Amongst the protected trees of the Gifford Pinchot National Forests, and far removed from tourist traffic, lies the gentle and majestic Mount Adams. It resides, almost as a hermit, in the south central Cascades of Washington State. Largely ignored by Washington�s population, due to its rural location and general lack of eruptive zeal, Mount Adams is an anomaly. While daydreamers discuss Mount St. Helens and Mount Rainer, Mount Adams is taken as a mere backdrop for everyday life, even among its locals. Why is this? What is Mount Adams, and what eruptive tricks might it play? This paper will address these questions in detail, looking for answers about this silent volcano. History of Volcanism in Western Washington
Beginning in the Jurassic era, Duncan and Kulm (1989) note, a massive oceanic plate, the Farallon Plate, began a slow motion collision with North America. This collision has continued through the present, as the Farallon Plate has gradually subducted under the North American Plate, reducing it into the small fragment now named the Juan de Fuca Plate. This motion brought microcontinents and island arc chains to the coast, adding them into the main continent. This action explains the diverse bedrock seen throughout Washington State. As the Plate of Juan de Fuca subducts, there is another reaction. The subducting plate begins to dehydrate (Francis, 1993), which helps to melt surrounding materials, such that they will begin to migrate towards the surface. The result of this is that surface volcanism will be observed. In the case of subduction at a continental boundary, a stratovolcano will be produced. Volcanism began in Washington 42 Ma, with such features as Mount Baker, Mount Rainer, Mount St. Helens, and Mount Adams being built from 18 to 9 Ma (Duncan and Kulm, 1989).
Mount Adams exists as a geographic feature, but it is also part of a larger volcanic area, in which several vents are present. These vents built separate, small volcanoes, which in part make up the base of Mount Adams. The separate fields are called Indian Heaven, Simcoe Mountains, and Goat Rocks. Mount Adams and Indian Heaven (Topinka b, 2002) have been the most active recently, and Mount Adams maintains the highest elevation as well as volume of erupted material.
Mount Adams Eruptive History
Mt. Adams, as its neighbors will be glad to hear, is a quiet volcano. Composed mainly of lava flows and fragmentary basaltic andesite and andesite (Topinka, 2002), Mount Adams does not boast extremely explosive materials. Scott et al (1995), write that highly explosive eruptions have been rare in Mount Adams� history. Much more frequently, eruptions were characterized by lava flows. These lava flows were several meters thick, covered in thick blocky chucks, under which the molten rock moved. Composed of 49-61% SiO2 (Topinka b, 2002), the lava has a viscosity of roughly 10^3.5-10^6. This viscous lava moves very slowly, very different from the glowing red streams of Hawaiian lava. Tephra events, in contrast to lava events, are infrequent for Mount Adams. Unlike its more active neighbors, Mount Adams rarely produces a large column, often only extruding a column capable of blanketing a local region. This is in contrast to an eruption such as Mount St. Helens in 1980, exploding enough ash and pumice to shower locations as far away as 120 miles with measurable debris (Scott et al, 1995). However, Scott et al (1995) explain that an explosion of the same magnitude as the 1980 Mount St. Helens eruption may be possible for Mount Adams.
In the last 15,000 years, Adams has erupted several times. Figure 1 provides the number and description of events. This history restates Scott�s et al (1995) statements that lava flows are more common than tephra events at Mount Adams. 30,000-20,000 years ago, during the highest period of recent activity, Mount Adams built its cone. Because of the thick lava, Mount Adams grew in the tall cone shape distinctive to stratovolcanoes. The flows seldom traveled more than 12 miles from any given vent on the volcano (Scott et al, 1995). However, Scott et al (1995) also note that Mount Adams did extrude about 50 km2 of lava in the past 10,000 years. Unfortunately for its volcanic interest value, this is less than 14% of the volume extruded by Mount St. Helens in the 1980 eruption.
Referring back to Figure 1, immediately it becomes evident that the most recent event was not an eruptive event. In 1921, a debris avalanche roared down the side of Mount Adams. Four million cubic meters of rock fell from the southwest flank, and traveled nearly 4 miles down into the Salt Creek Valley (Topinka b, 2002). This was not a new development in the history of the volcano. Throughout time, Mount Adams has sloughed off layers. The largest debris fall event took place only 6,000 years ago, when the mountain released roughly 70 million cubic meters into the Trout Lake drainage, extending debris as far as the location of the little town of Husum, some 35 miles from Mount Adams (Scott et al, 1995). Figure 2 illustrates the region around Mount Adams, including the Trout Lake area.
These avalanches are not related to eruptive activity, though they can be. Through a process of acid-sulfate leaching and deposition of alunite, the core of the volcano is in effect rotten, consisting of decomposed rock (Topinka b, 2002). The weak rock, exposed to the pressures of gravity and glaciers, breaks off.
The threat of a debris avalanche is constant on Mount Adams. In 1988 two small lahars, or mud flows, buried irrigation structures along Big Muddy Creek (Scott et al, 1995). In 1997, a small rock fall was noted on Mount Adams. Seismograph stations as far away as Corvallis, Oregon recorded the impact as rocks crashed down from the Castle rock formation (Norris, 1997). Figure 3 is a photo of the debris fall on the flank of the volcano.
Future Hazards
Figure 3 depicts a small debris fall on Mount Adams. Small. A mountain the size of Mount Adams, roughly 200 cubic kilometers of volume, has much more to offer in terms of debris falls and lahars (Topinka b, 2002). In fact, Scott et al (1995) state that they believe that debris falls are the greatest danger posed by Mount Adams. A large volume debris fall could fill rivers over brimming, extend outwards to populations by means of the rivers, and could possibly put the stability of downstream dams at risk. Scott et al (1995) further postulate that one cubic kilometer (only 1/200 of Adams volume) could transport enough sediment down river to cause the Bonneville Reservoir to overflow, possibly triggering dam collapse. This would cause extensive and devastating damage. Further, the debris could dam the river, and when it broke, similar problems would occur.
Though this is a worst case scenario, it is again worth noting that a debris fall does not have to be initiated by a volcanic event.
Conditions for such a far-reaching event do exist around Mount Adams. The southwest and east flanks are by far the weakest, due to steep angles and deep erosion (Scott et al, 1995). These flanks face into steep river valleys that would contain the distribution of the flood, and so force it further down the valley before its force could be spent. In contrast, the northwest and northeast flanks stand a lesser chance of an extensive lahar. The river valleys on those flanks are broad. If a large event were to occur on these flanks, it would spread across its own sides, and the rivers would have ample room for them. Also, the dams downstream of Mount Adams on the northwest and northeast side hold a great capacity of water, and would be able to withstand more volume of debris material without damage.
Debris flows are the greatest danger to the Mount Adams region. Eruptive activity has been so light in historical times that only a seismometer monitors the volcano, unlike the water temperature, gas composition, deformation monitored at other nearby volcanoes (Scott et al, 1995). They also state that �the maximum credible eruption at Mount Adams is�on the order of less than 1 in 100,000.� This is annually. And if the volcano does erupt again in historical times, it will likely follow its pattern of a small ash column and thick, viscous lava flows.
In Conclusion
Mount Adams remains silent. Her gentle slopes and glistening reflective pools do not seem to prophesize a fate like those of Mount St. Helens. Throughout the volcano�s long history, its neighbors have not had to fear the eruptions, but rather its potential for dangerous debris falls. Mount Adams does not have eruptive tricks, and she will not cause her visitors to speculate. At least not right now.
References
Duncan, R. A., Kulm, L. D., 1989, Plate Tectonic Evolution of the Cascades Arc Subduction Complex, in Winterer, E. L., Hussong, D.M., Decker, R. W., eds., The Eastern Pacific and Hawaii: Boulder, Colorado, Geological Society of America, The Geological Society of America, p. 413-438
Francis, Peter, 1993, Volcanoes, a Planetary Perspective: New York, Oxford University Press
Norris, Bob. Seismic Signals from the 10/20 Avalanche. (1997). Retrieved November 2, 2002, from http://www.geophys.washington.edu/USGS/DOCS/PROJECTS/adamsrk.html
Scott, William E. et al, 1995, Volcano Hazards in the Mt. Adams Region, Washington USGS Open-File Report 95-492: Menlo Park, California, US Dept. of the Interior, US Geological Survey
Topinka, Lyn. a CVO Website-Mount Adams, Washington-Map. (2002). Retrieved November 2, 2002, from http://vulcan.wr.usgs.gov/Volcanoes/Adams/Maps/map_adams_locale.html.
Topinka, Lyn. b CVO Website-Mount Adams Volcano, Washington. (2002). Retrieved November 2, 2002, from http://vulcan.wr.usgs.gov/Volcanoes/Adams/description_adams.html
Links to other sites on the Web
© 2000 [email protected]
