Plinian eruptions at Glacier Peak and Newberry volcanoes, : Implications for volcanic hazards in the

James E. Gardner* GEOMAR, Abteilung Vulkanologie und Petrologie, 1Ð3, Wischhofstrasse, 24148 Kiel, Steven Carey Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882 Haraldur Sigurdsson }

ABSTRACT (intensity) and composition, suggest- 121° ing that eruptions of these volcanoes relate to Several fall deposits from Glacier Peak and the accumulation rate of magma in their Newberry volcanoes, both located in the Cas- reservoirs. cade Range, United States, have been studied BC to determine eruptive column heights, intensi- INTRODUCTION WA ties, and volumes. The late Pleistocene erup- Lake Chelan tions of Glacier Peak ranged from small The most important facet of forecasting and 48° Glacier phreatic explosions to two Plinian eruptions mitigating hazards associated with volcanic erup- Peak that each erupted more than 1 km3 of magma tions is understanding the eruptive histories of at intensities >108 kg/s, generating plumes volcanoes. As an example, detailed stratigraphic Mt. Rainier Figure 2A with heights >30 km. At Newberry , examination and extensive dating of past events the last Plinian eruption (ca. 1300 14C yr B.P.) at Mount St. Helens, , led Mullineaux Mt. St. Helens had an intensity of 2.8 × 107 kg/s and a plume et al. (1975) and Crandell et al. (1975) to forecast 46° height of 18 to 21 km. About 0.1 km3 of that the volcano was likely to erupt before the end magma was erupted in the Plinian phase, fol- of the century. In fact, it erupted within 5 yr of lowed by eruption of a and an that forecast. Without their pioneering studies, obsidian flow. Combined with similar little would have been known about Mount St. data from Mount St. Helens, , Helens’ potential to erupt or the types of erup- ° Newberry and (), these tions that it would produce. 44 eruptions define the range of Plinian events Despite the importance of tephrochronological that have occurred in the Cascade volcanic arc studies, like those at Mount St. Helens, little has in the past 12 k.y. During this period there been done to quantify important aspects of past Figure 2B have been Plinian eruptions with plume eruptions, in particular their intensities (mass Crater Lake heights between 11 and 55 km, intensities be- eruption rates) and magnitudes (volume of ° OR 6 9 42 tween 10 and 10 kg/s, and volumes between erupted material). Important advances in model- CA NV 0.01 and >5 km3 of magma. All eruptions with ing of explosive volcanic eruptions now allow intensities ≥108 kg/s also produced large- these parameters to be inferred for prehistoric volume pyroclastic flows and surges. Monitor- events, especially for Plinian-style eruptions ing column height (intensity) during eruptions (Carey and Sparks, 1986; Sparks, 1986; Wilson 0 120 could help mitigate hazards because it may and Walker, 1987; Woods, 1988; Pyle, 1989; indicate pending generation of pyroclastic Fierstein and Nathenson, 1992). When combined kilometers flows. In the Cascade Range, there have been with detailed stratigraphic information, knowl- at least 12 eruptions of >1 km3 of in the edge of the intensities and volumes of past events past 12 k.y., suggesting that eruptions of such allows for a more complete picture of the erup- Figure 1. Location of recently active strato- magnitude occur about once every 1 k.y., tive history of a volcano and its likely future ac- volcanoes (triangles) in the Cascade Range vol- although such frequencies vary greatly at each tivity (Carey et al., 1995; Gardner et al., 1995a). canic chain of northwestern United States and volcano. The volume of magma erupted in In order to better constrain the eruptive activity . Volcanoes for which fall deposits are each event correlates with both column height of volcanoes in the Cascade Range, we examined discussed in this study are highlighted (filled some of the fall deposits of Glacier Peak and New- triangles). Locations for study areas of this re- *Present address: Department of Geological Sci- berry volcanoes, United States (Fig. 1). The explo- port are shown, as well as known distributions ences, Brown University, Providence, Rhode Island sive volcanic activity of Glacier Peak took place in of the Plinian-fall deposits from Glacier Peak 02912; e-mail: [email protected]. two intervals, one in late Pleistocene time and one (solid curve) and Newberry (dotted curve).

GSA Bulletin; February 1998; v. 110; no. 2; p. 173Ð187; 14 figures; 2 tables.

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in Holocene time (Porter, 1978; Beget, 1982, TABLE 1. COMPOSITIONS OF ERUPTIVE PRODUCTS OF GLACIER PEAK AND 1983). The late Pleistocene episode was domi- Deposit Glacier Glacier Newberry Paulina Big nated by explosive activity, including two of the Peak G Peak B Fall Ash Flow Obsidian largest Plinian eruptions in the Cascade Range, Flow which deposited the G and B fall layers (Porter, SiO2 67.65 65.40 73.05 73.76 73.25 TiO 0.39 0.61 0.25 0.23 0.23 1978). Both of these events erupted dacitic mag- 2 Al2O3 17.02 18.43 14.26 13.44 13.77 mas that are very similar in composition (Table 1). FeO 3.08 3.38 1.98 2.11 2.10 Between G and B, there are five other fall layers: MnO 0.09 0.09 0.07 0.06 0.06 MgO 1.76 1.87 0.25 0.20 0.10 N, C, F, M, and T (from oldest to youngest), with CaO 4.00 4.39 1.01 0.91 0.90 M being the thickest and coarsest (Porter, 1978). Na2O 3.98 3.90 5.06 5.05 5.39 K O 1.99 1.82 4.04 4.00 3.96 About 3 to 5 km3 of pyroclastic flows also erupted, 2 P O 0.13 0.17 0.04 0.00 0.01 some flowing more than 20 km away from source 2 5 Note: Major oxides in wt%; total Fe reported as FeO. Samples of G and B (Beget, 1982, 1983). Radiometric dates for layers fall layers are one from the base of each deposit. Both were G and B cluster around 11 200 14C yr B.P., in crushed, fused at 1300¡C and 1 bar for five hours, and analyzed by electron 14 microprobe (see Gardner et al., 1995b, for analytical details). Sample agreement with C ages for the pyroclastic flows of Newberry fall layer is the average of six pumices analyzed by X-ray (Beget, 1982, 1983; see Foit et al., 1993, for a sum- spectroscopy. Other data are from MacLeod and Sherrod (1988). mary). The mid-Holocene episode began about 5 ka, and the volcano has since erupted about every 1 k.y. until about 200 yr ago (Beget, 1982, 1983, 1984). Most of this activity consisted of mi- A nor explosions of ash fall and intermittent dis- charges of pyroclastic flows. Newberry volcano has been very active in the past 0.5 m.y., and has erupted more than 25 times during the Holocene (see MacLeod and Sherrod, 1988, for summary). During the Holocene Epoch, there have been six eruptive episodes in which either basaltic or rhyolitic magma was erupted. The four rhyolitic episodes, separated by reposes of 2Ð3 k.y., consisted of Plinian-style eruptions of pumice and ash and extrusions of domes and flows, totaling about 1 km3 of magma (MacLeod and Sherrod, 1988). The last episode occurred ca. 1300 14C yr B.P., and consists of a tripartite sequence of Plinian-fall, ash flow, and an obsidian lava flow (Big Obsidian flow), all very similar in composition (Table 1). MacLeod et al. (1982) suggested that the fall deposit erupted at or very near to the vent of the Big Obsidian flow, and because of their similar compositions, suggested that the three eruptive phases erupted closely in time (MacLeod and Sherrod, 1988). For this study we determined the intensities and B volumes of the Plinian eruptions of Glacier Peak (G and B) and Newberry (1300 yr B.P.) volcanoes and the volumes and possible eruptive style of the other late Pleistocene fall layers of Glacier Peak. Those data are used to assess the potential vol- canic hazards posed by future Plinian eruptions at those volcanoes. We also combine those data with similar data from other stratovolcanoes in the Cascade Range to determine the range in Plinian- style activity that has occurred and to make pre- liminary assessments of future volcanic hazards and likely volcanic activity.

METHODS Figure 2. Generalized maps showing locations of sample sites of this study. (A) Glacier Peak volcano. (B) Newberry volcano. Positions of specific sample sites in other figures and text are We examined the late Pleistocene fall se- shown with larger dots. Locations of Big Obsidian flow in Newberry and the vent of the quence of Glacier Peak at 24 localities (Fig. 2A). Plinian phase of the 1300 yr B.P. eruption are shown.

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At Newberry, we examined the 1300 yr B.P. fall the Newberry deposit display such two-curve be- the contour along the axis is related to both deposit at 23 localities (Fig. 2B). The thick- havior, and probably represent the volume of that plume height and velocity of the dispersal nesses of every tephra layer present at each site deposit. However, thickness data for the three winds. The model was calibrated so that when were measured. At most sites the sizes of the thickest fall layers at Glacier Peak (G, M, and B) the measured width and length of an isopleth maximum lithic fragments were also measured are from a relatively proximal area, and probably contour are entered, estimates for cloud height in the G, B, and Newberry fall layers; the sizes do not fully represent their volumes. The vol- and peak wind speed are returned. Carey et al. were determined by excavating a standard area umes of the layers were therefore calculated us- (1990) used the model to calculate the height of of 0.5 m2 of each deposit and measuring the ing the method of Carey et al. (1995), which the eruption cloud produced by the May 18, major axis of the five largest lithic fragments. modifies that of Pyle (1989) by scaling proximal 1980, Plinian eruption of Mount St. Helens at Where the deposits were sufficiently thick (usu- data to other Plinian-fall deposits for which more different times during the day. They showed that ally >30 cm), they were subdivided to determine widespread thickness measurements are avail- those heights agree to within 10% of radar mea- the variation in size as a function of stratigraphic able. We believe that most of the volumes of the surements made during the eruption. This sug- height. Our thickness measurements agree well thinner ash layers at Glacier Peak are encom- gests that the heights estimated in this study, with previously reported isopachs (Porter, 1978; passed by our isopachs. It is likely that errors on which are reported as the top of the plumes, MacLeod et al., 1982), so we used their dispersal the volume estimates are, at best, 50%. have an error of ±10% (Table 2). In order to cal- axes to contour our data for thickness isopachs The volumes of magma erupted (dense-rock culate wind speeds from isopleths, the vertical and clast-size isopleths. equivalents, DRE) were calculated from the profile for the variation of wind with height Isopach contours were constructed for most of tephra volumes by first assuming that 15% of the must be known. We did not attempt to calculate the fall layers of Glacier Peak in order to estimate total volume is accidental lithic fragments. The wind speeds, but note that contour shapes indi- their volumes. We used the isopachs of MacLeod remaining juvenile (pumice and ash) volume was cate relative wind speeds, regardless of actual et al. (1982) to estimate the volume of the New- converted to DRE using a particle density of values. Not knowing true wind speed does not berry fall deposit and those of Porter (1978) for 900 kg/m3 and a magma density of 2400 kg/m3 affect the estimate for column height. layer T because of their more complete data. (Table 2). A density of 900 kg/m3 was used be- The height of a Plinian eruption plume is Volumes were calculated using the method of cause most fall deposits are composed mainly of related to the energetics of the eruption, or its Pyle (1989), which utilizes the exponential decay ash, for which 900 kg/m3 is a reasonable estimate intensity (Sparks, 1986; Woods, 1988). It is there- in deposit thickness by plotting data on graphs of (Walker, 1980). fore possible to use models for the buoyant rise of ln (thickness) versus (isopach area)1/2, as modi- The aerial distribution of lithic sizes was con- plumes to calculate the intensity of a Plinian fied by Fierstein and Nathenson (1992). Many toured (isopleths) in order to estimate the col- eruption by knowing its column height. We have deposits, however, display two curves on such umn height of the eruption plumes from the calculated intensities from our estimated column plots, one defined by proximal data and the other model of Carey and Sparks (1986). Their study heights, using the model curves of Sparks (1986) by distal data; much of the volume is encom- showed that the width of an isopleth contour derived for a temperate climate (Table 2). It is passed by the distal curve (Pyle, 1989, 1995; perpendicular to the dispersal axis is sensitive to believed that such intensities have an error of Fierstein and Nathenson, 1992). The isopachs of the height of the plume, whereas the length of ~20%, although this is difficult to constrain.

TABLE 2. ERUPTION DYNAMICS OF RECENT FALL LAYERS IN THE CASCADE RANGE, UNITED STATES Volcano Tephra layer Age* Dispersal† Tephra Magma Column¤ Intensity¤ Duration# axis volume volume height (kg sÐ1) (hr) (km3) (km3) (km) Glacier Peak B ~11,200 N115¡E 6.5 2.1 31 1.3 × 108 11 T ~11,200 N125¡E 0.05 N.D. N.D. N.D. N.D. M ~11,200 N165¡E/N130¡E 1.1 0.4 N.D. N.D. N.D. N+C+F ~11,200 SE 0.3 N.D. N.D. N.D. N.D. G ~11,200 N95¡E 6.0 1.9 32 1.4 × 108 9 I ~11,200 N100¡E 0.05 N.D. N.D. N.D. N.D. Newberry 1300 yr B.P. ~1300 N80¡E 0.4 0.1 21 2.8 × 107 2 Mazama O ~6845 Two lobes >20 >5 ~55~ 2 × 109 2 (Crater Lake) Lower Pumice ~7015 N-E >9 >2 N.D. N.D. N.D. Mount St. Helens 1980 5/18/1980 N65¡E 1.2 0.3 19 2 × 107 10 T A.D. 1800 N45¡E 1.5 0.4 16 1 × 107 27 We A.D. 1482 N90¡E 1.5 0.4 21 2.5 × 107 11 Wn A.D. 1480 N40¡E 7.7 2.0 24 5 × 107 27 Pu ~2500 N45¡E 0.8 0.2 15 5 × 106 27 Ps ~2500 N85¡E 0.4 0.1 11 1 × 106 67 Ye 3350 N95¡E 3.5 0.9 23 4 × 107 15 Yn 3500 N15¡E 15.3 4.0 31 1 × 108 27 Yb 3900 N50¡E 1.2 0.3 22 3 × 107 7 Mount Rainier C 2200 N55¡E 0.12 0.04 N.D. N.D. N.D. D 6000 N90¡E 0.05 0.02 N.D. N.D. N.D. L 6400 N125¡E 0.02 0.01 N.D. N.D. N.D. Note: Data from this study, Williams and Goles (1967), Mullineaux (1974), Porter (1978), Sherrod and MacLeod (1979), Bacon (1983), Mullineaux (1986), Young (1989), Foit et al. (1993), Carey et al. (1995); N.D. = no data available. *Ages are in 14C years before present unless otherwise stated. †Axes for proximal isopachs; first axis for M is from pumice-clast isopleths (Porter, 1978), second is from isopachs (this study). Proximal deposit of Mazama fall layer has two lobes, one directed toward the east, the other toward the northeast. ¤Peak column heights and intensities (mass eruption rates) during Plinian eruptions. #Duration of Plinian event based on erupted volumes and peak intensities, rounded to nearest hour.

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STRATIGRAPHY AND ERUPTIONS OF ever, we found from one to seven ash layers; we (1978), as suggested from his pumice-clast iso- THE GLACIER PEAK FALL DEPOSITS refer to these collectively as set N+C+F in order to pleth map, but to a more southeasterly direction. avoid confusion. We also recognize a series of fall We observed erosional breaks between some Porter (1978) divided the late Pleistocene fall layers that lie directly beneath G that have not of the fall layers, as did Porter (1978). Spe- sequence into seven layers: G, N, C, F, M, T, and been described. We designate these fall layers col- cifically, we found set I, N+C+F, M, and T par- B, and also subdivided T into T1, T2, and T3. Our lectively as set I for their initial position in the se- tially or wholly eroded at several localities. Porter findings agree with Porter’s, and we retain his quence. Our thickness measurements of M do not (1978) reported that M is reworked in some nomenclature (Fig. 3). Between G and M, how- extrapolate to the N165¡E dispersal axis of Porter localities and we found evidence for reworking of the top of N+C+F before M was deposited. These breaks suggest that the sequence was produced by multiple eruptions rather than a single one. Paleosols were not found in the sequence, how- ever, suggesting that the time between eruptions was short. We cannot determine the length of the breaks, but studies of pollen accumulated between layers G and B suggest that they erupted in less than 200 yr of each other (Blinman et al., 1979; Mehringer et al., 1984).

Tephra Set I

The oldest fall unit in the late Pleistocene sequence is the previously unrecognized Set I (Fig. 3). This set consists of as many as five sep- arate layers, all pinkish gray to gray and moder- ately to poorly sorted, which grade rapidly into one layer away from source (Fig. 4). This tephra layer was present at 12 of 20 of our sites where the base of layer G was exposed (Fig. 5). Foit et al. (1993) described a thin (<1 mm) ash layer that is just below two Glacier Peak ash layers in a core collected from Kearns basin, Montana. A correlation between that thin layer and set I is possible, but unlikely, because Kearns basin is more than 1000 km away from Glacier Peak. Of the five separate layers of set I, the bot- tom two consist dominantly of oxidized lithics and crystals, there is little juvenile material. Rare glass shards are blocky and poorly vesicu- lar. In contrast, the fourth layer is composed mainly of highly vesicular, bubble-wall glass shards. Layers 3 and 5 contain both shard types. These variations suggest that the first explo- sions were mainly phreatic, whereas later ones were phreatomagmatic to dominantly mag- matic. About 0.05 ± 0.03 km3 of tephra was Figure 3. Composite stratigraphic column produced by these explosions (Table 2). This is of late Pleistocene fall units of Glacier Peak, six times the volume of phreatic ash erupted exposed at sites 1 and 2 (see Fig. 2A). Tephra from Mount St. Helens in the two months lead- designations are given to the left of the column ing up to its May 18, 1980, eruption (Sarna- and are from Porter (1978), except for unit I, Wojcicki et al., 1981a). from this study; note that N + C + F is referred to as one unit in this study. Tephra unit I is Layer G best exposed at site 1, whereas N + C + F, M, and T are best exposed at site 2. Layer G is recognized as a discrete fall layer throughout the northwestern United States and Canada (Porter, 1978). This deposit thins gradu- ally eastward within our study area, from 150 cm to 50 cm over a distance of 35 km, but thins rapidly to the south, down to <3 cm in

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Figure 4. Schematic stratigraphic columns illustrating thickness variations of Glacier Peak fall units, (A), along, and (B), across, the dispersal axes of G and B (see Fig. 2A for locations and Fig. 3 for descriptions).

20 km (Fig. 4). From our isopachs of the de- We estimate from the aerial distribution of lithic seven layers that are pinkish-gray to olive-gray, posit (Fig. 6A), we estimate that a total of fragments that the eruption plume that deposited rich in lithics and crystals, and generally show 6.0 ± 3.0 km3 of tephra (1.9 ± 1.0 km3, DRE) layer G reached 32 ± 3 km in height and was poorer sorting up section (Fig. 3). These layers was erupted during this event (Table 2); this dispersed by very strong winds (Table 2). The can be traced as far as 30 km east of the volcano, volume is significantly greater than that esti- limited size grading suggests that neither column but decrease in number to the south and farther mated by Porter (1978) because we attempt to height nor wind speed varied significantly during east (Fig. 4). Erosion of the upper layers indicates include the volume of ash deposited distally. the eruption. A 32-km-high column corresponds that these layers were produced by separate erup- Lithic fragments in layer G were measured at to an intensity of 1.4 × 108 kg/s. A relatively high tions. On the basis of our isopachs of the com- 14 localities (Fig. 6B) and were measured as a wind speed is supported by the narrow distribu- bined layers (Fig. 7), a total of 0.3 ± 0.15 km3 of function of stratigraphic height at 8 of those sites. tion of G in the northwestern United States tephra was deposited (Table 2). All layers in The deposit is slightly normally graded, sizes (Porter, 1978). N+C+F contain juvenile pumice clasts, glass decreasing by about 20% up section. A large pro- shards, and crystals with adhering glass. Both portion of the lithics are oxidized, giving them a Tephra Layers Between G and B blocky and highly vesicular glass shards are pres- distinctive reddish color, which allows layers ent in each layer, but the relative abundance of G and B (which contain many fewer altered lithic We found from one to nine layers between G vesicular shards increases upward to the final fragments) to be readily distinguished in the and B, all containing lithic clasts similar to those layer, which is composed dominantly of vesicular field. Most lithic fragments are from fine- in G, although their alteration appears less exten- shards. The presence of both shard types suggests grained, andesitic to dacitic lava flows or domes. sive. Set N+C+F can be divided into as many as phreatomagmatic explosions, but the increased

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Figure 4. (Continued).

Figure 5. Isopach map of the fall unit I of Glacier Peak, which is composed of as many as six layers (Fig. 3). Isopach contours in centi- meters. Sites where the correct stratigraphic level was not excavated are marked with a small dash. Those sites at which the correct level was reached but no layer was found are marked with zero thickness.

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more southeasterly dispersal (Fig. 8). Layer M is A partially reworked, however, making both inter- pretations suspect (Porter, 1978). On the basis of its thickness variations, the minimum tephra volume of layer M is about 1.1 ± 0.6 km3 (0.4 ± 0.2 km3, DRE) (Table 2). We interpret from the size of clasts in the deposit and the lack of blocky shards that the eruption was magmatic and Plinian. Set T was deposited southeast of the volcano. Its tripartite nature is best developed at southerly localities (Fig. 4). We estimate from the isopachs of Porter (1978) that 0.05 ± 0.03 km3 of tephra was deposited. Juvenile pumice and glass shards (both blocky and vesicular) dominate, suggesting that the explosions that produced set T were phreatomagmatic. The relatively fine grain size of the layer indicates that they were weak explo- sions. Set T is partially eroded at some localities, showing that a small break in time separated its deposition from the eruption of layer B.

Layer B

The late Pleistocene fallout sequence is capped B by layer B (Porter, 1978). The thickness of layer B decreases from ~110 cm to 20 cm over a distance of 30 km along its dispersal axis (Fig. 4A), but across axis it thins to 20 cm in less than 9 km (Fig. 9A). About 6.5 ± 3.3 km3 of tephra (2.1 ± 1.1 km3 DRE) erupted during this event (Table 2). The deposit exhibits a secondary thickness maximum 22 to 27 km from the source. Similar secondary maxima were recognized in Plinian deposits of Taupo, Mount St. Helens, Vesuvius, and Quizapu (Walker, 1980; Sarna- Wojcicki et al., 1981b; Sigurdsson et al., 1985; Hildreth and Drake, 1992). The thickening in the Taupo deposit results from erosion of proximal localities by pyroclastic flows, whereas the other maxima probably result from either strong wind shears (most likely in layer B) or premature deposition of particle aggregates (Carey and Sigurdsson, 1982; Brazier et al., 1983). The largest lithic fragments in layer B were measured at 18 sites, 14 of which were subdivided to determine the variation in size with strati- Figure 6. (A), Isopach and (B), lithic isopleth maps for the G Plinian-fall deposit of Glacier graphic height (Fig. 9B). Lithic fragments Peak. Isopach contours in centimeters, isopleth contours in millimeters. See Figure 5 for symbol decrease by 30% to 40% from base to top of the description. Isopleth data represent the average diameter of the five largest clasts found in a deposit; the most change occurring in the lower 2 0.5 m area, and are for those five found either in the bottom layer of the deposit, where lithic half. A large fraction of lithics in layer B are un- material was collected from different stratigraphic levels, or from the whole deposit. The deposit altered to slightly altered, medium- to-coarse- is normally size graded, so that the coarsest lithic materials in whole-deposit samples correlate grained plutonic rocks, in contrast to those in to the bottom level. layer G. The distribution of lithic fragments in layer B indicates that the reached 31 ± 3 km in height in the early part of the erup- abundance of vesicular shards suggests that they ular glass shards (no blocky shards observed), tion, but decreased to 28 ± 3 km by the end became more magmatic with time. thickens to the south (Fig. 4). On the basis of (Table 1). The winds were strong, but decreased Layer M is the coarsest fall deposit after layers pumice-size variations, Porter (1978) suggested in strength during the eruption. A 31-km-high G and B (Porter, 1978). This layer, composed that layer M was dispersed almost due south, column indicates an intensity of 1.3 × 108 kg/s, dominantly of juvenile pumices and highly vesic- whereas our thickness measurements indicate a which decreased during the eruption.

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In all, Glacier Peak erupted at least 4.4 ± 2.2 km3 of magma (DRE) as tephra fall in the late Pleistocene Epoch (Table 2). This does not include magma erupted during the I, N+C+F, or T events, but those probably represent, at most, 0.05 km3 of magma. Thus, about 90% of all magma erupted as tephra fall occurred during the two large Plinian events, which were remarkably similar in eruptive dynamics and volume (Table 2). Both eruptions tapped magma of very similar dacitic composition (Table 1). Because of that similarity and because they likely erupted within 200 yr of each other, it is possible that they tapped the same magma body.

STRATIGRAPHY AND ERUPTION DYNAMICS OF THE 1300 YR B.P. NEWBERRY FALL DEPOSIT

Sherrod and MacLeod (1979) indicated that the 1300 yr B.P. fall deposit of Newberry volcano forms a narrow lobe aligned in a nearly east direction, and our thickness measurements agree with that distribution (Fig. 10). Their 25 cm Figure 7. Isopach map of the fall unit N + C + F of Glacier Peak, which is composed of as isopach contour, for example, extends 60 km east many as seven layers. Isopach contours in centimeters. See Figure 5 for symbol descriptions. of the caldera yet is at most 10 km wide, a ratio of 6:1. In contrast, the same isopach for the Glacier Peak B deposit has a ratio of about 2:1 (Fig. 9A). We estimate that 0.4 ± 0.2 km3 of tephra (0.1 ± 0.05 km3 DRE) erupted during the Plinian phase of this eruption (Table 2). A total of about 0.19 km3 of magma (DRE) thus erupted during the entire event (0.002 km3 as a pyroclastic flow and 0.1 km3 as obsidian flow; Sherrod and MacLeod, 1979). The Plinian-fall deposit consists of highly vesicular, uniform rhyolitic pumice (Table 1), fresh chips of rhyolitic obsidian, and accidental fragments of domes (which we correlate to the Paulina Peak domes exposed just south of the Big Obsidian flow), and mafic scoria. Three ash lay- ers, each 1 to 2 cm thick, are present in the upper half of the deposit at site 1 (Fig. 2B). A single ash layer is found at other sites, off-axis or farther down the axis out to a distance of 18 km. The abundance of lithic and obsidian fragments in the deposit increases above the first ash layer. At proximal sites on or near the dispersal axis, the deposit is normally graded overall, and has a slightly reversely graded base and top (Fig. 11A). More distally along axis, the deposit is more Figure 8. Isopach map of the fall unit M. Isopach contours in centimeters. See Figure 5 for strongly normally graded (Fig. 11B). The deposit symbol description. is also normally graded north of the axis (Fig. 11B), yet south of the axis, the lower one- half to two-thirds is distinctly reversely graded, above which it is normally graded (Fig. 11C). We In order to understand the changes in dispersal deposition while another did not, which is reason- also find that the five largest pumices vary in size that occurred, we constructed lithic isopleth maps able because of the limited extent of our study just like the lithic fragments, indicating that the for the base, middle, and top of the deposit, area. Sites 15 and 16, which show very different size grading reflects differences in dispersal assuming the following. We assume that no part grading, are only 2 km apart (Fig. 2B). Because of rather than breakage. of the study area received a significant amount of the small area, we assume that deposition at all

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Figure 9. (A), Isopach and (B), lithic iso- pleth maps for the B Plinian-fall deposit of Glacier Peak. Isopach contours in centi- meters, isopleth contours in millimeters. See Figures 5 and 6 for descriptions of symbols and measurements. Isopleth data are for either the bottom layer of the deposit where lithic materials were collected from different B stratigraphic levels, or for the whole deposit. Normal size grading in the deposit indicates that the coarsest lithic material from whole- deposit samples correlates to the bottom level.

Figure 10. Isopach map of the Plinian-fall phase of the 1300 yr B.P. eruption of Newberry volcano. Isopach contours in centimeters.

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sites began and ended at the same time, so that the bottom and top layers at all localities are isoch- ronous. Although this is not strictly true, the dif- ference will be the time it takes a particle to travel in the eruption plume to a distal locality after another particle is deposited proximally (rise and fall times being about the same). The farthest dis- tance between our localities is ~18.5 km; if winds were 30 m/s (like that at Mount St. Helens during its eruption in 1980; Sarna-Wojcicki et al., 1981b) the difference in time will be ~10 min, which is short compared to the length of the eruption (see following). We assume that the level below an ash layer equals the coarse level in the southern sites, because the coarsest lithic fragments at the south- A ern sites are between the middle and two-thirds levels, which is below all three ash layers at site 1 (Fig. 11A). For sites where we did not subdivide the deposit, we assume that the coarsest lithic fragments at northern sites correlate to the base level, and the coarsest lithic fragments at southern sites correlate to the middle. Shapes of isopleths for the base level indicate that when the eruption began tephra was dis- persed to the east-northeast (Fig. 12A). Isopleths for the middle (Fig. 12B) and top (Fig. 12C) levels suggest that the plume had shifted to due east. Overall, the aerial extent of isopleths decreases from the base to top level. We estimate from the isopleths that the eruption column ini- B tially reached a height of 21 ± 2 km, but de- creased to 18 ± 2 km by the middle and end of the eruption. From the elongation of the iso- pleths, it appears that wind speed was very high at the beginning of the eruption, but decreased by the end. Therefore, the eruption column de- creased in height from the beginning to the mid- dle of the eruption, whereas winds decreased throughout. Together, these trends account for the overall normal grading of the deposit at proximal sites on the main axis (Fig. 11A). Off axis, deposition can be understood by considering two sites, equal distances from the vent, one located north of the axis, the other south. During the eruption, the northern site receives deposition as C the plume decreases in height, the winds decrease in speed, and the plume moves away toward the south. Together, these changes produce a strongly normally graded deposit (Fig. 11B). At the same time, the plume moves toward the southern site, so even though the column is lower the site is relatively closer to the axis and, hence, receives Figure 11. Variations in lithic sizes in the Plinian-fall phase of the Newberry eruption. (A) coarser material. After the plume stops moving, Proximal to the eruptive vent and on the dispersal axis. (B) Distally along the dispersal axis and the southern site receives finer material as the to the north. (C) South of the dispersal axis. Thicknesses of the deposit at each site are nor- effects of lower column height and wind speeds malized by dividing the stratigraphic height of a sample level by the total thickness of the deposit become important. These changes produce a at that site. Lithic sizes (average of five largest) at each site have been normalized by dividing the deposit in the south that is first reversely graded size of lithics at each sample level by the largest size at that site. Relative positions of ash layers and then normally graded (Fig. 11C). in the fall deposit at a site are shown by arrows next to their trends (and marked by the number Overall, the changes in lithic-size isopleths indi- of the site). See Figure 2B for locations of sites. cate that plume height decreased from 21 to 18 km.

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A Volcanic Hazards at Glacier Peak Volcano Glacier Peak erupted at least 4.4 ± 2.2 km3 of magma during its late Pleistocene eruptive period, as two larger Plinian deposits (G and B), a smaller Plinian deposit (M), and numerous small-volume ash falls. At the same time, another 1 to 1.5 km3 of magma erupted as pyroclastic flows (Beget, 1982, 1983). Erosional breaks between some of the fall layers, and the lack of paleosols, indicate that the sequence was pro- duced by several eruptions separated by brief time periods. Pollen studies suggest that the entire sequence was deposited in 200 yr or less (Blinman et al., 1979; Mehringer et al., 1984; Foit et al., 1993). Such a rapid outburst of explo- B sive activity raises questions concerning the haz- ards posed by future eruptions of the volcano. Because of Glacier Peak’s remote location, the most serious hazards that its eruptions pose are from wind-dispersed ash over distal populated areas and from pyroclastic flows and enter- ing drainage basins. Winds over Glacier Peak are usually directed to the east (Beget, 1983), and thus ash injected into the atmosphere would travel over the United States and/or Canada. Both of the large Plinian eruptions of Glacier Peak dis- charged about six times more tephra than the May 18, 1980, eruption of Mount St. Helens, and so the atmospheric loading of ash and its fallout could have more severe effects over North Amer- C ica. We did not investigate pyroclastic-flow or deposits generated during the late Pleisto- cene eruptive period, but the relatively sudden deposition of a large volume of hot tephra on ice fields and around the volcano could generate lahars, like those that occurred at Mount St. Helens, , and Pinatubo (Janda et al., 1981; Cummans, 1981; Naranjo et al., 1986; Rodolfo et al., 1993; Dolan, 1993). Overall, the pattern of explosive activity of Glacier Peak appears to be one of eruptions sepa- rated by long (thousands of years) hiatuses. The late Pleistocene episode may have been preceded Figure 12. Isopleth maps. (A) Bottom level. (B) Middle level. (C) Top level of the Plinian-fall by an earlier eruption, but its deposit has been ten- deposit of the 1300 yr B.P. eruption of Newberry. See Figure 2B for description of features and tatively identified at only one locality near the text for discussion of data. source (Beget, 1990, personal commun.). No other earlier explosive activity has been described. The Holocene eruptive period, which may be divided into eruptions separated by ~1 k.y., fol- A 21-km-high plume is produced by an eruption (Table 2). In some cases, volumes were estimated lowed as much as 5 k.y. of quiescence (Beget, intensity of 2.8 × 107 kg/s (Table 2). These are very using a model that approximates the total volume 1982, 1983). Glacier Peak may have erupted as similar to those of the May 18, 1980, Plinian erup- of the deposit, even though most tephra was de- recently as 200 yr ago, so the volcano is most tion of Mount St. Helens (Table 2). posited outside the study areas (Carey et al., likely still active (Beget, 1982, 1983). 1995). Consequently, we derive volumes for the DISCUSSION Plinian eruptions of Glacier Peak that are signifi- Volcanic Hazards at Newberry Volcano cantly greater than those previously estimated We examined fall deposits of Glacier Peak and (Porter, 1978). We now discuss the implications During the Holocene there have been four Newberry volcanoes in order to determine their of these data in terms of volcanic hazards at the rhyolitic eruptive episodes at Newberry volcano, volumes and the styles of their source eruptions individual volcanoes and in the Cascade Range. each separated by 2Ð3 k.y. The volcano is prob-

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ably still active because the time since its last in height, and an intensity of at least 2 × 109 kg/s more than 90 fall layers erupted from Mount St. eruption (~1.3 k.y.) is shorter than those reposes (Young, 1989), making it the largest known Helens in the past 40 k.y., only one was dis- (MacLeod and Sherrod, 1988). The eruptive epi- recent Plinian eruption in the Cascade Range, persed toward the west (Mullineaux, 1986). sodes have all been similar in volume and magma and one of the largest known worldwide. Mount Most ash layers from Glacier Peak and Mazama composition, and so the last event may serve as a Mazama experienced a large-volume Plinian were also dispersed to the northeast and south- model for future hazards at the volcano eruption (Lower Pumice fall) of about 2 to east (Williams, 1942; Powers and Wilcox, 1964; (MacLeod and Sherrod, 1988). In the Plinian 10 km3 of magma <200 yr earlier (Bacon, 1983). Porter, 1978). phase of the last event, 0.4 ± 0.2 km3 of tephra Carey et al. (1995) showed that nine of the Dispersal of tephra from Plinian eruptions is was dispersed across central from a Plinian-fall deposits erupted from Mount St. hazardous not only because of the large volumes plume that reached heights of 18 to 21 km Helens in the past 4 k.y. have volumes between of ash that can fall out over an area, but also (Table 2). Upper tropospheric and stratospheric 0.4 and 15.3 km3 (Table 2). Those eruptions had because those large volumes are injected into the winds over central Oregon are usually directed to plumes that reached 11 to 31 km in height, and atmosphere very rapidly. The amount of time that the east (Shipley and Sarna-Wojcicki, 1983), so intensities between 106 and 108 kg/s. Its largest an eruption lasted can be estimated by dividing the greatest hazard for fallout from future Plinian eruption in the past 4 k.y., which produced the Yn its erupted volume by its intensity (Table 2). eruptions of Newberry will be in that direction. deposit, was similar in eruptive dynamics to the Many of those times are minimum values, how- The Newberry deposit is a factor of three less two large Plinian eruptions of Glacier Peak ever, because they were calculated using peak voluminous than the May 18, 1980, Plinian erup- (Table 2). values for intensity. However, we calculated the tion of Mount St. Helens (Table 2). A major dif- There have been 11 fall layers erupted from May 18, 1980, eruption of Mount St. Helens to ference between those eruptions is that most of Mount Rainier in the past 10 k.y., but multiple have lasted 10 hr, whereas it actually lasted about the volume of the Newberry eruption is as fallout isopach contours are available for only 3: C, D, 9 hr (Criswell, 1987). The two large eruptions of and an obsidian lava flow, whereas most of that and L (Mullineaux, 1974). We estimated their Glacier Peak (G and B) are estimated to have of the May 18 eruption is pyroclastic flows and volumes using the same method (Table 2). Only lasted 9 to 11 hr. One of the shortest eruptions coignimbrite ash (Sherrod and MacLeod, 1979; two isopachs (10 cm and 5 cm) exist for layer L, would have been the largest one, the Mazama Carey et al., 1990). A pyroclastic flow was gen- but because it is relatively thin we believe that the Plinian eruption, but that event was only one erated at Newberry, but it covers an area of only line defined by those isopachs on a plot of ln phase of a complex caldera-forming eruption. about 5 km2 (Sherrod and MacLeod, 1979). As (thickness) versus (isopach area)1/2 approximates The total eruption probably lasted much longer. many as three ash layers are present in the fall closely the total. More isopachs are available for These times highlight how quickly large volumes deposit, however, and they probably represent the C and D deposits and each displays two of ash can be injected into the atmosphere by distal edges of pyroclastic surges or their related curves. Our estimates suggest that the Mount Plinian eruptions. cosurge ash falls. These surges would be a greater Rainier deposits are at least an order of magni- Another hazard from Plinian eruptions is the hazard than the pyroclastic flow, because they tude less voluminous than the May 18, 1980, generation of pyroclastic flows and surges. Carey covered a wider area. One factor that may lessen deposit of Mount St. Helens (Table 2). No grain- and Sigurdsson (1989) showed that a large per- hazards from pyroclastic flows and surges at size data are available, so we cannot model the centage of Plinian eruptions with intensities in Newberry is the volcano’s topography. The high dynamics of those eruptions. excess of 108 kg/s also produced pyroclastic walls around much of caldera may act as barriers The volumes of 21 fall deposits from 5 vol- flows and surges. Four of the known Plinian to smaller pyroclastic flows that lack sufficient canoes in the Cascade Range have been esti- eruptions in the Cascade Range had intensities momentum to surmount the walls. In the late mated; the eruption dynamics have been deter- that reached 108 kg/s, and, these had associated Pleistocene Epoch, however, Newberry volcano mined for 13 of those eruptions (Table 2). pyroclastic flows or surges (Bacon, 1983; Beget, erupted several large-volume pyroclastic flows Column heights range from 11 to 55 km, intensi- 1983, 1984; Carey et al., 1995). Low intensity that are distributed well beyond the caldera. ties range from 106 to >109 kg/s, and tephra vol- does not preclude the generation of pyroclastic umes range from 0.02 to >20 km3. From this flows and surges, as witnessed by the Newberry Plinian Eruptions in the Cascade Range and summary, we can briefly discuss the hazards that eruption, because other factors, such as exsolved Their Hazards are likely associated with future Plinian eruptions volatile content and plume density, play signifi- in the Cascade Range. cant roles in generating flows (Wilson et al., Our investigation of the Plinian activity at One of the direct hazards from Plinian erup- 1980; Carey et al., 1988; Woods, 1988). In any Glacier Peak and Newberry reveals the intensi- tions is fallout of tephra from the plumes case, the relationship between high intensity and ties and volumes of eruptions at those volcanoes. (Shipley and Sarna-Wojcicki, 1983; Casadevall, the generation of voluminous pyroclastic flows We summarize here similar data for other vol- 1992). The potential for tephra fallout to affect indicates that monitoring evolving Plinian erup- canoes in the Cascade Range and discuss their an area is dependent on many variables, one tions (column height and intensity) could be very eruptive behavior and associated hazards being the direction of the prevailing winds. The useful as a real-time hazard monitor. (Table 2). Mount Mazama (Crater Lake) erupted Plinian eruptions of the Cascade Range all had cataclysmically at 6845 14C yr B.P., sending a column heights in excess of 10 km, which is Future Explosive Activity in the Cascade large volume of tephra across the western half of generally higher than the tropopause over mid- Range the United States and Canada (Williams, 1942; latitude areas (Lutgens and Tarbuck, 1989). The Powers and Wilcox, 1964; Kittleman, 1973). dispersal of ash from such eruptions will thus be The best indicator of possible future explosive This fall deposit (Layer O) has a volume of controlled by upper tropospheric and strato- activity in the Cascade Range is the past history. >20 km3 (>5 km3 DRE), but that estimate prob- spheric winds, which are usually from the west In the past 12 k.y., there have been at least 12 ably includes coignimbrite ash (Williams and over the Cascade Range (Mullineaux, 1976; events that erupted more than 1 km3 of tephra Goles, 1967; Young, 1989). During the Plinian Crandell and Mullineaux, 1978; Shipley and (Table 2). This suggests that eruptions of that phase of that eruption, the plume reached 55 km Sarna-Wojcicki, 1983; Beget, 1983). Of the scale occur at least once every 1 k.y. Of those 12,

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5 exceeded 5 km3 in volume, suggesting that 60 eruptions of that magnitude occur once every 2.4 k.y. These rates differ from those of Shipley and Sarna-Wojcicki (1983) because they in- cluded several deposits that were estimated to be 50 larger than what is now known. These estimates of recurrence will likely change as more data become available on other fall layers. 40 The frequency with which each volcano erupts differs; for example, there have been at least 7 eruptions of more than 1.0 km3 of tephra from Mount St. Helens in the past 4 k.y. (1 every 30 600 yr), whereas there has been none of that magnitude at Mount Rainier in the past 7 k.y. (Table 2). It is misleading to infer eruption recur- 20 rences from these data, however, because large eruptions can occur in very brief time spans. For example, the two large eruptions of Glacier Peak 10 occurred within about 200 yr. Even more closely spaced were the Wn and We Plinian eruptions of Column Height (km) Mount St. Helens, which were separated by about 2 yr (Table 2). Hazard forecasts at indi- 0 vidual volcanoes along the Cascade Range must 0123456 take such variations into account. Another guide to possible future activity in the Cascade Range is to look at similarities between 3 volcanoes. When we compare eruptions, we find Magma Volume (km ) that column height (and intensity) correlates with the volume of the eruption (Fig. 13). This is sim- Figure 13. Variation of Plinian column height (km) as a function of volume of magma erupted ilar to the correlation that Carey and Sigurdsson (km3) for late Pleistocene and Holocene eruptions of Cascade Range volcanoes. (1989) found for a larger set of Plinian eruptions. They attributed the correlation to a larger body of magma being able to erupt for a longer time. This allows the walls of the eruption conduit to be more extensively eroded and thus increase in magma erupted in Plinian events at Mount between how fast magma (of whatever composi- width, which in turn allows higher intensities. Mazama is derived by crystallization of andesite tion) accumulates and when an eruption is trig- Erupted volume also correlates strongly with and can evolve to rhyodacite to (Bacon gered. If eruptions are triggered by injection of magma composition (Fig. 14); only the New- and Druitt, 1988). Hence, these two volcanoes magma into the reservoir or by gas exsolution, berry deposit plots significantly off this trend. accumulate magma of different compositions at both of these processes are tied to the rate at This relationship suggests that the size (and different rates, one controlled by partial fusion which magma accumulates. These processes intensity) of Plinian eruptions is at least partially and mixing, the other by crystal growth. almost certainly differ between volcanoes, and so controlled by magma composition, possibly The accumulation of magma at a volcano is the frequency with which they erupt differ. because the volatile content tends to be higher in interrupted when it erupts. An eruption is most Among the Cascade Range volcanoes, neither more silicic (Johnson et al., 1994). likely triggered when enough pressure builds up magma volume nor composition correlate with The above relationships suggest that the vol- in a reservoir to fracture the overlying crust and the length of the preceding hiatus. At individual umes and intensities of eruptions at a volcano are propagate a crack to the Earth’s surface. Such volcanoes, however, this is not necessarily true controlled by how much, and what composition, overpressure is achieved by either injection of (Carey et al., 1995; Gardner et al., 1995a). magma can accumulate in a reservoir before it new magma into the reservoir or by exsolution of erupts. For example, magma erupted in Plinian gas from the magma (Tait et al., 1989). The ACKNOWLEDGMENTS events at Mount St. Helens is derived from partial amount of overpressure required is likely to be melting of either the lower crust or the sub- less for shallower reservoirs because the crust is This research was partially funded by National ducting slab (Smith and Leeman, 1987; Defant more brittle nearer the surface. This implies that Science Foundation grant EAR-8804117. Gardner and Drummond, 1993). Its composition is influ- to start an eruption, a larger volume of magma thanks the Alexander von Humboldt Foundation enced, however, by mixing with basaltic magma, needs to accumulate in a deeper reservoir relative for financial support during the writing of this so the final composition remains dacitic (Gardner to a shallower one. The more voluminous erup- paper. We thank Helene Massol for field assis- et al., 1995b). The flux of magma to Mount St. tions of Mount St. Helens (Yn and Wn) were fed tance, Pam Smith (Wenatchee National Forest) for Helens has been relatively constant; therefore, from deeper reservoirs, compared to smaller logistical support, Joe Devine for electron micro- longer hiatuses allow greater amounts of magma eruptions (Gardner et al., 1995c). probe analyses, Don Hermes for X-ray fluores- to accumulate and, hence, larger explosive erup- We suggest that the volume and intensity of a cence analyses, and W. Hildreth and N. Foit for tions to follow (Carey et al., 1995). In contrast, Plinian eruption are controlled by the interplay critical reviews.

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10.0 lithosphere in a volcanic arc: Geology, v. 21, p. 547Ð550. Dolan, M. T., 1993, Buried in mud: A look at the sedimento- logical and morphological characteristics of lahars on Pinatubo Volcano’s eastern slopes: Eos (Transactions, 3 American Geophysical Union), v. 74, p. 671. Fierstein, J., and Nathenson, M., 1992, Another look at the cal- culation of fallout tephra volumes: Bulletin of Volcanol- 1.0 ogy, v. 54, p. 156Ð167. Foit, F. F., Mehringer, P. J., Jr., and Sheppard, J. C., Jr., 1993,Age, distribution and stratigraphy of Glacier Peak tephra in east- ern Washington and western Montana, United States: Canadian Journal of Earth Sciences, v. 30, p. 535Ð552. Gardner, J., Carey, S., Sigurdsson, H., and Rutherford, M. J., 1995a, Influence of magma composition on the eruptive 0.1 activity of Mount St. Helens, Washington: Geology, v. 23, p. 523Ð526. Gardner, J., Carey, S., Rutherford, M. 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