Classroom Supplements

Eruptive Periods in Context (*1)

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)
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
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
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.
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.
	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

Mount St. Helens Eruptive Activity, 1980-1984

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.

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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:

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.

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.

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:

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.

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.

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.

Mount St. Helens Revisited #1

Food and shelter are still not abundant, and the volcano continues to rumble, but many kinds of animals -- both survivors of the eruption and recent immigrants -- are making efforts to repopulate the mountain.

by James A. MacMahon

Reprinted from Natural History, Vol. 91 (May 1982)

For many years to come and certainly long after the second anniversary of its May 18, 1980, eruption, Mount St. Helens will continue to provide scientists with an incomparable opportunity to observe natural repair processes at work. Yet the task of deciphering the story of life on the volcano has turned out to be more difficult than many scientists ever envisioned. Instead of an affected area of about 160 square miles that could be subdivided into three or four neat devastation categories, researchers have found that nearly every patch of ground must be sleuthed to reconstruct the causes of its fate since the eruption. Was the site forested, clear-cut or above timberline before the big event? Was it snow covered on the morning of the May 18 eruption? Did it receive only a shower of pumaceous material or was it subjected to the full fury of the mixture of hot gas and rock material known as pyroclastics?

Adding to the difficulties of reconstructing the past are the problems of predicting the future. It is impossible to tell what the future activity of the volcano will be, and a sizable eruption would probably destroy many of the present scientific study sites. Even as we go to press, the mountain is rumbling, and scientists disagree as to whether the volcano's dome is about to erupt or whether Mount St. Helens is in for a period of intensive dome building. One thing is sure, however. If the volcano erupts now or at some future time, plants and animals -- and scientists -- will undoubtedly return to the mountain and start their work again. Certainly all three groups have been busy since the mountain blew two years ago.

Survival of plant and animal life in the areas devastated by the May 18 blast was often due to a chance set of circumstances, which no one could have modeled mathematically and which were independent of the myriad adaptations organisms possess as a result of thousands of generations of evolution. A few hundred feet above Spirit Lake, for example, the presence of a single rock outcrop in the middle of the most ravaged lands seems to have permitted the survival, virtually unchanged, of an oasis containing the normal mix of plants and animals. The isolation of these islands of survivors may lead to genetic differences between populations derived from the survivors and more widespread populations of the same species. Such areas might also form a source from which organisms can recolonize the landscape faster than if recolonization occurred only from the perimeter of the large devastated area.

Being alive on May 19, 1980, however, was no guarantee of continued survival. Today, two types of organisms occupy the affected areas of Mount St. Helens: survivors (or their offspring) and colonizers. Those that lived through the original catastrophe may have been protected by the deep snow that was still present on some sites, or they may have been dormant in belowground burrows or safely ensconced at the bottom of a montane lake. Continued survival depended on the nature of the changes in their environment and their ability to cope with those changes. Many of the animals seen on the earliest reconnaissance missions in the summer of 1980 were not present in the spring of 1981. Some of those that have persisted have gone on to recolonize other parts of the volcano. Colonizers have also come from outside the regions affected by the eruption.

Because chance may spare a site within a devastated area and because survivors at one site may be colonizers in another, neither the types of organisms nor the categories of devastation are completely clear. Nevertheless, these categories and types can help clarify what happened during -- and since -- the eruption.

As Mount St. Helens erupted, its north slope broke up. Great chunks of the mountain and glacial ice flowed northward and westward in landslides and a debris flow that eventually turned into mud flows. A major tongue of this flow was directed into Spirit Lake, causing it to slosh up the side of its basin and scour the hills down to the bedrock in some areas. A more westerly directed arm followed the valley up the North Fork of the Toutle River. This landform contains blocks of material standing more than 90 feet above the surrounding area. The flow, as thick as 300 feet in some areas, averages more than 120 feet thick over a total area of about 23 square miles. Apparently no organisms were able to survive the mechanical damage caused by an overburden of this magnitude.

Damage caused by the blast itself depended on the distance from the volcano. The affected areas are characterized, in a very general sense, as the blast, or near, zone where trees were completely removed; the blowdown, or intermediate, zone where trees were leveled; and a scorch, or outer, zone where trees still stand but were scorched. Close to the volcano, many areas were also subjected to the high temperatures of pyroclastic flows and surges (hot mixtures of gases, ash, and up to boulder-size pumice and rock). Temperatures beneath the surface of materials deposited by these flows often exceeded 480 degrees F -- precluding the survival of organisms and creating some of the most desolate vistas on the volcano.

The volcano also deposited ash and other airborne materials. Beyond the scorch zone this is the only effect of the eruption. In such areas, including places on the relatively unaffected south slope of the volcano itself, the list of plant and animal species remains nearly the same as during preeruption times, although each species is often represented by fewer individuals. For example, at Butte Camp, on the south slope, Roger del Moral, Larry Bliss, and others at the University of Washington have compiled a plant species list that is virtually identical to a typical list for alpine areas in that section of the Cascade Range. Notable animals at Butte Camp are a black bear that has persisted in the area since the time of the blast and pocket gophers that have reproduced within the last twelve months.

This is not to say that the ashfall had This is not to say that the ashfall had no consequences: John Edwards and Lawrence Schwartz of the University of Washington have demonstrated in the laboratory that ash, by causing physical scarification of insect cuticle and subsequent desiccation, can kill individuals much as some commercial silica-dust insecticides do. Ash certainly killed many insects, but after the eruption, numerous sites in the high-ashfall zones close to the blast area contained up to seven species of ants, a dozen spider species, and more than fifteen beetle species, all survivors. At one of the oases above Spirit Lake, where ash is more than six inches deep, I have seen ground-dwelling spiders, two millipede species, a centipede, three ant species, and several beetles.

(Bees, whose dense body hairs act as a trap for dust, were among the insects hardest hit by the abrasive ash. Populations of these beneficial insects, however, are recovering. Ladybird beetles probably did not survive the May 1980 eruption, but ladybugs were back on the mountain as early as the summer of 1980. Aphids, feeding on surviving fireweed, provided the beetles with food.)

Evidence for the effect of ash on vertebrates is also somewhat debatable. As far from the volcano as eastern Washington, David Pyke of Washington Sate University has found deer mice (Peromyscus maniculatus) with signs of eye swelling that may have been induced by ash. In contrast, Doug Andersen of Purdue University -- my collaborator on Mount St. Helens -- and I have found no eye swelling in deer mice from our Spirit Lake blast site or from any of our high-ashfall areas fifteen to twenty miles northeast of the crater. Furthermore, we found no lung damage in any of the dead animals we examined microscopically. Interestingly, in 1945, less than four inches of ash near Iliamna Lake, Alaska, reportedly led to the death of certain rodents, blindness in rabbits, and illness in reindeer.

Richard Mack of Washington State University has documented ashfall effects on vascular plants in Washington and Idaho more than ninety miles from Mount St. Helens. Ash deposits mechanically overloaded several species, killing some and burying the fruits and flowers of others. The ash may also have dusted the leaves of some plants in ways that altered the energy balance, causing the leaves to overheat and die. Mack believes that the most important effects of the ash may become more obvious in the future if compacted ash alters the nature of seed residence sites and the availability of seeds to seed predators.

The kind of disturbance experienced by an area influenced not only what survived but also what can now successfully colonize the site. Potential colonizers, however, must first get there. The animals and plants that have moved into affected areas of Mount St. Helens, whether from oases on the volcano or from outside, must have good dispersal mechanisms. The cysts and spores of myriad microorganisms, carried along by winds, fall on the volcano constantly. Some plant seeds and spiders are also passively dispersed by winds. Immature spiders of many species spin threads of silk that remain attached to their spinnerets. These parachutelike apparatuses carry the spiderlings "on gossamer wings" and are responsible for the presence of young orb-web weaving spiders on the debris flow, an area that still supports very little life. Highly mobile animals, such as birds and some large mammals, were able to move easily into many devastated areas within hours or days after the eruption.

For all these migrants, getting on the volcano is just the beginning of their problems. For most species, suitable home sites and food sources are not available. Wood-boring insects would appear to be an exception. The abundance of downed timber led to some anticipation of a boom of bark beetles (Dendroctonus sp.) and an infestation of nearby valuable timberlands, but to date this has not occurred.

Not surprisingly, colonizers of all kinds are scarce in the blast area. There are few food resources: few plants or plant remains and thus few animals. My impression is that many of the invading invertebrates are predatory forms, an observation that agrees with findings at other sites highly altered by such agents as fire. Colonizers often include a high proportion of carnivores because primary plant productivity is lower than in better developed ecosystems. Collecting insects and spiders in the blast zone with any quantitative certainty, however, has been difficult since there is so little life of any sort. The exceptional places, such as the series of protected oases near Spirit Lake, are so small that I do not want to alter them by intensive study techniques.

(Many factors, including chance, influenced the survival of plants and animals after the May 1980 eruption. In most of the blast zone, for example, devastation was nearly total, but the arms of Spirit Lake are dotted by a dozen or so oases -- patches of vegetation a few yards across -- that survived intact and now contain a normal mix of organisms. In other areas, only some species survived, and the composition of the community changed. Around Meta Lake, for instance, gophers survived the eruption but most plant life did not; unable to find food, the gophers had disappeared by the summer 1981.)

In some parts of the blowdown zone, moving among the tangled trunks is so hard that collecting, observing, or even laying out study plots is a challenge. Deer and elk, however, were seen in blowdown areas within days of the May 18 eruption, and they continue to make forays through the area. These large mammals have also been sighted in mountain basins just north of Spirit Lake, more than five miles from normal vegetation. When they find no food, they simply leave. If they do come across a patch of vegetation, the damage they cause by browsing can be traumatic for the plants and the creatures dependent on them.

America's cosmopolitan deer mouse occurs in a few blowdown sites. In some, where the Soil Conservation Service has attempted to stabilize the soil by seeding with grasses and forbs, high population densities have developed. Gophers, on the other hand, are on the decline in many places. These small mammals feed on bulbs and other belowground plant parts. The roots of such plants as bracken fern and fireweed, often the most conspicuous survivors, do not supply the necessary nutrition. Other, more suitable plants have not yet returned in great enough numbers to support gophers.

In the high-ashfall sites, insects and spiders are abundant and seem to represent a normal admixture, except for ground dwelling forms. Small wolf spiders (Lycosidae) are common in high-ashfall; other ground dwellers have not fared so well. Although in reduced numbers, the same species of small mammals, as well as plants and many other animal groups, occur in heavily ashed areas today as during preeruption times. Our pocket gopher data, for example, suggest population densities lower than at other places we have studied, and our marked population, lacking adequate food here as in blast and blowdown sites, may be declining slightly.

Birds are so mobile that their potential for reestablishment on the volcano is nearly unlimited. Some food exists, in the form of seeds and insects. In some areas, the reseeding projects have produced a considerable food supply for birds that can utilize grass seeds. Many of the bird species inhabiting Mount St. Helens' forests before the eruption, however, were conifer-seed eaters or insectivorous. For these birds, food is less abundant. Nevertheless, on a given day, a few birds can be seen flying overhead almost anywhere on the volcano. In the crater itself, helicopter pilots have reported that hummingbirds have dive-bombed their orange flight suits and the ubiquitous red plastic markers of geologists and biologists.

Establishing that birds are breeding on a site is more difficult than just compiling a list of sightings. Hole-nesting species, which require tree snags, might be expected to do well in the blowdown zone -- if they could find food. Similarly, ground-nesting species might not find the ash too much of a problem. We have noted refous hummingbirds, a raven, and a sparrow hawk in our plots in the blowdown area, but the birds we have seen most often on our early morning censuses of these sites are Oregon juncos (ground nesters) and mountain bluebirds (hole nesters.) In the high-ashfall area twenty miles northeast of the crater, we have found nests and eggs of the junco. For one such site, we estimated, in midsummer 1981, more than sixty birds per twenty-five acres, mostly Oregon juncos and robins but also orange-crowned warbiers, pine siskins, blue grouse, three species of woodpecker, varied thrush, and Townsend's solitaire. The site had been clear-cut some time ago and planted with conifers, which are now about six feet tall and growing out of six inches of ash. A mature forest adjacent to the clear-cut area, also heavily ash covered, contained eleven species and about thirty birds per twenty-five acres. These species lists and densities are not out of line with expectation for similar sites unaffected by the volcano.

I have not spent much time studying aquatic habitats on the mountain. In September of 1980, I visited temporary ponds in Mosquito Meadow in the high-ashfall area. They contained salamander and toad larva that were about to transform, as well as a garter snake. In May of 1981 , these ponds were full of breeding salamanders of two species, the long-toed salamander (Ambystoma macrodactylum) and the northwestern salamander (Ambystoma gracile.) Jim Seddell of the U.S. Forest Service Laboratory at Corvallis, Oregon, has been studying streams in the blast area and has, for some sites, lists of species similar to normal situations. Among the species are insects, fishes, the tailed frog (Ascaphus truei), and the Pacific giant salamander (Dicamptodon ensatus.) Muskrats, trout, mink, and newts (Taricha sp.) have persisted in some lakes -- for example, Ryan Lake, also in the blast zone.

In addition to determining what animals now reside on the mountain, and in what numbers, Doug Andersen and I are attempting to test some hypotheses about the role of animals in the process of ecosystem succession. Working in a clear-cut, high-ashfall site, we are particularly interested in the impact of small mammals and ants on plants and soil. Studies of succession in the subalpine habitats of Utah have generated some specific and some general hypotheses about the sequence of events following disturbance to the landscape. For example, animals, as dispersers of seeds and fungal spores, may influence the presence or absence of some plant species. Currently, I am also testing these hypotheses on sites altered during the process of strip mining. My assumption is that ecosystem recovery processes should be similar regardless of the disturbing agent -- fire, grazing, clear-cutting, or mining -- and that observing the processes in both managed and unmanaged situations should increase the ability to repair ecosystems effectively. When Mount St. Helens erupted, we recognized in the volcano a natural laboratory that might allow us to test some of our ideas, on a scale that no federal or private funding could underwrite and in areas much like the sites I was studying in Utah.

A final report of our finds would be premature, but the pocket gopher, our favorite beast, seems to be living up to our expectations. Gophers consume the underground portions of plants: bulbs, corns, roots, and rhizomes. In the winter, gophers fill snow tunnels with soil, and melting snowpack discloses ribbons of disturbed soil on the ground and a disk of freshly overturned soil around the burrow openings. These very characteristics were the basis for our postulation that gophers might have a positive role in the reestablishment of ecosystems on Mount St. Helens. First, gophers were good candidates for being survivors. Assuming that, even in the blast area, heat from deposited materials was not too great, some of these fossorial animals might persist in otherwise barren landscapes, at least until they exhausted their belowground food resources. This has been the case, and we have found gophers in areas representing most types of damage, including blast, blowdown, and high-ashfall areas.

Second, we thought the soil-turning proclivities of gophers might be beneficial. If well-developed soil was trapped under volcanic material, the gophers, just like a plow, might turn over the fresh and inert ash material and mix it with the old soil. This is occurring, but at a slow rate because so many gophers have died since the eruption for lack of the right plants to eat. Where mixing is taking place, our prediction seems to be borne out. In the summer of 1981, soil turned over by gophers had more seedlings growing on it than did adjacent volcanic material not mixed by the animals.

There are many reasons why this might be the case. The buried soil contains more organic matter and various chemical nutrients than does the new material deposited by the eruption. The tilling activities of gophers might also make possible an important association that many higher plants form with a group of fungi known as mycorrhizae. This association may increase the plants' ability to get nutrients and water from the soil and may be critical to the survival of some species. Mycorrhizal spores are not as easily disseminated as those of other fungi. Several studies implicate small mammals, such as rodents and rabbits, and insects, such as grasshoppers, as dispersing agents.

Most well-developed soils contain the desirable mycorrhizae, but covered by ash, these soils might be ineffective. In an analogous situation -- where topsoil stripped from coal mine sites is stored in large piles until mining is completed -- mycorrhizae have been shown to decline with the time stored in the pile. The stored soil thus becomes an even poorer potential growth medium. On Mount St. Helens, the gophers' digging activities appear to rescue spores from the old soil and mix them into the ash material, increasing the ability of the mixture to act as a growth medium. In tests, samples of soil processed by gophers contain forty to eighty spores per grams of dry soil, while the adjacent volcanic material had an average of less than one spore per gram of dry soil.

I do not mean to imply that gophers will be the main factor in the regeneration of the whole volcanically altered landscape, only that like all organisms, they have an influence, no matter how small. Mount St. Helens, recently reduced to a science fiction caricature of the surface of a planet, is very much a functioning, natural ecosystem in which the rules of the biosphere operate. Careful study of the varying abilities of different plants and animals to exist on the new Mount St. Helens is leading to insights on the varying roles these organisms have in the recovery of the volcano's ecosystems. Some of these insights. especially when combined with studies of undisturbed ecosystems, may help humans, users of various landscapes, approach management problems.

James A. MacMahon is a professor of biology and a member of the Ecology Center at Utah State University.

Mount St. Helens Revisited #2

With few exceptions, sportsmen should be able to enjoy their outdoor pastimes this fall much as they have in past years.

by Dick Bolding

Reprinted from Washington Wildlife, Fall 1980.

The blasted, monochromatic landscape littered with charred, denuded tree trunks has a kind of sterile beauty. But it's hard to look at the land around Mt. St. Helens without longing for the majestic forest that covered these ridges and valleys before the volcano's catastrophic eruption. Few who hunted elk in its forests or fished for steelhead in its pristine rivers can help but compare this biological desert with the magnificent vistas of a few months ago.

Yet the future for wildlife here is not quite as dim as it seemed during the first stunned weeks after the mountain's devastating May 18 explosion. Most recent observations by wildlife scientists around the state have been a source of optimism.

There's no denying that in the immediate vicinity of the volcano wildlife and wildlife habitat will take a long time to recover. But farther away from the mountain there have already been signs that nature is healing her wounds.

Wildlife biologist Rich Poelker recently flew into the blast zone about 10 miles north of the volcano, near the junction of Grizzly Creek and the upper Green River, just beyond Mount Margaret. He found a timber graveyard--tangled bodies of giant old-growth fir trees and occasional stands of dead, heat-seared trees. Flash-flooding left deposits of ash-laden mud in this part of the upper Green and its tributaries, and ash covers the ground in a layer that varies from two to six inches deep. While Poelker was there, he saw no living thing. But when he moved just three miles downstream, he found plants sprouting through the ash blanket at the rate of some 8,000 per acre and saw many deer and elk tracks.

Other observers have seen sprouting vegetation along mudflows that choke the north and south forks of the Toutle River. Streams are beginning to erode through the volcanic debris to the harder surfaces of the stream beds below. Upriver much of the fine silt has been washed away, and the water is already clear, although it gets progressively murkier downstream.

Fish biologists reported that some aquatic insects in the streams survived the disaster--a sign that bodes well for all the species that depend on these insects for food. Surprisingly, some observers have even reported seeing steelhead that have made their way upstream on the main Toutle, and fish are moving freely up the Cowlitz River, which was blocked by mud and volcanic debris just a couple of months ago.

Deer and elk are showing up in the valley bottoms with a nearly normal proportion of young. The animals seem to be in good condition, but biologists are quick to point out that they have yet to face the critical winter months, when food will be in short supply. And winter rains could reverse the recovery process, severely eroding the untimbered slopes and scouring the valleys. Floods could prove disastrous to the rebuilding fishery and jeopardize this year's wildlife production.

Many high lakes within the blast zone are filled with ash and debris from the eruption, and some even had most of their water blown out by the concussion. Although the fisheries in these lakes were obviously destroyed, biologist Bruce Crawford found plankton--the microscopic organisms that directly or indirectly support all aquatic life--still living in what was left of some of them. In Hanaford Lake, about 120 miles northeast of the mountain, he found a live female crawfish. He theorizes that, barring further destructive eruptions, the food chain may recover enough to support fish in some of these lakes in two or three years.

Because most ash from the eruptions drifted north and east from the volcano, lakes on the mountain's southwest side suffered little damage. Merrill Lake, Mc-Bride Lake, Kalama Springs, and Blue Lake all contain fine ash, but it has not, so far, significantly affected their fisheries.

High lake fisheries suffered the most damage in areas that received moderate to heavy ash falls. Ash smothered the bottom-dwelling insects on which fish feed and filled the spaces between the pebbles and rocks of the lake bottoms, eliminating the insects' habitat. No doubt many fish suffered fatal gill damage from the exposure to suspended ash particles in the water.

Outside the blast zone, toward the east, the depth of the ash blanket varies, as does the particle size. Rain has washed most of the gray mantle from trees and bushes, but ash still covers the ground. Blue Lake, 22 miles northeast of St. Helens, is surrounded by a heavy ash cover, but the lake water is still blue and clear. Ten miles farther east, Walupt Lake got a two- to four-inch accumulation of sand-like ash; here Game Department and Forest Service personnel found lake water temperatures and acidity to be normal. Bottom-dwelling invertebrates were reduced as a result of the ash, but not seriously. Biologists collected 32 rainbow trout, several cutthroat and 200 crayfish from the lake and found them all in good condition.

At Packwood Lake, 12 miles north of Walupt, conditions are much the same. The ash that fell here was finer, and it remained in suspension in the water longer, reducing the production of microscopic life. But enough aquatic insects survived to provide fish with an adequate food supply. Just southeast of Packwood Lake Poelker saw a band of 23 elk--10 of them calves--and on a high ridge above Smith Creek, a few miles to the west, he counted five ash-colored mountain goats.

Still farther east, ash fallout painted a 50-mile-wide band across the Cascade crest as it moved northeasterly toward Spokane. Most high lakes along the crest that got ash have not been scientifically monitored to assess the effects on their fisheries, but preliminary observations suggest that most have not been much affected. Some lakes without strong outlet currents seem murky from the suspended ash particles, but most lakes in this area have been providing their usual fishing--even those that are slightly off-color.

East of the Cascades, the Yakima County high country got an average of about an eighth of an inch of ash. The heaviest deposits were along the upper Tieton River south of White Pass, where the ash measures nearly an inch. Even so, regional Game Department biologist Ellis Bowhay has reported frequent big game sightings in the area, including observations of many young deer and elk.

After the May 18 eruption and ash fall, the first thing Bowhay and most other east-side observers had noticed was the loss of insects. Most young birds had just hatched or were about to hatch and were almost entirely dependent on insects for food. In some places, Bowhay said, hundreds of young cliff swallows were found lying dead on the ground below nest sites, apparently as a result of starvation, and the situation was much the same for many other songbirds.

As the summer progressed, wildlife inventory crews reported seeing fewer rodents and small animals than in past years. Their observations confirmed that, of all the wildlife, small birds and mammals were hardest hit by the eruptions.

Far more encouraging has been the frequency of upland bird sightings in eastern Washington. Immediately after the May 18 eruption, biologists reported that in some areas up to 85 percent of the pheasant hens were driven to abandon their nests. They feared that even if upland birds were to successfully renest, the survival of the newly hatched young would be limited by the reduced number of insects.

But counts of pheasant, quail and chukar broods in the Yakima region appeared to be as good as they were during some high population years. The counts were so good, in fact, that Bowhay confesses he's a little nervous about the effects of the reduction of insects and the habitat's capacity to support the birds through the winter. He attributed the good bird production to an apparently high rate of second nesting attempts. Heavy spring rains stimulated a more luxuriant growth of range grasses which lasted longer than usual into the dry summer months, providing good cover for nesting birds.

The high incidence of renesting by pheasants during the summer has resulted in more younger birds than most years, so hunters this fall will shoot greater-than-normal numbers of mature roosters

Similar fears about losing this year's upland bird production in the Columbia Basin have also proven unnecessary. High brood counts and the gradual comeback of insects in the Basin are good omens for this year's upland bird season.

Likewise, the lack of terrestrial insects during the height of eastern Washington's spring hatch apparently did little harm to waterfowl populations. Aquatic organisms furnished enough protein to carry small ducklings through their early days of life until they could forage for a wider variety of foods.

However, inventories of duck populations in nesting grounds throughout western Canada are a little lower than usual due to the effects of short water supplies on nesting and hatching survival. That means fewer ducks migrating through during hunting season. But although production is somewhat less than last year, it has not dropped enough to require any reduction in hunters' bag limits.

So, while there can be no doubt about the loss of wildlife and destruction of habitat near the volcano, short-term investigation seems to show that the effects of ash fallout beyond the blast area are not as severe as biologists had earlier feared. That's good news for sportsmen and other outdoor recreationists who should be able to enjoy wildlife-related activities this fall much as in past years.

Of course, there are some exceptions. The area around Mount St. Helens will be closed indefinitely to all recreation, as will most of the Gifford Pinchot National Forest. As for central and eastern Washington, the ash should prove not more than a discomfort and an inconvenience for outdoorsmen. It shouldn't seriously hamper their activities. Bird dog trainers, for instance, report that working their dogs in the volcanic dust has had no apparent ill effects on their dogs. Those living in ash-fall areas say the ash will be with us for a few years--we'll just have to put up with it.

As it turns out, the May 18 eruption was not the end of the world, nor will it seriously limit hunting and fishing opportunities in our state. And while there may be more eruptions in the future, we know now that we can deal with the aftermath, and so can our wildlife.

Mount St. Helens Fact Sheet

(Copied by permission from publication of Mount St. Helens National Volcanic Monument and Gifford Pinchot National Forest. Activity and Chronology are through March 1984.)

Background Information

Location:

Height:

eruption

Last Eruption:

Dormant

Other Active Volcanoes:

First volcanic eruption in continental 48 states since Lassen Peak in California last erupted from 1914 to 1917.

Activity

Eruptions:

pumice

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

Crater:

Earthquakes:

magnitude

Floods:

Resource Losses:

Potential Hazards:

Probable ash and steam eruptions.
Rapidly moving flows of mud (melting snow and ice mixed with ash).
Pyroclastic flows, which are masses of fiery gasses and light-weight volcanic particles that skim like an avalanche along and above the ground at speeds of up to 100 miles per hour.
Ashfall carried by prevailing winds as a result of a major eruption.
Flooding.

Chronology of Events

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.)

From Lava to Life

by Donna Gleisner and Bruce Crawford

Reprinted from Washington Wildlife, Fall 1980.

No one suspected that Mount St. Helens would utterly devastate 100,000 acres of the Cascade Mountains on that calm Sunday morning, or that it would happen with such force in just a matter of minutes. A 100-400 mile per hour lateral blast from the north face seared the surrounding countryside and alpine lakes with temperatures of up to 500 degrees F and engulfed all life in a deadly blanket of fallout.

All 37 lakes within the Mount St. Helens National Volcanic Monument received the full brunt of that natural, yet terrifying, phenomenon. The blast's destructive forces denuded most shorelines of anything green and growing. In their place settled a layer of hot, gray ash that choked off light and life and superheated the waters.

At that time the lakes, all managed by the Washington Department of Game for trout fishing, contained a mixture of eastern brook and cutthroat trout. Most lakes were planted by helicopter or plane on a three-year rotation basis. Fifty to 100 fish per acre was the norm, meaning that as many as 150,000 fish could have been in the lakes at eruption time.

Conditions looked extremely dismal immediately after the blast. The suspension of ash in the lakes along with super hot temperatures severely reduced the aquatic plants and animals abilities to survive. But to find out exactly what the conditions were in the blast zone, biologists from state and federal agencies decided to helicopter in as close as they dared, starting in July. This was the beginning of four years of research.

From July 1980 through August 1984, 21 lakes were surveyed by the Washington Department of Game. Surprisingly enough, live populations of trout were found in two-thirds of them: Elk, Fawn Hanaford, Island, Little Venus, Merrill, Meta, Obscurity, O'Connor, Panhandle, Shovel, Strawberry, Tradedollar and Venus. Of the seven remaining lakes where gill net surveys failed to produce fish, only three lakes -- Boot, Ryan and Spirit -- appear to have lost their populations due to the eruption of Mount St. Helens. Two of the seven barren lakes sampled probably didn't contain any fish prior to the eruption because they were too shallow.

Of those waters with surviving populations, eastern brook trout were either the dominant or the only fish species found in most. Cutthroat were found in only four lakes and lake trout in one. it seems that in the lakes where cutthroat were stocked on top of self-sustaining brook trout populations, only brooks survived the stress and competition imposed by the blast.

The good news is that many lakes still retain the habitat needed for natural spawning to take place in inlet or outlet streams. And evidence so far indicates that eastern brook trout are again successfully spawning in at least three lakes and cutthroat in one lake. The reason? Aquatic insects somehow managed to survive the blast's effects or quickly repopulated the devastated lakes. Just that little bit of life, plus the self-cleansing action of the water itself, enabled life in those lakes to respond rather quickly -- in one to three years. Now aquatic life, including fish, is rapidly approaching, and in some lakes even exceeding, its preeruption population levels.

Selected lakes outside the Monument, such as Hanaford, were stocked beginning as early as 1982 and receive quite a bit of use. But within the Monument itself, a five-year moratorium on stocking fish is in effect. This allows researchers time to complete their studies before fish are reintroduced on an artificial basis.

Bruce Crawford, regional fish biologist for the Department of Game, believes that all natural lakes, including Spirit lake with its abnormally high nutrient levels, are very capable of sustaining populations of trout. He also thinks the two newly-formed lakes, Castle and Coldwater, are capable of doing the same on an even larger scale.

The future of these alpine lakes and their fish populations depends upon proper management and adequate public access -- something that the Washington Department of Game and the Gifford Pinchot National Forest are working closely together on. We want to not only meet the recreational needs of our sport fishing public, but to also remain aware of the unique research opportunities and the fragile life within that seeming desert of ash.

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

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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Carithers, Ward. 1946. Pumice and pumicite occurrences of Washington: Washington Division of Mines and Geology Report of Investigations 15, 78 p.

Crandell, D.R., and Mullineaux, D.R., 1973, Pine Creek volcanic assemblage at Mount St. Helens, Washington: U.S. Geological Survey Bulletin 1383-A, 23 p.

_______ 1978, Potential hazards from future eruptions of Mount St. Helens volcano, Washington: U.S. Geological Survey Bulletin 1383-C, 26 p.

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_______ 1962, Recent lahars from Mount St. Helens, Washington: Geological Society of America Bulletin, v 73, no. 7, p. 855-870.

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Smith, H. W., Okasaki, Rose, and Knowles, C. R., 1977, Electron microprobe analysis of glass shards from tephra assigned to set W, Mount St. Helens, Washington: Quaternary Research, v. 7, no. 2, p. 207-217.

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