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. Fragmental deposits 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 between tephra sets C and M 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, which 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). -- (Web note: see full report) Its structure 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, of lava flows (Crandell, 1971). Subdivisions of Eruptive HistoryThe 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) -- (Web note: see full report).Old Mount St. Helens
Modern Mount St. Helens
Eruptive Stages and PeriodsBoth 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) -- (Web note: see full report). 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 as compared to 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 from the absence of eruptive products. Some evidence suggests that at least minor eruptions did occur during intervals between the first three stages, whereas no evidence as been seen that suggests any such activity between the latest two stages, the Swift Creek and Spirit Lake. Ape Canyon StageThe 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 IntervalAbout 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 StageThe 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 IntervalLittle 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 StageDuring 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 IntervalDuring 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 StageEruptions 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
Smith Creek-Pine Creek Interval
Pine Creek Period
Pine Creek-Castle Creek Interval
Castle Creek Period
Sugar Bowl Period
Kalama Period
Goat Rocks Period
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