Volcano Hazards in the Mount Adams Region, Washington

Volcano Hazards in the Mount Adams Region, Washington By William E. Scott, Richard M. Iverson, James W. Vallance, and Wes Hildreth INTRODUCTION than 200 km (120 mi) downwind with ash deposits Mount Adams, one of the largest volcanoes in several centimeters or inches thick, but those at Mount the Cascade Range, dominates the Mount Adams Adams have blanketed only areas a few kilometers away with a similar thickness of ash. Nonetheless, volcanic field in Skamania, Yakima, Klickitat, and despite their low level of explosivity, eruptions at Lewis counties and the Yakima Indian Reserva- Mount Adams are hazardous. More importantly, even tion of south-central Washington. The nearby during times of no eruptive activity, landslides of Indian Heaven and Simcoe Mountains volcanic weakened rock that originate on the steep upper flanks fields lie west and southeast, respectively, of the of Mount Adams can spawn lahars, which are watery 1250-km2 (500-mi2) Adams field (plate1). Even flows of volcanic rocks and mud that surge down- though Mount Adams has been less active during stream like rapidly flowing concrete. Lahars -- also the past few thousand years than neighboring known as mudflows or debris flows -- can devastate Mounts St. Helens, Rainier, and Hood, it assur- valley floors tens of kilometers from the volcano. edly will erupt again. Future eruptions will probably occur more frequently from vents on the summit and upper flanks of Mount Adams than Lava flows are the most likely type of from vents scattered in the volcanic fields beyond. future eruptive event Large landslides and lahars that need not be related to eruptions probably pose the most The dominant type of eruption at Mount Adams, destructive, far-reaching hazard of Mount Adams. as well as in the adjacent volcanic fields, produces The purpose of these maps and booklet is to (1) lava flows, or streams of molten rock. Plate 1 shows describe the kinds of hazardous geologic events lava flows that have occurred in the region during the past 10,000 years: eight flows issued from vents on that will likely occur in the future at Mount Mount Adams and covered an area of about 50 km2 Adams and at other volcanoes in the region, (2) (20 mi2); one large lava flow erupted in the Indian outline the areas that will most likely be affected Heaven volcanic field about 30 km (20 mi) southwest by these events, and (3) recommend actions that of Mount Adams and covered a similar-sized area; individuals and government agencies can take to another flow erupted from a vent about 25 km (15 mi) protect lives and property. west of Indian Heaven and covered about 10 km2 (4 mi2). These and older lava flows typically travelled less HAZARDOUS EVENTS AT MOUNT ADAMS than 20 km (12 mi) from vents, but in rare cases more voluminous flows reached lengths of 25 to almost 50 Volcanoes pose a variety of geologic hazards -- km (15 to 30 mi). Typical lava flows on the lower both during eruptions and in the absence of eruptive flanks of Mount Adams and elsewhere in the volcanic activity. During much of its history, Mount Adams fields spread onto gentle slopes and funnelled into val- has displayed a relatively limited range of eruptive leys. The moving flows were several meters (tens of styles. Highly explosive eruptions have been rare. feet) to more than 30 m (100 ft) thick and consisted of Compared to the tens of large explosive eruptions at crusty lava blocks enclosing a more fluid, molten nearby Mount St. Helens during the past 20,000 years, core. Their steep fronts advanced quite slowly--at eruptions of Mount Adams have been meek. Erup- most only about 100 m (330 ft) per hour, much more tions at Mount St. Helens have blanketed areas more slowly than people typically walk. Even so, lava 1 flows can bury, crush, or burn all structures in their Even minor tephra falls can be disruptive paths, and hot lava boulders cascading off flow mar- and hazardous gins can endanger spectators and start forest fires. A typical eruption consists of extrusion of a single lava Small explosions that accompanied past lava-flow flow over a period of days or weeks or of a sequence eruptions at Mount Adams and other volcanoes in the of several flows erupted over weeks to a few years. region were strong enough to hurl lava blocks from Intermittent eruptions over years to decades might vents, and probably created clouds of tephra that rose build a broad apron of lava flows on a flank of Mount thousands of meters into the atmosphere. In deposit- Adams or even build a separate small volcano several ing only a few millimeters of tephra for tens or, rarely, hundred meters high (300 m equals 1,000 ft) and 10 a few hundred kilometers downwind, such clouds km (6 mi) or more in diameter. offer little threat to life or structures. But tephra Explosions and gases streaming from the lava- clouds can create tens of minutes to hours of darkness flow vents can hurl upward fragments of rock and lava as they pass over a downwind area, even on sunny that range in size from large blocks to fine dust. Such days, and reduce visibility on highways. Deposits of material is called tephra by volcanologists. The tephra can short-circuit electric transformers and coarser particles fall to the ground around the vent and power lines, especially if the tephra is wet, which form a deposit that ranges from a blanket less than 1 makes it highly conductive, sticky, and heavy. Tephra meter (3 ft) thick to a pile, called a cinder cone, more injested by vehicle engines can clog filters and than 100 m (330 ft) high. Finer tephra fragments the increase wear. Tephra clouds often generate lightning size of sand and silt grains, called ash, can be carried that can interfere with electrical and communication downwind and form deposits that thinly mantle areas systems and start fires. Finally, and perhaps most tens to hundreds of kilometers away. importantly, even small, dilute tephra clouds pose a significant hazard to aircraft that fly into them. As More violent eruptions can occur from vents explained below, other volcanoes, especially Mount beneath lakes or in areas with high water tables as St. Helens, do produce large explosive eruptions, and magma interacts explosively with water. The explo- these volcanoes pose the greatest threat of significant sions can propel large rock fragments several kilome- tephra fall in the region. ters and generate pyroclastic flows--flows of hot The lessons learned in Washington communities rocks, ash, and gases, which can move rapidly out- such as Yakima, Ritzville, and Spokane during the ward more than 10 km (6 mi). 1980 eruption of Mount St. Helens are used through- out the Pacific Northwest and elsewhere in the world to prepare governments, businesses, and citizens for Lava flows can melt snow and ice and future tephra falls. All three communities experienced cause pyroclastic flows, lahars, and significant disruptions in transportation, business floods 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 Lava flows extruded on snow or ice-covered ter- tephra that fell, the longer a community took to rain can generate sufficient meltwater to cause small recover. As perceived by residents, tephra falls of less lahars and floods. More dangerous, however, is extru- than 0.5 cm (1/4 in) were a major inconvenience, sion on terrain so steep or icy that the lava flow breaks whereas falls of more than 1.5 cm (2/3 in) constituted apart and produces avalanches of hot lava fragments. a disaster. Nonetheless, all three communities recov- Such avalanches from near the summit of Mount ered to nearly normal activities within two weeks. Adams can form pyroclastic flows that may travel downslope as far as 15 km (9 mi). Pyroclastic flows can erode and melt large quantities of ice and snow, Landslides and lahars not necessarily transform into lahars, or produce water floods. These related to eruptions pose the most lahars or floods can then travel tens of kilometers far- serious threat ther down valleys. Pyroclastic flows formed by avalanches from hot lava flows have been rarer at Mount Mount Adams has erupted little during the past Adams during the past 10,000 years than at Mount 10,000 years, but most of the present volcanic cone Hood and other nearby volcanoes. was formed during a preceding 30,000-year period of 2 rapid, eruptive growth. During and since its forma- isolated blocks (some more than 5 m (16 ft) in diame- tion, much of the upper part of the cone has been ter) that protrude from fields and meadows (see cover eroded by glaciers to form steep, unstable slopes. photograph). Ground water warmed by residual heat and acidified No geologic evidence links Mount Adams' debris by volcanic gases has circulated through porous zones avalanches and lahars to eruptive activity. Therefore, and weakened the rocks that compose roughly two these extremely hazardous events can occur without cubic kilometers (one-half cubic mile) of Mount nature's warnings that typically signal the onset of Adams' summit by altering parts of the rocks to mud. volcanic unrest. However, the onset of volcanic These steep, weakened areas are prone to failures that unrest, with its earthquakes and steam explosions, spawn rapidly moving landslides called debris ava- unquestionably increases the likelihood of debris avalanches. Debris avalanches can attain speeds that lanches and enhances the probability of a catastrophic exceed 160 km/hr (100 mi/hr), and the largest ava- event that would have serious consequences in areas lanches can sweep down valleys more than 50 km (30 far downstream from the volcano.

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