Chicken Little Was Right - Sometimes the Sky DOES Fall

Phi Kappa Phi Forum, Winter 2006 by Cashman, Katharine

From my perch in the Pacific Northwest I see a volcanic landscape - an "in-your-face" volcanic landscape, a British friend of mine once commented after returning from a weekend trip to central Oregon where high Cascade peaks, barren lava flows, and pumice deposits dominate the scenery. It is a landscape that, to greater or lesser degrees, typifies many parts of the western United States, Alaska, and Hawaii. In the past two-thousand years, seven states have seen volcanic activity (Alaska, Arizona, California, Hawaii, Idaho, Oregon, Washington). Go back one-hundred-thousand years, and you can add Wyoming, Nevada, and New Mexico.

Why is the western United States subject to this activity? The reasons are many, but all relate to geologic location. Northern California, Oregon, Washington, and Alaska lie along what is termed an "active plate margin," meaning that they lie along the boundary between two tectonic plates. The interaction of those plates, specifically the subduction (thrusting) of one plate below another, generates magma that eventually makes its way to the surface and erupts, volcanically. Hawaii owes its existence to a different geologic setting - it lies above a hot spot, a region of enhanced melting that may reflect a disturbance thousands of kilometers deeper, at the boundary between the earth's core and mantle. Hruptions in Idaho, Arizona, and New Mexico appear to have a similar origin, except that here the hot spots lie under the continental United States.

Of the volcanically active states, Hawaii and Alaska have the most frequent eruptions. Kilauea volcano, Hawaii, has been erupting almost continuously since January 1983. Alaska has dozens of frequently active volcanoes, one of which - Augustine - is erupting as I write this. But in terms of the hazards that they pose, volcanoes that stretch from northern California to British Columbia, the Cascade Range, are the source of most concern. On average, one or two of these volcanoes erupt every century (Figure 1 ). For example, I.assen volcano in northern California erupted during World War I, and Mount St. Helens was active throughout much of the 1980s. Both Mount Hood and Mount St. FIelens also were active at the beginning of the nineteenth century, a few years before Lewis and Clark reached the Pacific Ocean. Moreover, some of the northwest's largest cities are located close to these peaks, most notably Seattle and Tacoma, northwest of Mount Rainier, and Portland, northwest of Mount Hood. For this reason, my focus here is on the recent (last several thousand years) eruptive history of the Cascades to provide perspective on the probability of future eruptive activity in this region.

THE HISTORICAL RECORD

Native American oral traditions from the Pacific Northwest abound with stories of volcanic eruptions, testifying to the frequency with which volcanic activity has punctuated the history of human occupation of this region. Dominant among these stories are those related to the approximately 7,700 years before present (ybp) eruption of Mount Mazama to create Crater Lake, Oregon. There are several versions of the eruption story, from the Klamath and Modoc tribes to the east of the volcano (Clark), as well as the Umpqua to the west (www.cowcreek.com). These stories refer back to "deep time" (Vansina), a time "long ago, before the stars fell" (Clark, p. 63). One story provides a vivid picture of the eruption, framed as a fight between the "Chief of the Below World" and the "Chief of the Above World" in which "[m]ountains shook and crumbled. Red-hot rocks as large as the hills hurtled through the skies. Burning ashes fell like rain. The Chief of the Below World spewed fire from his mouth. Like an ocean of flame it devoured forests on the mountains and in the valleys. On and on the Curse of Fire swept until it reached the homes of the people" (Clark, p. 54). The story then documents a pause in eruptive activity before the subsequent eruption, during which "the top of the mountain fell...the high mountain was gone" (Clark, p. 55). Since this devastating event, the resulting Crater Lake has been off-limits to many local tribes (www.cowcreek.com).

Geological investigations of the eruption do find evidence for two separate eruptions (Diller and Patton, Williams, Bacon). The first produced a high eruption column that deposited pumice and ash to the east. Explosive activity then was followed by more subdued effusion of a large lava flow of obsidian that probably lasted for weeks or even months. A second, and much larger, eruption then followed, its high column spreading pumice and ash over much of the western United States, covering parts of Oregon, Washington, Idaho, and California. As the eruption progressed and the magma reservoir beneath the mountain drained, the vent feeding the eruption widened, and the high column collapsed to form hot ash flows that swept down the flanks of the mountains for several kilometers in all directions. The eruption ended after more than 50 km^sup 3^ of magma had been evacuated, and the top of the mountain transformed into a hole 8 km across and 1.6 km deep (USGS) (Figure 2).