The American West - Tectonics and Volcanoes 

The American West is one of the most unusual volcanic provinces on Earth. It is an extraordinary wide region of contemporaneous volcanism, stretching 1,800 km from east of the Rocky Mountains to the west of the Cascades (103 degrees to 123 degrees West). It encompasses a very wide variety of volcanic landforms and diverse rock compositions, with the only consistency being small basalt fields that are scattered throughout the whole vast region.

Intriguingly, except for the Cascadian segment, there is no currently active plate subduction to drive the volcanism, although there are hot spots, persistently "leaky" faults, and rifting.

This cacophony of volcanism was nearly impossible to understand (e.g., Gilluly, 1965) until the breakthrough plate tectonic interpretation of western North America by Atwater (1970). Lipman et al. (1971) and Christiansen and Lipman (1982) immediately reinterpreted all of Cenozoic volcanism in North America in light of this new idea, creating an enduring framework for understanding American volcanism. As McKee and Noble (1986, p.39) point out, many "later papers have refined but not fundamentally changed these early concepts."

Atwater recognized from seafloor magnetic anomalies that an oceanic spreading center was located off western North America until mid-Tertiary time and that a subduction zone must have existed with concomitant arc volcanism on the continent.

Lipman, Christiansen, and colleagues showed that from approximately 40 to 18 million years ago subduction of the eastern limb (named the Farallon Plate) of the East Pacific Rise spreading center produced widespread calc-alkaline rocks, with very little basalt anywhere in western North America (McKee and Noble, 1986). At approximately 30 million years ago the East Pacific Rise collided with North America in the vicinity of northern Mexico. The collision zone (which Lipman and Christiansen believe destroyed both the spreading ridge and the trench) migrated northward to its present position near Cape Mendocino, California, progressively forming the San Andreas fault near the coast and cutting off the calc-alkaline volcanism further inland. By approximately 18 million years ago there had been a profound change in volcanism and tectonism, with arc-type volcanic activity replaced by abundant basaltic and bimodal, basaltic-rhyolitic lava fields.

Today''s volcanism is largely controlled by extensional tectonic regimes developed at that time.

In a sense, volcanism during the last 5 MY in the American West is the final dribble of activity in this much grander mid-Tertiary story of large ash-flow calderas with voluminous ignimbrite eruptions, extreme crustal thinning and extension, and high mountain building. The processes that produced these epic events are still disputed (e.g., Sonder et al., 1987) nearly 20 years after the revolutionary contributions of Atwater, Lipman, and Christiansen.

However, the relation of much of the younger volcanism to tectonism is generally understood, although the underlying reasons for the existence of particular tectonic structures are often uncertain. As a simplification, six interconnected volcano-tectonic provinces are described ( 1) Cascades, 2) Snake River Plain - Yellowstone, 3) Colorado Plateau, 4) Rio Grande Rift, 5) Jemez Zone, 6) Eastern California) that include most of the young (less than 5 million years) volcanic landforms in the western United States. These provinces combine many of 23 rectilinear volcanic zones defined by Smith and Luedke (1984).

Volcanoes of Canada and Western USA occupy tectonic environments ranging from the subduction volcanism that dominates the Cascade Range to the extensional tectonics controlling vast regions of the western interior, giving this region the largest number (and percent) of volcanoes consisting primarily of cinder cone fields.

Only Mount St. Helens and Lassen volcanoes in this region have had unequivocal eruptions in this century whereas 34 have earlier dated eruptions, a lower ratio than any other region and a striking contrast to a region like Indonesia where 83 percent of its dated eruptions were in this century. Globally, only 20 percent of dated eruptions are pre-historic, but his proportion is 79 percent in this region (Canada and Western USA), testifying to the strong attention paid to the recent geologic record.

This region has the largest number of Holocene eruptions dated by radiocarbon (126), by dendrochronology (10), and by magnetics.

Native American legends describe eruptions of Sunset Crater, Arizona, now dated to 1064-65 AD, and Canadian Indian legends record a British Columbian eruption in the 18th century. After the historic voyages of Columbus, Spain dominated exploration of North America in the 16th century, with the Grand Canyon first viewed by western eyes in 1540 and the Oregon coast only 4 years later. Permanent inland settlement of Santa Fe came in 1609, only 2 years after the first settlement on the east coast by the British.

The founding of the Massachusetts Bay colony in 1630 started the great emigration to eastern North America. Exploration of the west was slow, though, and it was not until the 1770s that Captain Cook closed the gap between Spaniards working north along the coast and Russians moving toward them from the far northwest. Cook brought publicity to the Pacific coast, and by the end of the century ships from 6 nations were busily trading furs along seacoasts that 20 years earlier had not been seen by Europeans. The first documented eruption in the region was California''s Shasta, in 1786. In the later half of the 18th century, while the US was gaining independence in the east, the Rocky Mountains were being explored by the British and French. In 1805 Lewis and Clark sighted the Pacific, and the first historical eruptions of Mount St. Helens were witnessed by settlers in the 1830s. In 1841 the first wagon train reached the Oregon Territories, and in 1848 gold was discovered in California. It was not until the end of the Civil War, though, that westward emigration exploded: the first transcontinental railroad was completed in 1869, and by 1890 the U.S. Census Bureau Director declared that the American frontier was at an end. The U.S. Geological Survey was founded in 1879 and from 1926 through 1931 operated a volcano observatory at Lassen, following that volcano''s 1914-17 eruption.

The second Cascade eruption of the century, Mount St. Helens, brought the founding of the Cascades Volcano Observatory in 1980.

And more - http://vulcan.wr.usgs.gov/Volcanoes/WesternUSA/description_western_volcanics.html

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