Archaeology and Volcanism|
VI. North American Archaeological Sites
A. Eruptions at Sunset Crater, Arizona
In the southwestern United States, native peoples pursued a sedentary existence and farmed land near the present site of Flagstaff, Arizona, since at least A.D. 600. Then, during the 1060s, Strombolian eruptions formed Sunset Crater, blanketing the landscape with tephra and emitting a series of lava flows that buried previously arable fields. Until recently, anthropologists believed that tephra ejected during the eruptions greatly enhanced the soil's fertility, causing large number of Native Americans from other territories to flock to the posteruption Sunset area. Reexamination of the archaeological evidence, however, indicates that the presumed population increase is illusory; instead of other cultural groups migrating to cultivate the ash-enriched fields, the indigenous peoples merely transferred their settlements to new locations near the cinder cone, readjusting their practices to cope with the volcanic changes wrought in the physical environment. Unusually heavy rains that characterized the three decades following the ash eruption probably accounted for the Sunset area's posteruption increase in crop productivity, not deposition of the ash itself.
B. Native American Site Near Mount Rainier, Washington
Most of the world's archaeological sites conserved by volcanic activity are those rapidly buried by tephra or pyroclastic flows. By contrast, lava flows typically crush, pulverize, and/or burn man-made structures in the process of covering them. Similarly, most volcanic debris flows and lahars, which resemble churning wet concrete as they stream downslope, also destroy more than they preserve. A rare exception is a Native American settlement in the Cascade Range foothills of western Washington, near the present towns of Enumclaw and Buckley (Fig. 4). Discovered in 1972, the prehistoric campsite contained numerous stone projectile points, scrapers, and other tools, as well as charcoal that helped to fix the date of its burial.
FIGURE 4 Map showing hazard zones for mudflows (lahars), lava flows, and pyroclastic flows from Mount Rainier, Washington State. About 5600 years ago, the volcano's former summit collapsed, generating the Osceola Mudflow that buried an Indian encampment near the present town of Buckley, preserving the oldest known human artifacts in the Puget Sound region. More recent mudflows followed the channels of the Puyallup and Nisqually Rivers, inundating areas where tens of thousands of people now live. (From U.S. Geological Survey Open-File Report 98-428).
The encampment, located on what was then a hilltop above a valley floor, was engulfed almost instantly by an enormous lahar, the Osceola Mudflow, generated when an explosive eruption caused the former summit of Mount Rainier to collapse (Fig. 5). Almost a cubic mile of rock, much of it hydrothermally altered, avalanched down the volcano's northeast flank and was transformed into a thick lahar as it traveled more than 65 miles down the White River Valley, beyond the Cascade mountain front, where it easily swept over the hilltop camp, and ultimately into an arm of Puget Sound. Although the Osceola Mudflow had transformed the area beyond recognition and many centuries elapsed before native peoples reoccupied it, a new settlement was eventually established directly above the buried site. Two thousand years older than Akrotiri, this collection of native artifacts is one of the oldest evidences of human culture in the Puget Sound region.
FIGURE 5 Mantled by a cubic mile of glacial ice, Mount Rainier (14,411 feet) towers above the densely populated Puget Lowland of western Washington State. Considered the most potentially dangerous volcano in the 48 contiguous states, Rainier has repeatedly produced massive debris flows, some of which have inundated areas where hundreds of thousands of people now live. (Photo by Austin Post, U.S. Geological Survey.)
C. Effects of the Mount St. Helens "Y" Tephra
As the most frequently and violently explosive volcano in the 48 contiguous states, Mount St. Helens in southwest Washington apparently affected the settlement patterns of some Native Americans. About 3600 years ago, St. Helens began a series of intermittent outbursts that produced the "Y" (yellow) tephra sequence, pumiceous deposits mantling much of the Pacific Northwest. Although the degree of environmental degradation to the affected areas is not known, comparisons with similarly voluminous eruptions at other volcanoes indicate that a heavy fall of coarse lapilli typically defoliates trees, smothers vegetation, pollutes water sources, and depletes both animal populations and the human predators who depend on them. Recent studies by the U.S. Forest Service and others indicate a hiatus in settlements throughout the southwest Cascade Range between about 3600 and 1600 years ago. During approximately the same period, there was a coincidental increase of native populations in the Columbia Plateau to the east, a shift perhaps attributable to St. Helens' repeated ejections of tephra.
D. Effects of the White River Ash on Native Inhabitants of the Yukon
The White River Volcano, located in southeastern Alaska near the headwaters of the White River, produced two cataclysmic eruptions, about A.D. 20 and A.D. 720, that covered most of the southwestern Yukon Territory with voluminous ashfalls. Anthropological studies indicate that the later and larger of the two outbursts, which deposited the east lobe of the White River Ash, caused profound disruption of the native population, possibly initiating a series of migrations that culminated in the formation of the Pacific Athapaskans in British Columbia and of the Apache and Navajo of the southwestern United States. Athapaskan lore, transmitted orally for more than 1200 years, refers to fiery explosions and a collapsing mountain that caused the people's ancestors to abandon their original homeland and, in small bands, eventually drift westward or southward to their present locations.
One of the world's most active volcanic regions, the Aleutian Range of Alaska hosts at least 45 historically active volcanoes. Studies of peoples inhabiting the Aleutian Islands during the 18th century A.D. indicate that eruptions, including submarine activity, have repeatedly influenced the movements of native inhabitants. Dependent exclusively upon marine fauna for their existence, the Aleuts have apparently been forced to abandon settlements destroyed by tsunamis generated during earthquake-induced underwater landslides or submarine eruptions. In some cases, it appears that the Aleuts have deserted settlements because underwater activity killed the sea life that constitutes their sole food supply.
Back to Contents