DESCRIPTION:
Western USA - Volcanic History and Historical Observations

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Map, Major West Coast Volcanoes - Washington, Oregon, and California
-- Topinka, 1998, basemap created at University of Virginia, USGS Digital Line Graph Data Browser

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Map, Potentially Active Volcanoes of the Western United States
-- Modified from: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication

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 kilometers (approximately 1,120 miles) 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 million years 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 (10).

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.

Mount St. Helens was recognized as a volcano at least as early as 1835; the first geologist apparently viewed the volcano 6 years later. James Dwight Dana of Yale University, while sailing with the Charles Wilkes U.S. Exploring Expedition, saw the peak (then quiescent) from off the mouth of the Columbia River in 1841. Another member of the expedition later described "cellular basaltic lavas" at the mountain's base.

The first documented observation of Mount St. Helens by Europeans was by George Vancouver on May 19, 1792, as he was charting the inlets of Puget Sound at Point Lawton, near present-day Seattle. Vancouver did not name the mountain until October 20, 1792, when it came into view as his ship passed the mouth of the Columbia River.

A few years later, Mount St. Helens experienced a major eruption. Explorers, traders, missionaries, and ethnologists heard reports of the event from various peoples, including the Sanpoil Indians of eastern Washington and a Spokane chief who told of the effects of ash fallout. Later studies determined that the eruption occurred in 1800.

The Lewis and Clark expedition sighted the mountain from the Columbia River in 1805 and 1806 but reported no eruptive events or evidence of recent volcanism. However, their graphic descriptions of the quicksand and channel conditions at the mouth of the Sandy River near Portland, Oregon, suggest that Mount Hood had erupted within a couple decades prior to their arrival.

Meredith Gairdner, a physician at Fort Vancouver, wrote of darkness and haze during possible eruptive activity at Mount St. Helens in 1831 and 1835. He reported seeing what he called lava flows, although it is more likely he would have seen mudflows or perhaps small pyroclastic flows of incandescent rocks.

On November 22, 1842, Reverend Josiah Parrish, while in Champoeg, Oregon, (about 80 miles or 130 kilometers south-southwest of the volcano), witnessed Mount St. Helens in eruption. Ash fallout from this event evidently reached The Dalles, Oregon (48 miles or 80 kilometers southeast of the volcano). Missionaries at The Dalles corroborated Parrish's account. Captain J. C. Fremont recounts the report of a clergyman named Brewer, who gave him a sample of ash a year later (Wilkes, 1845): "On the 23rd day of the preceding November, St. Helens had scattered its ashes, like a light fall of snow, over the Dalles of the Columbia." Other accounts of the same ashfall note that it was "like fine sand", its color "appeared like ashes", and the odor was "that of sulphur" (Majors, 1980).

Contemporary sketches and paintings by Paul Kane suggest the mountain was probably erupting at a point halfway down the north slope before or during 1847. The vent was apparently the Goat Rocks dome, which was removed by the 1980 eruption. On the basis of these and other observations, scientists think eruptive activity may have continued intermittently until 1857.

Small eruptions were reported in 1898, 1903, and 1921, but these events were not independently confirmed, nor have their deposits been identified. Judging by the nature of the post-May 18, 1980, activity at Mount St. Helens, it is likely that these events were steam emissions, small explosions, or large rockfalls.

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