Mount St. Helens, Washington
Summary of Eruptive Stages and Tephra Deposits

The eruptive history of the Ape Canyon and Cougar stages is relatively obscure; they occurred before and during the last major glaciation, and large parts of their deposits have been eroded away or strongly disturbed. Deposits of the Swift Creek stage are better preserved but not as well as those of the Spirit Lake stage, which are almost as well preserved as the deposits of A.D.1980.

Most tephras from Mount St. Helens are east of a north-south line through the volcano because of transport by prevailing westerly winds. Most tephras of the oldest two stages, Ape Canyon and Cougar, are preserved well enough to decipher their stratigraphic relations only at sites that are below about 600 meters in altitude. Almost all such sites are east or southeast of the volcano.

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.

The Goat Rocks period began about A.D.1800 with the eruption of the pumiceous layer T, which records a return to dacitic magma. Layer T was the last voluminous tephra ejected by Mount St. Helens before 1980, although several small-volume eruptions of lithic ash occurred later, near the middle of the 19th century. Only one ash bed from those events, probably erupted in A.D.1842, has been traced across multiple outcrops.

A thin ash bed was found in the pre-1980 forest duff at several places around Mount St. Helens. On and near the southeast flank of the volcano, a lithic ash probably erupted in A.D.1842 could be recognized in multiple outcrops over an area of a few square kilometers. It comprises a single, thin, greay bed of fine to coarse ash. No pumice was found in the deposits. The 1842(?) ash probably was produced by one of the small phreatic eruptions that ejected only particles of pre-existing rock. Many such eruptions were reported fomr 1831 through 1857. Although the deposit might include ash from any reported or unreported eruption, if probably resulted chiefly from the event of November 22 or 23, 1842. That eruption is the only one for which a distince bed has been described that reached beyond the volcano, and the bed extended southeastward. That eruption reportedly laid down a little more than a centimeter ("a half inch") of ash at The Dalles, Oregon, about 100 kilometers southeast of Mount St. Helens.

Layer T is a single, thick deposit of pumice lapilli, bombs, and ash. It forms a conspicuous light-gray stratum near the ground surface along a narrow lobe leading northeast from Mount St. Helens. It was not found on other sides of the volcano. Lawrence (1938, 1939, 1954) determined from growth rings in trees near Spirit Lake that layer T was erupted within a few years of A.D.1800. Yamaguchi (1982) confirmed an A.D.1800 date and suggested that the eruption occurred between the growth periods of A.D.1799 and 1800.

Tephra set X, erupted next during the early part of the 16th century, records a change to a more mafic composition within the Kalama period. This tephra set contains numerous fine-grained andesitic beds that are smaller in volume than set W deposits. Set X beds have been recognized only near the volcano.

Eruption of the dacitic set W began the Kalama period late in the 15th century, probably in A.D.1480. The initial event produced the large-volume, pumiceous layer Wn, the second largest Holocene tephra from Mount St. Helens. Layer Wn is overlain by several smaller pumiceous tephras, including the moderate-volume layer We. Both layers Wn and We have been traced for hundreds of kilometers downwind.

This period produced the only unequivocal laterally directed blast known from Mount St. Helens before the 1980 eruptions.

About 1,200 years ago, a small-volume dacitic tephra called layer D was ejected during an eruptive episode that emplaced the Sugar Bowl dome on the north flank of the volcano.

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.

Tephra set B includes andesitic, dacitic, and basaltic tephra accompanied by abundant lava flows but relatively few pyroclastic flows; all were erupted during the Castle Creek period between 2,500 and 1,600 years ago. Set B contains several small- to moderate-volume layers that are somewhat thicker and coarser than those of set P near the volcano. None, however, is as voluminous as the major layers of set Y or has been recognized as far downwind as ash layers of set P. No individual layers of set B were recognized beyond a few tens of kilometers from the volcano. The set B tephras record repeated changes in composition of magma discharged. Initial layers of set B are andesitic, and they are overlain by dacitic and finally basaltic tephras.

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. Lithic pyroclastic-flow deposits, probably derived from relatively nonexplosive erutpions of dacite domes, extend as far as 18 kilometers from the volcano.

The dacitic set P was produced by multiple small eruptions during the Pine Creek period between about 3,000 and 2,500 years ago. In contrast to Smith Creek time, the Pine Creek period is characterized by relatively few tephra layers but many pyroclastic flow deposits. Set P consequently includes only relatively small volume, fin-grained tephra layers. Ash beds that represent set P have been recognized several hundred kilometers downwind, but no specific layers of this set were traced farther downwind than a few tens of kilometers.

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 kilometers below Spirit Lake. The tick deposits of Smith Creek age that crip out downvalley indicate that an early Spirit Lake was formed or expanded at that time.

The Smith Creek period began the Spirit Lake eruptive stage with eruption of the dacitic set Y. this period is characterized by abundant and varied tephras but relatively few pyroclastic flows. Set Y eruptions started shortly after 4,000 years ago and continued until at least about 3,300 years ago. The tephra set consists chiefly of two voluminous, coarse pumice layers interbedded with many smaller layers. One of the coarse layers, layer Yn, is the largest volume Holocene tephra known from Mount St. Helens; it and the similar, but smaller volume layer Ye have been found several hundred kilometers downwind.

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.

Tephra set J was erupted between about 12,000 and 10,500 years ago during the late part of the Swift Creek stage. The set is characterized by a few large-volume dacitic pumice layers that consist chiefly of lapilli near the volcano. Layers have been recognized hundreds of kilometers east of Mount St. Helens.

Tephra set S was erupted about 13,000 years ago during the early part of the Swift Creek stage. The set is characterized by a few large-volume dacitic pumice layers that consist chiefly of lapilli near the volcano. Layers have been recognized hundreds of kilometers east of Mount St. Helens.

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, one or more lava flows of dacite or siliceous andesite, and probably one or more dacite domes of similar composition. 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.

Tephra set K was produced during the latter part of the Cougar stage about 19,000 years ago; it consists of multiple thin beds of dacitic pumice and ash. Set K is small in volume, and no layers in it are separately described. It was not recognized beyond the immediate vicinity of the volcano.

Set M was erupted during the early part of the Cougar stage, which began about 20,500 years ago. The set is characterized by several moderate-volume dacitic layers of pumiceous and lithic lapilli and ash, none of which is more than a few tens of centimeters thick near the volcano or as voluminous as the major layers of set C. Nevertheless, one ash bed that probably represents part of this set has been recognized as far away as Nevada.

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. 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. 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.

Tephra set C was produced during the Ape Canyon stage, which began about 40,000 or perhaps even 50,000 years ago and continued until about 36,000 years ago. The set contains at least two large-volume dacitic pumice layers and other layers of smaller volume. The voluminous layers consist chiefly of lapilli and small bombs near the volcano and initially must have formed recognizable strata far downwind. One of them, erupted near the end of Ape Canyon time, records one of the largest volume tephra eruptions known from Mount St. Helens and has been recognized as far away as Nevada.