1. 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 Baker is an active volcano. Its most recent activity was in the mid-1800's at a time when permanent populations around its base were few and infrastructures, such as roads, powerlines and other structures, were virtually non-existent. Although most of the area adjacent to Mount Baker is still largely unpopulated (much of the mountain is in the Mt. Baker-Snoqualmie National Forest), population patterns and infrastructure are much different than 150 years ago, and each year greater and greater numbers of people live and play in areas that could be affected by future volcanic activity. This report discusses the types of volcanic events that are likely to affect the region.
The primary purpose of this report is to provide planners, emergency management personnel, and federal and state agencies with information regarding eruptive and other hazardous geologic processes that will likely occur at Mount Baker in the future. Hopefully it will also be of interest to the general public. A hazard-zonation map accompanies this report and designates areas that will most likely be affected by such processes. Much of the geologic rationale for the hazard designations is from work by Hyde and Crandell (1978) and from ongoing hydrologic and geologic investigations by K. M. Scott and W. Hildreth.
Throughout this report a distinction is made between magmatic and nonmagmatic volcanic activity. Magmatic activity involves magma (molten rock and associated gases) reaching the surface whereas nonmagmatic activity does not. The reason for this distinction is that the movement of magma can usually be detected through volcano monitoring; therefore, there is generally some warning prior to a magmatic event. In the case of nonmagmatic events, such as the generation of debris flows, there is generally no movement of magma and an event may not be detected until it occurs. Thus volcanic activity not directly related to an eruption also poses a serious threat.
Mount Baker (3285 m; 10778 ft.) is an ice-clad volcano in the North Cascades of Washington State about 50 km (31 mi) due east of the city of Bellingham. After Mount Rainier, it is the most heavily glaciated of the Cascade volcanoes: the volume of snow and ice on Mount Baker (about 1.8 km3; 0.43 mi3) is greater than that of all the other Cascades volcanoes (except Rainier) combined. Isolated ridges of lava and hydrothermally altered rock, especially in the area of Sherman Crater, are exposed between glaciers on the upper flanks of the volcano: the lower flanks are steep and heavily vegetated. The volcano rests on a foundation of non-volcanic rocks in a region that is largely non-volcanic in origin.
The present-day cone is relatively young, perhaps less than 30,000 years old, but it sits atop a similar older volcanic cone called Black Buttes volcano which was active between 500,000 and 300,000 years ago. Much of Mount Baker's earlier geologic record was eroded away during the last ice age (which culminated 15,000- 20,000 years ago), by thick ice sheets that filled the valleys and covered much of the region. In the last 14,000 years, the area around the mountain has been largely ice free, but the mountain itself remains heavily mantled with snow and ice.
Deposits which record the last 14,000 years at Mount Baker indicate that Mount Baker has not had highly explosive eruptions like those of Mount St. Helens or Glacier Peak, nor has it erupted frequently. During this time period only four episodes of magmatic eruptive activity can be definitively recognized Magmatic eruptions have produced tephra, pyroclastic flows, and lava flows from summit vents and from the Schriebers Meadow cinder cone. However, the most destructive and most frequent events at Mount Baker have been debris flows and debris avalanches, many, if not most, of which were not related to magmatic activity but may have been induced by steam emissions, earthquakes, heavy rainfall, or in some other way.
Historical activity at Mount Baker includes several explosions during the mid-19th century, which were witnessed from the Bellingham area, and since the late 1950s, numerous small- volume debris avalanches. In 1975, increased fumarolic activity in the Sherman Crater area caused concern that an eruption might be imminent. Additional monitoring equipment was installed and several geophysical surveys were conducted to try to detect the movement of magma. The level of Baker Lake was lowered and people were restricted from the area due to concerns that an eruption- induced debris avalanche or debris flow might enter Baker Lake and displace enough water to either cause a wave to overtop the Upper Baker Dam or cause complete failure of the dam. However, few anomalies other than the increased heat flow were recorded during the geophysical surveys nor were any other precursory activities observed to indicate that magma was moving up into the volcano. An increased level of fumarolic activity has continued at Mount Baker from 1975 to the present, but there are no other changes that suggest that magma movement is involved.
Tephra consists of fragments of molten or solid rock which are ejected into the atmosphere and then fall back to the earth's surface. The fragments are usually carried away from the volcano by the wind. During magmatic eruptions, a volcano blasts the fragments into the atmosphere with tremendous force, forming a vertical eruption column. Eruption columns can be enormous in size and grow rapidly, reaching tens of kilometers (miles) in height and width in 30 minutes or less. As particles in the eruption column are carried downwind they form an eruption cloud or tephra plume (figure 1). Particles in the tephra plume begin to fall out of the plume almost immediately, with the larger and heavier particles falling out close to the volcano and progressively smaller and lighter particles falling out with increasing distance downwind. Thus, the distribution of tephra is largely controlled by the strength and direction of the wind during an eruption, whereas particle size and deposit thickness are largely controlled by how explosive the eruption is and the volume of material ejected.
Tephra hazards vary from a nuisance to life-threatening. Tephra plumes pose a serious hazard to aviation because particles in plumes can damage aircraft systems and jet engines, resulting in loss of power and damage to equipment. In addition, particles in a plume can sandblast aircraft windshields such that visibility is lost. On the ground, the hazards to life from tephra vary depending upon the amount that falls and the health of individuals. In general tephra hazards diminish downwind. High concentrations of tephra can make breathing difficult for people and livestock, and thick accumulations, especially if wet, can cause roofs of buildings to collapse, endangering inhabitants within. Minor amounts of tephra pose little threat to healthy individuals but may affect people with respiratory problems, the elderly, infants, and the infirm. Even minor tephra falls, however, can be detrimental to machinery (cars, lawn mowers, computers, etc.), can short out power transformers and electric lines, can be a nuisance to remove from roads and airports, can cause panic due to darkness during daylight hours, can cause traffic accidents because of reduced visibility, and can cause respiratory and eye problems for pets and livestock.
Data for wind direction and speed (figure 2) show that winds at an altitude between 3,000-16,000 m (10,000-50,000 ft) in the Mount Baker area are dominantly from the west with the percentage of time when winds are blowing from the north or south being fairly even. Winds blow from the east less than 10 percent of the time so that tephra from Mount Baker will normally be carried to the east away from major communities. Wind direction can be unpredictable however; wind patterns for Mount St. Helens are similar to those at Mount Baker, yet during 1980 two of the six major eruptions of Mount St. Helens took place during easterly winds, resulting in tephra fallout at both Olympia and Portland. Wind speeds are generally stronger from the west than from the east, so that tephra plumes may be carried farther downwind during times of westerly winds.
Volumetrically, tephra has been a minor component of eruptions from Mount Baker, and although definitive forecasting is impossible, it seems likely that future tephra eruptions will also be relatively small in volume. Three of the four known tephra deposits from Mount Baker are related to magmatic eruptions (table 1). Two of these tephras are from vents on Mount Baker and the other one is from an eruption of the Schriebers Meadow cone. Tephra from the fourth and youngest event consists mainly of altered and older volcanic rocks and it may not be related to a magmatic eruption, but to a steam blast associated with the formation of Sherman Crater (K. Scott, work in progress, 1995).
The largest tephra event at Mount Baker is poorly constrained in age (between 550 and 7600 years ago; table 1) and has an estimated volume on the order of 0.1-0.2 km3 (0.02-0.04 mi3) or about one-tenth the volume of tephra from the May 18, 1980 eruption of Mount St. Helens. Other tephra events at Mount Baker have been considerably smaller. To illustrate the amount of tephra an area downwind from Mount Baker might receive, a thickness versus distance plot for different sized eruptions is shown in figure 3. The plot shows that at distances of 50 km (31 mi), or about the distance of Bellingham from Mount Baker, thicknesses of tephra from a 0.1 km3> (0.02 mi3 event are on the order of 6 cm (about 2 in). For an event of 0.01 km3 (0.002 mi3;) thicknesses at 50 km are less than 2 cm (about 0.5 in). Figure 4 illustrates the possible distribution of tephra from an eruption with a volume of 0.08 km3. In this example, the data are transposed from Mount Rainier where details regarding thickness and distribution of a tephra deposit of this size are well known. During this eruption, the winds were from the west, but during a future eruption the winds could be from any direction. (The shaded area in figure 4 can be rotated around the summit to see what the thickness and distribution would be like if winds came from some other direction.) It should be noted that tephra accumulations would occur beyond the shaded area, but would be less than 1 centimeter (less than 0.4 in) in thickness.
There are two sources of tephra hazards for people living in the vicinity of Mount Baker: one is from eruptions of Mount Baker itself, the other is from eruptions of more distal and more explosive volcanoes in the Cascades. Figures 5a and 5b shows the annual probability of an area receiving tephra from Mount Baker or from an eruption from another Cascade volcano in the United States, respectively. As can be seen from the plots, residents in the Bellingham area have a greater chance of receiving tephra from a distant volcano as from Mount Baker. Both probabilities, however, are relatively low - on the order of 1 chance in 5,000 to 1 chance in 100,000 for any given year (however, still better than the odds of winning the lottery jackpot).
Assessment of volcano hazards at Mount Baker is based on the philosophy that future volcanic activity is most likely to be similar to what has happened in the past. The time period since settlers have come to the area is too brief to serve as the basis for estimating the future behavior of the volcano which is hundreds of thousands of years old. Fortunately, at least some of the record of prehistoric eruptions and events is preserved in the deposits they produced. Such deposits can be mapped, studied, and dated in order to learn about the types and frequencies of past events and then to identify areas that could be affected by future events. At Mount Baker, many of the deposits older than 14,000 years were eroded away by ice sheets and so the past 14,000 years is assumed to be representative of the type of activity that has occurred throughout the volcano's lifetime.
Areas designated as hazardous are delineated on the basis of past eruptive events as well as topography, degree of alteration of the volcano (to help determine the likelihood of a debris avalanche), and knowledge of comparable eruptive phenomena at other volcanoes. Hazards are depicted in all drainages that begin high on Mount Baker - whether or not deposits of past events are preserved there. Thus, unless protected by topographic barriers, any valley starting high on Mount Baker could be affected during the next eruption.
The accompanying hazard maps shows areas that could be affected by future flowage hazards such as debris flows, debris avalanches, lava flows, pyroclastic flows, and pyroclastic surges. Tephra hazards are shown in figures 5a and 5b and a lateral blast hazard map is shown in figure 6. It is important to recognize that the degree of hazard does not change abruptly at the hazard-zone boundaries. Rather, the level of hazard typically decreases gradually as one moves away from the source area, or in the case of debris flows, as one moves above the valley floor. Areas immediately outside hazard-zone boundaries should not be regarded as hazard free, because many of the boundaries can only be approximately located, especially in areas of low relief. Too many uncertainties exist about the size, mobility, and source of future events to definitively locate hazard-zone boundaries.
Tephra hazard maps, shown in figures 5a and 5b, show the annual probabilities of a tephra fall of 1 cm (about 0.4 in) or more from an eruption at Mount Baker or another Cascade volcano. The data base for figure 5a (an eruption from Mount Baker) includes all tephra falls from Mount Baker in the last 10,000 years and assumes present day wind directions. The data base for figure 5b includes tephra falls for all U. S. Cascade volcanoes during the last 10,000 years, and again assumes present day wind directions. The patterns for both figures are keyed to scales shown at the right of each map. A 0.002% probability means that there is 1 chance in 50,000 (1/50,000 x 100) that the area shaded with that pattern will experience an accumulation of 1 cm (about 0.4 in) or more of tephra during any given year.
VOLCANIC MONITORING AND ERUPTION RESPONSE
Future magmatic eruptions at Mount Baker are likely to be preceded by changes at the volcano that can be detected by modern volcano-monitoring techniques. Magma moving up into a volcanic edifice causes rock fracturing, deforms the ground surface, and releases magmatic gases. Therefore, volcanic seismicity (earthquakes), deformation, and gas studies are the principal monitoring tools that the U. S. Geological Survey (USGS) employs to detect magma movement. In conjunction with the University of Washington's Geophysics Program, the USGS operates and continuously receives data from a network of seismometers on and around Mount Baker. Deformation measurements, that could detect magma movement within the volcano, is done to provide baseline information on the state of Mount Baker. Gas measurements and fumarole temperatures have been measured sporadically at Mount Baker since the early 1970's to detect changes in gas composition or increases in temperature, both of which may accompany movement of magma to shallow levels.
If one or more of these techniques were to show consistently anomalous behavior indicative of magma movement, additional seismic, deformation, and gas monitoring would be initiated. If the evidence indicated that conditions were developing that might lead to an eruption, USGS crews would begin monitoring the volcano on a round-the-clock basis and the status of the volcano would be communicated as often as necessary to appropriate officials at Federal, State, County, and local levels - usually through a coordinating agency. If an eruption appeared imminent and during an eruptive crisis, updates regarding the status of the volcano and anticipated tephra plume paths based on wind forecasts would be issued by the USGS at least daily to the above groups and to the aviation community. Hazard maps and delineation of hazard zones would be updated as new information dictates. If an eruption occurred, notification of the eruption would be sent out immediately to the coordinating agency and other concerned groups. Equally important, these groups would be notified of the cessation of an eruption as soon as practical; monitoring of the volcano and tracking of the tephra plume would continued for as long as the hazards persisted. Such full-scale monitoring and hazard communication would continue throughout any period of intense volcanic unrest until the monitoring evidence indicated that further activity was no longer a threat.
The onset of eruptive activity differs from volcano to volcano. The range in lead time from the start of anomalous (mostly seismic) behavior to an eruption for some well-monitored volcanoes was 2 months for the 1980 eruption of Mount St. Helens; 24 hours for the 1989-1990 eruption of Redoubt, Alaska; 2.5 months for the 1991 eruption of Pinatubo Volcano in the Philippines; and 10 months for the 1992 eruption of Crater Peak (Mount Spurr), Alaska. Because lead times prior to volcanic crises may be on the order of only a day to a few months, it is important that coordination among officials occur and decisions regarding the roles of the various agencies be made before a crisis begins.
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