Approximate Dates | Stages/periods |
Approx. Age in Years Before 1980 |
Volcanic Activity/Effect |
Historical Event(s) |
1980- | Most recent eruption | Laterally directed blast from cryptodome followed by eruption of dacite tephra and pyroclastic flows. Heat unleashed during the eruption melted glacial snow and ice. The melt water combined with rock and mud to form a liquid sandpaper-like mixture called a lahar. Shattered stumps remain, testifying to the abrasive force of the lahar. The flow surged up a 120-foot high section of hill, scouring away the forest in its path. The lahar raced down the Muddy Tiver and Pine Creek drainages into Swift reservoir. A dome developed in a new crater. | Man on the moon (1970)
Alaska granted statehood (1959) WWll (1941) WWl (1919)
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1776-1857 | Dormant interval of 123 years.
Goat Rocks eruptive period. |
180-123 (*2) | Eruptions of daicite, tephra, andesite lava flow, dacite dome.
X Tephra layer: fiery red arteries of andesitic lava pulsed down the southeast side of the volcano. These lava flows can be seen from the lahar viewpiont and are called the worm flows. Pasty, sticky dacite lava oozed out of the volcano, crowning the pre-1980 summit with a lava dome. |
Declaration of Independance (1776) |
1480-1650 | Dormant interval of about 200 years
Kalama (*3) eruptive period |
350-500 (*4) | Eruptions of daicite, andesite tephra, dacite dome(s) and pyroclastic flows, andesite lava flows.
We Tephra layer: Scorching hot pyroclastic flows tumbled down the volcano. Winds blew mushroom shaped ask plumes to the east. The Kalama, with Goat Rocks and Sugar Bowl periods, built the symmetrical cone shape of Mount St. Helens. Most rocks visible on the surface originated during the Kalama. |
Columbus sets sail (1492) |
800 | Dormant interval of about 650 years
Sugar Bowleruptive period |
1150 | Eruption of dacite dome. laterally directed blast, pyroclastic flow(s), air-fall tephra.
Wn Tephra Layer: Swirling plumes of ash rushed skyward. Prevailing winds deposited tephra in southeast British Columbia. |
Viking explorations |
300 BC-250 AD | Dormant internal of about 600 years
Castle Creekeruptive period |
1700-2200 | Eruptions of andesite, dacite, and basalt tephra, andesite and basalt flows, andesite and dacite pyroclastic flows.A variety of material was ejected from the volcano, marking a significant change in the eruptive behavior of Mount St. Helens. These constructive eruptions built most of the modern volcano. Prior to this time, dacitic lava had been the primary lava type. During this period, pasty dacite lava was accompanied by fluid basaltic lava, and andesitic lava (lava with properties between basalt and dacite).
Bh Tehpra Layer: fluid red ribbons of basaltic lava spilled down the volcano and formed Ape Cave. Bo Tephra Layer: Dacitic pyroclastic flows and rivers of basaltic lava surged down the slopes of the volcano. Bi Tephra layer: Andesitic lava and pyroclastic flows spilled down the volcano. Bu Tephra layer: Glowing rivers of basaltic lava flowed from the volcano. |
Roman Empire in power
Christ lives |
1200 BC-700 BC | Apparent dormant internal of about 300 years
Pine Creekeruptive period |
3000-2500 | Eruptions of dacite tephra, dacite domes, pyroclastic flows. Mount St. Helens awoke from a brief slumber with a series of small explosive eruptions. Searing hot pyroclastic flows tumbled down all sides of the volcano. Airfall tephra from these eruptions lift four distinctive deposits on Mount Rainier!
Tremendous lahars scoured down the volcano into the Lewis and Toutle river valleys. Spirit Lake over topped the natural dam created during the Smith Creek period and formed a huge mudflow that roared down the North Fork of the Toutle River Valle. This mudflow blocked a stream tributary (similar to the formation of spirit Lake) and formed Silver Lake. |
Greek civilization begins
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2600 BC-1600 BC | Apparent dormant internal of about 300 years
Smith Creekeruptive period |
4000-3300 | Eruptions of dacite, tephra, dacite domes, pyroclastic flows; probably included dormant intervals as long as several centuries. Billowing clouds of tephra rocketed towards the sky during what was likely the largest eruption in the hitory of the volcano. Nearly 2.5 cubic miles of tephra was ejected from the volcano, in contrast to 0.1 cubic miles deposited during the May 18, 1980 eruption.
Yn Tephra layer: Pumice from this eruption has been found in Canada up to 500 miles away from the volcano. Ye Tephra Layer: Intense heat from the erupting volcano melted snow and Ice. The melt water mixed with rock and mud, forming cement-like slurries called lahars. A lahar plugged the North Fork of the Toutle river, blocking a stream drainage. Spirt Lake formed behind this natural dam. |
Egypt builds pyramids
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12000 BC-9000 BC | Apparent dormant internal of about 5000 years(?)
Swift Creekeruptive stage |
13000-10000 | Eruptions of dacite tephra, dacite domes, litihic and pumiceous pyroclastic flows; probably included dormant intervals of at least a few centuries. Repeated powerful explosions hurled ash and pumice into the air. Many superheated rock avalanches called pyroclastic flows raced down the slopes of the volcano.
S Tephra layer: large eruptions of pumice and ash called tephra were blasted into the air. Tephra from this eruption was found in central Washington! J Tephra Layer: Winds blew towering clouds of tephra to the west. This was the only time that pumice was deposited to the west in any significant amount. |
First migrations across Bering land bridge into North America.
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19000 BC-16000 BC | Apparent dormant internal of about 5000 years(?)
Cougareruptive stage |
21000-18000(?) | Eruptions of pumice tephra, one or more dacite domes and lava flows, lithic and pumiceous pyroclastic flows; probably included dormant intervals of at least several centuries. | Crater Lake is formed.
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Apparent dormant internal of about 15000 years(?)
Ape Canyon eruptive stage |
50000-36000(?) | Eruptions of tephra and pumiceous pyroclastic flows. |
(*1) - Derived from Crandell, Dwight R., Deposits of pre-1980 Pyroclastic Flows and Lahars from Mount St. Helens Volcano, Washington (USGS Professional Paper 1444: U.S. Fovernment Printing Office, Washington D.C., 1987)
(*2) - Years before 1980, based on tree-ring dates and historic records
(*3) - Boldface periods/stages visible at stratigraphic bands study site
(*4) - Years before 1980, based on tree-ring dates and 14-C dates
The Two-Month Precursory Period
The Mount St. Helens volcano reawakened in March 1980 after more than a century of quiet. A magnitude 4.0 earthquake on March 20 was followed by two months of intense earthquake activity, and phreatic "steam-blast" eruptions which began on March 27. Ejecta from these phreatic eruptions were composed of fragments of pre-existing rocks; no magma was tapped during these eruptions. These events were caused by the intrusion of viscous magma into the volcano, shoving the north flank outward more than 300 feet and creating the famous `bulge.' Repeated surveys during April and May showed that the bulge was growing northward at an average rate of about five feet per day.
The Eruption
A magnitude 5.1 earthquake on May 18 (8:32 a.m. PDT) shook loose the steepened bulge on the volcano's north flank, resulting in the largest known landslide in historic time, 2.3 cubic km (0.56 cubic miles). The entire north flank was described by an aerial observal as "rippling" and "churning" moments before "the north side of the summit began sliding north along a deep-seated slide plane."
As the avalanche reached the north base of the cone, the topography it encountered caused it to be divided into three sections:
1.
Part of the avalanche slid into Spirit Lake, raising the lake bed roughly 180 feet, and damming its natural outlet. Water displaced by the avalanche surged up the surrounding hillslopes, washing the blown-down timber from the lateral blast into the lake.
2.
Part of the avalanche "ramped" up and over a 1,200 foot high ridge five miles north of the volcano (Johnston Ridge) depositing debris on top of the ridge and in the South Colwater Creek drainage.
3.
The bulk of the avalanche was deflected westward down the North Fork of the Toutle River valley. The front of the avalanche traveled a distance of 15 miles in about 10 minutes. The resulting deposit covers the valley floor to an average depth of 150 feet, but it is more than 500 feet deep in a few places (such as 1.5 miles west of Harry Truman's Lodge).
The hummocky avalanche deposit covers a total area of about 24 square miles. It consists of intermixed volcanic debris of various sizes, including blocks, pebbles, sand and silt, and blocks of glacial ice.
Lateral Blast
The sudden removal of the volcano's north flank released pressure on the hydrothermal and magmatic system within the volcano, triggering a devastating lateral blast to the north. The abrupt pressure release, or "uncorking," of the volcano by the avalanche can be compared in some ways to the removal of the cap from a vigorously shaken bottle of soda pop, or to punching a hole in a boiler tank under high pressure.
The northward-directed lateral blast of rock, ash, and hot gas devastated an area of about 150 square miles. The blast stripped trees from most hill slopes within six miles north of the volcano and leveled nearly all vegetation for as far as 13 miles in a 180-degree arc north of the mountain. The blast deposited blocks and smaller rock fragments and organic debris over the devastated area in layers to more than three feet in thickness. Surrounding this zone of toppled vegetation is a narrow band of scorched but standing timber in which sandy deposits are as thick as four inches; this zone has an area of about 25 square miles.
Plinian Column (Vertical Eruption)
A vertically-directed ash column erupted from the newly formed horseshoe-shaped crater within minutes of the lateral blast. Within ten minutes, the ash column reached an altitude of more than 12 miles. Ash from this eruption cloud was rapidly blown east-northeastward by the prevailing winds, producing lightning and starting hundreds of small forest fires, and causing darkness eastward for more than 125 miles. Ash fell visibly over the Great Plains, and fine ash was detected by systems used to monitor air pollution in several cities of the northeastern United States. Some ash drifted around the globe within about two weeks. The eruption subsided by late afternoon on May 18; by early May 19 the eruption had stopped.
The air-fall ash deposited during the nine hours of vigorous eruptive activity amounted to about 540 million tons distributed over an area of more than 22,000 square miles. The volume of uncompacted ash is equal to about 0.05 cubic mile of solid rock, or only about ten percent of the amount of material that slid off the volcano during the avalanche.
Lahars (Mudflows)
Lahar is an Indonesian term used to describe dense, viscous flows of volcanic debris and water resembling wet concrete that form during a volcanic eruption or originate on the slopes of a volcano. These mixtures typically contain 60 percent sediment and 40 percent water by volume. Lahars occurred on nearly all streams draining the volcano during the eruption and were formed in three major ways:
1.
Within minutes of the eruption's onset, hot pyroclastic surges mixed with snow and ice on the upper flanks of the cone, forming major lahars in the South Fork Toutle River, Pine Creek, and Muddy River drainages. Subsequent calculations indicate the surges, which are air-mobilized, low density, turbulent clouds of volcanic debris, were moving initially up to 120 ft./sec. (80 mph), but slowed considerably as they transformed into denser, water-mobilized lahars on the lower slopes of the volcano. The Pine Creek lahar reached Swift reservoir by about 9 a.m.; the Muddy River lahar arrived at about 9:40 a.m.
2.
The largest lahar originated in the slumping and flowing of water saturated parts of the debris avalanche deposit during the afternoon. This lahar peaked near the mouth of the Toutle River at midnight, flowing at velocities between 25 and 40 feet per second, and left deposits three feet thick on parts of the flood plain, and 15 feet thick in the channel. The mudflow in the Toutle River drainage area deposited more than 65 million cubic yards of sediment along the lower Cowlitz and Columbia rivers. The water-carrying capacity of the Cowlitz River was reduced by 85 percent, and the depth of the Columbia River navigational channel was decreased from 39 feet to less than 13 feet, causing disruption of river traffic and temporarily choking off ocean shipping.
3.
The smallest lahars formed from the erosion and turbulent mixing of snow and ice by small, hot pyroclastic flows on the afternoon of May 18, and from small landslides of water-saturated tephra that liquefied.
Pyroclastic Flows
The term "pyroclastic" -- derived from the Greek words "pyro" (fire) and "klastos" (broken) -- describes materials formed by the fragmentation of magma and rock by explosive volcanic activity. Pyroclastic flows are composed of hot gas, entrapped air, and different-size particles of fragmented magma and old volcanic rock (ash, blocks, bombs). Pyroclastic flows travel at great speeds in response to gravity (up to 60 to 100 miles per hour).
Pyroclastic flows were first directly observed at 12:17 p.m. and continued intermittently during the next five hours of strong eruptive activity. Smaller pyroclastic flows were erupted during the first few minutes of the avalanche lateral blast sequence. The successive outpourings of pyroclastic material consisted mainly of pumice and ash derived from new magma. Fragments of preexisting rocks were minor components.
The pyroclastic flow deposits formed a fan-like pattern of overlapping sheets, tongues, and lobes that extend five miles north of the crater. Temperature measurements made in these pyroclastic flows were still 780 degrees Fahrenheit two weeks after the eruption. Many "rootless" steam-blast explosions formed small craters on the northern margin of the deposits near Spirit Lake, as encroaching ground water was flashed into steam by the hot material. These steam-blast explosions continued intermittently for several weeks or months after the emplacement of the pyroclastic flows.
Explosive Eruptions
Following May 18, Mount St. Helens erupted explosively five times during 1980. None of these eruptions was as large as the events on May 18, but each eruption produced ash columns 25,000-50,000 feet above sea level and hot, dry pyroclastic flows of pumice and ash that swept down the north flank as fast as 60 miles per hour. These pyroclastic flows deposited ash and pumice fragments in fan-like patterns of sheets, tongues, and lobes in an area extending up to five miles north of the vent. Individual pyroclastic-flow units were generally less than 15 feet thick, and maximum temperatures recorded several hours after their deposition ranged from about 570 to 1,350 degrees Fahrenheit. The thickness of air-fall deposits ranged from one-third to one-fortieth that of the May 18th air-fall deposit at a given distance from the volcano.
Lava extruded from the vent and formed lava domes within a few days after the June 12, August 7, and mid-October explosive eruptions. The June and August domes were blown away by subsequent explosive eruptions, but the October dome survived to form the core of the present dome.
Domes are formed by thick, pasty masses of lava too sticky to flow very far from the vent. Lava of the Mount St. Helens dome is dacite. It contains a higher percentage of silica than the Hawaiian basalts and is about one million times more viscous.
Dome-building Eruptions
Eleven eruptions after October 1980 were dominantly nonexplosive events that built a composite lava dome about 800 feet high and 2500 feet in diameter in the crater. Each eruption extruded near the top of the dome and crept three to 15 feet per hour down one side over a period of several days; between 100 and 150 million cubic feet of new lava was added to the dome during each of these episodes.
Dome-building eruptions in 1981-82 were episodic, occurring every one to five months. Between February 1983 and February 1984, the dome grew continuously both by the intrusion of magma into it and by the extrusion of lava onto its surface. As of May 1984, it appeared that Mount St. Helens had returned to the episodic style of dome growth. At the then current rate of dome growth, which averaged about 35 million cubic feet per month, it was estimated that some 150 to 200 years would be needed to build Mount St. Helens to its former height. However it was considered unlikely that such a simple scenario would prevail.
Small Explosions
Small explosions sometimes precede or accompany the dome-building eruptions at Mount St. Helens. If they occur when snow mantles the crater floor, they can produce mudflows and snow avalanches. The explosive onset of the March 19, 1982, eruption hurled hot pumice and dome rocks against the 2,000-feet-high south crater wall, dislodging snow and rock that avalanched through the crater and down the north flank of the volcano. Deep snow in the crater melted quickly from the volcanic heat, forming a temporary small lake from which a destructive flood swept down the north flank and into the North Fork Toutle River. About a day later a new lava lobe began to flow down the southeast flank of the dome.
Tephra Emissions
In addition to the dome-building eruptions, vigorous emissions of gas and tephra have occurred from fractures and small craters on top of the dome since late 1980. These periodic outbursts usually last several minutes, occasionally sending ash plumes as high as 15,000 to 20,000 feet above the volcano. Most of the tephra consists of fragmented pieces of dome rock, not new liquid magma, in contrast to the more hazardous magmatic explosions of 1980. These events are intermittent, sometimes occurring several times per day, and at other times not occurring at all for several weeks.
by James A. MacMahon
Reprinted from Natural History, Vol. 91 (May 1982)
James A. MacMahon is a professor of biology and a member of the Ecology Center at Utah State University.
by Dick Bolding
Reprinted from Washington Wildlife, Fall 1980.
Location:
Height:
Last Eruption:
Other Active Volcanoes:
Eruptions:
The eruptions of May 25, June 12, and October 16, which were accompanied by some pyroclastic flows, left 1/8 to 1/2 inch of ash on Vancouver, Southwestern Washington, and Portland, Oregon.
The first dome of crusty, volcanic lava was observed after the June 12 eruption. This dome was destroyed by the July 22 eruption. A second dome, observed on August 8, was destroyed by the October 16 eruption and a third was observed forming on October 18. A non-explosive event occurred December 27, 1980 - January 4, 1981, adding two additional lobes to the October dome. Non-explosive eruptions beginning February 5, April 10, June 18, September 6, and October 30, 1981, added new extrusions to the pre-existing composite dome. The next eruption began March 19, 1982, with moderate explosive activity, accompanied by mudflows, followed by the extrusion of two additional lobes of lava on the dome and further small explosive events. Two subsequent non-explosive events, May 14 and August 17, added new lobes to the existing dome. The mountain remains calm with minor steam plumes and low-level seismic activity.
Fatalities: 36
Missing People: 21
Earthquakes:
Floods:
Resource Losses:
Potential Hazards:
March 20, 1980
First earthquake of magnitude 4 reported. The number of earthquakes gradually increased during the succeeding week suggesting impending volcanic activity.
March 27, 1980
First eruption of steam and ash. Similar explosions continued until about April 22.
May 7, 1980
Eruptions of steam and ash resumed and continued until May 14.
May 18, 1980
Huge landslides and associated violent explosive eruption, accompanied by mudflows, pyroclastic flows, flooding, and extensive ash deposits.
May 25 through October 16
Series of 5 separate ash eruptions accompanied by pyroclastic flows. Lava dome destroyed by eruption on October 16.
October 18 & 19
A new lava dome grew in crater.
December 27, 1980, February 5, 1981, April 10, 1981, June 18, 1981, September 6, 1981, October 30, 1981
Last major ash eruption occurred October 16, 1980, but since that time there have been several non-explosive (minor ash) eruptions, each adding a portion to the pre-existing composite dome formed during the October eruption.
March 19, 1982 Dome-growth eruptive phase with minor explosive events and small mudflows, adding two new lobes to the pre-existing dome. Continued until April 12, 1982.
May 14, 1982, August 17, 1982
Non-explosive (one minor and one no-ash) eruptive phases, May 14-20 and August 17-23, added new lobes on the northwest and south-southwest sides of the composite dome.
February 1983 to March 1984
This period was marked by a slow, continuous addition of lava to the dome. Size of the dome, March 1984: 800 feet high, by approximately 2500 feet across. (Check with Monument Headquarters for latest dimensions.)
Reprinted from Washington Wildlife, Fall 1980.
Donna Gleisner is a Department of Game Conservation Education Programs Specialist and Bruce Crawford is a Regional fish Biologist, both in Region Five, Vancouver.
The Eruptive History of Mount St. Helens
by Donald R. Mullineaux and Dwight R. Crandell
ABSTRACT
The eruptive history of Mount St. Helens began about 40,000 years ago with dacitic volcanism, which continued intermittently until about 2,500 yr ago. This activity included numerous explosive eruptions over periods of hundreds to thousands of yr, which were separated by apparent dormant intervals ranging in length from a few hundred to about 15,000 yr. The range of rock types erupted by the volcano changed about 2,500 yr ago, and since then, Mount St. Helens repeatedly has produced lava flows of andesite, and on at least two occasions, basalt. Other eruptions during the last 2,500 yr produced dacite and andesite pyroclastic flows and lahars, and dacite, andesite, and basalt airfall tephra. Lithologic successions of the last 2,500 yr include two sequences of andesite-dacite-basalt during the Castle Creek period, and dacite-andesite-dacite during both the Kalama and Goat Rocks periods. Major dormant intervals of the last 2,500 yr range in length from about 2 to 7 centuries.
During most eruptive periods, pyroclastic flows and lahars built fans of fragmental material around the base of the volcano and partly filled valleys leading away from Mount St. Helens. Most pyroclastic flows terminated with 20 km of the volcano, but lahars extended down some valleys at least as far as 75 km. Fans of lahars and pyroclastic flows on the north side of the volcano dammed the North Fork Toutle River to form the basin of an ancestral Spirit Lake between 3,300 and 4,000 yr ago during the Smith Creek eruptive period, and again during the following Pine Creek eruptive period.
ERUPTIVE PERIODS AT MOUNT ST. HELENS
The eruptive history of Mount St. Helens is subdivided here into nine named eruptive "periods," which are clusters of eruptions distinguished by close association in time, by similarity of 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 yr is believed to be reasonably 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 PERIOD
The first stratigraphic evidence of the existence of Mount St. Helens consists of voluminous dacitic 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. 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 15,000-yr interval coincided with climates which, at times, were evidently somewhat cooler than those of the present (Alley, 1979, p. 233).
COUGAR ERUPTIVE PERIOD
The 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. 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 Frasier 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, 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 modern 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 stratigraphic record of volcanism at Mount St. Helens between about 18,000 and 13,000 yr ago.
SWIFT CREEK ERUPTIVE PERIOD
The third eruptive period was characterized by repeated explosive eruptions that initially produced many pyroclastic flows as well as pumiceous air-fall tephra deposits, 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. 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. The 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 PERIOD
Multiple 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 Y are 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. 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 extended 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 PERIOD
Although 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. 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 dactic domes. Eruptions of dactic 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 locking a tributary valley (Mullineaux and Crandell, 1962). Similar deposits also formed a contiguous fill across the floor of the Cowlitz River valley near Castle Rock that was about 6 m above present river level; this fill probably extended 209 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. A17-A18).
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 PERIOD
The next period of activity marked a significant change in eruptive behavior and variety of rock types being erupted at Mount St. Helens. 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.
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:
Lava flow of olivine basalt (youngest)
Lava flow of hypersthene-augite andesite
Tephra deposit of olivine-augite andesite scoria (layer Bo)
Pyroclastic-flow deposits of hypersthene-dacite pumice
Tephra deposit of hypersthene-augite andesite scoria (layer Bh)
Lava flow and lahars of hypersthene-augite andesite (oldest)
The pumiceous pyroclastic-flow deposits have a radiocarbon age of 2,000-2,200 yr. Deposits and rocks of Castle Creek age on the south and east flanks of the volcano include pahoehoe basalt lava flows whose radiocarbon age is about 1,900 yr, and pumiceous dacite tephra whose age is about 1,800 yr (layer Bi.). East of the volcano, layer Bi overlies a pyroclastic-flow deposit of pyroxene andesite, and directly underlies thin olivine basalt lava flows which probably are correlative with the uppermost unit in the Castle Creek valley. The Dogs Head dacite dome was extruded before those thin olivine basalt flows, probably during the Castle Creek eruptive period. Layer Bu is the youngest tephra of Castle Creek age; it underlies a deposit whose radiocarbon age is about 1,620 yr. This tephra is basaltic and probably was formed when thin olivine basalt lava flows were erupted near the end of the Castle Creek period.
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 35,000-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 PERIOD
During 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-homblende 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 50 degrees wide and to a distance of at least 10 km. The resulting deposits are as much as 50 cm 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-homblende 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 any time between the Castle Creek and Kalama eruptive periods, a time span of about 1,200 yr.
KALAMA ERUPTIVE PERIOD
Most 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. 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-falltephra 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 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 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 350 yr (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-1980 size and shape. The eruptive period was followed by a dormant interval of about 200 yr.
GOAT ROCKS ERUPTIVE PERIOD
The Goat Rocks eruptive period began about A.D. 1800 with the explosive eruption of the dacitic pumice of layer T. This pumice was carried northeast-ward 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) 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, 1989, 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.
DISCUSSION
One of the most interesting features of Mount St. Helens' history is the change in eruptive behavior that occurred about 2,500 yr ago. Eruptions of dacite had characterized the volcano for more than 35,000 yr. Then, with virtually no interruption in eruptive activity, andesite and basalt began to alternate with dacite, and not always in the same order. The chemical composition of eruptive products changed gradually during some episodes and abruptly during others. Thus, basalt followed dacite and dacite succeeded basalt; andesite followed dacite of considerably different SiO2 content, and vice versa. Some of these changes in composition of eruptive products are not adequately explained as results of eruption of cyclic sequences of compositionally different magmas derived from successively deeper levels in a larger magma body that differentiated at shallow depth, as proposed by Hopson (1971) and Hopson and Melson (19800. An alternative explanation that fits the stratigraphic record better, suggested by R.E. Wilcox (oral commun., 1974), is that some changes resulted from repeated contributions from more than one magma body, or from different parts of an inhomogeneous magma.
Explosive eruptions of volumes on the order of 0.1 to 3 km have occurred repeatedly at Mount St. Helens during some eruptive periods in the past. This record suggests that a similar sequence could occur during the present period of activity and could result in one or more explosive magmatic eruptions of similar or larger volume than the eruption of May 18. If the lengths of the last two eruptive periods are a valid guide to the future, we might expect intermittent eruptive activity to continue for several decades.
Eruptive History References
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