IntroductionMount St. Helens is a young volcano by comparison with its neighbors in the Cascade Range, and it has been characterized by intermittent explosive behavior since its birth about 40,000 years ago. Before the May 18, 1980, eruption, the volcano was as large or larger than at any previous time in its history, much of its bulk and shape having been acquired during just the last 2,500 years. This report summarizes the eruptive behavior of the volcano throughout its lifetime in order to provide a perspective for the eruptive events of 1980 which so drastically modified the shape of the volcano and devastated broad areas beyond it.Modern studies of Mount St. Helens started with Verhoogen (1937), who outlined the general geologic history of the volcano. He noted that the volcano is young, that it consists of a wide variety of rock types, and that the modern cone overlies an older volcano. Lawrence (1939, 1954) was able to date, by growth-ring studies of trees on the flanks of the volcano, a major pumice eruption of Mount St. Helens that occurred about A.D. 1800, as well as another such eruption nearly 300 years earlier. The two pumice deposits he dated are now know as layers T and Wn, respectively (Mullineaux, et.al., 1975, p.329,331). Carithers (1946, p.16, 18) described two thick pumice layers from Mount St. Helens, a coarse, light-yellow layer and an overlying coarse, light-gray deposit. In addition, he noted scattered pumice granules that are younger than the light-gray deposit. These three deposits have since been designated layers Yn, Wn, and T (Mullineaux and others, 1975, p.329,331). Our studies began in the late 1950's, with an investigation of the origin and age of mudflow and fluvial deposits that dammed Silver Lake, 50 kilometers west-northwest of the volcano (Mullineaux and Crandell, 1960, 1962). These studies confirmed the relative youth of Mount St. Helens, inasmuch as the youngest rocks of Verhoogen's "old Mount St. Helens" were found to be only about 2,000 years old. Shortly thereafter, we discovered that two conspicuous layers of volcanic ash in Mount Rainier National Park came, not from Mount Rainier, but from Mount St. Helens (Crandell, et.al, 1962, p.D64). These deposits, the tephra layers Yn and Wn, have been subsequently identified in Canada by Fulton and Armstrong (1965, p.89) and Smith and others (1977, p.209), respectively. In the late 1960's Hopson (1971) began to prepare a geologic map of the volcano, and to study evidence for possible cyclic eruptive behavior of the modern volcano. Also in the late 1960's, Hyde (1975; Greeley and Hyde, 1972) started a detailed investigation of volcanic deposits on the southern flank of Mount St. Helens. We began detailed stratigraphic studies in 1970 in order to reconstruct the eruptive history of Mount St. Helens. Our chief objective was to gather data for a volcanic-hazards assessment that could serve as a data base for long-range land-use planning; a second goal was to collect information useful for hazards mitigation, should an eruption occur. Although these studies dealt with the entire history of the volcano, we (Crandell and Mullineaux, 1978) based on assessment of potential hazards on the events of the last 4,000 radiocarbon years (about 4,500 calendar years) because this time span included a representative variety of kinds an scales of eruptions. Furthermore, this time span was preceded by an interval of similar length of little or no activity. We consluded that Mount St. Helens had been frequently active and highly explosive during the last 4,000 years and could erupt within the next 100 years -- possibly even before the end of the 20th century (Crandell, et.al., 1975, p.438; Crandell and Mullineaux, 1978, p.C25). A study by Hoblitt (Hoblitt, et.al., 1980), which was virtually complete before eruptions began in March, examined the stratigraphic sequences of rock types produced during the last 1,500 years and evaluated the evidence for cyclicity or other systematic trends in the eruptive behavior of the volcano. Information presented here concerning deposits more than 4,000 years old is largely summarized from inpart unpublished work done by Crandell and by Mullineaux. Ages of eruptive events and eruptive periods are based primarily on 65 radiocarbon dates determined in the radiocarbon laboratory of the U.S. Geological Survey under the supervision of Meyer Rubin.
Eruptive Periods at Mount St. HelensThe eruptive history of Mount St. Helens is subdivided here into nine named eruptive "periods" -- (Web note: a "re-classification" has since occurred, see: Millineaux, 1996, USGS Professional Paper 1563), which are clusters of eruptions distinguished by close association in time, by similarity or rock types, or both. The term "eruptive period" is used in an informal and largely arbitrary sense to divide the volcano's history into convenient units for the purpose of discussion. The periods are as much as several thousand years in duration, and include what may have been a single group of eruptions as well as extended episodes of volcanism, during which there were tens or possibly hundreds of eruptions. Eruptive periods are separated by apparently dormant intervals, which are inferred chiefly from buried soils and absence of eruptive deposits. However, some dormant intervals may span times of minor activity that did not produce deposits which can now be recognized. Fine-grained, air-laid volcanic detritus was deposited during some dormant intervals but these deposits are not known to have originated directly from eruptions; they might be material reworked from the flanks of the volcano. The stratigraphic record of eruptive activity during the last 13,000 years is believed to be reasonable complete. Parts of the older record, however, apparently are missing because of glacial and stream erosion during the last major glaciation (the late Pleistocene Fraser Glaciation) of the region. Ape Canyon Eruptive PeriodThe first stratigraphic evidence of the existence of Mount St. Helens consists of voluminous dacitic (table 3) deposits of slightly vesicular to pumiceous air-fall tephra and pyroclastic flows, and at least one pumice-bearing lahar. These deposits overlie extensively weathered glacial drift formed during the next-to-last alpine glaciation of the Cascade Range. The volcanic deposits were formed during at least four episodes, separated by intervals during which very weak soils developed (table 2). The entire eruptive period may have extended over a time span as long as 5,000 yr. One pumiceous tephra deposit produced during the period probably had a volume as great as that of any subsequent tephra erupted at Mount St. Helens.The Ape Canyon eruptive period was followed by a dormant interval which may have lasted from about 35,000 to 20,000 yr ago. Most of this 15,OOO-yr interval coincided with climates which, at times, were evidently somewhat cooler than those of the present (Alley, 1979, p. 233). Cougar Eruptive PeriodThe second eruptive period probably began about 20,000 yr ago, and was characterized by the eruption of small volumes of pumiceous dacite tephra; it also produced lahars, pyroclastic flows of pumiceous and lithic dacite, a few lava flows of dacite or high-silica andesite (C.A. Hopson, written commun., 1974), and perhaps one or more dacite domes. Several different eruptive episodes can be identified during the period. At least one pumiceous pyroclastic flow moved southward to at least 16 km from the center of the present volcano about 20,350 yr ago (Hyde, 1975, p. B11-B13). Two sequences of air-fall tephra that followed (sets M and K) are separated by a two-part deposit of. fine air-laid sediment that locally is a meter or more thick, and that contains at least one weakly developed soil (table 2). After another quiet interval during which there was a small amount of soil development, at least two more pyroclastic flows moved south and southeast from the volcano between about 19,000 and 18,000 yr ago. The Cougar eruptive period occurred during the Fraser Glaciation when alpine glaciers in the Cascade Range were at or near their maximum extents, and the products of eruptions generally are poorly preserved.One lahar that apparently occurred early in the Cougar period is of special interest because of some similarities to the debris avalanche of May 18, 1980, that swept down the North Fork Toutle Valley. The lahar of Cougar age consists of an unsorted and unstratified mixture of gray dacite fragments in a compact matrix of silt and sand as much as 20 m thick. Locally, it contains discrete texturally similar masses of red dacite many meters across. The iron-magnesium mineral content of rocks in the lahar is similar to that of the Ape Canyon period (table 2), suggesting that the lahar might have been derived from older parts of the volcano, The lahar was recognized in the Kalama River drainage 8 km southwest of the center of the modem volcano, and on both walls of the Lewis River valley near Swift dam (Hyde, 1975, p. B9-B11). It has not been recognized elsewhere; thus, little is known of its original extent. Its local thickness and heterolithologic character suggest that the lahar might have originated in a large slope failure on the south side of the Mount St. Helens of early Cougar time. There is no known stratigraphic record of volcanism at Mount St. Helens between about 18,000 and 13,000 yr ago. Swift Creek Eruptive PeriodThe third eruptive period was characterized by repeated explosive eruptions that initially produced many pyroclastic flows as well as pumiceous air-fall tephra deposits (table 2, set S), some of which had large volumes and extended at least as far east as central Washington. These eruptions of dacite pumice were followed by many lithic pyroclastic flows, which are believed to have been derived from domes; at least one of these pyroclastic flows reached a point 21 km from the center of the present volcano. The pyroclastic flows were followed, in turn, by another series of explosive eruptions that produced the voluminous tephra set J (table 2). One coarse pumice layer of set J extends west-southwest from Mount St. Helens, and is as much as 20 cm thick as far as 20 km from the volcano. This layer represents the only coarse and thick pumice known to have been carried principally in a westerly direction. The sequence of explosive eruptions that formed set J apparently ended the Swift Creek eruptive period sometime before 8,000 yr ago, and was followed by a quiet period of at least 4,000 yr.Smith Creek Eruptive PeriodMultiple explosive eruptions of the Smith Creek eruptive period, which began about 4,000 yr ago, initiated at least 700 yr of intermittent and at times voluminous eruptive activity. Three coarse pumice layers at the base of tephra set Yare overlain by layers of denser, somewhat vesicular tephra. Deposition of these units was followed by an interval during which a soil began to develop on the tephra. The next eruption of the period produced the most voluminous and widespread tephra deposit of the last 4,000 yr; it is one of the largest, if not the largest, in the history of the volcano, and has an estimated volume of at least 3 km 3 . The resulting pumice layer, Yn, has been found nearly 900 km to the north-northeast in Canada (Westgate and others, 1970, p. 184). The formation of this layer was followed shortly by another voluminous eruption of tephra, which resulted in layer Ye (Mullineaux and others, 1975, p. 331), then by a pumiceous pyroclastic flow and a coarse lithic pyroclastic flow. The lithic pyroclastic flow was accompanied by clouds of ash that spread at least a kilometer beyond the sides of the flow and as much as 2 km beyond its front. Many smaller eruptions of lithic and moderately vesicular ash and lapilli followed, perhaps within a few years or tens of years.Lahars and pyroclastic flows of Smith Creek age formed a fan north of the volcano, and lahars ex- tended down the North Fork Toutle River at least as far as 50 km downvalley from Spirit Lake. An ancestor of the lake probably came into existence at this time, dammed in the North Fork valley by the fan of lahars and pyroclastic-flow deposits. It is not known if the lake ever existed before Smith Creek time. A dormant interval of apparently no more than a few hundred years followed the Smith Creek eruptive period. Pine Creek Eruptive PeriodAlthough only a short time elapsed between the Smith Creek and Pine Creek periods, eruptive products of Pine Creek age contain an iron-magnesium phenocryst assemblage that is distinctly different from those of Smith Creek age (table 2). During the Pine Creek eruptive period, large pumiceous and lithic pyroclastic flows moved away from the volcano in nearly all directions. The lithic pyroclastic flows, some of which extended as far as 18 km from the present center of the volcano, are believed to have been derived from dacite domes. Eruptions of dacitic airfall tephra were of small volume, but at least four formed recognizable layers as far away as Mount Rainier (Mullineaux, 1974, p.36).During this time, lahars and fluvial deposits aggraded the valley floors of both the North and South Fork Toutle River, and created the basin of Silver Lake 50 km west-northwest of the volcano by blocking a tributary valley (Mullineaux and Crandell, 1962) . Similar deposits also formed a continuous fill across the floor of the Cowlitz River valley near Castle Rock (fig. 1) that was about 6 m above present river level; this fill probably extended 20 km farther to the mouth of the Cowlitz River. Lahars and fluvial deposits formed a similar fill in the Lewis River valley which, near Woodland, was about 7.5 m higher than the present flood plain (Crandell and Mullineaux, 1973, p. AI7-AI8). The eruptions of Pine Creek time extended over a period of about 500 yr. No single eruption of very large volume has been recognized from deposits of Pine Creek age, and the period seems to have been characterized by many tens of eruptions of small to moderate volume and the growth of one or more dacite domes. Some radiocarbon dates on deposits of Pine Creek and Castle Creek age overlap, and if the two eruptive periods were separated by a dormant interval, it must have been short. Castle Creek Eruptive PeriodThe next period of activity marked a significant change in eruptive behavior and variety of rock types being erupted at Mount St. Helens (tables 1, 3). During the Castle Creek eruptive period, both andesite and basalt were erupted as well as dacite, and these rock types evidently alternated in quick succession. The overall sequence includes, from oldest to youngest, andesite, dacite, basalt, andesite, dacite, basalt (table 1).Thus, the stratigraphic sequence of Castle Creek time is complex, and not all stratigraphic units are represented on all sides of the volcano. Northwest of Mount St. Helens, in the Castle Creek valley, the sequence preserved includes the following:
Castle Creek time marked the start of eruptions that built the modern volcano. It is interesting to note that the change in eruptive behavior from that of the preceding 35,OOO-plus years did not follow a long period of dormancy like several that occurred during Mount St. Helens' earlier history, The dormant interval that followed Castle Creek time apparently lasted about 600 yr. Sugar Bowl Eruptive PeriodDuring the next 1,200 yr, the only eruptions recorded at Mount St. Helens are those associated with the formation of Sugar Bowl, a dome of hypersthene-hornblende dacite at the north base of the volcano. During extrusion of the dome, a directed blast carried rock fragments laterally northeastward in a sector at least 50o wide and to a distance of at least 10 km. The resulting deposits are as much as 50 em thick and consist of ash, lapilli, and breadcrusted blocks of dacite from the dome, fragments of charcoal, and stringers of material eroded from the underlying soil. A single fragment of charcoal from within the deposit has a radiocarbon age of about 1,150 yr, whereas a sample of wood charred and buried by the deposit has an age of about 1,400 yr (Hoblitt and others, 1980, p. 556). We provisionally assign an age of about 1,150 yr to the blast deposit; the older date may have been obtained from a fragment of a mature tree that was overridden by the blast.A pyroclastic-flow deposit of breadcrusted blocks, as well as prismatically jointed blocks of dacite of the same composition as the dome, was found on the north slope of Mount St. Helens downslope from Sugar Bowl; this pyroclastic flow may have occurred at the time of the lateral blast. Three lahars containing breadcrusted blocks of similar dacite were formerly exposed in the North Fork Toutle River valley west of Spirit Lake. These lahars may have been caused by melting of snow by the lateral blast or by the pyroclastic flow. East Dome, a small dome of hypersthene-hornblende dacite at the east base of the volcano, may have been formed at about the same time as the Sugar Bowl dome. East Dome is overlain by tephra of the Kalama period but not of the Castle Creek period, and could have been formed anytime between the Castle Creek and Kalama eruptive periods, a time span of about 1,200 yr. Kalama Eruptive PeriodMost of the rocks visible at the surface of the volcano before eruptions began in 1980 were formed during the Kalama eruptive period, Although the range in radiocarbon dates and ages of trees on deposits of Kalama age suggest that the eruptive period lasted from nearly 500 to 350 yr ago, all the events described here probably occurred during a shorter time span, perhaps less than a century. The Kalama eruptive period began with the explosive eruption of a large volume of dacite pumice (layer Wn), which forms the basal part of tephra set W (table 2). Layer Wn was deposited northeastward from the volcano across northeastern Washington and into Canada (Smith and others, 1977, p. 209) and was followed by additional pumice layers. At about the same time, pyroclastic flows of pumiceous and lithic dacite moved down the southwest flank of the volcano. The relative timing of these events is poorly known because most of the air-fall tephra was carried eastward and northeastward, whereas the pyroclastic flows have been found only on the southwest flank of Mount St. Helens.A short time later, scoriaceous tephra of andesitic composition (table 2, set X) was erupted. In addition, andesite lava flows extended down the west, south, and east slopes of the volcano, and andesite pyroclastic flows moved down the north, west, and south flanks. These eruptions of andesite were followed by the extrusion of the dacite dome that formed the summit of the volcano (fig. 4) before the May 18, 1980, eruption. Avalanches of hot debris from the dome spilled down over the upper parts of the preceding lava flows, and some of this hot debris partly filled channels between levees of the andesite lava flows on the south side of the volcano (Hoblitt and others, 1980, p. 558). Late in this eruptive period, a pyroclastic flow of pumiceous dacite moved northwestward from the volcano down the Castle Creek valley and covered lahars of summit-dome debris. Charcoal from the pyroclastic-flow deposit has a radiocarbon age of about 3SOyr (Hoblitt and others, 1980, p. 558). The Kalama eruptive period was characterized by frequent volcanism of considerable variety; rock types being erupted alternated from dacite to andesite and back to dacite, and the volcano grew to its pre-198O size and shape. The eruptive period was followed by a dormant interval of about 200 yr. Goat Rocks Eruptive PeriodThe Goat Rocks eruptive period began about A.D. 1800 with the explosive eruption of the dacitic pumice of layer T. This pumice was carried northeastward across Washington to northern Idaho (Okazaki and others, 1972, p. 81) and apparently was the only eruptive product of that time. Many minor explosive eruptions of the Goat Rocks period were observed by explorers, traders, and settlers from the 1830's to the mid-1850's. The Floating Island Lava Flow (andesite) (fig. 2) was erupted before 1838 (Lawrence, 1941, p. 59) and evidently was followed by extrusion of the Goat Rocks dacite dome on the north flank of the volcano (Hoblitt and others, 1980, p. 558),The last eruption of the Goat Rocks eruptive period was in 1857, when "volumes of dense smoke and fire" were noted (Frank Balch, quoted in Majors, 1980, p. 36). A recent study of old records has suggested that minor eruptions of Mount St. Helens also occurred in 1898, 1903, and 1921 (Majors, 1980, p. 36-41). The published descriptions of these events suggest that they were small-scale steam explosions, and none produced deposits that were recognized in our studies. |
Return to:
[Report Menu]
[Mount St. Helens Eruptive History Menu]
