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Michael C. Malin (2), Department of Geology, Arizona State University, Tempe, AZ 85287
(1) Now at: Science Applications International Corporation, SAIC/LMEPO/XM, Nova Building, 16904 Buccaneer Rd., Houston, TX 77058(2) Now at: Malin Space Science Systems, P. O. Box 910148, San Diego, CA 92191-0148
Table 1. Times of known events during the May 1915 eruptions
Date Time Description 14 May All day "New crater. Large eruption. Smoke-east ...Fire seen for first time by 8 per- sons...lasted all night...seen just be- fore daybreak [observations made from Manton]" (Day and Allen 1925) 15 May 2:30 A.M. "Light seen at top of Lassen at 2:30 A.M." (Day and Allen 1925) 17 May Evening Persons on all sides of the volcano re- ported a glow on clouds above the sum- mit crater (Day and Allen 1925) 19 May Evening "...after dark a steady glow of light was seen shining on cloud of smoke for several hours...Fire lava seen on top at 9:00 P.M .... " (Day and Allen 1925) Personnel at power station in Manton report seeing incandescent boulders rolling down west side of Lassen around sunset (Loomis 1926) These observations indicate that the dacite lava flow that now covers a portion of the summit was erupted between 14 and 19 May 20 May ~1:00 A.M. Ranchers in Hat Creek Valley forced to move to high ground because of exten- sive flooding of the creek (Loomis 1926) The extensive flooding was apparently caused by the emplacement of a major lahar into Hat Creek 22 May 4:45 P.M. "Terrific eruption. Incomparable with any former eruption...column of steam reached 30,000 feet" (Day and Allen 1925) This eruption caused the most damaqe of any preevious eruptions in the series; details in Diller (1916), Day and Allen (1925), Eppler (1987)
Of principal concern in this report is the nature of the eruption that occurred on the night of 19 May. Diller (1916) investigated the effects of the May 1915 eruptions during the following summer. He concluded that a steam blast, associated with the eruption of a dacite lava flow, had melted the residual snow pack on the northeast flank of the Peak and had triggered the large lahar that occurred during the early morning of 20 May. Day and Allen (1925) drew similar conclusions, although they felt that the snow was melted by the deposition of tephra, rather than by a hot steam cloud. Finch (1929, 1930) disputed the claims of Diller (1916) and Day and Allen (1925), primarily on the basis of the absence of tree blow down in the photographs taken by B.F. Loomis on the afternoon of 22 May 1915. Finch instead proposed that the lahar had been triggered by snow melted during the extrusion of the dacite lava. Finch (1930) specifically argued against the occurrence of a volcanic blast on the night of 19 May 1915. Williams (1932) adopted Finch's conclusions, and the understanding of these eruptions has remained essentially unchanged since his work (E. Richards, pers. commun., 1982). The characteristics of the eruption that occurred late on the afternoon of 22 May 1915 are discussed in Eppler (1987), and a more expanded treatment of the eruptions of May 1915 and a discussion of the lahars that they generated may be found in Eppler (1984).
Gorshkov (1963) introduced the term volcanic blast to describe destructive, laterally-directed explosive events that are distinct from vertical eruption columns. Two distinctive characteristics are important in the conceptual model of directed blast events: (1) horizontal or near-horizontal ejection angles, and (2) lack of a strong channelization by topography (Gorshkov 1963). For this work, a volcanic blast is defined as the effect of a violent eruption that causes significant destruction to the area surrounding the volcanic edifice, with an inferred significant radial component to the forces associated with the event. Any sediment, either juvenile or accidental in origin, that is emplaced during a volcanic blast will be termed "blast deposit."
The re-evaluation presented here is prompted by continuing concerns about future volcanic hazards in the Cascade Range, and as a test of recent concepts and techniques developed to examine processes in physical volcanology through field and laboratory analyses. Several factors complicate this evaluation: the larger eruption of 22 May covered essentially the same area as was affected on 19 May; the 19 May pyroclastic deposits were relatively thin and were immediately covered by a volumetrically larger lahar; and all the deposits have been subjected to 70 years of erosion and modification.
Three sources of information were examined in this study: (1) limited eyewitness observations of Lassen Peak made prior to and on the night of 19 May, which permit a rough estimation of the setting and chronology of events; (2) photographs taken by B.F. Loomis immediately after the 19 May eruption, which provide some spatial constraints on the type of eruptive activity; and (3) deposits emplaced during the May 1915 eruptions that can constrain the sediment transport characteristics of the events that occurred on 19 May.
Figure 1: Map of locations around Lassen Peak from which the 1914-1917 eruptive activity was observed
Eyewitness accounts are notoriously difficult to decipher, as the descriptions are usually biased toward the observer's previous experience and degree of scientific literacy. The qualitative nature of these observations (e.g., "medium-sized eruption" or "one of the largest eruptions seen") make them difficult to use for constraining eruption phenomena and processes. However, they can help establish the timing of events and document changes in the style of eruption, especially when such changes are seen by many observers.
One such change occurred during the third week of May, 1915. For the first time in the year since the start of the eruption cycle, "fire" was reported at the summit of Lassen Peak during the early afternoon of 14 May. This was noted initially by a group of eight people in Manton, including the postmistress, who recorded the event. It was a sufficiently anomalous event for her to telephone as many people as she could, and over 100 people saw the eruption during the night and morning hours of 14-15 May (Day and Allen 1925).
No report was made of events on the 16th, presumably because of poor visibility. A "red glow" was observed on the clouds above the summit on the evening of 17 May (Day and Allen 1925). On 19 May, at about 9:00 P.M., observers in Manton reported seeing incandescent material at the summit, and people in Viola observed incandescent boulders rolling down the west flanks of the volcano, presumably derived from the dacite lava flow that erupted at the summit and was emplaced partly down the west and northeast flanks of the volcano (Day and Allen 1925; Loomis 1926). These are the only reports of incandescence for the Lassen Peak eruptions, and they become more important when considered in light of subsequent events. No observations subsequent to those of 19 May mention "fire" or summit "glows" other than those attributed to a glow on the clouds above the summit at sunset or sunrise.
At around 1:00 A.M. on 20 May, about 4 h after the evening observations of incandescent boulders described above, ranchers about 30 km from Lassen Peak in the lower reaches of Hat Creek were forced to move to high ground due to a rapid increase in the discharge of the creek (Loomis 1926). This increase in discharge was presumably caused by the emplacement of a major lahar in the upper reaches of the creek. No other real-time eyewitness observations were made of the 19 May eruptions.
On 22 May, Loomis traveled to Emigrant Pass, about 5 km northeast of the summit of the volcano. At this location, he photographed and recorded his observations of the effects of the 19 May eruptions, just prior to the eruption of 22 May. Figure 2a is a photograph of the northeast flank of Lassen Peak taken from Emigrant Pass by Loomis on 22 May, and Fig. 2b is a sketch drawn from the photograph, delineating the areas discussed below. The time of day is estimated to be between 12:00 and 3:00 P.M., on the basis of shadow orientations. Prior to 19 May, the Peak was largely covered by snow, with some admixed ash from eruptions that occurred during late winter and early spring. Contemporary comparison of snow pack at Chester, California (~30 km southeast of Lassen Peak; see Fig. 1), and at Mineral (~20 km south; see Fig. 1) with that observed at Lassen Peak suggest that in May 1915 there was probably ~15 m snow on the upper slopes of the peak (Day and Allen 1925) and as much as 5 m in the vicinity of Emigrant Pass (E. Richard, pers. commun., 1983). Accordingly, the surface above the tree line that was unaffected by the 19 May eruption should appear white except around rock outcrops, as it does in Fig. 2.
Figure 2a: The northeast flank of Lassen Peak photographed on 22 May 1915 by B.F. Loomis from the position marked on Fig. 5, several hours prior to the 22 May eruptions. Peaks labeled A and B can be used as registration points when comparing Fig. 2a and Fig. 3. The label "hot rock" was written on the original glass plate by Loomis over the image of a piece of 19 May dacite lava and alludes to the fact that the rock was still too hot to touch ~48 hr after it was erupted.
Figure 2b: Sketch drawn from Fig. 2a, showing areas discussed in text: 1 - steam cloud rising from the summit area; 2 - lahar deposit emplaced on the night of 19 May; 3 - area inundated by the 19 May lahar; 4 - area believed to have been deforested on the night of 19 May; 5 - area unaffected by the 19 May eruption; 6 - area apparently blanketed by tephra and partially cleared of snow during the 19 May eruption
In Fig. 2, the summit is obscured by vapor cloud (Area 1), but several important features can be seen on the flank of the volcano. Area 2 extends downslope from the summit into gullies that form the head of Lost Creek. This area has sharp boundaries that appear to follow, and be confined by, Lost Creek. In the photograph, this area has a complex variation of gray tones, suggesting a rugged surface morphology. This area is interpreted as coarse-grained, bouldery lahar deposits that were emplaced in the upper reaches of Lost Creek on the night of 19 May.
In the foreground of Fig. 2 is a meadow, locally called Jessen Meadow, that has been inundated by the lahar deposit that was triggered by the 19 May eruption. The lahar appears to have been confined generally within the channel of Lost Creek until reaching the break in slope where Lost Creek enters the meadow. At this point, a portion of the lahar left the channel, spread out into the meadow and continued onto the divide between Lost and Hat Creeks.
Area 4 is a ridge to the southeast of Lost Creek that is clear of tree cover, in sharp contrast to slopes to the north of Lost Creek. It is not clear whether this ridge was deforested as a result of the 19 May eruption. A Loomis photograph taken in 1910 of Lassen Peak from Jessen Meadow (Fig. 3) shows that the ridges surrounding the volcano were heavily forested at that time. Due to the position of the camera, Fig. 3 does not show most of the area designated as Area 4 in Fig. 2b, but the density of the forest throughout the photograph suggests that this ridge supported a normal population of trees in mid-1910. There is no record of a violent eruption occurring prior to 19 May, nor was the area inhabited or subject to logging operations. Accordingly, it seems probable that the ridge was deforested by the 19 May eruption.
Figure 3: The area affected by the 19 May 1915 eruptions photographed by B.F. Loomis in 1910. Lost Creek debouches into the meadow at the break in the tree line (see arrow) and flows from left to right along the base of the ridge beyond the fence line. Peaks labeled A and B are the same peaks labeled in Fig. 2a
Extending outward from the summit is a light gray area (Area 6) that appears to be shaped like a fan with its apex at the summit, extending northeast toward the camera. This gray area has well-defined though feathery edges that do not appear to be confined by topography, and appears to be overlain in the summit area by the lahar deposits of Area 2. The margin of Area 6 is generally darker than the interior. The interior is also more variable in tone than the margin. This area is interpreted as snow covered by a blanket of tephra at the edges, with bare ground exposed in the interior. This interpretation is based on the apparent overall shape of the area, its lack of topographic confinement, and the feathery nature of its lateral boundaries. The variation of gray tones may be the result of progressively greater clearing of snow toward the axis of the fan, which allowed more of the surface texture previously covered by snow to show through.
First, a print of Loomis' 22 May photograph was digitized using a video camera attached to a Gould IP8400 image array processor operating on a VAX 11/750 minicomputer. This turned the print into a digital image that could be mathematically processed by the computer to change the viewing geometry of the original image, and to enhance contrast and improve the detail of the image. The resulting 512 x 512 x 8 bit image was processed to enhance detail and scene contrast. The results are shown in Fig. 4a.
Figure 4 (a-d): Computer processed images of Lassen Peak: (a) digitized Loomis photograph taken 22 May 1915 from the same location as Fig. 2; (b) digitized terrain model of the Lassen Peak area rotated down to the viewing perspective of a; (c) digitized Loomis photograph after projection onto the terrain model of the Lassen Peak area and rotation of both data sets to an overhead perspective. Superimposed on the image are topographic contours with 200-ft contour interval. The strong lineation seen on the processed image is the result of foreshortened boundaries in the unprocessed image being stretched as it is rotated to an overhead perspective; (d) boundary of the area affected by the 19 May 1915 eruption superimposed on the digitized terrain model
Next, a digital version of the 1:63,360 scale topographic map of the Lassen Peak area was created by tracing the contour lines on a GTCO X-Y digitizing table. Algorithms were applied to create an artificial three-dimensional surface from these contours, and to artificially illuminate this surface to produce a synthetic shaded relief image seen from a position approximately equal to the one from which Loomis photographed Lassen Peak on 22 May 1915. This image is shown in Fig. 4b.
Finally, identical positions were located on both the shaded relief image and the digitized version of Loomis' photograph. This allowed the digital photograph to be meshed with the terrain model, and allowed the photograph to be geometrically manipulated along with the digital terrain model. Both data sets were geometrically reprojected to produce an image of the photograph seen from an overhead, orthographic perspective. This, in effect, allowed viewing of Loomis' photograph as if it were taken from a position above the volcano, rather than looking obliquely across the terrain. Although this causes stretching and distortion of the photograph, it permits viewing of the geometrical relationships between the areas described in the previous section. The reprojected photograph is shown in Fig. 4c, with the digital topography overlain to relate the features seen on the photograph to the topography. There is a sufficiently good spatial correlation between known topographic features seen on the photograph and the contour expression of these features to give confidence in geometric relationships seen on the reprojected photograph. The exterior boundary of Area 6 was outlined and the digital photographic image was then removed, in order to show only its shape and relationship to the topography (Fig. 4d and Fig. 5).
Figure 5: Sketch map of the area affected by the 19 May eruption prepared using Fig. 3d. The question marks lie along the boundary defined by the edge of Loomis' photograph, and represent the limit of data available from the photograph. It is uncertain how far laterally this boundary extended to the northeast and southwest. Note the location of UL85-285 northwest of Hat Lake, and the approximate location from which B.F. Loomis photographed the area
Of particular interest is the strong linear shape of Area 6. The relatively large length to width ratio (>4:1) is unlike other volcanic blasts, such as that which occurred at Mount St. Helens (e.g., see Kieffer 1981, p 392, and Crandell and Hoblitt 1986). Area 6, which in oblique views appears consistent with the fan-shaped blast zones seen around other volcanoes, actually resembles more the planimetric form of a region affected by more topographically confined flows, such as lahars. The path also resembles that taken by many snow avalanches that begin along the eastern ridge crest at the summit of Lassen Peak. The area lies entirely within the zone delineated by tree blow down associated with the 22 May eruption, reinforcing field experience that there remain few localities where 19 May tephra deposits may be preserved without having experienced the effects of the 22 May volcanic blast. In addition, the distal portion of Area 6 lies within the area that was inundated by the lahar that was triggered by the 19 May activity and was emplaced at ~1.00 A.M. on 20 May (see Table 1). The lateral extent of the affected area in the lower meadow (i.e., close to the camera) is uncertain, and is marked by question marks in Fig. 5. This queried boundary represents the lateral limit of data available in Loomis' photograph. At present, we have been unable to find either photographic or written records that might indicate how far outside the queried boundary the area affected by the 19 May eruption extended.
Although it is possible that rain fell after the 19 May eruption, it seems unlikely that sufficient precipitation occurred between the early morning hours of 20 May and the morning of 22 May to allow complete erosion of the 19 May deposits.
Fortunately, a location about 250 m north of Hat Lake (Fig. 5, labeled UL85-285) preserves a sequence of deposits associated with the May 1915 eruptions. Included is a tephra deposit whose stratigraphic position, described below, suggests that it was emplaced on 19 May. This location was protected by a small ridge from experiencing the full force of erosion by lahars within the Lost Creek drainage basin, and received only thin marginal overflow from the 19 May lahar. The specific locality is within an inlier surrounded but not covered by a lahar deposit emplaced late in the 22 May eruption sequence. These lahar deposits appear to have subsequently protected the site and have preserved a record of events that occurred on 19 May. Excavation of the inlier to a depth of 50 cm reveals four distinct deposits (Fig. 6). The lowermost is a soil formed from a mixture of fluvially- and glacially-deposited detritus derived from Lassen Peak, and pyroclasts erupted from the Chaos Crags ~1080 years B.P. (D. Trimble, pers. commun. 1982). Organic debris associated with the pre-1915 forest cover is no longer visible as a discrete layer, and was probably incorporated into the tephra deposit from 19 May.
Figure 6: Photograph of the pit excavated at UL85-285, showing the relationship between May 1915 eruption products. The uppermost deposit is the 22 May volcanic blast deposit (UL85-285A); the intermediate deposit is the marginal deposit from the 19 May lahar (UL85-285B); the lower deposit is interpreted as the volcanic blast deposit from the 19 May eruption (UL85-285C). The bottom of the pit is in pre-May 1915 soil.
Above the old soil are three May 1915 deposits. The uppermost (UL85-285A in Fig. 6) has a flat upper surface an irregular lower contact. The deposit ranges from 10-15 cm in thickness. No sedimentary structures were visible in the limited exposure of the pit. Median grain size (Mdø) is 0.5ø and the sorting coefficient (Sø) is 2.1ø, indicating poor sorting. Binocular and scanning electron microscope examination of grains indicate that the pyroclasts are similar in composition and morphology to those found in many locations affected by the 22 May volcanic blast (Eppler 1987). This uppermost deposit was most likely emplaced by the 22 May blast cloud (Eppler 1987).
Immediately below UL85-285A is another layer (UL85-285B in Fig. 6) that has irregular upper and lower contacts and is also 10-15 cm thick. The deposit has poorly developed normal grading. Mdø for an aggregate sieved sample is 1.1ø, and it is poorly sorted with a Sø of 2.4ø. The deposit includes black, glassy grains with large plagioclase phenocrysts that appear to have been derived from the 19 May dacite lava flow; altered grains of dacite erupted from the Chaos Crags and Lassen Peak prior to the 1914-1917 eruptive cycle; and tephra derived from the underlying deposit. The 19 May dacite lava grains are distinguished from the older Lassen dacite by fresh, glassy surfaces and lack of hydrothermal alteration. Scanning electron microscope examination of the grains from UL85-285B show the deposit to be composed mostly of lithic and mineral fragments that have been partially rounded by grain to grain collisions during transport (Fig. 7a). Rare pumices present in the deposit are also partially rounded and have a sizable population of smaller particles, most likely clay minerals or clay-sized particles, within the vesicles (Fig. 7a). The rounding of the grains, and the presence of fine-grained sediment in void spaces, suggests transport within a wet slurry composed of abundant clay- and siltsized particles. The presence of grains derived from the 19 May dacite lava flow, the condition of constituent grains in the deposit, and the similarity of this deposit to the 19 May lahar deposits seen at many localities along Lost and Hat Creeks (Eppler 1984) make it reasonable to conclude that this unit is either material emplaced by splash from the 19 May lahar, or by runoff associated with the dewatering of the same lahar deposit after emplacement.
Figure 7 (a-f): Scanning electron photomicrographs of samples from UL85-285: (a) pumice and lithic grains that have been partially rounded during transport. Also note the fine-grained sediment filling void spaces and adhering to individual grains. (b) Plagioclase grain coated with vesicular glass. Note the stretched vesicles and general lack of chipping and edge modification. (c) Pumice grain with small, primarily spherical vesicles. (d) Pumice grain with spherical vesicles substantially larger than in (c). (e) Glass fragment that has undergone some chipping and edge modification during transport. (f) Plagioclase grain that has been partially rounded during transport.
The lowermost deposit of interest at this locality (UL85-285C in Fig. 6) unconformably overlies the soil horizon. It is coarser grained than the upper deposits, and has a speckled appearance caused by the presence of large plagioclase phenocrysts in clasts >0.5 cm. The deposit is generally ungraded, although it occasionally appears to have poorly developed reverse grading. Additionally, in some portions of the pit it is matrix supported, while in others it appears to be clast supported. The deposit ranges in thickness from 4-15 cm. Grain size analyses yield an Mdø of 0.4ø and Sø of 1.7ø, indicating poor sorting.
Macroscopically, UL85-285C consists of clasts of the 19 May dacite lava; fine-grained, dark gray pumice; and intimately intermixed pieces of charred and uncharred organic material (Table 2). Binocular and scanning electron microscope examination of the deposit indicates that a portion of the finegrained sediment is fresh, glassy pumice with a wide range of vesicle sizes and proportions (Fig. 7 b-d). The vesicles in the pumice are often flattened and stretched, and in thin section this deformation gives the appearance of flow banding. There do not appear to be any juvenile pumices that strongly resemble the juvenile pumices erupted on 22 May; neither does there appear to be a continuum between the vesicular grains and more massive lithic grains. Mineral grains as well as pumices show some indication of grain to grain collision during transport (Fig. 7 e,f), but less than seen on similar clasts within the 22 May deposit (Eppler 1987). Either collisions were less vigorous on 19 May than on 22 May (Eppler 1987) or there may have been fewer collisions. There is no evidence of transport by fluvial processes, and there are few adhering silt- or claysized particles within the pumice vesicles.
Table 2. UL85-285C grain count data
Component Number of Percent Description grains 19 May dacite 199 40.0 Subhedral plagioclase phenocrysts, lava zoned,rounded and embayed; smaller broken plagioclase phenocrysts, rounded biotite, with opaques at rim, also very rare biotite pseudo- morphic after olivine;rounded and embayed clinopyroxene some with rims of amphibole; rare subhedral hornblende; all set in vuggy/vesi- cular groundmass with devitrified glass, feldspar needles and opaque oxides; vugs appear to be undeformed Fresh pumice 111 22.3 Phenocrysts of euhedral-subhedral plagioclase,euhedral amphibole; set in vesicular-vuggy, glassy groundmass with minor needles of feld-spar, opaque oxides; voids have been flattened and deformed, giving the pumice a flow-banded ap-pearance in plane light Pre-1915 dacite 104 20.9 Similar mineralogy to 19 May dacite lava; distinguished by the absence of glass and extensive hydrothermal alteration causing Fe-staining of groundmass Mineral grains 84 16.8 Plagioclase 56 11.2 Quartz 26 5.2 Opaque oxides 2 0.4 Total 498 100.0
The organic material in UL85-285C consists of small red fir needles, pieces of bark, and segments of twigs. The majority of fir needles retain their shape but have been completely charred. The pieces of bark are only partially charred, and in most cases, the area of charring is greatest at the surface, decreasing toward the interior. Several pieces of bark are completely charred but retain their internal structure. Such effects indicate temperatures in excess of 250 deg C applied for periods of at least several minutes Winner and Casadevall 1981) under oxygen-poor conditions.
The stratigraphic position, composition, and the presence of fresh, glassy pyroclasts and carbonized organic material suggest that the lowermost unit is a deposit emplaced by eruptive activity on 19 May 1915. The mechanism of emplacement of the deposit is unclear. Waitt (1981) described a basal gravel facies emplaced by the 18 May 1980 blast deposit (layer Ai) at Mount St. Helens that resembles UL85-285C. Waitt's layer AI is massive to weakly stratified and in general consists of a sandy to gravely layer of juvenile dacite, accidental lithic fragments, and wood fragments. This deposit ranges from ungraded to normally graded in some locations to reversely graded in others. Waitt (1981) attributed this layer to deposition by a highly turbulent pyroclastic density flow. We suggest that the deposit at UL85-285C may have been deposited by a similar type of flow. However, inability to document areal variations due to the lack of additional exposures makes determination of such a specific mechanism of emplacement impossible. For the purposes of the following discussion, we prefer to use the term "blast deposit" to describe UL85-285C. In this case, the term is used to describe a deposit for which a specific fluid dynamic model of emplacement cannot be determined, but which was emplaced as part of a violent, probably explosive eruption.
Table 3. Summary of characteristics of the 19 May eruption
Characteristic Implication Two juvenile components are The eruption involved produc- present in sample UL85- tion of juvenile pumice as well 285C: 19 May dacite lava as disruption of the 19 May da- and fresh, glassy pumice cite lava flow Mineralogy of the juvenile Both components may have been components appears to be derived from the same source extremely similar Vesicles in the pumices were Pyroclasts were plastic enough deformed to deform when subjected to stress during the eruption Organic material intimately Organic material was mixed into mixed with pyroclasts in the deposit during transport/ the deposit deposition by the emplacement process Needles pyrolized Deposit was >250 deg C immediately after deposition (Winner and Casadevall 1981) Needles charred in an oxygen- poor environment after burial Large pieces of bark not There was sufficient heat in completely charred the deposit to char small, slender needles, but insuffi- cient to completely char large, equant pieces of bark Pyroclasts not strongly Grain to grain collisions were abraded either rare or of a low kinetic energy Boundary of the disturbed The eruption did not occur as a area is dissimilar to dis- point-source type of explosion, turbed areas from point- but has the characteristics of source explosions a mass-movement event
The exact mechanism by which the 19 May volcanic blast was generated is unknown. The shape of the affected zone is not like that of other volcanic blasts attributed to directed events (e.g., Mt. Pelee, 1902, in Fisher and Heiken 1982), or where hydrodynamic pressures have dominated the distribution of effects (e.g., Mount St. Helens, 1980, in Kieffer 1981). Rather, the resemblance of the affected zone to a mass movement suggests that the blast behaved as a "ground hugging" flow. However, no geomorphic evidence of the source of this flow remained on 22 May, suggesting that its generation did not result from the collapse and subsequent avalanching of a major part of the volcanic edifice, nor from the excavation of a crater on the east face of Lassen Peak.
It seems plausible that the blast was in some way associated with the emplacement of the dacite lava flow. Two possibilities can be readily suggested. First, the flow may have collapsed, producing a landslide which traveled down the steep eastern slope of Lassen Peak, similar to, but with a significantly greater volume than the incandescent boulder falls that were observed moving down the gentler western slopes. Under such conditions, the lava would have broken up and become intimately mixed with the more than 15-m-deep snowpack, generating an explosive production of steam in a fuel-coolant reaction similar to that described by Colgate and Sigurgeirsson (1973), and produced under experimental conditions by Wohletz and McQueen (1984). The large volume change brought about by the water to steam reaction would have fragmented the lava further, enhancing the heat transfer and steam production. The shape of many of the vesicles indicates that some vesiculation and deformation took place in the pyroclasts after their formation. It is not clear, however, what contribution the continued growth of the vesicles made to the magnitude of the blast. A similar set of circumstances (i.e., one involving the breakup of the hot dacite cryptodome and its intimate mixing with the water in Spirit Lake) has been proposed by Moore and Rice (1984) to explain the more energetic pulse of the 18 May 1980 eruption of Mount St. Helens that occurred nearly 8 km north of the volcano and almost 2 min after the initial summit explosion.
The second possibility, suggested by the presence of pumice, the degree of vesiculation of that pumice, and the range in shapes and sizes of the vesicles, is that the dacite lava flow produced a pyroclastic flow. Rose et al. (1976) reported on the eruption of a pyroclastic flow from the toe of an active dacite lava flow at Santiaguito Volcano in Guatemala. The pyroclastic flow, which was triggered when the lava flow collapsed during extrusion (Rose et al. 1976), produced many features resembling those described above, particularly strong similarities in the shape of the area devastated, the proportions of various grain types in the flow deposit, and similarity in vesicle shape and degree of vesicularity in the two deposits. It is clear from the Santiaguito eruption that silicic lava flows can produce violent events that emplace volumetrically significant pyroclastic deposits without the involvement of an underground magma chamber. The additional factor at Lassen Peak of mixing of hot dacite pyroclasts with snow probably increased the violence of the event.
The temperature inferred by the presence of pyrolized organic material in UL85-285C, the fragmentation characteristics of materials within the deposit, and evidence of transport with some energetic collisions between particles, is consistent with either of these hypotheses. A third possibility, that the deposit could have been generated by a phreatic explosion at the summit, seems much less likely, given the shape of the affected area as seen in Figs. 4 and 5, the presence of juvenile pumice, and the lack of a crater that would have formed in such an explosion.
The data on the 19 May eruption that are presently available do not allow the determination of the exact genetic mechanism for the eruption of 19-20 May, and for the subsequent emplacement of the pyroclastic deposit at UL85-285. However, they strongly suggested that a violent, high temperature event took place on the night of 19-20 May, as was suggested originally by Diller (1916) and Day and Allen (1925).
*Originally submitted 16.6.86; Resubmitted 3.2.1987
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