By Donal R. Mullineaux 

U. S. Geological Survey Professional Paper 1563


Mount St. Helens is much younger and has been more explosively active recently than other major Cascade Range volcanoes. In addition, its sequence of eruptive products records more changes in chemical and mineral composition than is typical of the other volcanoes. These fluctuations reflect many differences in physical or chemical conditions, or both, in the source magma or magmas.

Much of the known eruptive record of Mount St. Helens has been determined from study of fragmental deposits, including study of flowage deposits (Crandell, 1987) and tephras described herein. Fragmental deposits are particularly useful for study of eruptive histories because they form series of strata that are highly susceptible to erosion, which exposes their stratigraphic sequences. They are also widespread, and they commonly char, bury, and preserve carbonaceous material, which provides radiocarbon ages for the eruptive events.

Even the complex eruptive history recognized for Mount St. Helens does not include all of its past eruptive events. For example, several indistinct, poorly preserved strata of unknown origin lie between tephra sets C and M; they suggest additional, unidentified eruptions. Moreover, comparison of small observed eruptions in October 1980 with the resulting obscure deposits indicates that comparable eruptions in the past might not have been detected during this study of tephra.

The youth of Mount St. Helens is demonstrated by the recency of both its oldest deposits and its visible cone. The oldest known deposits are only about 40,000 or perhaps 50,000 years old, in contrast to rocks at other major Cascade Range volcanoes that are more than a hundred thousand years old. The cone of Mount St. Helens is also young compared to other large Cascade Range volcanoes. Before 1980, Mount St. Helens was a relatively symmetrical, little-eroded structure emplaced over older ridges and valleys (fig. 1). Its symmetry and smooth slopes were recognized by early explorers and scientists alike as evidence that the edifice was young. Just how young was surprising; most rock formations visible on the cone before 1980 were less than 2,500 years old, and those on the upper part were less than 1,000 years old.

Mount St. Helens has also been much more active, especially explosively active, during the last 40,000 years than its sister Cascade Range volcanoes. During the past 4,000 years, for example, it produced more than 50 identifiable tephra strata, numerous pyroclastic-flow and hot lahar deposits, several domes, and lava flows that flowed down all sides of the volcano. In contrast, during that same 4,000-year period Mount Rainier produced only a few tephra deposits (Mullineaux, 1974) and relatively few deposits of pyroclastic flows, hot lahars, or lava flows (Crandell, 1971).

Subdivisions of eruptive history

The eruptive history of Mount St. Helens can be subdivided into (1) old and modern segments according to major compositional changes and (2) several stages and periods according to the episodic nature of its eruptions (table 7).

Table 7. Classification of pre-1980 eruptive history, Mount St. Helens.

Old Mount St. Helens

Compositionally, an early silicic volcano that existed until about 2,500 years ago can be distinguished from the modern, more mafic cone. The early volcano, termed "old Mount St. Helens" by Verhoogen (1937), consisted of dacite and silicic andesite. It produced abundant tephra, pyroclastic-flow deposits, domes, and short lava flows, some of which in turn generated other pyroclastic flows and lahars. Close to the volcano, pumiceous tephra deposits of old Mount St. Helens commonly are as thick as several meters (fig. 3). Pyroclastic-flow deposits from the old volcano are major constituents of thick, flat-topped stacks of strata that partly fill valleys leading away from the cone (Crandell, 1987). Although remnants of old domes and lava flows are sparse, lithic pyroclastic-flow deposits provide evidence of their existence (Crandell, 1987). The dominantly fragmental deposits of the old volcano include the rocks of the "old Mount St. Helens series" of Verhoogen (1937, p. 268) and the "ancestral volcanic center" of Hopson (1971).

Modern Mount St. Helens

The modern volcano began with the eruption of mafic magma shortly after 2,500 years ago. Most eruptions of the modern volcano occurred during one of two main episodes: between 2,500 and l,500 years ago, and between about 500 years ago and the present. A brief eruptive episode occurred about 1,200 years ago.

The mafic eruptions that initiated the modern volcano produced andesitic and basaltic tephra, pyroclastic flows, and hot lahars as well as lava flows; however, not all magma erupted by the modern volcano was mafic. Even after the mafic magma appeared, dacite continued to be erupted intermittently, producing tephra, pyroclastic flows, and domes. The resulting modern cone is a complex pile of mafic lava flows, dacitic domes, and dacitic and andesitic deposits of pyroclastic flows, lahars, and tephra, all of which have been intruded by dikes and plugs of basalt, andesite, and dacite.

Eruptive Stages and Periods

Both the old and modern volcanoes were strongly episodic as well as variable in composition, and thus the eruptive history can be divided into many parts. The pre-1980 history of the volcano as now recognized includes four eruptive stages, the Ape Canyon, Cougar, Swift Creek, and Spirit Lake; the Spirit Lake is further divided into six eruptive periods (Crandell, 1987) (table 7). Each stage and each period represents an episode of multiple eruptions characterized by close association in time, similarity in composition, or both.

Initially, Mount St. Helens' eruptive record was subdivided into nine eruptive periods (Crandell and others, 1981; Mullineaux and Crandell, 1981). In that classification, eruptive periods before 4,000 years ago had durations of thousands of years, in contrast to younger ones that had durations of only centuries. To remedy that disparity, Crandell (1987, p. 12-13) reclassified the first three periods as stages, and combined the last six periods into a single stage. In the revised classification, each stage includes repeated episodes of activity and intervening dormancy spread over more than a thousand years.

Deposits of the first two stages, Ape Canyon and Cougar, are relatively poorly preserved, and we have only sketchy knowledge of their eruptive history. All deposits of these stages were subjected to severe erosion and other disturbances during the last major glaciation and have been weathered for a long time compared with other Mount St. Helens products. Deposits of the following Swift Creek stage are much better preserved; however, they have been subjected to rigorous processes caused by a cold, late-glacial climate and more than 10,000 years of postglacial weathering. In contrast, products of the youngest (Spirit Lake) stage are remarkably well preserved, and the eruptive history of that stage is much better known.

Each stage was separated from the next by a long dormant, or at least relatively quiet, interval that can be inferred from buried weathering profiles or absence of eruptive products. Some evidence suggests that at least minor eruptions did occur during intervals between the first three stages, whereas no evidence has been seen that suggests any such activity between the latest two stages, the Swift Creek and Spirit Lake.

Ape Canyon Stage

The Ape Canyon stage began with the small-volume eruptions that apparently record the birth of the volcano. The first evidence known of a Mount St. Helens is in the multiple, thin beds of layer Cb, which record small, mild to moderately explosive eruptions. These eruptions probably also created domes and perhaps pyroclastic flows, but tephra of layer Cb is their only identified product. The outbursts that produced layer Cb may have occurred in rapid succession because no evidence of a pause long enough to form even an incipient soil was found within the layer.

The volcano then was dormant long enough for an oxidation profile to form in the upper part of layer Cb. The length of time represented by the profile is not known, but comparison with profiles in younger, better dated deposits suggests more than a thousand years. Evidence from studies of Mount St. Helens tephras far downwind suggests the possibility that a much longer time, perhaps as much as 10,000 years, passed between deposition of layer Cb and younger layers of set C (Busacca and others, 1992).

Small-volume eruptions of pumice also began the next, main series of Ape Canyon events. These were followed, without evidence of a pause, by large-volume eruptions that produced layers Cw and Cm. Those were followed in turn by repeated outbursts that produced thin beds of probable tephra and thicker deposits of probable ash-cloud origin. That series of eruptions was interrupted at least three times by pauses long enough for weakly oxidized soils to form. The Ape Canyon tephra eruptions then culminated in highly explosive outbursts that produced the voluminous layers Cy and Cs. Layer Cs, which may be correlative with layer Cy, is the largest volume tephra known of Pleistocene age.

During that main series of Ape Canyon events, the volcano also produced pyroclastic flows and surges and their associated ash clouds as well as mudflows (Hyde, 1975; Crandell, 1987). Prismatically jointed lithic blocks in one mudflow deposit suggest the presence of one or more domes (Crandell, 1987, p. 19).

The main series of Ape Canyon eruptions probably spanned at least 2,000 years. Two radiocarbon ages from just underneath tephras erupted during that time (W-2661 and W2976, table 2) and one from a volcanic mudflow (Hyde, 1975) at Mount St. Helens are between 38,000 and 36,000 years B.P. A radiocarbon age of about 33,650 years B.P. from 25 cm above tephra of Ape Canyon age in Nevada (Davis, 1978, p. 45) is consistent with the ages obtained from deposits near the volcano.

Thick flowage deposits of the Ape Canyon stage extended down the North Fork Toutle valley and probably aggraded it to at least as far downstream as the Cowlitz River valley (Crandell, 1987; Scott, 1988). Those deposits must have also dammed the North Fork Toutle valley north of Mount St. Helens to produce the first of many versions of Spirit Lake.

Ape Canyon-Cougar Interval

About 15,000 years passed between Ape Canyon and Cougar stage eruptions. No unequivocal primary eruptive products have been identified at Mount St. Helens that represent that period, although some evidence suggests that the volcano was not completely dormant. Fine, ash-rich detritus accumulated on large areas of uplands during at least three separate episodes during this interval. In addition, thin, discontinuous lenses of small pumice lapilli within those deposits suggest that some tephra was erupted; however, these pumice deposits are not sufficiently voluminous or well preserved to be identified satisfactorily as to origin, and thus unambiguous evidence of eruptions is lacking. That lack of evidence, however, could be due only to lack of preservation because voluminous tephras may have been erupted and carried only northeasterly. In the Cascade Range to the northeast, any such tephra would be severely eroded and not easily found. Farther to the northeast in Canada, two tephras that are between 35,000 and 20,000 years old and are characterized by cummingtonite have been identified (Westgate and Fulton, 1975).

Cougar Stage

The Cougar stage, which apparently lasted only 2,000-3,000 years, is characterized by tephra eruptions that were less voluminous than those of Ape Canyon time but show more compositional variation. Those eruptions also produced large pyroclastic flows and lahars (Crandell, 1987), one or more lava flows of dacite or siliceous andesite (C.A. Hopson, written commun., 1974), and probably one or more dacite domes of similar composition. Two episodes of tephra production are identified within the Cougar stage and are separated by enough time to form an oxidized soil profile.

The Cougar stage apparently began with mildly explosive or nonexplosive events that produced lahars, a debris avalanche, and a siliceous-andesite lava flow (Crandell, 1987, p. 24). Explosive eruptions then created large pumiceous pyroclastic flows that travelled down the southeast, south, and west sides of the volcano (Crandell, 1987); deposits of those flows are characterized by hypersthene and hornblende. These eruptions were followed by outbursts that produced the multiple pumiceous layers of tephra set M and some coeval ash deposits that probably were derived from pyroclastic flows. The first set M tephras are characterized by cummingtonite, which is progressively replaced upward in the set by hypersthene.

Eruptions of now recognizable tephra layers stopped temporarily after deposition of set M. Ashy fine material that accumulated on top of the set suggests, however, that at least minor eruptions, perhaps of tephra or pyroclastic flows, continued.

The next eruptive episode of this stage produced a sequence of tephras very different from earlier ones. Eruptions that produced tephra set K as recognized were small volume, intermittent, and repetitive in terms of scale and composition. Some larger volume eruptions may have occurred at about that time, as suggested by strata in an outcrop northeast of the volcano, but the age relation of these strata to set K tephras is not known. Set K was followed by eruption of voluminous pyroclastic flows that moved down the south and southeast flanks of the volcano. Those flows are the last known products of the Cougar stage.

During Cougar time, large pyroclastic flows and lahars filled the Lewis River valley south of Mount St. Helens to a depth of more than a hundred meters and aggraded the valley far downvalley (Hyde, 1975; Crandell, 1987). Detritus from the volcano so overwhelmed the Lewis River that pyroclastic-flow deposits of Cougar age have surface gradients of as much as 25 m/km from north to south across the Lewis River valley directly south of Mount St. Helens.

The ferromagnesian mineral suites in products of Cougar time record different magmatic conditions at various times during this stage. Experiments of several investigators (see Geschwind and Rutherford, 1992, and references therein) indicate that magmas characterized by cummingtonite last equilibrated at lower temperature and perhaps higher water content than those characterized by hypersthene. Several differences or changes between relatively cool and hot magmatic conditions are recorded in the Cougar-stage products. It is not known, however, whether these changes represent different source magmas, different parts of a nonhomogeneous magma, or changes in a magma with time.

The hypersthene-rich ferromagnesian suites of early products suggest that the source magma had equilibrated at a higher temperature than that of last magma erupted at the end of the previous, Ape Canyon, stage. But, by the time the first layers of set M were erupted, the dominance of cummingtonite suggests that the source magma or magma batch for the set M tephras had equilibrated at a lower temperature. No intervening products characterized by both cummingtonite and hypersthene are known.

During set M eruptions, temperatures of the source magma apparently became progressively higher, as indicated by hypersthene substitution for cummingtonite upward in the sequence. Such a change could have resulted from progressive heating of the source magma by injection of new magma or perhaps by tapping of successively deeper parts of a magma body (Hopson and Melson, 1990). Presence of olivine in layer Mo suggests that a new magma, if present, was mafic.

The sequence of set K tephras and the pyroclastic-flow deposits that followed set M indicates again a relatively low-temperature source magma and a return to a higher temperature magma.

Cougar-Swift Creek Interval

Little is known about the relatively quiet interval of about 5,000 years that followed the Cougar stage. Accumulation of ash-rich fine sediments on uplands suggests some volcanic activity, but no deposits containing pumice lapilli were seen in those sediments. Because this interval occurred during the latter part of the last major glaciation, eruptive products of the time could have been so severely eroded or altered that they were not identified in this study.

Swift Creek Stage

During the Swift Creek stage, between about 13,000 and 10,500 years ago, the volcano produced large volumes of pyroclastic flows and moderate to large volumes of tephras that extend hundreds of kilometers downwind. The Swift Creek stage includes two distinct episodes of tephra production, one about 13,000 years ago and the second between about 12,000 and 10,500 years ago. Between these episodes, multiple pyroclastic flows and lahars built extensive valley fills (Crandell, 1987).

The Swift Creek stage began with the production of tephras and ash-cloud deposits, and ash-cloud deposits continued to be produced throughout the time of deposition of set S tephras. The final two eruptions of set S produced the two largest volume tephras since Ape Canyon time. Although only the last two tephras of set S were of such large volume, as many as three beds of set S have been recognized at multiple sites in eastern Washington as far as 300 km east of the volcano (Foley, 1976, 1982; Hammatt, 1976; Moody, 1977; Mullineaux and others, 1978; Busacca and others, 1992). The presence of ash-cloud deposits interbedded with tephra of all parts of set S indicates that pyroclastic flows were produced repeatedly during that part of the Swift Creek stage.

Set S tephra was followed by many lahars and lithic pyroclastic flows; some lithic pyroclastic flows probably were derived from domes (Crandell, 1987). Flowage deposits were voluminous enough to produce extensive valley fills, especially southeast of the volcano (Crandell, 1987, p. 36). As the pyroclastic flows and lahars filled valleys, ash-rich deposits accumulated on upland surfaces.

The next Swift Creek events produced thin ash deposits at the base of set J. The volcano then erupted at least three large-volume tephras but apparently no associated pyroclastic flows. Ash from these highly explosive outbursts extended far to the east of the volcano. The resulting ash beds have been identified not only in eastern Washington but also as far as Montana (Carrara and others, 1986).

The final eruptions of Swift Creek time produced the thick, coarse tephra layer Jg, which extends toward the west. Limited distribution relative to thickness and grain size suggests that either the eruptive column was low or the winds were of low velocity during its eruption.

As in the Cougar stage, the ferromagnesian minerals in Swift Creek deposits record differences in source-magmas conditions from one eruptive episode to another. The cummingtonite in set S suggests a magma that had equilibrated at a lower temperature than the source magma of the last-known eruptions of Cougar time. The disappearance of cummingtonite and its replacement by hypersthene in tephras of set J indicate again a relatively high temperature in the source magma. The last Swift Creek tephra erupted is also the least silicic and may record mixture of invading mafic magma and preexisting dacitic magma.

Swift Creek-Spirit Lake Interval

During this interval, from about 10,500 to 4,000 years ago, Mount St. Helens apparently was completely dormant. No evidence has been found that indicates that the volcano erupted at all during that time; a search during many years has turned up no evidence of eruptive products attributable to that interval. Nor, in contrast to the earlier intervals between stages, is there any oxidized, ash-rich bed that would suggest unrecognized events during this interval. Obviously, the possibility of minor eruptions cannot be ruled out because their products could have filtered down into the disturbed soil zone at the top of the deposits of Swift Creek age. This interval, however, is the longest in the history of the volcano for which we have no evidence whatever of eruptive activity.

Spirit Lake Stage

Eruptions during the Spirit Lake stage were responsible for building the volcano generally recognized as Mount St. Helens. The six eruptive periods of the Spirit Lake stage produced the rocks that make up the visible cone and record the compositional change from the older Mount St. Helens to the more mafic and variable modern volcano. Highly explosive eruptions of voluminous, pumiceous tephra are notable features of the earliest period. During later periods, domes, pyroclastic flows, and lava flows became more important.

Smith Creek Period

The first period of Spirit Lake time consists of three eruptive episodes that differ distinctly in character. The Smith Creek period began with several explosive pulses of small to moderate volume that produced the thin but widespread pumiceous layer Yb. The distribution of these beds shows that wind directions changed between or during the eruptions. Deposition of layer Yb was followed, with no apparent pause, by numerous but probably less gas rich and less vigorous ejections of lithic to somewhat vesicular tephras of layer Yd.

Mount St. Helens then was dormant for a few hundred years. During that time, surficial processes formed a thin, incipient soil that is commonly topped by sparse carbonaceous material. This soil is weak but persistent and can be identified from northeast to southeast of the volcano.

Highly explosive and voluminous eruptions characterize the second phase of Smith Creek time. The second phase started with the discharge of layer Yn, the largest volume tephra of Holocene time. Layer Yn is present along a narrow lobe to the north-northeast that extends for at least several hundred kilometers across the northwestern United States and into Canada (Crandell and others, 1962; Westgate and others, 1970). The tephra eruption was closely followed by one or more pyroclastic flows that produced thick ash-cloud deposits; these deposits directly overlie layer Yn close to the volcano. Another highly explosive and voluminous pumice eruption then produced the Ye tephra, which was carried eastward across Washington State and at least as far as Idaho (Smith and others, 1977). Eruption of the Ye tephra was also followed by pyroclastic flows, which produced ash-cloud deposits that cover the tephra layer near the east base of the volcano.

Tephra eruptions that followed layer Ye were much smaller and are characterized by less vesicular pumice and higher proportions of lithic fragments. Interbedding of ash-cloud deposits and abundance of lithic clasts suggest that the eruption of tephra was interspersed with formation of pyroclastic flows and domes. This activity apparently continued with no pauses long enough to form recognizable soils until the end of the Smith Creek period. One conspicuous, hot lithic pyroclastic flow of late Smith Creek time entered the Smith Creek valley, picking up and charring logs (Crandell, 1987); its associated ash cloud spread over adjacent slopes and formed the charcoal-bearing ash bed ya.

Pyroclastic-flow and lahar deposits of the Smith Creek period raised the level of the fan on the north flank of the volcano, and lahars extended down the North Fork Toutle River Valley at least 50 km below Spirit Lake. The thick deposits of Smith Creek age that crop out downvalley indicate that an early Spirit Lake was formed or expanded at that time.

Radiocarbon dates show that the Smith Creek period began shortly after 4,000 years ago and ended at some time after 3,350 years ago. Ages at the base of layer Yn of about 3,500 years demonstrate that the dormant time between eruption of layers Yb and Yd lasted no more than a few hundred years.

Smith Creek-Pine Creek Interval

This interval lasted at most about 300 years and may have been much shorter. Some eruptions occurred during the interval, but apparently none were strong enough to produce lapilli-size clasts on the lower flanks of the volcano or nearby. Only the ash bed pbp, with its faint bedding, records activity during the Smith Creek-Pine Creek interval. The thin lamina within ash bed pbp exhibit successive changes in ferromagnesian mineral composition that record a series of some kind of small-volume eruptive events. They also apparently record a progressive change from a relatively low temperature magma, characterized by cummingtonite, to a higher temperature magma characterized by hypersthene. The magma under Mount St. Helens at the time may have been heated by injection of mafic magma that did not reach the surface until several hundred years later, during the eruptions of Castle Creek time.

Pine Creek Period

Pine Creek time, relative to Smith Creek time, was marked by much lower explosivity. It was characterized by discharge of small-volume tephras and large-volume pumiceous and lithic pyroclastic flows (Crandell, 1987). Lithic pyroclastic-flow deposits, probably derived from relatively nonexplosive eruptions of dacite domes, extend as far as 18 km from the volcano (Crandell, 1987).

The earliest eruptions of the Pine Creek period probably formed a dacite dome or cryptodome. The first tephra eruption, of layer Pm, was strongly explosive but not voluminous. The tephra contains abundant fresh, lithic fragments of hypersthene-hornblende dacite. Evidently, degassed magma or rock of Pine Creek composition was emplaced on or near the surface before being fragmented during the eruption of layer Pm. Several successive small-volume tephra eruptions produced the thin, multiple beds that make up layer Ps; these beds contain abundant lithic clasts as well as clasts of pumice. The eruptions of layers Pm and Ps presumably were interspersed with pyroclastic flows and domes that provided a source of fresh, lithic clasts.

Although no recognized tephra deposits are present between layers Ps and Pu, ash of probable ash-cloud origin suggests that pyroclastic flows formed during that time.

When tephra eruptions resumed, they produced the thin, multiple beds of layer Pu, which are remarkably similar to those of layer Ps. As during the early part of Pine Creek time, abundance of fresh lithic clasts indicates that domes or cryptodomes formed between tephra eruptions.

The final tephra of Pine Creek time records at least three explosive events, during which gas-rich magma was erupted along with lithic fragments. Yellow-gray ash of probable ash-cloud origin that is associated with beds of layer Py indicates that at least one pyroclastic flow occurred during the eruptions of tephra.

Pine Creek time spanned most of an interval from about 3,000 to 2,500 years ago. During that time, lahars and fluvial deposits consisting mostly of newly erupted material strongly aggraded the valley floors of the North Fork Toutle River and other rivers that headed on the volcano. They raised the surface of the fan that impounds Spirit Lake and, 40 km down the North Fork Toutle valley, blocked a small tributary valley to create the basin that holds Silver Lake (Mullineaux and Crandell, 1962). They also formed a continuous fill across the floor of the Cowlitz River Valley near Castle Rock and another fill in the Lewis River valley, which, near Woodland, was about 7.5 m higher than the present floodplain (Crandell and Mullineaux, 1973).

Pine Creek-Castle Creek Interval

This interval was short, lasting at most about 300 years and very likely much less. Evidence of volcanic activity is limited to that provided by ash bed v, which suggests that some volcanic events of small volume occurred. The lack of charred vegetation in or at the top of ash bed v suggests that no dense stand of vegetation was established during this interval.

Castle Creek Period

The defining feature of this period is the first appearance of mafic magma at the surface, which initiated the modern Mount St. Helens and its variety of rock composition. Andesite, dacite, and basalt all were erupted during Castle Creek time, producing pyroclastic flows, surges, and tephra as well as lava flows. Volumes of explosively erupted deposits were moderate or small.

The first known eruptive products of the period are andesitic and are characterized by ferromagnesian suites of hypersthene and augite. The early Castle Creek eruptions resulted in at least one lava flow and created lahars (Crandell, 1987). Next, explosive eruptions produced the tephra layer Bh. Similar explosions that followed resulted in layer Bo, in which olivine dominated ferromagnesian suites for the first time in Mount St. Helens history. Repeated explosions created several different shower beds within layer Bo, as well as pyroclastic flows and hot lahars. Intercalation of tephras with pyroclastic-flow and lahar deposits suggests frequent changes in eruptive behavior, which probably included initial fountaining of tephra during eruptions that later produced pyroclastic flows, lahars, and lava flows.

After layer Bo, milder eruptions of undetermined character produced mafic ash, followed by ash that was more silicic. These deposits, which formed ash bed ba, probably include detritus from both small tephra eruptions and from ash clouds derived from pyroclastic flows. As ash bed ba accumulated, the small-volume, slightly vesicular dacitic tephra layer Bd was erupted. Dacitic pyroclastic flows of the same composition as layer Bd provide radiocarbon dates of 2,200-2,000 years for those deposits (Crandell, 1987). After a pause of 200-400 years, another explosive eruption produced the dacitic tephra layer Bi, which is highly vesicular and has a ferromagnesian mineral suite slightly different from that of layer Bd. Radiocarbon ages of approximately 1,800 years, obtained from both below and above layer Bi, indicate that eruption of layer Bi was distinctly later than that of layer Bd.

After formation of layer Bi, only olivine basalt is recorded by the tephra sequence; the basaltic scoria of layer Bu records at least two mildly explosive events. At one site, however, an andesite lava flow overlies dacitic pyroclastic-flow deposits and underlies a basalt lava flow (Crandell, 1987, p. 57-59). Thus, at least one andesitic eruption occurred between the eruptions of dacite and basalt.

Basaltic lava flows as well as tephra were produced at the end of Castle Creek time. Multiple thin flow units of basalt overlie at least part of layer Bu on the north flank of the volcano; the lava flows were emitted after at least the first eruption of layer Bu. Fountaining that produced tephra may have been followed by quieter emission of the lava flows.

The largest basalt flows of late Castle Creek time, however, are pahoehoe flows on the south flank of Mount St. Helens. There is no physical overlap of the basalt flows on the north with the pahoehoe flows on the south, and radiocarbon dates do not firmly determine which is younger (Crandell, 1987). Consequently, the age relation of the pahoehoe flows and other basalts of Castle Creek time is uncertain.

Castle Creek eruptions followed those of Pine Creek time after only a short period and continued for almost 1,000 years.

Sugar Bowl Period

The Sugar Bowl episode was short and markedly different from other periods in Mount St. Helens history. It produced the only unequivocal laterally directed blast known from Mount St. Helens before the 1980 eruptions (Crandell and Mullineaux, 1978).

During Sugar Bowl time, the volcano first erupted quietly to produce a dome, then erupted violently at least twice producing a small volume of tephra, directed-blast deposits, pyroclastic flows, and lahars (Crandell and Hoblitt, 1986; Crandell, 1987). The Sugar Bowl blasts, described initially as only one (Crandell and Mullineaux, 1978), were much smaller than the one in 1980, extending only about one-third as far as that of 1980. In composition, fragmental deposits of the Sugar Bowl period are similar to rock of Sugar Bowl dome.

Radiocarbon analyses and stratigraphic relations indicate that the Sugar Bowl eruptions occurred about 1,200 years ago, about midway between the Castle Creek and Kalama periods. The morphogically similar East dome, in contrast, has no fragmental deposits associated with it, and its age is not closely known. Absence of set B tephras on East dome indicate that it was emplaced after the Castle Creek period, and presence of set W tephras on it show that it existed before Kalama time. It could have risen at any time between those two periods, and whether or not it is coeval with the Sugar Bowl dome is not known.

Except for the dome itself, Sugar Bowl eruptions produced only deposits of small volume and probably occurred within a short time, perhaps a few years. Sugar Bowl eruptions caused no major changes in the shape or size of the volcano but did form a prominent dome on the north flank of Mount St. Helens.

Kalama Period

In A.D. 1480, or possibly late in A.D. 1479, a highly explosive pumice eruption opened the Kalama period with discharge of tephra layer Wn. The Kalama period continued for perhaps 300 years and comprise a wide variety of magma compositions and kinds of eruptions. It included highly explosive ejection of voluminous dacitic and andesitic tephras and pyroclastic flows, dacitic pyroclastic flows, nonexplosive eruption of andesite lava flows, and rise of the dacitic summit dome.

Eruption of layer Wn produced the most extensive and most voluminous tephra of Kalama time; layer Wn has been identified as far as 500 km northeast of Mount St. Helens (Smith and others, 1977). Reworking of that pumice layer, where thick, produced fluvial and laharic deposits locally at the northeast and south bases of the volcano (Crandell, 1987). A dome probably formed next, before eruption of the relatively small tephra layer Wa. The dome provided layer Wa with abundant fresh lithic fragments of the same mineral composition as the new magma. The presence of more than one bed or zone in both layers Wa and Wb, along with abundant fresh lithic clasts, indicates that multiple small explosive pulses and dome formation occurred between A.D. 1480 and A.D. 1482, when the next voluminous and highly explosive eruption produced layer We. One or more pyroclastic flows probably were created during that event because ash probably derived from pyroclastic flows is intermingled with and overlies the upper part of the layer. Layer We also extended far downwind and has been identified eastward more than 300 km from Mount St. Helens (Smith and others, 1977).

Perhaps about the time of eruption of layer We, pyroclastic flows of pumiceous and lithic dacite began to move down the southwest flank of the volcano (Hoblitt and others, 1980). The magma erupted gradually became more mafic. Explosive, dacitic eruptions were followed by weaker eruptions of andesitic tephra and the quiet emission of andesitic lava flows, which mantled much of the south flank of the volcano. Some beds of tephra set X probably were ejected during early phases of eruptions that produced lava flows.

The andesitic eruptions were followed by rise of a large dome, perhaps starting during the first half of the 17th century (Hoblitt, 1989); that dome made up the summit of Mount St. Helens until early 1980. Avalanches of hot debris from the dome swept down all sides of the volcano, formed pyroclastic flows and lahars, and covered the upper parts of many Kalama lava flows (Hoblitt and others, 1980; Crandell, 1987). The rounded summit dome and smooth profiles of the avalanches and lava flows were primarily responsible for the smooth, youthful appearing cone of the pre-1980 Mount St. Helens.

The principal growth of the volcano during the Kalama period probably occurred within about 200 years, but debris avalanches from the dome continued for about another 100 years (Hoblitt, 1989; Pallister and others, 1992).

Changes in composition during Kalama time were gradual but distinctive, from dacite to andesite and back to dacite. Patterns of trace elements along with other compositional changes indicate that the changes resulted from injection of mafic magma into a preexisting dacite (Hoblitt, 1989; Pallister and others, 1992)

Goat Rocks Period

The Goat Rocks eruptive episode was short and produced relatively small volumes of magma. Its first eruptive event was a single, highly explosive, moderate-volume eruption in A.D. 1800 that produced the pumiceous, dacitic layer T. That layer spread along a narrow path northeastward across Washington, Idaho, and into Montana (Okazaki and others, 1972, p. 81). The tephra eruption apparently was not accompanied by other events but was followed later in the same year (Yamaguchi and others, 1990) by the andesitic lava flow called the "floating island" flow by Lawrence (1941).

No eruptions have been documented for the next three decades, but minor ones were reported from A.D. 1831 to A.D. 1857 (Holmes, 1955). In A.D. 1842, the largest volume of those eruptions dumped a reported half-inch of ash on The Dalles, Oregon. Goat Rocks dome was extruded at about that same time (Crandell, 1987, p. 88). The dome probably was constructed incrementally through several years during the 1840's, during which time many eruptions were observed (Holmes, 1955). Growth of the dome caused dome-rock avalanches, pyroclastic flows, and lahars; the flowage deposits created a broad fan below the dome on the north flank of the volcano (Hoblitt and others, 1980, p. 558; Crandell, 1987). The last eruption usually attributed to the Goat Rocks period occurred in 1857, although a study of old records suggests that minor eruptions of Mount St. Helens might have occurred in 1898, 1903, and 1921 (Majors, 1980, p. 36-41).

The relatively small volume eruptions of Goat Rocks time produced no major changes in the size or shape of the volcano.


Future eruptions of Mount St. Helens are certain, and some of those eruptions will endanger lives and property. Tephra generally is less of a threat to lives than flowage events such as pyroclastic flows and lahars. Nevertheless, tephra can cover such large areas that it can cause even greater overall loss to people and property than flowage events.

Although future eruptions certainly will occur, their specific character and timing cannot be predicted. Only generalized forecasts can be made, based on patterns of past activity, and even those are necessarily uncertain. Probabilities of various kinds and magnitudes of events can be estimated, however, from the volcano's eruptive record, and hazard maps based on these probabilities can be drawn to guide response planning. And, although the timing of future eruptions cannot be predicted, seismic monitoring would be expected to detect movement of large amounts of magma at Mount St. Helens, which in turn would provide warning before another large eruption occurs (Pallister and others, 1992).

Tephra causes physical, medical, psychological, and social problems, especially close to the volcano. The severity of most effects decreases rapidly with increasing distance because of decreasing thickness of tephra deposited, yet the overall damage beyond the immediate vicinity of the volcano can be great because of the vast areas affected.

Tephra endangers people and property chiefly by impact and heat of large fragments, burial and load from thick accumulations, ash inhalation, and many other effects resulting from dispersion of ash in air and water (see Wilcox, 1959b, Blong, 1984, and Scott, 1989, for more detailed descriptions). Close to a volcano, the chief dangers are large fragments and thick ash. Most deaths from tephra eruptions are caused by fragment impact or collapse of roofs caused by the weight of tephra. Large fragments can also kill or injure people and animals by heat, penetrate and otherwise damage structures, and start fires. In addition to crushing roofs, thick ash deposits commonly disable power and communication lines, block transportation routes, and bury crops. People have been killed or injured by inhaling large amounts of ash, especially if it is hot, and injury to eyes and respiratory systems can be serious for both people and animals. Ash also commonly overloads water supply and sewer systems, and development of a cemented crust on ash deposits can lead to excessive runoff that in turn causes secondary mudflows.

At great distances, where tephra is fine grained and dispersed, it can still cause serious injuries to eyes and respiratory systems. The thin deposits can also severely hamper transportation and communication facilities. Even at hundreds of kilometers from a volcano, ash suspended in the air can limit transportation by causing darkness during daylight hours and otherwise reducing visibility and by damaging engines and other equipment. Growing crops can be set back or destroyed by thin ash, but future crops typically benefit from the addition to the soil.

Recent events make it clear that ash is extremely dangerous to jet aircraft, even at great distances from a source volcano. On several occasions, highly dispersed ash has damaged jet engines sufficiently to cause temporary failure and accompanying loss of altitude and risk of a crash. In most of these encounters, ash particles have also caused extensive damage to other parts of the aircraft (Casadevall, 1991).

Medical, psychological, and social effects (see Blong, 1984) are commonly long term as well as immediate. Depression, anxiety, and stress were high in areas affected by tephra in the first two years after the 1980 Mount St. Helens eruptions (Shore and others, 1987), and ensuing political disruption, anger, and litigation continued for many more years.

Fortunately, losses from tephra effects can be greatly minimized, except for those from large-volume eruptions and near the volcano. In some cases, the fallout can simply be avoided, and in many others protective measures can limit adverse effects. Avoidance is feasible except close to the volcano primarily because of the time wind-carried tephra takes to reach any given location after its eruption has begun. That time can allow people to move out of the path of the tephra; obviously such a response is increasingly feasible with increasing distance from the volcano. Avoidance by evacuation before an eruption begins, however, generally is not appropriate. Neither the character nor timing of such an eruption could be specifically predicted, a large number of people might have to be moved, and the area predicted to be at risk would change with each anticipated change in wind direction.

In contrast, protective measures are relatively straightforward, and many are easy to implement. Taking shelter in sturdy structures, and using masks or covers to protect people and equipment from ash, can significantly reduce damage. For best results, mitigative measures should be both planned well in advance and easy to implement on short notice. Obviously, early notification that tephra has been erupted and information about its direction and speed are critical in order for protective measures to be taken before the tephra arrives.

In practice, effective reponse to eruptions, even when the hazards have been assessed and described, has been difficult (see Peterson, 1988, and Peterson and Tilling, 1993, and references therein). An important factor is that predictions (and thus warnings) of future eruptions must be probabilistic; they are not specific as to either time or character of eruptions. Sorenson and Mileti (1987) pointed out that warnings need to be not only specific, but also certain, consistent, accurate, and clear. In the absence of predictions that are specific and certain, response plans must be drawn to cover many possible times and situations, perhaps too many to be readily understood and accepted. Those same uncertainties make it difficult to maintain a high state of readiness.

To date, volcanologists' mitigation efforts have consisted chiefly of hazards assessments and monitoring to predict eruptions. Recently, the importance of effective communication of that information has been emphasized (Peterson, 1988). Yet another important factor is that of detection of hazardous events when they actually occur so that civil officials and the general public can be warned. Such warnings are more desirable than conditional warnings of "possible" or "probable" events, which usually are all that can be provided in advance. Warnings of real events in progress meet the criteria of being specific and certain (Sorenson and Mileti, 1987) and enable civil officials and the public to react with greater confidence.

The risks posed by tephra are particularly appropriate for mitigation by warnings based on detection of events already in progress. In many cases, erupted tephra can be detected and tracked visually from the ground or air, and satellites can provide primary or supplemental information. Another method developed recently (Hoblitt, R.P., written commun., 1992) tracks the downwind progress of erupted tephra by detecting the lightning strikes that it causes.

Thus, tephra can be dangerous, but mitigation can be relatively successful because of its characteristics. Success, however, requires adequate advance planning, detection of hazardous events when they actually occur, and the capability of prompt response to those events.


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