by: W.E.Scott, R.M.Iverson, J.W.Vallance, and W.Hildreth, 1995, USGS Open-File Report 95-492
U.S.Geological Survey, David A. Johnston Cascades Volcano Observatory, 5400 MacArthur Blvd., Vancouver, WA 98661
This report is preliminary and has not been reviewed for conformity with U.S.Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S.Government.
Mount Adams, one of the largest volcanoes in the Cascade Range, dominates the Mount Adams volcanic field in Skamania, Yakima, Klickitat, and Lewis counties and the Yakima Indian Reservation of south-central Washington. The nearby Indian Heaven and Simcoe Mountains volcanic fields lie west and southeast, respectively, of the 1250 square kilometers (500 square miles) Adams field (plate 1) -- (Web note: not available in this format) -- . Even though Mount Adams has been less active during the past few thousand years than neighboring Mounts St. Helens, Rainier, and Hood, it assuredly will erupt again. Future eruptions will probably occur more frequently from vents on the summit and upper flanks of Mount Adams than from vents scattered in the volcanic fields beyond. Large landslides and lahars that need not be related to eruptions probably pose the most destructive, far- reaching hazard of Mount Adams. The purpose of these maps and booklet is to (1) describe the kinds of hazardous geologic events that will likely occur in the future at Mount Adams and at other volcanoes in the region, (2) outline the areas that will most likely be affected by these events, and (3) recommend actions that individuals and government agencies can take to protect lives and property.
Volcanoes pose a variety of geologic hazards -- both during eruptions and in the absence of eruptive activity. During much of its history, Mount Adams has displayed a relatively limited range of eruptive styles (figure 1). Highly explosive eruptions have been rare. Compared to the tens of large explosive eruptions at nearby Mount St. Helens during the past 20,000 years, eruptions of Mount Adams have been meek. Eruptions at Mount St. Helens have blanketed areas more than 200 km (120 mi) downwind with ash deposits several centimeters or inches thick, but those at Mount Adams have blanketed only areas a few kilometers away with a similar thickness of ash. Nonetheless, despite their low level of explosivity, eruptions at Mount Adams are hazardous. More importantly, even during times of no eruptive activity, landslides of weakened rock that originate on the steep upper flanks of Mount Adams can spawn lahars, which are watery flows of volcanic rocks and mud that surge downstream like rapidly flowing concrete. Lahars -- also known as mudflows or debris flows -- can devastate valley floors tens of kilometers from the volcano.
Small explosions that accompanied past lava-flow eruptions at Mount Adams and other volcanoes in the region were strong enough to hurl lava blocks from vents, and probably created clouds of tephra that rose thousands of meters into the atmosphere. In depositing only a few millimeters of tephra for tens or, rarely, a few hundred kilometers downwind, such clouds offer little threat to life or structures. But tephra clouds can create tens of minutes to hours of darkness as they pass over a downwind area, even on sunny days, and reduce visibility on highways. Deposits of tephra can short-circuit electric transformers and power lines, especially if the tephra is wet, which makes it highly conductive, sticky, and heavy. Tephra injested by vehicle engines can clog filters and increase wear. Tephra clouds often generate lightning that can interfere with electrical and communication systems and start fires. Finally, and perhaps most importantly, even small, dilute tephra clouds pose a significant hazard to aircraft that fly into them. As explained below, other volcanoes, especially Mount St. Helens, do produce large explosive eruptions, and these volcanoes pose the greatest threat of significant tephra fall in the region.
The lessons learned in Washington communities such as Yakima, Ritzville, and Spokane during the 1980 eruption of Mount St. Helens are used throughout the Pacific Northwest and elsewhere in the world to prepare governments, businesses, and citizens for future tephra falls. All three communities experienced significant disruptions in transportation, business activity, and community services during fallout of from 0.5 to 8 cm (1/4 to 3 in) of tephra and for several days after the eruption. The greater the amount of tephra that fell, the longer a community took to recover. As perceived by residents, tephra falls of less than 0.5 cm (1/4 in) were a major inconvenience, whereas falls of more than 1.5 cm (2/3 in) constituted a disaster. Nonetheless, all three communities recovered to nearly normal activities within two weeks.
During the past one million years, numerous volcanic vents were active throughout south-central Washington, from Vancouver to Goldendale (plate 1) -- (Web note: not available in this format) -- . Most were probably active for relatively short times ranging from days to tens of years. Unlike Mount Adams, which has erupted repeatedly for hundreds of thousands of years, these vents typically did not erupt more than once. Rather, each erupting vent built a separate, small volcano, and over time a field of numerous overlapping volcanoes was created. Clusters of these vents define the Mount Adams, Indian Heaven, and Simcoe Mountains volcanic fields. In addition, the Goat Rocks volcanic center lies 30 km (18 mi) north of Mount Adams. The Mount Adams and Indian Heaven fields have been the most active recently; the Simcoe field and the Goat Rocks center have not erupted for hundreds of thousands of years.
Because the numerous volcanoes in these fields were active for geologically brief times, they are much smaller than Mount Adams. Underwood Mountain, which lies west of the mouth of the White Salmon River, is one such volcano. It is about 8 km (5 mi) in diameter and less than 800 m (2,600 ft) high. About 9,000 years ago, the Big Lava Bed (plate 1) -- (Web note: not available in this format) -- . issued from a small volcano less than 300 m (1,000 ft) high and partly filled the northwest part of the Little White Salmon River drainage basin with a thick lava flow almost 16 km (10 mi) long. A few ancient lava flows were sufficiently large to flow down tributary valleys, spread out on the floor of the Columbia River Gorge, and dam the river to form a lake. The river then cut a new channel around or through the lava flow.
Mount Adams and nearby vents are not the only sources of volcano hazards in the region. For example, volcanic ash produced by explosive eruptions at Mount St. Helens (50 km or 30 mi to the west of Mount Adams) and Mount Mazama (site of Crater Lake, 370 km or 230 mi to the south) form the thickest and most conspicuous ash layers of the past 15,000 years around Mount Adams. Ash of the Mazama eruption is 2 to 8 cm (1 to 3 in) thick, while ash of several St. Helens' eruptions ranges from a fraction of a centimeter to 40 cm (16 in) thick.
Lahars generated at Mounts St. Helens and Rainier have inundated the lower reaches of the Lewis and Cowlitz Rivers, which also drain Mount Adams. Even a lahar from Mount Hood that flowed down Hood River inundated the lower White Salmon River. It was generated by a large debris avalanche that crossed the Columbia River and flowed a few miles up the lower White Salmon drainage.
The accompanying map (plate 2) -- (Web note: not available in this format) -- . shows areas that could be affected by future debris avalanches and lahars, as well as by eruptions. Hazard zones are based largely on the type and scale of events that have occurred in the recent geologic past at Mount Adams and in the surrounding volcanic fields. The zonation for debris avalanches and lahars also takes into account the distribution and volume of weakened, altered rock on Mount Adams' summit and upper flanks, which could spawn events much larger than those of the recent past. Hazard zonation for tephra falls is based chiefly on contributions of tephra from other major Cascade volcanoes, especially Mount St. Helens.
Although we show boundaries of hazard zones for lava flows, debris avalanches, and lahars by lines or edges of patterns, the degree of hazard does not change abruptly at these boundaries. Rather, the hazard decreases gradually as distances from the volcano increase and as elevations above valley floors increase. Areas immediately beyond outer hazard zones should not be regarded as hazard-free, because the boundaries can only be approximately located, especially in areas of low relief. Too many uncertainties exist about the source, size, and mobility of future events to locate zero-hazard zones with confidence.
Even minor tephra eruptions can disrupt daily life and commerce and greatly endanger aircraft. The most serious tephra hazards in the region are due to the proximity of Mount St. Helens, the most prolific producer of tephra in the Cascades during the past few thousand years. The accompanying maps (figure 2) provide estimates of the annual probability of tephra fall affecting the region. The maps are based on the combined likelihood of tephra-producing eruptions occurring at Cascade volcanoes, the relationship between thickness of a tephra-fall deposit and distance from its source vent, and regional wind patterns. Probability zones extend farther east of the range because winds blow from westerly directions most of the time. One map shows probabilities for a fall of 1 cm (about 0.5 in) or greater and the other for a fall of 10 cm (about 4 in) or greater. Northern Skamania County has an annual probability of a tephra fall of 10 cm or more of about 1 in 100 (1%) to 1 in 500 (0.2%). A much larger area, including most of Skamania, Klickitat, and Yakima counties and the Yakima Indian Reservation, has a similar probability of a tephra fall of 1 cm or more. In fact, the maps demonstrate that even though Mount Adams is a meager tephra producer, the region around Mount Adams has the highest probability of tephra fall of anywhere in the western conterminous United States, owing to its location just downwind of Mount St. Helens!
The maximum credible eruption at Mount Adams is an event of very low annual probability -- on the order of less than 1 in 100,000 -- but one that would have very serious consequences. Although preparing for an event of such low probability is problematic, we should nonetheless understand the worst-case scenario.
Two types of large-scale eruptions occur at volcanoes like Mount Adams. Caldera-forming eruptions are the type that created Crater Lake, Oregon, about 7,600 years ago. During this eruption, 50 km3 (12 mi3) of magma erupted explosively. Tephra fell from western Oregon to British Columbia, Alberta, and Wyoming. Pyroclastic flows swept out 30 to 60 km (20 to 40 mi) from the volcano. Fortunately, recent studies of the Mount Adams system conclude that conditions there are not conducive to such an eruption. Another type of eruption that is possible at Mount Adams is one like the 1980 eruption of Mount St. Helens -- specifically the large lateral blast that devastated more than 500 km2 (200 mi2) north of the volcano. Were it to occur at Mount Adams, a lateral blast of similar size and mobility could engulf a broad sector of the hazard zone outlined on plate 1 -- (Web note: not available in this format) -- . The irregular shape of the hazard zone reflects the topography of the surrounding area. The zone extends farthest from the volcano where elevations drop most steeply.
Scientists recognize several signs of impending volcanic eruptions. The upward movement of magma, or molten rock, into a volcano prior to an eruption causes changes that can be detected by geophysical measurements and visual observation. Swarms of small earthquakes are generated as rocks break to make room for rising magma or as heating of fluids causes underground pressures to increase. Heat from the magma can increase the temperature of ground water and boost temperatures and steaming from fumaroles, which are vents that emit volcanic gas; it can also generate small steam explosions. The composition of gases emitted by fumaroles can change as magma nears the surface. Injection of magma into the volcano can cause swelling or other types of surface deformation.
The regional seismic network operated jointly by the U.S. Geological Survey and the Geophysics Program at the University of Washington can detect earthquakes around Mount Adams. Currently, the Mount Adams region has very few small earthquakes compared to Mounts St. Helens, Rainier, and Hood. Thus an increase in the level of earthquake activity at Mount Adams would be noticed quickly. At monitored volcanoes similar to Mount Adams, a notable increase in seismicity typically has occurred days to months before the onset of eruptions.
Owing to Mount Adams' low level of eruptive activity in the recent geologic past, scientists have not deployed instruments to detect other types of precursory activity at the volcano. But an increase in seismicity would prompt the deployment of appropriate monitoring systems. Such a response would be conducted by scientists from the U.S. Geological Survey and other institutions.
Periods of unrest at volcanoes are usually times of great uncertainty. Although outstanding advances have been made in volcano monitoring and eruption forecasting over the past few decades, scientists are often able to make only very general statements about the probability, type, and scale of an impending eruption. Precursory activity can go through accelerating and decelerating phases, and sometimes die out without leading to eruption. Government officials and the public must realize the limitations in forecasting eruptions and be prepared for such uncertainty.
Some hazardous events at Mount Adams, including debris avalanches and related lahars, may have little or no advance warning. Onset of earthquakes and deformation related to eruption precursors would, however, increase the probability of debris avalanches, especially those of large size that have the greatest chance of impacting developed and settled areas.
Communities, businesses, and citizens can undertake several actions to mitigate the effects of future eruptions, debris avalanches, and lahars. Long-term mitigation includes using information about volcano hazards when making decisions about land use and siting of critical facilities. Development can avoid areas judged to have an unacceptably high risk. Or developments can be planned to reduce the level of risk, or even institute engineering measures to mitigate risk. For example, a real-estate development along a valley could set aside low-lying areas at greatest risk from lahars for open space or recreation, and use valley walls or high terraces for houses and businesses. In the Mount Adams region, much of the area in hazard zones for eruptive events lies in the Gifford Pinchot National Forest or remote areas of the Yakima Indian Reservation. Areas of greatest concern are located along the channels and flood plains of rivers that are subject to lahars. The relatively low population density in these higher- hazard areas simplifies the often complex economic and social aspects of hazard management. But these conditions also may increase the need for individuals who are at risk to know about volcano hazards and to make informed decisions on their own.
When volcanoes erupt or threaten to erupt, short-term emergency responses are needed. Such responses will be most effective if citizens and public officials have an understanding of volcano hazards and have planned the actions needed to protect communities. Because the time can be short between onset of precursory activity and an eruption (days to months), and because some hazardous events can occur without warning, suitable emergency plans should be made beforehand. Public officials need to consider issues such as public education, communications, and evacuations. Emergency plans already developed for floods may be applicable, with modifications, to hazards from lahars in valleys that head on Mount Adams.
Businesses and individuals should also make plans to deal with volcano emergencies. Planning is prudent because once an emergency begins, public resources can often be overwhelmed, and citizens may need to provide for themselves and make informed decisions. The Red Cross recommends numerous items that should be kept in homes, cars, and businesses for many types of emergencies that are much more probable than a volcanic eruption. Other items that will help include a map showing the closest access to high ground.
The most important additional item is knowledge about volcano hazards and, especially, a plan of action based on the relative safety of areas around home, school, and work. If your house is within a hazard zone for debris avalanches and lahars, a neighbor's house on a nearby hillside may be a good place to go if you learn that a hazardous event may be in progress. If you feel that you should evacuate, be certain that you don't move into a more hazardous area. Be aware of the location of the volcano and valleys that may be swept by lahars. The best strategy for avoiding the path of a lahar is to move to the highest possible ground. A safe height above river channels depends on many factors including size of the lahar, distance from the volcano, and shape of the valley. For areas around and downstream from the Trout Lake lowland, all but the largest lahars will probably rise no more than 50 m (160 ft) above river level. Once a lahar is flowing down a valley beyond the flanks of the volcano, its speed will likely be less than 50 kilometers per hour (30 miles per hour). Thus if higher ground is not accessible, a second option is to drive downvalley at a reasonable speed.
Hildreth, Wes, and Lanphere, M.A., 1994, Potassium-argon geochronology of a basalt-andesite-dacite arc system: The Mount Adams volcanic field, Cascade Range of southern Washington: Geological Society of America Bulletin, v. 106, p. 1413-1429. Hildreth, Wes, and Fierstein, Judy, 1995, Geologic map of the Mount Adams volcanic field, Cascade Range of southern Washington: U.S. Geological Survey Miscellaneous Investigations Series Map I-2460. Scott, K.M., and Vallance, J.W., 1995, Debris flow, debris avalanche, and flood hazards at and downstream from Mount Rainier, Washington: U.S. Geological Survey Hydrologic Investigations Atlas HA-729. Vallance, J.W., 1994, Postglacial lahars and potential hazards in the White Salmon River system on the southwest flank of Mount Adams, Washington: U.S. Geological Survey Open-File Report 94-440, 51 p.
U. S. Geological Survey Branch of Distribution P.O. Box 25286 Denver, CO 80225 (303) 202-4210
