Cascades

The Cascade mountain range extends from Northern California through western Oregon and Washington into British Columbia in Canada. The loftiest peaks of this range are stratovolcanoes, the highest being 4,392-meter Mount Rainier in Washington. The history of the Cascades goes back about 25 million years, but the periodic eruption of once-dormant volcanoes such as Mount St. Helens shows that the range is still evolving.

Location and Characteristics

The Cascade mountain range, named for the Columbia River's great cascades (waterfalls), spans 1,139 kilometers through the northwestern United States and southwestern Canada. Its highest peaks are a string of lofty volcanoes extending from Northern California through Oregon and Washington and ending in British Columbia. The Cascades lie about 160 to 240 kilometers inland from the Pacific Coast and are separated from the northwestern coast ranges by a broad valley called the Puget Trough in Washington and the Willamette Valley in Oregon. The range is divided into western and eastern sections; the western side consists of older, eroded volcanic rocks of relatively low topographic relief, but the east side, popularly known as the “high Cascades,” sports more spectacular—and much younger—volcanic peaks. Because the Cascade Range creates a major climatic barrier, the western slopes are heavily forested, nourished by abundant rainfall. In contrast, the eastern slopes are dry scrublands covered mostly by grass and low bushes. The rugged scenery of the Cascades, including numerous deep valleys, lakes, and ridges, has been sculpted both by stream erosion and by glaciation, which began in the Pleistocene ice age but has continued in recent times.

The high Cascades give the range its highest peaks and grandest mountain scenery. They consist of a string of stratovolcanoes that extend from Northern California (Mount Shasta, 4,317 meters; Lassen Peak, 3,187 meters) to British Columbia (Meager Mountain, 2,679 meters; Mount Garibaldi, 2,678 meters). Other important volcanoes in the Cascades include Mount Hood (3,426 meters; near Portland) and Mount Jefferson (3,199 meters) in Oregon, and Mount Adams (3,751 meters), Mount Rainier (4,392 meters; near Seattle), and Mount Baker (3,284 meters) in Washington. Mount St. Helens (2,500 meters) in southwestern Washington is one of the youngest and the most active Cascade volcano, having erupted violently in May 1980 and more than sixty times during the last 50,000 years. Oregon's famous Crater Lake was created by a volcano that blew itself out of existence nearly 7,000 years ago. The crater, which comprises the remains of once-mighty Mount Mazama (estimated elevation of more than 3,500 meters), is now an eight-kilometer-wide bowl-shaped collapse depression (caldera) filled with water.

Stratovolcanoes (also known as composite volcanoes) occur worldwide, mostly along continental margins and in near-continent island arcs such as the Aleutian Islands, Japan, and the Philippines. They tend to form lofty, cone-shaped peaks, commonly snow-packed because of their high elevations above sea level. The Andes in western South America, like the Cascades, also exist as a coast-hugging, linear chain of stratovolcanoes.

Stratovolcanoes owe most of their impressive height to the composition of the volcanic magma they emit. This magma is rich in silica (silicon dioxide), the most common chemical component of rocks and minerals in the Earth's crust and mantle. High-silica magmas have high viscosities and thus are generally thick and sticky upon eruption. As a result, erupted concentrations of high-silica volcanic materials tend to pile up in a single area, producing tall mountain peaks. Less viscous basaltic magmas tend to spread out, creating broad, low shield volcanoes such as those that make up the islands of Hawaii.

The volcanic rocks produced by the high-silica magmas of stratovolcanoes are called andesite. Dacite is an especially siliceous (silica-rich) variety of andesite; stratovolcanoes commonly eject one or both of these magma types. Stratovolcanoes are noted for their explosive eruptions, and this proclivity, too, can be attributed to the nature of their magmas. Andesite and dacite magmas generally contain significant quantities of water, which, when trapped as steam within this sticky magma, may suddenly explode with the power of several atomic bombs. For example, the 1980 eruption of Mount St. Helens released the estimated explosive energy of about five hundred Hiroshima-type atomic bombs. For this reason, stratovolcanoes, including those in the Cascades, are the most dangerous volcanoes in the world, at times causing catastrophic death tolls and property damage.

Geologic History

The geologic history of the present Cascade range is complicated, especially as it relates to other major geologic terrains in the Pacific Northwest. For example, the Cascades can be considered a younger northern extension of the Sierra Nevada range in eastern California. The thick, layered accumulations of basalt (a dark, low-silica, high-iron rock) lava flows of the Columbia River Basalt Plateau also have a relationship to the Cascades, as do the Willamette and Puget Valleys and the Pacific Coast ranges. The Cascades, especially the high Cascade volcanoes, represent one of the most recent additions to the geology of the Northwest; most of these volcanoes have initial eruption ages of less than one million years, making them very young by geologic standards. The eruptions of Mount Lassen between 1914 and 1917 and Mount St. Helens in 1980 show that these volcanoes are still actively adding to the mass of the range. By comparison, the north-south-trending volcanic field on which the current volcanoes are built was first established about twenty-five million years ago, during the Miocene epoch. Much of the western Cascades consists of the eroded remnants of volcanoes that once dominated the region's skyline but are now extinct.

To appreciate the origin of the Cascade range fully, it is necessary to go back in geologic time about 150 million years, during the era when dinosaurs dominated life on the Earth. At that time, much of northwestern Oregon and southwestern Washington were covered by an embayment of the Pacific Ocean that extended nearly to the present state of Idaho. Paralleling the coastline of this embayment was a subduction zone, a linear terrain representing the collision line of two lithospheric plates. The Earth is broken up into “plates,” averaging about 100 kilometers thick, that consist of the crust and a part of the upper mantle. These plates move horizontally over the Earth's surface, carrying the continents with them. Where they collide, one plate will slide under the other (a process known as “subduction”). When this process involves a thinner but denser oceanic plate colliding with a thick, buoyant continental plate, the oceanic plate always dives under the continental plate. This movement creates a narrow trench in the ocean off the coast of the continent that fills with sediments scraped off the ocean floor as the plate descends. These crumpled sediments eventually are pushed upward to become coastal ranges. Inland from the coastal ranges about 100 kilometers will be volcanoes produced by the melting of former surface materials sinking to ever deeper and hotter depths. Magma produced by the partial melting of subducted rocks powers the volcanoes of the present Cascades and most other stratovolcanoes.

Approximately 150 million years ago, subduction was occurring along the Oregon coast, parallel to the northeast-trending ocean embayment. This movement produced a northeast-trending mountain range consisting of coastal ranges (including the current California, Oregon, and Washington coast ranges) and a string of volcanoes, now mostly eroded away. These old mountains are now represented by the Blue and Wallowa ranges in eastern Oregon and southeastern Washington and the Klamath range in southwestern Oregon and Northern California. Also at this time, a subduction zone paralleled much of the California coast, producing the present California coast ranges and active andesite stratovolcanoes in the area of the Sierra Nevada in eastern California. As in Oregon, these old volcanoes in the Sierra have eroded away, but the underlying magma chambers, now crystallized to granitic rocks, provide evidence of their former existence. Yosemite National Park is an excellent place to view the massive granitic plutonic bodies that once fed volcanoes similar to those in the present-day Cascades.

About thirty-five million years ago, a strange event altered the pattern of mountain building and volcanic activity in the Pacific Northwest. The subduction zone trending northeast across Oregon straightened out into a north-south trend, an orientation it has maintained. About twenty-five million years ago, a new north-south-trending line of volcanoes sprang up parallel to the subduction zone. These volcanoes were similar to the present Cascade volcanoes, but they spewed out an even more siliceous and explosive variety of magma called rhyolite, the volcanic equivalent of granite. This activity began a very impressive era of explosive volcanism, with the widespread deposition of volcanic ash, that continued for about ten million years. The remnants of these ash deposits can still be observed in the John Day formation of northern Oregon. During this time, the volcanic range was gradually being uplifted by the buoyancy of the hot crustal rocks below. To the east, the areas of the present Willamette and Puget Valleys were down-warped as they were squeezed between the uplifting coast ranges to the west and the volcanic arc to the east. These valleys originally were shallow seas that eventually filled with sedimentary rocks and later were covered by basalt lava flows.

During much of the time that volcanoes were erupting in Oregon and Washington, they were also erupting in a nearly continuous band down the length of California in the present area of the Sierra Nevada. About thirty million years ago, however, volcanism suddenly ceased in the Sierra as the former subduction zone evolved into the current San Andreas fault system. The San Andreas, famous for the earthquakes it has wrought in California, is another form of plate boundary called a transform fault. In a transform fault, the lithospheric plates slide past each other but do not subduct. Thus, volcanic activity is limited at such boundaries. The Sierra volcanoes, which had once formed a continuous chain with the early Cascade volcanoes, died with the formation of the San Andreas system. They have since eroded away, leaving behind the granitic rocks that crystallized below them.

About twenty million years ago, volcanism in the present western Cascade area suddenly stopped, replaced later by the extrusion of voluminous quantities of very fluid basalt lava that flooded wide areas in Oregon and Washington. This was the event that created the present Columbia River Basalt Plateau. About twelve million years ago, volcanism resumed in the western Cascades but never achieved the level of activity of the area's explosive early history. The current eastern Cascade volcanoes have resulted from a renewed burst of volcanism over only the last one million years. However, the infrequent rate of volcanism in the Cascades probably is signaling a decline in subduction activity, which may eventually lead to the extinction of the Cascade volcanoes in a manner similar to what befell the Sierra Nevada volcanoes.

Study of the Cascades

The Cascades are among the best-studied volcanoes in the world. Scientifically, they reveal how continental subduction-zone volcanoes originate and evolve. They also have important implications for how continents, in general, grow and evolve. From a human interest standpoint, the study of Cascade-type volcanoes may lead to methods of predicting their potentially devastating eruptions. Volcanologists, geophysicists, and other geologists study volcanoes by first performing field studies of their lava flows or pyroclastic (explosively ejected) ash deposits. These rock units are investigated by field parties who chart their surface distribution on geologic maps. This information may be used to correlate volcanic materials with tectonic features such as faults, folds, uplifted or down-warped areas, and plate boundaries, or it may be used to estimate the energy of eruptions and the amount of material involved. The history of a volcanic area can be determined by noting the number and order of interlayered deposits of volcanic ash (indicating an explosive event) and lava flows (indicating a relatively quiet event). Samples are also analyzed by radiometric means (potassium-argon, carbon-14, and other techniques) to estimate the ages of the rocks. Rock ages allow the precise determination of when volcanoes were most active and for how long. Other chemical data collected from volcanic rocks can be used to suggest which kinds of parent rocks were melted to produce the magma and under which conditions.

Predicting volcanic eruptions is difficult because volcanoes seldom produce warning signals in a reproducible pattern that can be universally applied. Previous experience with volcanic eruptions shows that earthquakes generally precede eruptions, but the strength and frequency of quakes vary from one eruption to the next. Many volcanoes show an actual rise in elevation (caused by rising magma) shortly before an eruption, and such changes can be measured with precision laser transits. Also, the Earth's magnetic field around volcanoes may show a change in intensity shortly before an eruption. These measurements rely on having sophisticated instruments and well-trained operators observing the volcanoes on a nearly constant basis. Highly technical prediction strategies may not be financially or practically feasible in all circumstances in which the threat of volcanic eruptions is a constant concern.

The May 18, 1980, eruption of Mount St. Helens provided an excellent opportunity for intense, close-up study of an active volcano. Not only did this volcano provide a spectacular show of volcanic fury when it finally erupted (with the loss of fifty-seven lives), but it also was located in a uniquely accessible area, close to established government and university facilities with expertise in volcanology. The U.S. Geological Survey made thorough scientific studies of the volcano before, during, and after the catastrophic eruption. One encouraging result of these studies was the discovery that a certain frequency (vibration rate) of earthquake wave called a “harmonic tremor” provided a fairly reliable warning that an eruption was imminent. Harmonic tremors are believed to be generated by magma rising within the central feeder conduit of the volcano. As the magma moves upward, it rubs against the rocks lining the conduit, producing the characteristic vibration of the harmonic tremor. Detection of this special earthquake wave allowed the prediction of five subsequent minor eruptions of Mount St. Helens between 1980 and 1982.

Principal Terms

andesite: a type of volcanic, igneous rock, typically gray in color; the predominant lava expelled by stratovolcanoes such as those in the Cascades

basalt: a dark, iron-rich, silica-poor volcanic rock

dacite: a volcanic rock similar to andesite but containing more silica, typically gray in color

lithospheric plates: massive horizontal slabs of the Earth consisting of crust and the uppermost upper mantle, capable of movement by sliding over a semifluid layer in the mantle

rhyolite: a light-colored, silica-rich volcanic rock, commonly violently erupted as ash deposits

stratovolcanoes (composite volcanoes): lofty volcanoes composed of alternating layers of volcanic ash and lava or mudflows

subduction zone: a linear area representing the line of collision between two lithospheric plates

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