diverse, but is described as basalt with minor basaltic andesite, andesite, dacite, and rhyodacite (USGS, 2002a). Middle Sister was probably active around 20 millennia ago (Jacobs, 1996).
South Sister is one of the youngest composite volcanoes of this area. It is composed mostly of andesite with minor dacite and rhyodacite (USGS, 2002a). The cone is composed of basaltic andesite and forms a well preserved crater at the summit (USGS, 2002d). South Sister is more recently active, believing to have erupted twice around 2,000 years ago (Jacobs, 1996).
This area developed in this manner because of the geologic situation and activity of the area. The Cascade Range has been an active arc for around 36 million years due to plate convergence (USGS, 2002a). The Cascadia subduction zone is located off the coast of Washington, Oregon, and northern California and is the site of the convergence of the North America plate and the Juan de Fuca plate (USGS, 2002c).
diagram illustrates this convergence of the
two plates. The small Juan de Fuca plate is moving northeastward and then is subducted beneath the large North America plate that is moving westward. This convergence is at about 4 centimeters per year but was at a rate of 6-7 centimeters per year around 7 million years ago (USGS, 2002a). As this denser plate of oceanic crust is subducted beneath the continental plate, it encounters high temperatures and pressures that will partially melt it. As this magma is created, some of it will rise to the surface in volcanic eruptions (USGS, 2002c). This is the process that has formed the chain of volcanoes now known as the Cascade Range (USGS, 2002a). The resulting volcanoes parallel the curved trend of the subduction zone.
Also associated with this area are some earthquakes. Earthquakes occur at plate boundaries as do volcanoes. The Cascade area experiences shallow earthquakes in Washington and deep earthquakes in the western parts of Washington and Oregon (USGS, 2002c).
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