Eruptive Histories and Volcanic Hazards Assessments at Select Cascade Range Volcanoes.
Mount Baker, Washington
Eruptive HistoryMount Baker is a Pleistocene stratovolcano of chiefly andesite lava flows and pyroclastic debris that overlaps rocks of an older eruptive center (Coombs, 1939), which are dated at about 400,000 yr (Easterbrook and Rahm, 1970). Construction of the present cone was largely completed prior to late Wisconsin glaciation. Postglacial events are dominated by debris avalanches and lahars, which repeatedly flowed down valleys that head on the volcano. Eruptions also produced tephra falls, lava flows from both summit and satellite vents, and pyroclastic flows and accompanying pyroclastic surges (Hyde and Crandell, 1978). Many of the debris avalanches and lahars are not directly associated with evidence of magmatic eruptions, and may have been triggered by minor phreatic eruptions, earthquakes, or in other ways.
Historical activity of Mount Baker includes several explosions during the mid-19th century (Coombs and Howard, 1960) and numerous small-volume debris avalanches between 1958 and 1979 (Frank and others, 1975; Frank, 1983). Beginning in 1975, heightened thermal activity manifested by increased fumarolic emission and by melting of ice and snow near the summit caused concern that an eruption might be imminent (Frank and others, 1977).
Volcanic-hazards assessmentThe postglacial record of activity at Mount Baker indicates that the greatest potential hazard is from debris avalanches and lahars of hydrothermally altered material and related floods in the valleys draining the volcano (Frank and others, 1977; Hyde and Crandell, 1978; Frank, 1983). Large-volume events of these types have repeatedly affected the valleys; the mean frequency of these events during the past 600 yr has been about once per 150 yr (Hyde and Crandell, 1978). Parts of valleys on and close to the volcano have been affected much more frequently. Large floods related to lahars or other types of eruptive activity could inundate the flood plains of the Nooksack and Skagit Rivers to their mouths. However, the hydroelectric dams and reservoirs on the Baker River (a tributary of the Skagit) could trap debris avalanches, lahars, and floods and reduce hazards in downstream areas (Hyde and Crandell, 1978). An analysis by Shreve (in Frank and others, 1977) indicates that debris avalanches and lahars of similar size to those of postglacial age would probably not generate waves high enough to overtop the dam of Baker Lake, if the reservoir were at a low to intermediate level.
An event of low probability, but of potentially serious consequences, would be a debris avalanche or lahar of a volume unprecedented at Mount Baker in postglacial time (Frank, 1983). Such an event would require disrupting a large part of the edifice at one time. The potentially huge volume of hydrothermally altered, clay-rich material present in the volcano and the continual production of such material, coupled with steep slopes and great topographic relief, suggest that such an event is possible. An avalanche and lahar at Mount Baker could be as large as the 600-yr-old Electron Mudflow at Mount Rainier, which had a volume of more than 0.15 km3 and inundated valley bottoms more than 50 km away (Crandell, 1971). If an avalanche or lahar of similar or larger volume were to descend the east flank of the volcano and enter Baker Lake while the reservoir level were high, a wave large enough to overtop the dam might be generated, which would have catastrophic consequences downstream. Lava flows, pyroclastic flows, and pyroclastic surges have occurred less frequently at Mount Baker than debris avalanches and lahars and therefore are considered less likely to occur in the future (Hyde and Crandell, 1978). During postglacial eruptions, lava flows have been confined to within 15 km of the volcano (Fig. 4-4; Hyde and Crandell, 1978). Future lava flows are unlikely to extend much farther unless they erupt from satellitic vents on the distal flanks of the volcano. Hyde and Crandell (1978) show areas that could be affected by future pyroclastic flows and surges limited to within 15-25 km of the volcano based on the extent of postglacial deposits they recognized. Heller and Dethier (1981) found a postglacial pyroclastic-flow deposit in the lower Baker River valley which suggested to them that the hazard zone of Hyde and Crandell (1978) should be extended by 5 km.
One tephra eruption at Mount Baker in postglacial time had an estimated volume of 0.1-0.2 km3;
two or three others had volumes
less than 0.1 km3. Hyde and Crandell (1978) estimate that beyond 50 km from the volcano,
tephra thicknesses from future eruptions of similar size will probably not exceed 5 cm. Based on its past
activity (one tephra eruption equal to or exceeding 0.1 km3 in the past 12,000 yr), the probability of such an
explosive eruption in any one year is about 1 X 10-4. The dominantly andesitic composition of Mount Baker
products suggests that tephra eruptions of more than a few tenths of a cubic kilometer are much less likely
than at volcanoes that erupt more silicic magmas.
Glacier Peak, Washington
Eruptive HistoryGlacier Peak, geographically the most remote of the Cascade volcanoes, is a Pleistocene and Holocene composite volcano composed chiefly of dacite, with a minor amount of basalt erupted from satellitic vents (Tabor and Crowder, 1969; Beget, 1982, 1983). Large explosive eruptions about 11,000- 12,000 yr ago produced: (1) two tephra-fall deposits of large (>1 km3, dense-rock equivalent) volume, which are widely distributed east of the volcano (Lemke and others, 1975; Porter, 1978; Sarna- Wojcicki and others, 1983; Mehringer and others, 1984), (2) seven tephra falls of small (0.01-0.1 km3) volume (Porter, 1978), and (3) many pyroclastic-flow deposits and lahars that form thick (locally >100 m) fills in the valleys that head on the volcano (Tabor and Crowder, 1969; Beget, 1982, 1983). The two large tephra eruptions were separated in time by probably no more than a few centuries (Mehringer and others, 1984). Tephra of each eruption is about 1 m thick at a distance of 50 km downwind from the volcano, and about 0.5 m thick at a distance of 70 km (Porter, 1978). These deposits represent two of the largest Cascade tephra eruptions of postglacial time, although they are less voluminous than the tephra fall that accompanied the climactic eruption of Mount Mazama (about 34 km3, dense-rock equivalent).
Pyroclastic flows associated with the eruptive period of 11,000-12,000 yr ago traveled as far as 15 km from the volcano, and lahars reached areas along the Stillaguamish and Skagit Rivers more than 100 km from the volcano (Beget, 1982, 1983).
Beget (1982, 1983) also describes Holocene eruptions, associated with dome extrusion near the summit, which produced lahars, pyroclastic flows, and minor tephra. The tephra and pyroclastic flows were less extensive than those of the eruptive period of 11,000-12,000 yr ago. Several Holocene lahars extended tens of kilometers downvalley, and two reached distances of more than 100 km.
Volcanic-hazards AssessmentBeget (1982, 1983) proposed tephra-hazard and flowage-hazard zones for future eruptions at Glacier Peak that were based on postglacial volcanic activity and the frequency with which areas had been affected by volcanic events. The zones of highest hazard from lahars and pyroclastic flows extend 25-30 km from the volcano down valleys that have been affected by these phenomena at least as often as once per 1000-3000 yr. A zone of intermediate hazard from lahars extends another 30 km downstream to the Skagit River. This zone has been inundated several times during the past 3000-6000 yr. Zones of low hazard from lahars include the lower 70 km of the Skagit River valley and its delta, and the entire Stillaguamish River valley. These areas were affected by lahars and floods during the major eruptive period of 11,000-12,000 yr ago; the lower Skagit was also affected by lahars about 5000 yr ago.
Future eruptions of large volume are likely to form thick fills of lahars and pyroclastic-flow deposits in the upper parts of valleys that head on the volcano. Subsequent incision of these deposits would aggrade valley floors farther downstream with sediment for many years after the eruption, thereby affecting the capacity of stream channels and locally increasing heights of floods. These effects would be especially significant for the extensive low-lying areas of the Skagit River flood plain and delta.
The tephra-hazard zones of Beget (1982, 1983) suggest that the most likely volume of future tephra eruptions would deposit no more than a few centimeters of tephra at a distance of 50 km. A large tephra eruption similar to those of 11,000-12,000 yr ago, which could deposit 1 m of tephra at 50 km, is less likely. On the basis of postglacial activity, the annual probability of such an eruption is about 2 X 10-4. Glacier Peak's past behavior and composition of eruptive products suggest that it could erupt enough tephra to affect hundreds of thousands of square kilometers.
The risks to people and property from catastrophic lateral blasts, which could affect broad areas
around the volcano out to 35 km, are low at Glacier Peak because of its remote location.
Mount Rainier, Washington
Eruptive historyThe construction of Mount Rainier volcano probably began in early or middle Pleistocene time (Crandell, 1963; Crandell and Miller, 1974). Multiple lahars, alluvium, and layers of volcanic ash, all presumably derived from Mount Rainier or its ancestor, are interbedded with glacial deposits in the southeastern Puget Sound lowland. Some of these deposits have reversed magnetic polarities and are older than an 840,000-yr-old layer of volcanic ash (Easterbrook and others, 1981; Easterbrook and others, 1985). The bulk of the present cone consists of andesite lava flows (Fiske and others, 1963) with normal magnetic polarity, and are therefore probably less than 730,000 yr old; limited K-Ar dating supports this conclusion (Crandell and Miller, 1974). Erosion, chiefly by glaciers and debris avalanches, has predominated over cone building during the past 100,000 yr at Mount Rainier.
Postglacial eruptive activity at Mount Rainier produced 11 layers of tephra, at least one pyroclastic flow that was restricted to within 10-15 km of the summit, and the summit lava cone (Crandell and Mullineaux, 1967; Crandell, 1969; Mullineaux, 1974). More than 60 debris avalanches and lahars swept down valleys heading on the volcano; the largest reached an arm of Puget Sound more than 100 km away (Crandell, 1971). Many of the large debris avalanches and lahars contain much hydrothermally altered material (Crandell, 1971); such material is still being produced by the volcano's active hydrothermal system (Frank, 1985).
The last major eruptive period at Mount Rainier occurred between about 2,500 and 2,200 years ago and produced a lava cone at the summit of the volcano, many lahars in several valleys, a pyroclastic flow on the west side of the volcano, and the most voluminous Mount Rainier tephra of postglacial time (Crandell and Mullineaux, 1967; Crandell, 1969). The tephra is about 15 cm thick 12 km east of the summit of the volcano and 8 cm at 25 km. Mullineaux (1974) estimated its original volume to be about 0.30 km3.
The largest known explosive eruption of Mount Rainier occurred between about 30,000 and 100,000 years ago and is recorded by a pumice deposit that has been recognized northeast, east, and southeast of the volcano. The deposit is about 2 m thick at a site 12 km northeast of the present summit (D.R. Crandell, written commun., 1986). Its distribution and thickness farther east are not known, nor is it known whether the thickness of 2 m occurs along the axis of the lobe. This thickness at 12 km is greater than that of tephra layer Yn at a similar distance from Mount St. Helens (Mullineaux, 1986) but less than that of layers B and G from Glacier Peak (Porter, 1978). Layers Yn, B, and G all have estimated volumes equal to or more than 1 km3 (Crandell and Mullineaux, 1978; Porter, 1978). The limited thickness data for the tephra layer at Mount Rainier suggest that it may have had a comparable volume (1-10 km3). Therefore, the volume of this late Pleistocene tephra is probably at least one order of magnitude greater than that of the most voluminous tephra of postglacial age.
Volcanic-hazards assessmentEleven pumice-producing eruptions occurred at Mount Rainier during postglacial time (Mullineaux, 1974). The most likely future eruptive event, based on the Holocene history of the volcano, is a tephra eruption of small volume (Crandell, 1973), probably between 0.01 and 0.1 km3. The probability of such an eruption in any one year is about 1 X 10-3. The effects of such an eruption would be minor beyond a distance of 50 km. The annual probability of an explosive eruption producing more than 0.1 km3 of tephra, which would have serious effects beyond 50 km, is about 1 X 10-4.
An explosive eruption like that between 30,000 and 100,000 yr ago at Mount Rainier is even less likely. Other explosive eruptions may have occurred during that period, but their products have been removed or buried. The likelihood of such an eruption in any one future year is less than 1 X 10-4, and may be as low as 1 X 10-5.
The most likely future hazardous events at Mount Rainier are debris avalanches, lahars, and floods like those of the past that have repeatedly swept down the valleys heading on the volcano (Crandell and Mullineaux, 1967; Crandell, 1973). The frequency with which lahars have affected areas more than 20 km from the volcano, suggests that the annual probability of such an event is about 1 X 10-3. Larger, more hazardous events extending to distances of more than 50 km have an annual probability one order of magnitude less (1 X 10-4). Large-volume events such as these could reach beyond the mountain front into the Puget Sound lowland and could inundate tens to hundreds of square kilometers in relatively densely populated areas.
The largest debris avalanches and lahars at Mount Rainier originated from parts of the volcano that contained large volumes of hydrothermally altered material (Crandell, 1971). Frank (1985) concludes that the upper west flank and the summit provide the largest potential sources of this material. This distribution suggests that any side of the volcano could be affected, but that the valleys that head on the west and northeast sides of the volcano are particularly vulnerable to large debris avalanches and lahars. Debris avalanches of large volume probably are most likely during eruptions but could also occur during dormant periods (Crandell, 1971; Frank, 1985).
All of the major rivers that drain Mount Rainier, except the Puyallup-Carbon, are dammed at
distances that range from 40-80 km downvalley from the summit. If reservoirs were empty or nearly so,
these dams could contain all but the very largest expectable lahars and floods.
Mount St. Helens, Washington
Eruptive HistoryMount St. Helens is among the youngest of the major Cascade volcanoes, has been the most active of these in postglacial time, and is currently the only erupting volcano in the range. The following summary of the eruptive history of the volcano is taken Mullineaux and Crandell (1981), Mullineaux (1986), and Crandell (need REF!), who in part summarize numerous previous reports. From its inception about 40,000 yr ago until about 2500 yr ago, Mount St. Helens erupted chiefly dacite and minor silicic andesite; since 2500 yr ago the volcano has produced a more diverse suite ranging from basalt to dacite.
Three eruptive stages are recognized before about 4500 yr ago (Crandell, NEED REF!). Each lasted several thousand years and was separated from the preceding stage by an apparently dormant interval that lasted several thousand years. The current, or Spirit Lake, eruptive stage began about 4500 yr ago. It consists of seven eruptive periods from several decades to several centuries long separated by apparently dormant intervals as long as 650 yr. The current eruptive period followed a dormant interval of 123 yr. Eruptive stages that preceded the Spirit Lake stage also included alternating eruptive periods and dormant intervals.
Although somewhat different patterns and character of activity occurred during each of Mount St. Helens eruptive periods, they share many similarities. All included explosive eruptions of tephra. These varied from eruptions whose effects were negligible beyond a few tens of kilometers to eruptions that deposited several tens of centimeters of tephra 50 km from the volcano and several centimeters hundreds of kilometers away. Tephra layer Yn, which was erupted about 3500 yr ago, is, after Mazama ash, the most voluminous Holocene tephra in the Cascade Range. Most eruptive periods included pyroclastic flows and related pyroclastic surges that swept several kilometers to almost 20 km down valleys.
Lahars inundated valley bottoms tens of kilometers from the volcano and, during a few eruptive periods, reached the Columbia River more than 100 km away. Extrusion of lava domes probably occurred during each eruptive period. Dome building was restricted to the summit and flanks within 3 km of the summit; however, the indirect effects of dome building, such as pyroclastic flows and lahars, extended far beyond the flanks of the volcano. Lava flows were produced mainly within the last 2500 yr; these affected areas chiefly within 10 km of the volcano, although two lava flows reached almost 20 km away.
The initial highly explosive activity of the present eruptive period, which began on May 18, 1980 following several months of small phreatic explosions, included a great debris avalanche and lateral blast that devastated areas in a broad sector 20-30 km north of the volcano (Christiansen and Peterson, 1981). Lahars reached the Columbia River by way of the Toutle and Cowlitz Rivers and obstructed the deep-water shipping channel. Smaller lahars were generated in all of the other drainages heading on the volcano. Tephra was deposited over a broad region east of the volcano, adversely affecting people as far away as eastern Montana. Pyroclastic flows extended northward as far as 7.5 km from the vent. Activity after May 18 included small explosive eruptions of tephra and pyroclastic flows, generation of lahars, and growth of a composite lava dome in the volcano's new crater.
Volcanic-Hazards AssessmentThe initial volcanic-hazards assessment for Mount St. Helens (Crandell and others, 1975; Crandell and Mullineaux, 1978)--based on the eruptive events of the past 4500 yr--accurately anticipated the effects of most of the eruptions of the present eruptive period. Exceptions were the unprecedented magnitude (for Mount St. Helens) of the debris avalanche and lateral blast of May 18, 1980 (Miller and others, 1981). The events of May 18, which formed a large crater open to the north, drastically altered the form of the volcano. This prompted a revised hazard assessment (Miller and others, 1981), which pointed out the relatively greater hazard from flowage events and lateral explosions on the north flank of the volcano than in other sectors. Subsequently, Scott (1986) determined lahar-inundation frequencies for the Toutle-Cowlitz flood plains. In addition, Newhall (1982, 1984) developed a method for quantifying the risks to people in areas around the volcano from future eruptions.
Based on the past behavior of Mount St. Helens, the most likely events to occur during the course of the present eruptive period include dome building with related pyroclastic flows, minor tephra eruptions, and generation of lahars and floods. These events will chiefly affect areas along the valley of the North Fork Toutle River. Other likely events include extrusion of lava flows and explosive eruptions of tephra and related pyroclastic flows.
During past eruptive periods, such as the Kalama period of 500-350 yr ago (Hoblitt and others, 1980), the mean annual frequency of lahars and pyroclastic flows affecting areas within 10-20 km of the volcano increased greatly over the longer term mean annual frequency, which included repose intervals as well as eruptive periods. Annual probabilities of such events estimated from the Kalama period, which we infer are applicable to the present eruptive period, are more than 1 X 10-1.
Voluminous lahars, which occur less often, could affect areas far beyond a distance of 20 km. A recent study of the Toutle River valley (Scott, 1986) concludes that lahars or lahar-runout flows large enough to inundate flood plains 50 km or more from the volcano have an annual probability of at least 1 X 10-2. Generation of lahars in the Toutle River basin and erosion of deposits of the current eruptive period will continue to aggrade river channels and flood plains farther downstream and will increase flood peaks in the lower Toutle and Cowlitz River valleys and in the Columbia River near the mouth of the Cowlitz (U.S. Army Corps of Engineers, 1984).
In the near term (the next 1-10 yr), large explosive eruptions with widespread tephra fall and pyroclastic flows are less likely than continued dome building. The annual probability of a large (>0.1 km3) explosive eruption is about 1.5 X 10-3 for the past 4500 yr and 8 X 10-3 for the past 500 yr. It should be noted that two large tephra-producing eruptions occurred just 2 yr apart, in 1480 and 1482 A.D., during the Kalama eruptive period (Yamaguchi, 1985). Voluminous tephra deposits would result from high eruption columns; tephra falling back onto the volcano from such columns would increase the likelihood of pyroclastic flows and lahars affecting all flanks of the volcano.
The Kalama and Lewis River valleys could also be affected by eruptions if the present dome continued to grow and eventually filled the crater. Pyroclastic flows from the upper part of the dome could then descend other flanks of the volcano. If such pyroclastic flows were as large as ones of the Swift Creek period, those on the south side could move down the valley of Swift Creek and enter Swift Creek Reservoir. Pyroclastic flows could also melt snow on and beyond the south flank of the volcano and generate lahars and floods that could reach the reservoir.
The Lewis River is dammed at three sites to form large reservoirs, which could be used to trap all
but the very largest expectable floods and lahars provided they were drawn down in time (Crandell and
Mullineaux, 1978). The flood effects of a failure of the Lewis River dams have been assessed for the Trojan
Nuclear Plant (Portland General Electric, 1980), which lies on the west bank of the Columbia River across
from the mouth of the Kalama River. That report concludes that the flood height at the plant site would be
less than the design flood height.
Mount Adams, Washington
Eruptive HistoryMount Adams is composed of lava flows and fragmental rocks of basaltic andesite and andesite; numerous satellitic vents on the flanks of the volcano have erupted rocks ranging from basalt to dacite (Hildreth and others, 1983). Most of the main cone is younger than 220,000 yr. Seven postglacial lava flows issued from flank vents (Hildreth and others, 1983), the youngest of which is between 6850 and 3500 yr old (J. W. Vallance, personal commun., 1986). Debris avalanches and lahars affected several valleys around the volcano during postglacial time; the longest lahar extended at least 52 km from the volcano (Hopkins, 1976; Vallance, 1986). A large amount of hydrothermally altered material in this and one other lahar and in one debris avalanche implies they originated as avalanches of wet, altered, clay-rich debris from near the summit (Vallance, 1986). The youngest such event was a debris avalanche that descended the southwest flank in 1921 A.D. Numerous debris flows generated by glacial and meteorologic processes occur frequently at Mount Adams, but typically affect areas within only a few kilometers of the volcano (J. W. Vallance, personal commun., 1987). Postglacial eruptions and weak, diffuse fumarolic emissions in the summit area suggest that the volcano is capable of erupting again (Hildreth and Miller, 1984).
Volcanic-Hazards AssessmentThe postglacial record of volcanic activity at Mount Adams indicates that the most likely events to occur in the future are eruptions from flank vents to form scoria cones and lava flows, and debris avalanches and lahars (Vallance, 1986). Lava flows would probably extend no farther than 10 km from their vents, which might lie as far as 10 km from the summit of the volcano. Therefore, the direct effects of lava flows are likely to be restricted to within 20 km of the summit of the volcano. Lava-flow eruptions, especially those on the upper part of the cone, could generate lahars and floods by melting ice and snow, and thereby affect valley bottoms far downstream.
Debris avalanches and lahars have originated on the southwest flank of Mount Adams with a mean frequency of about one per 1500 yr during the past 6000 yr (Vallance, 1986). No evidence indicates that any were accompanied by eruptive activity. The presence of hydrothermally altered material on the steep southwest and east flanks near the summit suggests that these areas are probable sources of future avalanches and lahars, similar in size to those of postglacial age.
A less likely event on Mount Adams is a debris avalanche and lahar perhaps an order of magnitude
more voluminous than the largest of postglacial age (Vallance, 1986). The greater the quantity of
hydrothermally altered material lying on the upper slopes of the cone, the higher would be the probability
for such an event. Hydrothermally altered material does exist on the upperslopes, but its volume is
poorly known because of extensive ice cover (Vallance, 1986). If a very large
debris avalanche were to occur on the east or southwest flank, lahars could reach the Columbia River, at a
point that is impounded by Bonneville Dam. Similar events on the northwest or north flank could send
lahars down the Lewis and Cispus-Cowlitz River valleys. In both of these drainages, dams 50-60 km
downstream from Mount Adams could trap the lahars and associated floods, particularly if the reservoirs
had been drawn down prior to the event.
Mount Hood, Oregon
Eruptive HistoryMount Hood is a composite volcano of chiefly andesite and dacite lava flows and pyroclastic debris (Wise, 1969). The cone is probably less than 730,000 yr old because of (1) the absence of rocks with reversed magnetic polarity (White, 1980) and (2) a few K-Ar dates (Keith and others, 1985). Its past eruptive behavior has been dominated by lava flows and domes and by the generation of lithic pyroclastic flows and lahars. Investigators have not found any major tephra-fall deposits erupted from Mount Hood.
The first eruptive stage, which consists solely of the Polallie eruptive period, occurred late in the late Wisconsin glaciation (perhaps 15,000-12,000 yr ago) while glaciers larger than those at present existed on the volcano. Pyroclastic flows, related pyroclastic surges, and lahars spawned by emplacement of dacite domes near the summit descended all flanks of the volcano; lahars probably reached more than several tens of kilometers from the summit along all of the valleys that head on the volcano.
A better known eruptive stage occurred during late Holocene time and consists of three periods; each lasted for decades to centuries and was separated by apparent repose intervals of several centuries. The Timberline eruptive period occurred between about 1800 and 1400 yr ago (Crandell, 1980; Cameron and Pringle, 1986); the Zigzag occurred about 500 yr ago (Cameron and Pringle, 1986); and the Old Maid occurred during the 18th and early 19th centuries (Crandell, 1980; Cameron and Pringle, 1986). Dacite domes were extruded just south of the summit during each of these periods. The summit and prominent ridges extending northwest and east of the summit deflected pyroclastic flows and lahars into drainages on the southeast, south, and west flanks of the volcano. However, one (Crandell, 1980) or two (Major and Burnett, 1984) lahars of Holocene age flowed 34 km to the north down the Hood River Valley. The farthest reaching effects of the late Holocene eruptions were a few lahars and lahar runouts that extended 50-60 km from the volcano down the Sandy River during the Timberline period and the White River during the Old Maid period.
Only one lava flow was erupted during postglacial time. The vent for the 6-km-long Parkdale andesite flow lies about 11 km north of the summit of Mount Hood. It is not known whether this lava flow is related to the magmatic system of Mount Hood or is part of the basaltic volcanoes and volcanic fields of the Cascades.
Evidence of numerous lahars and floods formed by glacier outburst floods and intense storms occurs in most drainages heading on Mount Hood; the evidence lies within 15 km of the volcano (P. Pringle, written commun., 1986). The 1980 lahar and flood described by Gallino and Pierson (1985) along Polallie Creek is probably similar to many of these events.
Volcanic-Hazards AssessmentThe Holocene activity at Mount Hood provides a good example of the most likely types and sizes of eruptions that will occur in the future, chiefly extrusion of dacite lava domes near the summit and associated pyroclastic flows and lahars (Crandell, 1980). During these events, the flanks of the volcano and valley bottoms within 20 km of the volcano could repeatedly be affected by pyroclastic flows and lahars. The largest lahars could inundate areas as far as 60 km from the volcano. An even more extensive consequence of voluminous lahars would be the deposition of large amounts of sediment in the Columbia River at and downstream from the mouth of the Sandy River, or in the lakes behind Bonneville and The Dalles Dams, as a result of the subsequent flushing of sediment from upstream areas.
Because of the current vent position on the south side of the summit, lahars and pyroclastic flows related to future dome extrusions are less likely in the Hood River Valley than along other valleys. The opening of a new vent on the north flank, the collapse of the summit, or the formation of a vertical eruption column, however, would increase the probability of hazardous events affecting the Hood River Valley.
Great relief, steep slopes, and large masses of hydrothermally altered rock near the summit imply that the greatest potential hazard at Mount Hood is a catastrophic debris avalanche and lahar. A lahar thought to have occurred in late Pleistocene time descended the Hood River Valley north of Mount Hood and crossed the Columbia River (Vallance, 1985; 1986). Such an event would cause total destruction along valley bottoms for many tens of kilometers from the volcano. In addition to the direct effects, secondary floods and lahars related to possible catastrophic draining of debris-dammed lakes could cause even greater downstream hazards, including voluminous sedimentation in the Columbia River. An explosive lateral blast accompanying a large debris avalanche could also affect broad sectors extending as far as 35 km from the volcano.
Past Mount Hood eruptions have not produced voluminous tephra falls. However, the dacitic
composition of the recent eruptive products suggests that large, explosive tephra-producing eruptions are
possible, although the estimated annual probability of these is very low.
Mount Jefferson, Oregon
Eruptive HistoryMount Jefferson is a composite cone of basaltic andesite, andesite, and dacite erupted on several overlapping basaltic shield volcanoes (Sutton, 1974; White and McBirney, 1978). The lava flows of both Mount Jefferson and the shield volcanoes have normal magnetic polarities and thus are probably less than 730,000 yr old.
The youngest known eruptive activity of Mount Jefferson included explosive eruptions of dacitic to rhyolitic tephra and pyroclastic flows (Fig. 4-31) and the extrusion of lava domes (Beget, 1981; Yogodzinski and others, 1983). Relationship of the tephra to glacial deposits provides only broad minimum ages for this activity. The eruptions pre-date the late Wisconsin glaciation, which culminated between 22,000 and 18,000 yr ago, and probably also pre-date a glacial advance that occurred between 40,000 and 150,000 yr ago. Other evidence that bears on the age of the tephra comes from east-central Idaho. The lower of two tephra layers near Arco, Idaho, (Pierce, 1985) is chemically similar to the Jefferson tephra on the basis of preliminary work by A. M. Sarna-Wojcicki (written commun., 1986). The younger of the tephras near Arco has a fission-track age of 76,000 +/- 34,000 yr and is probably between 70,000 and 110,000 yr old (Pierce, 1985). The tephras occur in close stratigraphic succession in a loess deposit, which implies that the Jefferson-like tephra is not greatly older than the upper ash. This evidence suggests that if the Jefferson- like tephra near Arco and the Jefferson tephra described by Beget (1981) and Yogodzinski and others (1983) are equivalent, the Jefferson tephra is between 70,000 and 120,000 yr old.
The only known postglacial activity is small lahars and floods on the lower flanks of the volcano (Fig. 4-32), which probably resulted from non-eruptive processes. Postglacial eruptions at vents in an area south of Mount Jefferson produced scoria cones and lava flows. They are included in the discussion of the basaltic volcanoes and volcanic fields of the Cascades.
The hazard implications of Mount Jefferson\"s quiescence during the past several tens of thousands of years is unclear. Is the volcano now in a long repose interval that will end with an explosive eruption, or are future eruptions unlikely? Owing to evidence of previous explosive eruptions, a conservative assessment would regard Mount Jefferson as a potentially explosive volcano. On the basis of currently available data, we estimate that the annual probability of a major explosive eruption is no more than 1 X 10- 5. However, this value would be revised upward if future work determines that the behavior in the past few hundred thousand years has been characterized by short eruptive periods separated by repose intervals as long as the present one.
The great relief and steep slopes of the volcano imply a potential for voluminous debris avalanches
and related lahars even in the absence of eruptions. Such events could affect valley floors 50 km or more
away from the volcano. A more likely type of activity would be small debris avalanches and lahars, similar
to those that occurred in postglacial time, which would chiefly affect valley bottoms within 10 km of the
Three Sisters, Oregon
Eruptive HistoryThe Three Sisters area contains 5 large cones of Quaternary age-- North Sister, Middle Sister, South Sister, Broken Top, and Mount Bachelor. North Sister and Broken Top are deeply dissected and probably have been inactive for at least 100,000 yr. Middle Sister is younger than North Sister (Taylor, 1981), and was active in late Pleistocene but not postglacial time (Wozniak, 1982). South Sister is the least dissected; its basaltic andesite summit cone has a well preserved crater (Wozniak and Taylor, 1981). Most of South Sister predates late Wisconsin glaciation and is therefore older than 25,000 yr; however, eruptions of rhyolite from flank vents have occurred as recently as 2000 yr ago (Taylor, 1978; Wozniak, 1982; Scott, 1987).
The type and scale of eruptive activity in the Three Sisters area have varied widely. Major explosive eruptions during middle Pleistocene time (Sarna-Wojcicki and others, 1987; A. M. Sarna-Wojcicki, personal commun., 1987) produced several large pyroclastic flows and a pumice fall, whose deposits are exposed on the east margin of the Three Sisters area from south of Bend to Sisters (Taylor, 1978, 1981; Mimura, 1984; Hill, 1985). The pumice-fall deposit is as thick as 13 m about 15 km from the closest suspected vent area (Hill, 1985) and together with the overlying pyroclastic-flow deposit records the eruption of at least 20 km3 of magma (Mimura, 1984). Eruptions of this size have formed calderas at other volcanoes (Smith, 1979) and suggest that buried calderas may be present in the Three Sisters area. Subsequent eruptions were smaller.
Holocene activity at South Sister consisted of two eruptive periods between 2000-2300 yr ago separated by no more than a few centuries (Taylor, 1978; Scott, 1987). Eruptions occurred at numerous flank vents and produced rhyolite tephra, pyroclastic flows, and lahars that were followed by extrusion of lava domes and flows. Tephra fall was negligible beyond 30 km downwind from vents; the other eruptive products were restricted to within a few kilometers of vents (Figs. 4-34 to 4-36). Similar eruptions, but somewhat more explosive and voluminous, accompanied emplacement of dacite and rhyolite lava domes and flows during late Pleistocene time at both South and Middle Sister. At least two such eruptions probably occurred between 15,000-25,000 yr ago (W. E. Scott, unpublished data).
Much of South Sister is composed of lava flows and pyroclastic ejecta of basalt, basaltic andesite, andesite, and dacite formed by eruptions that were presumably less explosive than those involving more silicic magmas (Taylor, 1978, 1981; Wozniak, 1982; Clark, 1983). Although the evidence has largely been buried or eroded by glacial processes, Pleistocene eruptions at South Sister were probably accompanied by lahars and floods that affected areas many tens of kilometers downstream from the volcano.
Volcanic-Hazards AssessmentThe most likely type of future activity at South Sister is a dacitic or rhyolitic eruption at one or more flank vents, similar to the eruptions of Holocene time. Areas within 10 km of the volcano would be affected by tephra fall, pyroclastic flows, and lava flows and domes; thin (<1 cm) tephra might fall as far away as 30 km. A few dacitic and rhyolitic tephra-fall deposits of late Pleistocene age from vents on the flanks of South and Middle Sister are an order of magnitude thicker than the Holocene tephras at comparable distances from vents (W. E. Scott, unpublished data). This suggests that pyroclastic eruptions more voluminous and explosive than the Holocene eruptions are possible, although of much lower probability. Eruptions of basaltic andesite and andesite tephra and lava flows are also possible and would likewise affect areas chiefly within 10 km of vents. Because of the large volume of snow and ice on the volcano (Driedger and Kennard, 1984), any future eruption could generate lahars and floods, which might affect valley floors tens of kilometers from the volcano.
The least likely, but nevertheless possible, event in the
Three Sisters area is a major explosive
eruption of silicic magma like those of middle Pleistocene age. Such an eruption would affect areas tens of
kilometers from vents with pyroclastic flows and areas to distances of hundreds of kilometers with thick
tephra fall. Whether a silicic magma chamber of sufficient volume exists in the Three Sisters area to
produce such an eruption is unclear. The distribution of late Pleistocene and Holocene vents of silicic and
mafic magmas and the chemical composition of the magmas suggest a silicic magma chamber of several
tens of cubic kilometers may be present (Scott, 1987). However, Bacon (1985) reasons that a shallow,
silicic magma chamber of large volume (>50 km3) is probably not present based on vent distributions.
Nevertheless, the Three Sisters area has had a long history of silicic eruptions and of explosive eruptions of
large volume. Therefore, a conservative assessment would regard the Three Sisters area as a possible site
for a future large-volume eruption.
Newberry Caldera, Oregon
Eruptive HistoryNewberry volcano lies 65 km east of the crest of the Cascade Range and is one of the largest Quaternary volcanoes in the United States--its broad shield-like shape covers more than 1300 km2 (MacLeod and others, 1981). A summit caldera 6-8 km across contains numerous rhyolite lava flows and domes and related pyroclastic debris; lava flows and tuffs of basalt and basaltic andesite are less common. The flanks of the volcano are composed of rhyolitic to andesitic pyroclastic-flow deposits, dacitic and rhyolitic lava flows and domes, and hundreds of scoria cones and lava flows of basalt and basaltic andesite (MacLeod and others, 1982). An active hydrothermal system is evidenced by warm springs in the caldera and by drilling that has penetrated zones with temperatures in excess of 265 degrees C. at depths of 932 m (MacLeod and Sammel, 1982).
Three Holocene rhyolitic eruptive episodes at vents within the caldera have produced one tephra- fall deposit that extends far to the east, several tephra-fall deposits of limited extent, a pyroclastic-flow and pyroclastic-surge deposit that is confined to the caldera, and numerous lava flows and domes (MacLeod and others, 1981). Holocene activity at flank vents, some as far as 30 km from the caldera, was chiefly the formation of numerous scoria cones, local tephra-fall deposits, and lava flows of basalt and basaltic andesite (Peterson and Groh, 1969; Chitwood and others, 1977; MacLeod and others, 1982). Rhyolitic eruptions of small volume may also have occurred on the upper flanks of the volcano (MacLeod and others, 1981).
The most voluminous eruptions of Newberry volcano occurred chiefly during middle Pleistocene time; however, one of these may have occurred as recently as several tens of thousands of years ago (MacLeod and others, 1981; MacLeod and Sammel, 1982). These eruptions produced voluminous tephra- fall and pyroclastic-flow deposits and were probably accompanied by caldera collapse. The sizes of these events are not well known, but the volume of one pyroclastic-flow deposit probably is more than 25 km3 (MacLeod and others, 1981).
Volcanic-Hazards AssessmentThe past behavior of Newberry volcano suggests that the most likely type of eruption in the future is a small- to moderate-volume (<0.1-1 km3) rhyolite eruption in the caldera or near the caldera rim. This activity probably would begin with the eruption of tephra and pyroclastic flows and surges and would culminate with the extrusion of lava domes or flows. Flowage phenomena would chiefly affect the caldera floor and areas within several kilometers of the rim. Depending on wind strength and the character of the eruption, significant tephra fall could occur many tens of kilometers from the caldera. The last rhyolitic tephra eruption at Newberry deposited 25 cm of tephra at a distance of 60 km (MacLeod and Sammel, 1982). The effects of Holocene lahars were apparently restricted to the caldera floor (MacLeod and others, 1982). However, if explosive eruptions were to occur at vents in Paulina Lake, as they did in East Lake during Holocene time, floods and lahars could descend the lake's outlet stream, Paulina Creek, westward to the Little Deschutes River.
An eruption of basalt and basaltic andesite is also possible at Newberry volcano, either from a pre- existing or new vent on the flanks. Such an eruption would be less explosive than a rhyolitic eruption in the caldera; however, the resulting lava flows could be as long as 10 km and cover tens of square kilometers. Multiple eruptions of this type might occur over a relatively short period of time at vents arrayed along a system of fissures or faults that could extend as far as 30 km from the caldera. Such activity occurred about 6000 yr ago (MacLeod and others, 1981, 1982). An eruption of basalt and basaltic andesite could also occur in the caldera. During such eruptions in the past, interaction of magma with ground and surface waters caused violent explosions that resulted in generation of pyroclastic surges. The effects of such surges would probably be limited to within 15 km of vents.
A catastrophic silicic eruption of large volume (>10 km3) like those that have occurred very
infrequently in the past is possible, but very unlikely. Such an eruption would produce pyroclastic flows
and tephra fall that could devastate the flanks of the volcano and areas tens of kilometers beyond, and
seriously affect areas many hundreds of kilometers downwind.
Crater Lake, Oregon
Eruptive HistoryCrater Lake occupies a caldera formed 6850 yr ago during the climactic eruption of Mt. Mazama, which was a cluster of Pleistocene stratovolcanoes (Williams, 1942; Bacon, 1983; Druitt and Bacon, 1986). A period of 15,000-40,000 yr was required to form the silicic component of the climactic magma chamber (Bacon, 1983). During that period, eruptions of basalt, andesite, dacite, and rhyolite occurred in the Mount Mazama area.
During the few centuries preceding the climactic eruption, at least two small- to moderate-volume (<1 to several cubic kilometers) eruptions of rhyolite occurred in the area underlain by the magma chamber (Bacon, 1983; C. R. Bacon, written commun., 1986). Tephra from one of these eruptions extended into southeastern Oregon and western Nevada (Davis, 1978, 1985; Blinman and others, 1979) and the same tephra, or one or more others, fell as far away as eastern Washington (Blinman and others, 1979; Mack and others, 1979). The explosive eruptions were followed by the extrusion of rhyolite lava flows.
The climactic eruptions 6850 yr ago produced voluminous tephra-fall and pyroclastic-flow deposits. The tephra deposits are about 40 cm thick at points 200 km northeast of the volcano and 4-5 cm thick at 1000 km; layers have been found in 8 western states and 3 Canadian provinces. The tephra fall was followed by two episodes of pyroclastic-flow formation. The first was of small extent, but it was followed by voluminous pyroclastic flows that moved outward in all directions to distances of as much as 60 km (Bacon, 1983). The total volume of magma erupted during the climactic eruption was about 50-60 km3 (Bacon, 1983), which is an order of magnitude larger than that produced during any other explosive eruption in the Cascade Range during postglacial time.
Following the climactic eruption, an andesite scoria cone and lava flows were erupted within the caldera to form Wizard Island (Bacon, 1983; C. R. Bacon, written commun., 1986). The initial postcaldera eruptions probably occurred shortly after the climactic eruption, prior to the development of the lake. Other eruptions occurred after the lake had begun to form. A rhyolite dome on the flank of the Wizard Island volcano records the youngest known eruptive activity.
Volcanic-Hazards AssessmentA comprehensive assessment of potential volcanic hazards at Crater Lake is premature and can only be attempted when the current state of the magma chamber is better known. A critical question is whether sufficient silicic magma is now present or will be present in the near future to sustain an eruption similar to that of 6850 yr ago. One line of reasoning suggests that another such eruption may not occur for thousands of years, because 15,000-40,000 yr were required for the climactic magma chamber to evolve. This reasoning is consistent with Smith's (1979) estimate of the periodicity of large-volume (101-102 km3) eruptions at other volcanoes. However, it is not known that these estimates are applicable to the existing magmatic system of Crater Lake. Is the present magma chamber in such a state now that the most likely events are small- to moderate-volume eruptions (10-2 to 101 km3) of andesite, dacite, and rhyolite? If so, what is the probability of such eruptions over the next 50-100 yr? These questions cannot yet be answered. We use an annual probability of 2 X 10-4 for explosive eruptions exceeding 0.1 km3 in the tephra-hazard assessment in Chapter 5 (in OFR87-297). This estimate is based on the frequency of such eruptions at Mount Mazama in postglacial time. However, during this time period the magma chamber underwent major changes. Thus this estimate has a high degree of uncertainty.
Small- to moderate-volume eruptions of andesite, dacite, or rhyolite within the caldera could produce waves in the lake, pyroclastic flows and surges, and lava flows and domes whose major effects probably would be confined to the caldera. Explosive rhyolite eruptions similar to those that preceded the climactic eruption could result in deposition of several centimeters of tephra hundreds of kilometers downwind. Eruptions at vents outside the caldera could produce pyroclastic flows and lahars tens of kilometers in length along valleys that head near the vent.
The large volume of water in Crater Lake implies a high probability that water will interact with magma during future eruptions. Such interaction would be expected to increase the explosivity of eruptions that occur in relatively shallow water. Violent explosions from silicic magma interacting with water would greatly increase the fragmentation of the products, producing finer grain-sized tephra which would lead to greater dispersal (Self and Sparks, 1978; Walker, 1981). Depending on the location of the eruption and the amount of water expelled from the lake, floods and lahars of various volumes could be generated along one or more drainages leading away from the caldera rim.
Landslides are probably not capable of generating large floods and lahars, either by causing
spillover or by breaching the rim. The minimum height of the caldera rim above the lake (160-180 m on
the northeast and southeast rims) indicates that a great wave would be required to accomplish spillover. A
landslide from the steep caldera wall would generate waves. However, using the waves generated by
landslides into Lituya Bay, Alaska, and other historic examples for comparison (Miller, 1960; Slingerland
and Voigt, 1979), it is unlikely that a wave high enough to overtop the lowest part of the caldera rim could
be generated in Crater Lake at its present level, even by a landslide as large as several hundred million cubic
meters. The gentle slopes outside the caldera imply stability and little likelihood of a major outward-moving
collapse that would lower the caldera rim and permit catastrophic outflow of lake water.
Medicine Lake Caldera, California
Volcanic-Hazards AssessmentEruptions of the past 10,000 yr form a reasonable basis for assessing hazards from future eruptions. Similar eruptions of silicic magma are likely from vents within and just outside of the summit caldera, which is thought to be underlain by silicic magma (Heiken, 1978; Eichelberger, 1981), part of which could still be molten. These eruptions probably will produce tephras that could fall as much as several hundred kilometers downwind and mostly east of the volcano (Christiansen, 1982; Miller, in press). Such eruptions could also produce pyroclastic flows that could endanger areas within about 10 km of the active vent, although such phenomena are not known to have occurred during Holocene time. Silicic eruptions are likely to culminate with eruption of dacite to rhyolite lava flows or domes that could reach as far as several kilometers from their vents.
Eruptions of basalt and basaltic andesite lava may also occur from vents on the flanks of the Medicine Lake volcano (Christiansen, 1982). Such eruptions may begin by forming cinder cones and dispersing mafic tephra as far as 20 km from the active vent and culminate with the production of lava flows that may extend for tens of kilometers downslope from their vents.
Eruptions of both mafic and silicic magma may be fed by dikes. As a consequence, eruptions of basalt and rhyolite may occur simultaneously, or nearly so, from multiple, probably aligned vents.
Eruptions of volumes larger than those of Holocene time are possible, including a caldera- forming eruption (Christiansen, 1982), because of the inferred existence of a large body of silicic magma beneath the Medicine Lake volcano, (Heiken, 1978; Christiansen, 1982). Future eruptions of this type could deposit thick accumulations of tephra over wide regions and produce pyroclastic flows that could affect areas more than 50 km from the vent.
Debris avalanches and laterally directed blasts are not known to have occurred in this region in
the past. Owing to the limited relief of the Medicine Lake volcano, debris avalanches are not
considered likely in the future. Due to the absence of permanent snow and ice, future eruptions are not
likely to generate large-volume lahars and floods, although lahars and floods of moderate volumes are
possible if eruptions occur when snow covers the ground.
Mount Shasta, California
Eruptive HistoryMount Shasta in northern California, is a massive composite volcano consisting of overlapping cones centered at four main vents. The volcano was constructed during the last several hundred thousand years (Christiansen and Miller, 1976; Miller, 1980; Christiansen, 1985). Each of the cone-building periods produced andesite lava flows and pyroclastic flows, mainly at the central vents, as well as numerous lahars on and beyond the flanks of the volcano. Construction of each cone was followed by eruption of domes and pyroclastic flows of more silicic composition at central vents, and of domes, cinder cones, and lava flows at vents on the flanks of the cones.
Two of the main eruptive centers at Mount Shasta, the Shastina and Hotlum (summit of Mount Shasta) cones, were formed during the last 10,000 yr (Miller, 1980). Holocene eruptions also occurred at Black Butte, a composite dacite dome about 13 km west of Mount Shasta (Miller, 1978). Geologically recent eruptions at these two main centers and at flank vents form the principal basis for assessing the most likely kinds of future eruptive activity and associated potential hazards.
Streams that head on Mount Shasta drain into the Shasta River to the northwest, the Sacramento River to the west and southwest, and the McCloud River to the east, southeast, and south. The lower flanks of Mount Shasta consist mostly of broad, smooth coalescent fans formed by pyroclastic flows, lahars, and streams that descended the volcano along canyons and then spread out. As a result, pyroclastic flows and lahars at Mount Shasta have traveled a shorter distance from the volcano than they would have if they had been confined to narrow valleys. Their paths, on the fans, however, are less predictable.
Mount Shasta has erupted on the average at least once per 800 yr during the past 10,000 yr, about once per 300 yr during the past 3,500 yr, and about once per 250 yr during the past 750 yr (Miller, 1980; Crandell and others, 1984). The last known eruption occurred about 200 radiocarbon years ago (Miller, 1980) and may have occurred in 1786 A.D. (Finch, 1930).
Eruptions during the last 10,000 yr produced lava flows and domes on and around the flanks of Mount Shasta. Lava flows issued from vents near the summit and from flank vents as far as 9 km away, and individual flows are as long as 13 km. Only about 33 percent of past lava flows reached more than 10 km from the summit and none reached as far as 20 km.
Some pyroclastic flows originating at the summit vent and at the Shastina vent extended more than 20 km (Miller, 1978; Miller, 1980). Pyroclastic flows from the Black Butte vent extended about 10 km southwestward.
Eruptions at the Hotlum and Shastina vents produced many lahars, about 20 percent of which reached more than 20 km from the summit of Mount Shasta, and spread out on fans around the base of the volcano. Even larger lahars and floods extended beyond the base of the volcano and entered the McCloud and Sacramento Rivers (Hill and Egenhoff, 1976; Miller, 1980). During Holocene time Mount Shasta erupted pumiceous dacite tephra twice about 10,000 yr ago (Miller, 1980). One deposit is more than 0.1 km3 in volume and the other is less than 0.1 km3; both lie mainly east and within about 50 km of the volcano. Lithic ash has been erupted at Mount Shasta many times during the last 10,000 yr and the deposits mantle the ground surface within about 25 km of the summit (Miller, 1980).
No known debris avalanches have occurred at Mount Shasta during Holocene time, but a catastrophic debris avalanche occurred at there between about 300,000 and 360,000 yr ago (Crandell and others, 1984; Ui and Glicken, 1986). According to D. R. Crandell (personal commun., 1986), the Shasta debris avalanche flowed more than 64 km through the Shasta valley, covers more than 675 km2, and has a volume that exceeds 45 km3.
Volcanic-Hazards AssessmentFuture eruptions like those of the last 10,000 yr will probably produce deposits of lithic ash, lava flows, domes, and pyroclastic flows, and could endanger works of man that lie within several tens of kilometers of the volcano.
Lava flows and pyroclastic flows may affect low areas within about 15-20 km of the summit of Mount Shasta or any satellite vent that might become active. Lahars could affect valley floors and other low areas as much as several tens of kilometers from Mount Shasta.
Owing to great relief and steep slopes, a portion of the volcano could also fail catastrophically and generate a very large debris avalanche and lahar. Such events could affect any sector around the volcano and could reach more than 50 km from the summit. Explosive lateral blasts could also occur as a result of renewed eruptive activity, or they could be associated with a large debris avalanche; such events could affect broad sectors to a distance of more than 30 km from the volcano.
On the basis of its Holocene behavior, the probability is low that Mount Shasta will erupt large
volumes of pumiceous ash in the future. The distribution of Holocene tephra and prevailing wind
directions suggest that areas most likely to be affected by tephra are mainly east and within about 50
km of the summit of the volcano. However, the andesitic and dacitic composition of its products
suggests that Mount Shasta could erupt considerably larger volumes of tephra in the future. Moreover,
Christiansen (1982) has suggested that because it is a long-lived volcanic center and has erupted only
relatively small volumes of magma for several thousand years, Mount Shasta is the most likely Cascade
Range volcano to produce an explosive eruption of very large volume (101 -102) km3. Such an event
could produce tephra deposits as extensive and as thick as the Mazama ash and pyroclastic
flows that could reach more than 50 km from the vent. The annual probability for such a large event
may be no greater than 10^-5, but it is finite.
Lassen Peak, California
Eruptive HistoryThe Lassen volcanic center consists of a chain of vents aligned roughly north-south that extends about 8 km north from Lassen Peak. Although volcanism began at the center between about 600,000 and 350,000 yr ago (Clynne, 1984), events of the last 35,000 yr are the most thoroughly studied and form the basis for assessing hazards from future eruptions in the region.
The stratigraphic record of late Pleistocene and Holocene eruptions in this region contains evidence for many episodes of eruptive activity during the last 35,000 yr (Day and Allen, 1925; Crandell and Mullineaux, 1970; Crandell and others, 1974; Heiken and Eichelberger, 1980; Christiansen and Clynne, 1986; Clynne and Christiansen, 1987). Eruptions about 35,000 yr ago (Trimble and others, 1984; M. A. Clynne, written commun., 1986) produced two pyroclastic flows from a vent east of Sunflower Flat near the north end of the chain. These eruptions were followed by extrusion of one or more domes at vents in the same area.
Eruptions at Hat Mountain about 25,000-35,000 yr ago (M. A. Clynne, written commun., 1986) produced andesitic lava flows that reached up to 6 km from their vents. At about the same time, eruptions at a vent now buried by the Lassen Peak dome produced at least four pyroclastic flows and several short rhyolite lava flows (M. A. Clynne, written commun., 1986). These pyroclastic flows overlie a 31,000-yr-old peat deposit (M. A. Clynne, written commun., 1986).
Eruptions about 20,000 yr ago (M. A. Clynne, written commun., 1986) formed an ancestral dome, now buried by the Lassen Peak dome, which is thought to have erupted shortly before 11,000 yr ago (Crandell and Mullineaux, 1970). During late Wisconsin deglaciation, lahars formed on the slopes of Lassen Peak and flowed at least several kilometers, primarily northeastward.
The Chaos Crags eruptive episode, about 1000 to 1200 yr ago, began with eruption of a pumiceous tephra with a volume less than 0.1 km3. At least two pyroclastic flows traveled west down Manzanita Creek about 4 km, and a similar distance north down Lost Creek (Crandell and others, 1974; Heiken and Eichelberger, 1980; M. A. Clynne, written commun., 1986). Following a short hiatus, explosive activity destroyed a small dome soon after it formed, at the site of the Chaos Crags, generating pyroclastic flows that extended at least 12.5 km down Manzanita Creek and 21 km down Lost and Hat Creeks (Crandell and others, 1974). This eruption also deposited a lobe of pumiceous tephra at least 40 km northeastward (M. A. Clynne, written commun., 1986). Shortly thereafter, extrusion of five dacite domes with an estimated combined volume of about 1 km3 (Crandell and Mullineaux, 1970) formed the Chaos Crags.
About 300 yr ago, three or more rockfalls from the Chaos Crags generated high-velocity avalanches of rock debris that traveled as far as 4.3 km westward from the Chaos Crags (Crandell and others, 1974). Evidence for eruptive activity that might have triggered these rockfalls has not been found. The falls may have resulted from earthquakes, steam explosions, or intrusion of a dome into the central part of the Chaos Crags (Crandell and others, 1974).
The most recent eruptive activity occurred at Lassen Peak in 1914-1917 A.D. (Diller, 1914; Day and Allen, 1925; Loomis, 1926). This eruptive episode began on May 30, 1914, when a small phreatic eruption occurred at a new vent near the summit of the peak. More than 150 explosions of various sizes occurred during the following year (Williams, 1928). By mid-May 1915, the eruption changed in character; lava appeared in the summit crater and subsequently flowed about 100 m over the west and probably over the east crater walls. Disruption of the sticky lava on the upper east side of Lassen Peak on May 19 resulted in an avalanche of hot rock onto a snowfield. A lahar was generated that reached more than 18 km down Lost Creek (R. L. Christiansen and M. A. Clynne, written commun., 1986). On May 22, an explosive eruption produced a pyroclastic flow that devastated an area as far as 6 km northeast of the summit. The eruption also generated lahars that traveled more than 20 km down Lost Creek and floods that went down Hat Creek (Day and Allen, 1925; R. L. Christiansen and M. A. Clynne, written commun., 1986). A vertical eruption column resulting from the pyroclastic eruption rose to an altitude of more than 9 km above the vent and deposited a lobe of pumiceous tephra that can be traced as far as 30 km to the east-northeast (Day and Allen, 1925; R. L. Christiansen and M. A. Clynne, written commun., 1986). The fall of fine ash was reported as far away as Elko Nevada, more than 500 km east of Lassen Peak. Intermittent eruptions of variable intensity continued until about the middle of 1917.
Volcanic-Hazards AssessmentThe record of late Pleistocene and Holocene eruptive activity at the Lassen volcanic center suggests that the most likely hazardous future events include pyroclastic eruptions that produce pyroclastic flows and tephra. Christiansen (1982) regards the Lassen volcanic center as one of the principal candidates in the Cascade Range for future silicic, probably explosive, eruptions. Based on the eruptive history cited above, pyroclastic flows could endanger areas within several tens of kilometers of an active vent. Lahars and floods caused by these events could affect low-lying areas even farther from the vent, particularly if eruptions occur during periods of thick snow cover. Eruptions that produce lava flows are generally less dangerous, although both lava flows and domes can become unstable and produce pyroclastic flows and rockfall avalanches that could affect areas as far as several kilometers away. Mixing of hot debris with snow can generate lahars that could inundate valley bottoms for tens of kilometers as in 1915.
Areas of high relief within the Lassen volcanic center such as the Lassen Peak dome could also collapse and generate rockfalls and/or debris avalanches that could endanger areas within about 10 km of the source.
The late Pleistocene and Holocene eruptive history of the Lassen volcanic center suggests that large volumes of pumiceous tephra are not likely to be produced in the future. Areas subject to the greatest hazard from tephra falls resembling those of the past 35,000 yr are mainly east and within about 50 km of the center.
The older eruptive history of the volcanic center suggests that considerably larger and more
devastating eruptions are possible (Christiansen, 1982). The presence of a vigorous hydrothermal system
(Muffler and others, 1982), the early-20th-century eruption, continuing seismicity, and the cluster of young
domes suggested to Christiansen (1982) the existence of an active silicic magmatic system. This system lies
within a large negative gravity anomaly (LaFehr, 1965), which suggests the presence of a large pluton
(Heiken and Eichelberger, 1980). Christiansen (1982) suggested that future eruptions at vents within the
Lassen volcanic center could produce voluminous air-fall tephra and pyroclastic flows that could devastate
broad areas. This suggestion is supported by evidence of three caldera-forming events in the Lassen region
(M. A. Clynne, written commun., 1986). The youngest of these is about 400,000 yr old and was probably
of similar volume to the climactic eruption of Mount Mazama (Sarna-Wojcicki and others, 1987). Clynne
estimates that about 50 km3 of rhyolite pyroclastic flows and air-fall pumice was erupted from the Lassen
volcanic center during each of these episodes. Although the consequences of such a large eruption would
be severe, the annual probability of such a large event is small but finite.
Other Basaltic Volcanoes and Lava Fields of the Cascade Range
Distribution and Eruptive CharacterAlthough the 13 volcanic centers discussed above
are the most prominent volcanic features of Quaternary age in the Cascade Range, most of the range between Mount Rainier and Lassen Peak is composed of more than a thousand volcanoes, chiefly of basalt and basaltic andesite (White and McBirney, 1978; Hammond, 1980; Taylor, 1981; McBirney and White, 1982; Luedke and others, 1983; Sherrod, 1986; Hughes and Taylor, 1986). In order to simplify terminology in this report, we refer to these as basaltic volcanoes. Individual basaltic volcanoes have a limited compositional range, typically were active for only brief periods of time, and sometimes occur in fields of numerous, nearly coeval volcanoes. Basaltic volcanoes have formed throughout the past few million years in the Cascade Range; the youngest erupted in 1851 A.D. in the Lassen area. Eruptions of these basaltic volcanoes are, in general, much less explosive than eruptions of composite volcanoes, and therefore rarely affect areas more than 15 km away. However, the wide distribution in the Cascades of basaltic vents less than 1 million years old suggests that, although any one eruption would affect only a limited area, such an eruption could occur almost anywhere in the range.
The eruptions of Cascade Range basaltic volcanoes, despite a wide range in volumes of products, have not typically been highly explosive. Locally, however, interaction between magma and shallow ground water or surface water has caused violent hydromagmatic explosions. Lava flows have been by far the most voluminous product of eruptions of these volcanoes. The lavas flows of latest Quaternary age typically are less than 15 km long; 96% of lava flows are less than or equal to 10 km long. The longest of the lava flows plotted is 29 km; however, a few Pleistocene lava flows in the southern Washington Cascades are 40-80 km long (Warren 1941; Hammond, 1980).
Taylor (1965) described Holocene eruptions between North Sister and Three-Fingered Jack, Oregon, that exemplify the activity of Cascade basaltic volcanoes, although the density of Holocene vents there is much greater than in other areas of the range. One type of activity was characterized by initial scoria and ash eruptions that formed cinder cones and by later extrusion of lava flows. The most recent such eruption in the Cascades occurred in 1851 A.D. at Cinder Cone, east of Lassen Peak (Williams, 1928; Finch, 1930). In many areas, this type of activity has formed fields of numerous scoria cones, which are typically arranged in a linear zone, and lava flows.
Another type of basaltic activity is characterized by the concentration of many tephra and lava-flow eruptions at a central vent and several flank vents. This type of activity has built shield volcanoes typically 5-15 km in diameter and several hundred meters to more than 1000 m high. Many have summit cinder cones. Belknap in central Oregon is the youngest such shield volcano in the Cascades and has lava flows as young as 1400 yr.
Several large basaltic shield volcanoes along the range have steep-sided summit cones, such as Three-Fingered Jack (Davie, 1980), Mount Washington and North Sister (Taylor, 1981), Mount Bachelor (Taylor, 1978; Scott and Gardner, 1985), Diamond Peak, Mount Bailey, and Mount Thielsen (Sherrod, 1986), and Mount McLoughlin (Carver, 1972; Maynard, 1974). A few of these volcanoes contain rocks as silicic as andesite and may have been constructed during several eruptive episodes. These peaks rival the major composite cones in size but contrast with them in origin and structure. Most are composed of central scoria and tuff cones intruded by numerous dikes and one or more plugs. Thin lava flows intertongue with the scoria and mantle the central cone, and more voluminous lava flows typically extend beyond the base of the central cone. No evidence suggests that these volcanoes formed during highly explosive eruptions. Most lava flows and thick tephra-fall deposits are restricted within a few kilometers of vents, and scoriaceous tephras are typically not traceable farther than 20 km from vents (Taylor, 1965; W. E. Scott, unpublished data). Mount Bachelor, which is between 11,000 and 15,000 yr old (Scott and Gardner, 1985; W. E. Scott and C. A. Gardner, unpublished data), is the youngest of these volcanoes in the Cascades.
Volcanic-Hazards AssessmentHazardous effects of eruptions at basaltic volcanoes are chiefly restricted to areas within 15 km of vents; however, a few lava flows of postglacial age have extended almost 30 km, and some Pleistocene lava flows have extended 80 km. The most serious hazards result from faulting and fissuring near vents, burial by lava flows and thick tephra, and impact by ballistic fragments and base surges. Beyond 15 km, lahars and floods might be expected downvalley from some eruptions in ice- or snow- covered areas. Also, lava flows and related clastic debris could dam streams and form lakes that could spillover catastrophically and generate lahars and floods extending downvalley for tens of kilometers. Such lakes would also inundate upstream areas. The damming of some of the larger streams in the Range such as the Cowlitz or Deschutes, or even the Columbia, would pose the greatest hazard. In addition, owing to their great relief and steep slopes, some of the large basaltic volcanoes could produce large debris avalanches and related lahars that could extend tens of kilometers away.
The large number of postglacial basaltic volcanoes in the Cascade Range implies that the birth of a basaltic volcano has an annual probability of about 3 X 10-3. We determine this probability by dividing the number of basaltic centers in (about 40) by 15,000 yr. However, the prediction of specific locations for such events is probably not possible except a few days or weeks before an eruption. Figure 4-58 -- web note: not currently available -- shows the basaltic vents that have been active in the Cascades over the past 1 million years and provides a basis for defining a hazard zone for future eruptions.
A potential hazard at the large basaltic volcanoes, such as Mount McLoughlin and Mount Bachelor, is the possibility of more explosive volcanism if magmas become more silicic. Some of the composite volcanoes in the range were initially basaltic volcanoes that evolved to more silicic and explosive activity. We have no way to estimate the probability of such a change at a given center.
The probability that an area in the Cascades will be covered by a lava flow in a given year is related generally to the long-term rate at which lava is extruded. Few well-supported rates have been published because of the lack of detailed mapping and chronologic control. However, Sherrod (1986) has calculated Quaternary extrusion rates in the area of the Oregon Cascades between latitudes 43 and 44 degrees N of about 3- 6 km3/1 million yr for each 1-km-long segment of the range. This figure implies that, on the average, a volume equivalent to that of the Belknap shield volcano (5 km3) is extruded each 1 million yr in each 1-km- long segment of the range. As the basal area of a shield of this size is approximately 80 km2 and the width of the range is typically 25-50 km, the mean frequency of coverage for any point in this part of the range is roughly 2-3 times per million years. This frequency translates to a mean annual probability of 2.5 x 10-6 that a point will be covered by a lava flow. If the 5 km3 of magma were erupted from many vents, as would be the case for a field of scoria cones and lava flows, rather than largely from a central vent, the area covered could be several times greater than 80 km2. Hence, by combining both modes of eruption, the annual probability would be greater than 3 x 10-6. The reliability of this estimate for other areas and for time periods less than 1 million years is difficult to evaluate. Data for the central Oregon Cascades indicate a frequency much higher than 2-3 times per million years. During the past 15,000 yr, the 110-km-long segment of the range from Odell Lake to Mt. Jefferson has had a mean extrusion rate of at least 30 km3/my per 1 km of range length (W. E. Scott, unpublished data), which is about 10 times greater than the long-term, mean rate for latitude 43-44 degrees N. This rate implies that the annual probability of areas in the central Oregon Cascades being covered by lava flows from basaltic volcanoes is on the order of 10-5 and may locally be as low as 10-4.
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