PRE-1980 TEPHRA-FALL DEPOSITS ERUPTED FROM MOUNT ST. HELENS, WASHINGTON

 By Donal R. Mullineaux 

U. S. Geological Survey Professional Paper 1563

ABSTRACT

Eruptions of Mount St. Helens, a highly explosive and frequently active volcano in the Cascade Range have, within the past 40,000 years, produced more than 100 tephra deposits now recognizable as distinct strata. The volcano has also erupted abundant pyroclastic flows, surges, and ash clouds, as well as lava flows and domes. Tephra deposits and those other products record a complex eruptive history of Mount St. Helens and provide information about the hazard it poses to people and property. The tephra strata also serve as time-stratigraphic marker beds that are widespread in Pacific Northwest States.

Mount St. Helens' eruptive history consists of a long silicic phase followed by a shorter, more complex episode that included mafic as well as silicic eruptions. Until about 2,500 years ago, the volcano produced only dacite and silicic andesite. At that time, mafic andesite appeared, and since then eruptions of andesite have alternated irregularly with those of dacite and even basalt.

The volcano's eruptive record is divided into four named stages, Ape Canyon (oldest), Cougar, Swift Creek, and Spirit Lake (youngest). The Spirit Lake stage is subdivided into six named periods, Smith Creek (oldest), Pine Creek, Castle Creek, Sugar Bowl, Kalama, and Goat Rocks (youngest). 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 of the tephra strata are classified into ten major groups called sets. Each set includes more than one named layer, and each layer represents a different eruptive event or group of events. In addition, three single tephra strata are described separately. At least one tephra set or separately described layer was erupted during each named eruptive stage and period.

Tephra sets are distinguished chiefly on the basis of evidence of elapsed time, ferromagnesian mineral composition, and grain size. Most sets and many layers are characterized by distinctive combinations of ferromagnesian minerals. Many of those combinations can be recognized in assemblages of heavy-mineral particles obtained by crushing and washing small pumice fragments.

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 m in altitude. Almost all such sites are east or southeast of the volcano.

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.

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.

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.

Sets S and J were erupted between about 13,000 and 10,500 years ago during the early and late parts of the Swift Creek stage, respectively. Both are characterized by a few large-volume, dacitic pumice layers that consist chiefly of lapilli near the volcano. Layers of each set have been recognized at hundreds of kilometers east of Mount St. Helens.

A dormant period of more than 6,000 years, between about 10,500 and 4,000 years ago, followed eruption of set J. It is the longest time span known for which no evidence has been found of any eruptive activity at the volcano.

The Smith Creek period began the Spirit Lake eruptive stage with eruption of the dacitic set Y. This period was 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.

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, fine-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.

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.

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.

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.

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.

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.

Mount St. Helens will surely erupt in the future. Its eruptive history as determined before 1980 strongly indicated future activity, and the 1980 eruptions erased any doubts. Tephra from future eruptions will, as in 1980, affect distant as well as nearby commumities. Although no way to prevent such eruptions is known, recognition of their potential hazards and appropriate planning can significantly reduce damage.