Volcanic Events, pg. 2
Mount St. Helens History, pg. 3-15
Eyewitnesses, pg. 53-67
Absolute Times, pg. 81-82, 86
Activity Sequence, pg. 127-134
Gas Studies, pg. 190-191
Ash Clouds, pg. 323-333
Blast Dynamics, pg. 379-400
Rapid Deposition, pg. 466-478
Phreatic Explosions, pg. 509-511
New Lava Dome, pg. 540-544
Ash-Fall Deposits, pg. 568-584
Water Chemistries, pg. 659-664
River Water Quality, pg. 719-731
THE 1980 ERUPTIONS OF MOUNT ST. HELENS, WASHINGTON
SUMMARY OF EYEWITNESS ACCOUNTS OF THE MAY 18 ERUPTION
By J. G. ROSENBAUM and RICHARD B. WAITT, JR.
Many individuals who were
in the vicinity of Mount St. Helens on May 18 were interviewed to gain
information on phenomena associated with the eruption. The observed phenomena
include an earthquake, a massive avalanche of the volcano's north flank,
a directed blast, development of the vertical eruption, a mudflow in the
South Fork Toutle River valley, and the fall of early eruptive products.
The eyewitness accounts, although somewhat subjective, provide information
about the sequence of events, descriptions of the directed blast (including
velocity, temperature, and a preceding pressure change), and the timing
and nature of early air-fall material. These observations supplement a
variety of subsequent scientific investigations.
On May 18, 1980, a large number of people at many locations in the vicinity of Mount St. Helens witnessed the awesome eruption of the volcano (fig. 35). Over the following months, many of these individuals were interviewed. In presenting summaries of geologically pertinent observations from these interviews we have attempted to retain the descriptive words and phrases used by the eyewitnesses. In like manner we have retained the measurement units used by the witnesses, converting them to the metric system in our discussion.
Locations of the various eyewitnesses are shown in figure 35 and described in table 5. For brevity, testimony by one or more witnesses at a specific location is referenced in the text only by site number. The site numbers consist of a distance and a direction relative to the preemption summit of Mount St. Helens. For example, site 17NE is located approximately 17 km northeast of the old summit.
The eyewitnesses observed a wide variety of phenomena associated with the eruption, including an earthquake, a massive landslide, various aspects of a directed blast (such as its movement, temperature, and the material deposited), noises, winds, lightning, and the nature of air-fall material. Observations of the various eruptive events and their products are retold here in approximate chronological order.
Although eyewitness accounts provide a unique source of information
about the eruption, they are subjective. Two individuals recalling the
same phenomena may differ markedly in their perceptions as well as their
descriptions. In particular, unless otherwise stated, all quantitative
data (such as time intervals and distances) specified in these accounts
are estimates of the witnesses and are often based on observations made
under extremely stressful conditions.
EARTHQUAKE, LANDSLIDE, AND INITIATION OF ERUPTION
At 0832 PDT, May 18, 1980, several witnesses felt a shallow earthquake that emanated from Mount St. Helens (Endo and others, this volume). A "solid jar" and noticeable ground motion lasting for 3-5 s were noted (12Wa) at about this time. Others who reportedly felt the quake include members of a tree-planting crew on the north side of Marble Mountain (8SE) and individuals to the north (23N, 29N) and northwest (17NW). Significantly, one observer felt the shock and then saw the north side of Mount St. Helens begin to move (12Wb). This sequence is consistent with a report of avalanching into the summit crater (presumably triggered by the earthquake) prior to failure of the north flank (0).
The massive failure of the bulging north side of Mount St. Helens was viewed from a variety of locations. None of those viewing the start of the avalanche were able to observe its progress for more than a few tens of seconds because of topographic obstructions and rapid development of the eruption cloud. Fortunately, the initial events were recorded by photographs from several locations (0, 12W, 15E, 17NE, 27NE). Two of the following accounts (17NEc, 15E) are based largely on these photographs rather than on visual observations.
0. --Within about 15 s after the start of avalanching into the summit crater "everything north of a line drawn east-west across the northern side of the summit crater began to move as one gigantic mass. * * * The entire mass began to ripple and churn up, without moving laterally. Then the entire north side of the summit began sliding to the north along a deep-seated slide plane * * * a huge explosion blasted out of the detachment plane. We neither felt nor heard a thing, even though we were just east of the summit at the time. * * * the southern portion of the summit crater began to crumble and slide to the north just after the initial explosion. From our view point, the initial cloud appeared to mushroom laterally to the north and plunge down. Within seconds, the cloud had mushroomed enough to obscure our view" (Stof-fel, 1980).
17NEc.--One observer watching the mountain through binoculars saw the north side start to get "fuzzy, like there was dust being thrown down the side." Several seconds later the north face began to slide. The lower part of the north slope seemed to slide away more quickly than the upper. The first dark-colored cloud appeared near the middle of the north slope in an area vacated by the slide, and a light-colored cloud formed near the summit. A short time later, a very black cloud emerged from the summit area. The upper part of the north flank moved downward and the flank eruption seemed to explode through the moving material. The flank eruption grew rapidly, especially to the north.
15E.-- "As the avalanche reached the halfway point on the mountain,
the summit eruption began with a dense black cloud followed by lighter
gray material. A second eruption halfway down the slope occurred moments
later * * *." At this time the avalanche appeared to consist of upper and
lower parts. The flank eruption was between the two. Seconds later the
upper slide overrode the flank eruption and material was hurled far down
slope onto the lower slide. About 45 s after the landslide began, the eruptive
centers merged and the rapidly expanding cloud overtook the avalanche.
17NEa.--The "bulge was moving * * * the whole north side was sliding down." The first cloud appeared to form at the bottom of a "cirque-like" wall from which the bulge had moved. Within about 20 s the landslide was out of view behind a ridge. At this time, there were two distinct clouds which seemed to be emanating from separate vents; an extremely dark one rising vertically from the summit, and a lighter one from the north flank.
12Wa.--Goat Rocks (a volcanic dome at the 2,000-m level on the
north flank) slid down to the north. Two eruption clouds formed, one from
the summit and the other from the north side. The cloud from the north
flank grew very quickly and discrete objects could be seen being hurled
to the north.
The above accounts agree remarkably well and reveal the following sequence.
The earthquake apparently preceded, and therefore probably caused, the
catastrophic failure of the volcano's bulging north flank. The initial
landslide followed the quake within seconds and initially consisted of
at least two large slump blocks. Moments later, eruptive activity began
near the middle of the north flank above the lower slump block, and near
the summit. The upper slump block was blown apart as it descended across
the rapidly growing flank eruption.
DIRECTED BLAST ACCOUNTS
50Ea.--Climbers on Mount Adams saw material moving to the north less than 1 min after the beginning of the eruption. One of the climbers reported seeing what seemed to be a shock wave, similar to that associated with a nuclear explosion, traveling to the north 30 s or less after the start of the eruption.
75N.--Climbers on Mount Rainier observed two distinct "flows,"
which began very shortly after the eruption started. These "flows" were
described as clouds 1,000-2,000 ft thick that appeared to hug the ground.
The heads of the "flows" disappeared into alleys and reappeared as they
"hopped" over ridges. The earlier flow traveled to the west, perhaps down
the North Fork Toutle River, and was followed almost immediately by a "flow"
that seemed to travel to the east.
OBSERVATIONS SOUTH, EAST, AND WEST OF DEVASTED AREA
15E. --About 45 s after it began, the avalanche was hidden by the developing eruption cloud. For a short time, a steep, smooth front of the eruption cloud could be seen rushing down the north slope of the volcano. In contrast, as the cloud spread eastward the front became billowy rather than smooth. The base of the cloud seemed to turn under and no basal avalanche was visible. There was no sign of a shock wave; trees were not knocked down ahead of the cloud, but disappeared under it.
17NEa.--A dark cloud grew vertically from the summit. A lighter cloud, which seemed to come from the area vacated by the landslide, grew more or less spherically except that a large "arm" shot out to the north in the direction of the avalanche. The spherical cloud seemed to approach from the mountain--not as part of the northward arm--and reached the ridge closest to the mountain 25-30 s after the start of the landslide. When the cloud hit the ridge, it rose and boiled upwards.
17NEb.-- Shortly after the vertical eruption began, a large horizontal blast occurred. Just before the top of the mountain became obscured, the south side of the summit crumbled into the hole formed by the landslide. As the cloud grew, what appeared to be a shock wave similar to that associated with a nuclear explosion moved ahead of the cloud. About 1 1/2 min after the start of the landslide and perhaps 45 s after the start of the blast, a noise like a clap of thunder accompanied some sort of pressure change. The initial noise was followed by a continuous rumbling "like a freight train."
17NEc.--A rumbling noise began within 7-8 s of the start of the landslide. One member of the group sensed a pressure decrease at about the same time. A "shock wave," which looked like heat waves, formed ahead of the blast cloud.
12Wa.-- Spherical black clouds seemed to rise simultaneously from the summit and from the north flank. The cloud boiling out of the north side was larger than that from the summit and grew very quickly. After perhaps 1/2 min, discrete objects could be seen being hurled to the north. A barely audible rumbling was the only noise accompanying the eruption.
12Wb.--A very short time after the start of the landslide, a black jet developed at the top of the moving bulge. A light-gray, ground-hugging cloud moved down slope to the north. The west edge was about 45 m thick. This initial part of the directed blast was not particularly dense: "You could look into it but not through it."
13NW. --Following the collapse of the north side and the appearance of a black and white jet, a white ring, like campfire smoke, came down as an apron around the mountain. This foglike ring descended very quickly and expanded out from the mountain. A black cloud, which seemed to rise near the summit, followed and caught up with the ring near timberline. The leading wall of the black cloud could be seen through a white haze as it climbed over a ridge. A warm wind began to blow from the mountain. It increased in intensity to 30-40 knots, causing trees to bend and some branches to break.
About 2 min after the beginning of the eruption, the witnesses began driving west at about 70-75 mi/hr. At this speed, they did not seem to get any farther from the cloud. The wind blowing into the car through an open sun roof was warm enough to give the impression that the car heater was on. At State Highway 504 they increased their speed to about 85 mi/hr and began outdistancing the cloud. About 2.5 mi farther west on State Highway 504 they stopped and could not see the black cloud. After a short time the cloud reappeared, moving down valley at about 45 mi/hr. The white mist still preceded the dark cloud by about 1/2 mi. The base of the black cloud looked "like avalanches of black chalk dust--first, one part of the black cloud would shoot out in front, then another, then another, like waves lapping up on a beach." Pulling back onto the highway they outran the cloud at about 65 mi/hr.
17NW.--A short time after the landslide, the view toward the mountain was replaced by a "big black inky waterfall." This "waterfall" appeared to hit a ridge a few miles up valley within 15-30 s. At this time the witnesses began driving west on State Highway 504 at about 100 mi/hr. After 3-4 min the black cloud had almost reached them. The cloud front was shell shaped with the leading edge of the cloud 100 yd to either side and 50 yd behind. The material in the cloud was "coming down with much authority," then bubbling or bouncing back up. One witness compared the cloud to boiling oil with huge bubbles 6 or more feet in diameter. They continued driving at speeds between 80 and 105 mi/hr and about 5 min later began pulling away from the cloud.
9W.--Soon after the start of the eruption, a rock prominence high on the north flank exploded down toward Spirit Lake. As a big, black cloud started rolling over the hills, the witnesses jumped into their truck and began driving west at 75-80 mi/hr. After they had traveled about 4 mi, a dark-gray cauliflower cloud billowed above the ridge to the northwest ahead of them. They realized that something had moved down the North Fork Toutle River valley faster than they were able to drive parallel to it down the South Fork Toutle River valley.
27NE.--Following the landslide, two clouds, one from the summit
and the other from the north flank, grew rapidly. About 25 s later, the
cloud on the north began to billow near the base of the north side. After
45 s, the cloud seemed to be spreading laterally, with a tremendous amount
of material coming from low on the north side and perhaps rising from the
Spirit Lake area. A rumbling noise, which lasted only a few seconds, began
1 1/4 - 1 1/2 min after the start of the eruption.
OBSERVATIONS NORTH OF DEVASTATED AREA
22N.--A noise similar to, but which "didn't sound quite right" for, a propeller-driven aircraft occurred for 10-20 s before a rapid pressure change, which caused ears to pop numerous times over a period of about 10 s. One person also felt as if she was being squeezed gently over her entire body. A short time later, an immense ash cloud approached that seemed to consist of a lower vertical wall and upper overhanging part.
20N. --A pressure change which caused ears to pop 10-15 times over an interval of 1/2 - 1 min was followed a half minute or so later by a rumbling which sounded very much like a jet plane. At this time the eruption cloud could be seen in the direction of Mount St. Helens. Two or three minutes later, as the upper part of the eruption cloud passed overhead, a wind, like the updraft around a large forest fire, began blowing toward the mountain.
25N.--The whole north side of the mountain seemed to explode out over Spirit Lake 5-10 s after a couple of small puffs of steam came from the summit. Almost simultaneously there was a noise like a distant cannon. A loud roaring noise--"like a train going by"--began 30-45 s after the first puffs of steam. The eruption cloud seemed to spread horizontally and then later rose vertically. As the cloud moved northward, it could be seen as it passed each ridge and seemed to follow the slope down into the next valley. The cloud reached the ridge north of Coldwater Creek about 1 1/2 min after the initial puffs of steam were observed. About 5 min after the beginning of the eruption, a "wave front" moved southward across Riffe Lake. Then a 2- to 3-s, 50-mi/hr gust of wind from the north hit the wall of ash and "stood it up," slowing it greatly.
29N. --Following some white steam and a small black cloud from the summit, a much larger cloud rose from the north flank. At this time, a large piece of the north flank seemed to be airborne. Ten to fifteen seconds after the earthquake, light-colored material appeared over the high country around Mount Margaret. A gentle rumbling was heard about this time. An instant later, a huge dark wall appeared over the Mount Margaret area. Within seconds it had covered the area around Fawn, Hanaford, and Elk Lakes, and in another 10-15 s reached Elk Rock. "When it hit that point it seemed to split--you could see a wall of ash going down the North Fork of the Toutle." The other branch moved more northerly and there seemed to be standing green timber in between. At first timber was knocked down far ahead (estimated 2-3 mi) of the ash wall. The ground where the trees had been knocked down was no longer green. A wall of greenery "a mile high" could be seen in the air ahead of the wall of ash. The ash cloud overtook the green material before it reached Black Mountain, and trees were no longer flattened ahead of the ash. As the wall of ash moved down slope toward the Green River, it was preceded by a surflike wave of ash. When the wall reached the Green River, it seemed to be stopped, as if the river was a firebreak. The cloud seemed to be "stood up" by something, then curled over so that the upper part preceded a lower vertical wall to the north. At about the same time a wind began blowing toward the mountain at 40-50 mi/hr.
23N. --About 20 s after the earthquake there was a loud roar and possibly a second quake. At this time a small plume of ash coming from the mountain began to grow so rapidly that within 5 or 6 s the mountain was completely obscured. Shortly after this, some ash appeared at some point to the west and then at some point near the headwaters of the Green River to the east of Mount Margaret. About 20 s after the roaring noise began, the front of the blast cloud appeared atop the ridge north of Coldwater Creek, and was described as follows: "All of a sudden the whole thing seemed to come up all in one big line and just loom right up in front of us." One photo taken at this time shows the cloud front to be no more than a few thousand feet high. Before the blast cloud appeared, the region around the mountain was visible, although the mountain itself was obscured. The cloud moved toward the witnesses at high speed, and seemed to follow topography--rolling over everything like a track on a caterpillar tractor. After driving several miles to the north, the witnesses noted "at least 80 mi/hr winds" were blowing off Riffe Lake to the south. The wind seemed to stand the blast cloud straight up.
30N.--Five to seven minutes after a rumbling noise, part of the
eruption cloud passed high overhead. At about this time, a sudden 40-knot
wind blew toward the mountain lasting about 10 s and was followed by an
80- to 90-knot wind that blew away from the mountain. The second gust was
sufficiently strong to knock down one of the witnesses.
OBSERVATIONS WITHIN DEVASTATED AREA
19N.--A photo taken shortly after the eruption was first noticed shows the eruption cloud above and in the area of Mount St. Helens, viewed over the ridge north of Coldwater Creek. Seconds after the blast cloud crossed this ridge, "I looked east toward Hanaford Lake and Fawn Lake and that area--it looked like that whole mountain range had just exploded." As the blast cloud approached, "it looked like a boiling mass of rock--and just as high as you could see." Trees were picked up and thrown into the air at the leading edge of the cloud. The cloud seemed to follow the topography. As the blue-black cloud approached, there was a rumbling noise. No wind preceded the cloud's arrival.
As soon as the blast cloud enveloped the area, it became totally dark and got very hot. One witness received burns on the right side of both legs a little above the ankles. Although they did not blister, these burns were quite deep and not entirely healed at the time of the interview (3 mo after the eruption). A doctor said that the burns were similar to microwave burns. This person also received burns on the upper back of the thighs and another witness was burned along the forearms. Why the burns occurred in these places is not clear. In addition, the heat deformed the grill of the truck the witnesses were in, and a styro-foam cooler in the truck bed was melted.
For a short time after the blast cloud arrived, "chunks" of material could be heard hitting the truck. At this time the right side of the truck, which was toward the volcano, received extensive damage, including breaking of the vent window, stripping of the chrome trim and outside rear view mirror, and severe sandblasting of the paint. Shortly after the blast cloud arrived, the inside of the truck's windshield was found to be so cool and wet that the witnesses felt certain that the material deposited outside must have contained ice. In a short time it became lighter for a few seconds, then became totally dark again.
18N.--An external-frame tent which had been tipped on its side to dry was suddenly blown over by several gusts of wind. This was immediately followed by noises like three rifle shots in the distance and then by an apparent pressure change which seemed to force the witnesses to the ground. A black cloud shot overhead 10-15 s after the noises. "Golf-ball-sized" and smaller pieces of rock dropped from this cloud (some of these were collected and are a gray dacite). The cloud moved some distance to the north and then pulled back to the south (so that blue sky appeared overhead) in a span of about 5 s. Although the cloud pulled back, it did not completely disappear from sight. The cloud reapproached with a "roaring noise." As it passed overhead, a cedar tree began to fall and within seconds "there were no trees left." Seconds later it was totally dark and ash was falling so heavily that visibility, with a flashlight, was no more than a foot. Although thousands of trees fell, the witnesses did not hear them, nor did they feel a blast or wind at that time: "whatever happened, it happened over our head." The first material to fall "was cold and was like mud," and may have contained some ice. The particles seemed to be somewhat larger than sand sized and appeared to fall vertically. A minute or two after the trees blew down, it became uncomfortably hot, as if a forest fire was nearby. The heat did not last long.
2lNb.--One of the witnesses may have sensed a pressure change; however, the other did not recall such a change. Nevertheless, for some reason they simultaneously sensed something unusual. One of the witnesses looked to the west (or southwest) and observed a "red column" extending to some height. He quickly "dove" out of the tent and rolled under a previously downed tree. As soon as he got under the tree "something hit--I could just hear this big rumble. I was just instantly buried." Trees came down on either side of the log he was under. His leg or hip was broken at this time. The other witness, in the open when the blast cloud arrived, was knocked to the ground immediately. He quickly reached forward and felt several logs which he used to drag himself more or less upright. The first material to hit him was "mud and ice. There were chunks hitting me in the back and melting." He was very wet after being hit with this material. After about 3 s, it got extremely hot. He became suddenly dry and was then burned on his hands, scalp, and several other places. The witness who was buried by downed trees and the initial blast material was not burned and perceived the heat only as being "real warm." Although it stayed very warm (estimated 130° F) for quite some time the intense heat which caused the burns was short lived. Material, which seemed to be "dirt clods," fell for about 7 min. During this time visibility ranged from 0 to 1 ft. This material quit falling quite suddenly. "Then it was almost sunny." At this time the deposit was 3-4 in. deep. Several minutes passed before ash began falling again.
21Na.--A very strong wind, which blew flames from the campfire flat along the ground and held braids of hair out horizontally, preceded the blast cloud by about 10-15 s. The witnesses were able to move about in the wind with little trouble, and no trees were toppled by it. No noise was associated with the approach of the cloud, and no concussion or loud noise was noted prior to the cloud's arrival. When the cloud arrived, it became totally black and all of the trees seemed to come down at once. The witnesses were instantaneously buried in a combination of timber and "ash" and probably fell into a hole left by the root ball of a blown-over tree. They could talk to each other but could see nothing. After perhaps 10 s, it got very hot. At this time they could hear their hair "start to sizzle" as it was singed. One witness, who is a baker, estimated the heat to be like an 300°F oven. "Pitch boiled out of trees" and remained hot enough to cause minor burns several minutes later. The sky cleared suddenly after several minutes and remained clear for a few more minutes. Then a dense ash fall began again.
20NW.-- The witness and two companions were felling timber with
chainsaws. Mount St. Helens was hidden by a ridge and the three men neither
heard nor felt anything unusual until they were alerted to the eruption
by a fourth man. About 10 s later, "a horrible crashing, crunching, grinding
sound" came through the trees from the east. Suddenly it became totally
dark: "I could see absolutely nothing." It immediately got very hot, and
almost impossible to breathe. While the men were gasping for air, the inside
of their mouths and their throats were burned. The witness was knocked
down although he does not recall being hit by rocks or other projectiles.
He arose with his back to searing, painful heat that lasted about 2 min.
At about this time visibility began to return. All trees had been knocked
down, and everything was covered with about a foot of drab gray ash. None
of the men's clothing had been burned, but their bodies had been burned
extensively. Three of the men subsequently died. Heavy ash fall resumed
after about 20 min.
With few exceptions, the eyewitness accounts agree that: (1) close to the mountain no loud concussive noise was associated with the directed blast; (2) no shock wave preceded the blast cloud; (3) the blast cloud expanded horizontally, especially to the north, much faster than it rose vertically; (4) the velocity of this horizontal movement was great; and (5) the blast involved a brief period of intense heat that followed the blast front by a short time.
Although loud noises due to the eruption were heard hundreds of kilometers away, the blast seems to have been surprisingly quiet in the vicinity of Mount St. Helens. A similar phenomenon was reported for the climactic eruption of Krakatoa in 1883, which was heard 5,000 km away but not on neighboring islands (Symons, 1888). Close to Mount St. Helens, a variety of noises was associated with the early stages of the eruption. The most commonly described noise was a rumbling or roaring sound, which was variously described as from lasting only a few seconds and being barely audible to being a continuous loud roar. The source of this noise is not totally clear. One group, located approximately 17 km from the mountain (17NEc), thought that this sound began less than 10 s after the beginning of the landslide. Another estimate from the same locality (17NEb) places the onset of noises about 45 s after the start of the blast. The first estimate does not seem reasonable, because if the noise originated at the mountain, its onset would have been well before the beginning of the landslide. The second estimate seems more reasonable if the rumbling was generated by the onset of vigorous venting (that is, by the blast itself). Some rumbling noises may have been due to the landslides and debris avalanche. Also, the roaring may in part be due to the felling of thousands of trees by the blast, although most observers did not connect the two. Only one person's graphic description--"a horrible crashing, crunching, grinding sound"--seems to fit the destruction of a forest. In fact, the lack of reported noises associated with trees falling is striking.
Although three individuals reported seeing what appeared to be a shock wave preceding the blast cloud, no witness reported feeling one. The phenomenon described as a shock wave may have been something entirely different, perhaps similar to the white mist that led the blast cloud down the North Fork Toutle River valley (13NW). Several individuals sensed a pressure change. To some this was a rather vague feeling, while others experienced very definite sensations such as ear popping, being squeezed all over, or being pushed to the ground. This rather mild baric phenomenon certainly caused no damage.
As the blast cloud expanded, one might expect it to have displaced the air ahead of it, causing a strong wind away from Mount St. Helens. Surprisingly, only a few observers reported such a wind. One report (13NW) describes a warm 50-60 km/hr wind down the North Fork Toutle River valley. This wind was strong enough to break some branches, but not to fell trees. Two other accounts (18N, 2lNa) report a wind preceding the blast cloud by 10-15 s in the Green River valley. This wind was strong enough to blow braids of hair out horizontally and knock over a propped-up tent, but it did no damage.
The absence of a shock wave or hurricane wind preceding the blast cloud is consistent with the fact that very few witnesses were able to observe trees being knocked down. Those who witnessed the cloud's advance from the east, the northeast, the North Fork Toutle River valley, and most localities to the north, did not see any damage done. Even some of those within the blow-down zone could not see trees fall (20NW, 21Na, b). The people within the blow-down zone reported that the trees fell suddenly at the moment of the cloud's arrival, and that it became completely dark at the same instant. Another report (19N), confirmed that the trees fell at the cloud's leading edge, although in this case the trees were seen thrown into the air. Of those within the blow-down zone, only one witness, located in the valley of Miners Creek (18N), saw trees fall and noted that the entire forest had been flattened before he was enveloped in total darkness. This seems to be the result of his being in a steep-sided valley and apparently having the blast cloud move overhead, hitting the upper parts of trees while momentarily permitting light from the north.
Only one account is not easily reconcilable with all destruction occurring at, or after, the arrival of the blast cloud (29N). This report indicates that, at least in some localities, the forest was destroyed a substantial distance ahead of the blast cloud. In the absence of a shock wave or preceding high wind, it is difficult to explain what mechanism might have caused destruction in advance of the cloud.
Photographs from the east, northeast, and west show the top of the blast cloud to be quite low (12Wa, 15E, 17NEa, c). From Mount Rainier, the head of the blast was seen disappearing into valleys and reappearing as it topped ridges (75N), and just north of the Green River, witnesses watched the cloud pass over each ridge and follow the slope into the next valley (25N). Impressive testimony to the ground-hugging nature of the blast cloud is provided by those north of the Green River who did not see its approach until it suddenly topped the ridge north of Coldwater Creek (23N, 29N). These witnesses looked over the top of the blast cloud to the vicinity of Mount St. Helens while the cloud traveled 13 km to the north. The northward progress of the cloud from the top of this ridge down to the Green River was witnessed by a number of people. All of these felt that the cloud closely followed topography. However, in at least one case, the blast cloud traveled some distance above ground level (18N).
All eyewitness accounts agree that the blast cloud moved at a high velocity. Although the information in these accounts is highly subjective, it is possible to use it to make some crude estimates of the cloud's speed. Two reports, one from a location roughly 22 km north (23N) and the other about 27 km north-northeast (29N) of the north flank of Mount St. Helens, allow a rough calculation. The first describes a rumbling noise approximately 20 s prior to the cloud's topping the ridge north of Coldwater Creek; the latter a rumbling noise only a few seconds before the cloud reached this location. Both observers could first see the cloud after it had traveled approximately 13 km. It would take a sound from the vicinity of Mount St. Helens about 66 s to reach the first location and 81 s to reach the second. Therefore, if the roaring began at the time the blast began, the time for the cloud to travel 13 km can be estimated as:
Rounding this to 1 1/2 min because of the approximate nature of the data yields an average velocity of 140-150 m/s (310-335 mi/hr) for the 13 km.
This high velocity applies to the "northward arm" of the blast (17NEa). In other directions the blast moved much more slowly. In particular, testimony from those who drove away from the blast cloud down the North Fork Toutle River valley (13NW, 17NW) indicates that the cloud's velocity may have been as low as 20 m/s and certainly was no higher than 45 m/s in that part of the valley west of Elk Rock.
Although material at and near the leading edge of the blast cloud flattened trees and damaged vehicles, movement of high-velocity material did not last very long. In two cases, individuals who had apparently been knocked down by the arrival of the blast cloud were able to regain their feet quickly (20NW, 21Nb).
The majority of those hit by the blast reported that the first material in it was cold and that a short time elapsed before it became hot (18N, 21Na,b). Several of these people suggested that this cold material contained ice. The difference between the blast's arrival and that of the heat was estimated to be 3 s by one individual who was directly exposed (2lNb). Other estimates were somewhat longer (18N, 2lNa). Only one individual reported the simultaneous arrival of the blast and heat with no evidence of preceding cold material (20NW). The interviewees found the duration and intensity of the thermal event extremely difficult to judge. Most thought that the intense heat lasted a few minutes or less. This short duration explains why those who were completely unprotected received severe burns (20NW, 2lNb), whereas those who were somewhat protected by a vehicle or fallen timber escaped with few ill effects from the heat.
As the blast cloud reached the limits of its destruction, a number of
observers to the north witnessed a rather sudden slowing of its movement.
Many descriptions refer to strong winds towards the mountain, to the cloud
front being "stood up," and to the cloud's curling over so that the upper
part preceded a lower vertical wall to the north. These descriptions are
extremely similar to the following description of the 1951 eruption of
Mount Lamington, Papua, New Guinea (Taylor, 1951): "The black cloud whirling
and billowing like an oil fire could be seen advancing * * *. The summit
of the cloud appeared to curl over like a wave about to break on shore,
but when its front was less than a mile away a brisk breeze sprang up and
the cloud rolled back again."
DEVELOPMENT OF THE VERTICAL COLUMN
Most of those in the immediate vicinity of Mount St. Helens lost sight of the mountain very shortly after onset of the eruption. Hence, for the most part, they were unable to observe the development of the vertical column. Cool winds blowing toward the mountain were noted at many locations minutes after the beginning of the eruption, during or soon after the development of the vertical column (8W, 12Wb, 20N, 23N, 25N, 29N, 30N, 50Ea, 50Eb).
15E.--It was impossible to tell precisely when the vertical eruption began. However, it could be determined that no major vertical eruption took place for several minutes after the initial eruptive events, and that a well-developed column was present within about 10 min.
40W.--The pilot of a commercial airliner, located about 25 mi west-southwest of Mount St. Helens at an altitude of about 35,000 ft, spotted the eruption at about 0838. The enormous energy of the eruption sent the ash column from about 25,000 ft to the plane's altitude in about 2 min. In another 2 min it had risen to 60,000 ft. The column then spread into a mushroom-shaped top reaching a diameter of about 35 mi in about 4 min. The column below the mushroom top was about 15 mi in diameter. A thistlelike spike projected above the top of the mushroom cloud. The top of the mushroom expanded so rapidly that it soon passed over the airliner. A short time later, the airline pilot heard another pilot flying on the east side of the mountain complain over the radio of being pelted by rocks, presumably falling from the mushroom cloud.
75N.--The column grew rapidly and assumed a classic mushroom shape in about 3-5 min. The mushroom shape continued to develop for about 15 min; then the top of the cloud began to drift to the east.
100SE.--When the eruption was first noted, the mountain was obscured,
and a vertical tube of ash could be seen disappearing into a layer of haze
about 1,000 ft above the summit. The witness' watch read 0835 at this time.
A few minutes later the column began to grow, reaching a diameter of about
10 mi in 5 min.
Lightning and other electrical phenomena associated with the eruption were so spectacular that many witnesses mentioned them. Many witnessed displays of "ordinary lightning" impressive only in the size and frequency of bolts. To some the lightning appeared to be mostly from cloud to cloud (8SE). A heavy concentration of vertical lightning at altitudes of 25,000-30,000 ft was entirely within the vertical eruptive column (40W). Others noted many cloud-to-ground strikes (9W, 15E), some of which started forest fires (12Wb) and one of which struck an individual (20N). Beneath any part of the eruptive cloud, radios became useless because of static (8SE, 12Wa, 27N). On Mount Adams, climbers noted that the air became electrically charged as the ash cloud moved overhead, and one climber received an electrical discharge upon raising his ice axe (50Ea).
Some observers witnessed unusual forms of lightning. Some of the lightning
(9W, 15ENE, 17NEa) appeared red. 25N: It wasn't normal lightning--"first
a white dot appeared in the cloud, and then a bolt would shoot out from
it." 100SE: The lightning was in ball form "streaking toward the ground,
connected neither with the cloud nor with the ground. It was like a group
of balls all going in the same direction, but going much too fast to have
been projectiles." 29N: After the cloud passed overhead, lots of lightning
started 600-800 ft in the air and formed "big balls, big as a pickup and
just started rolling across the ground and bouncing."
MUDFLOW ON THE SOUTH FORK TOUTLE RIVER
8W.--Less than 10 min after the beginning of the eruption, a huge mass of water, mud, and trees crashed down a small tributary within the South Fork Toutle River valley. It snapped off trees and "exploded" when it hit lows, bursting as much as 60 ft when it hit obstacles. A similar flow then swept across the road about 100 yd to the north, moving trees, rocks, and stumps. A few minutes later a substantial but much smaller flow, containing numerous trees and other debris, was still moving down the valley. About 8 min after the first flood, the witness crossed the flow path by wading through material like "warm concrete" and a flow of very muddy, cool water. The river returned to and stayed within its deep channel until about 1400, when there occurred a second smaller flood deep enough to spill out of the channel.
40NW.-- Sometime after 0900, the South Fork Toutle River began
rising and quickly rose 2 or 3 ft. The river was slightly muddy and carried
numerous logs. About 2-3 min after the first logs moved downstream, a railroad
trestle moving at about 25 mi/hr appeared about 1/2 mi upstream with an
enormous log jam behind it. Trees were being snapped off about 30 ft beyond
the original streambanks. As the trestle went by, large logs behind it
rolled up the bank. The mass of thick mud and logs pushed the witnesses
and their car off the bank and into the river. They were swept along for
about 5 min at about 25-30 mi/hr in very thick, warm (70°-80°F)
mud. The witnesses jumped from log to log toward shore where the flow was
moving more slowly, and waded the last 40 yd in warm mud over 1 1/2 ft
deep. The mud continued to rise slowly for a short time. After perhaps
15-30 min the mud and logs stopped moving.
8SE.--Although a dark cloud passed overhead several minutes after the start of the eruption and remained overhead for 10-15 min, the area received no ash.
12Wb.-- About 30 min after the start of the eruption, ash began to fall at a point somewhere between the initial observation point and Goat Mountain (southwest of the volcano; fig. 35). The ash fall lasted about 10 min.
8W. --About 15 min after the start of the eruption, a dark cloud descended from the mountain and sand-sized ash began raining down vertically. The witness sensed pressure pulses on his face and in his ears that seemed to correspond to periods of more intense ash fall. He experienced difficulty in breathing during the heavy ash fall and clearly attributed this difficulty to a lack of air, not to clogging of his nose and mouth with ash.
15ENE.-- At roughly 0900, chunks of ice as much as 1 or 1 1/2 in. in diameter began to fall. The ice fell for 2 or 3 min and was followed by "ice-cold mud-balls" as much as 3 1/2 in. across, which splattered when they landed. About 45 min after the eruption started, "B-B-like" ash began to fall. Warm pumice as much as 1 1/2 in. in long dimension soon joined the ash fall. The witnesses drove south on road 125 and quickly left the ash fall.
15E.-- Mud, which one of the witnesses (a geologist) recognized as accretionary lapilli, began to fall at about 0915. About an hour later (1015), the falling material included pumice.
20NW. --About 20 min after the arrival of the first cloud, it began to get dark again and quickly became "pitch black" as ash fell heavily. Forty-five minutes later it began to get light, but a dense vertical rain of dust-sized gray ash continued, gradually decreased, and finally stopped after about 4 hr.
19N.--A few minutes after the blast and a brief period of light, the ash fall was so dense that the truck's headlights produced only a dull diffuse glow. Even with the lights on the witnesses could not see the truck's hood. Breathing was difficult due to a "talcum powderlike" ash. By 0900 the witnesses were able to follow the edge of the road by using a flashlight held within 6 in. of the ground. "Pretty good sized" particles continued to fall with enough horizontal velocity that the witnesses were able to maintain some sense of direction from the angle at which these particles fell. It began to get light a little after 1000, and by 1020 visibility was roughly 30 ft.
21Na.--After the blast and a few minutes of clear sky, hot ash began to fall. Again it became totally dark. This ash seemed to fall vertically. It was "like someone pouring a bag of it over your head." This extremely heavy ash fall lasted about 15 min. The ash fall was so intense that the witnesses had to use their fingers to dig the ash out of their mouths. They put their shirts over their heads in an effort to keep the ash out of their mouths and noses. After 15-20 min "stuff started coming out of the sky." They could hear this material hitting trees. One witness was hit on the head by something large enough to raise a lump. During the heavy ash fall, they became cold, sleepy, and nauseous, but did not suffer from headaches. After an hour and a half visibility began to return.
2lNb.--After a period of clearing following the blast, powdery ash fell heavily for about 1/2 hr and at a lesser but significant rate for another half hour. The witnesses experienced difficulty in breathing in that their mouths and noses became clogged and they had to repeatedly spit out ash. The intensity of the ash fall varied somewhat so that visibility ranged from essentially zero to about 20 yd. The witnesses sat with their backs to the southwest and the ash seemed to be hitting them from that direction, although the horizontal velocity was not great.
18N.--Fifteen or twenty seconds after the blast cloud moved overhead, ash was falling so intensely that, even with a flashlight, visibility was no more than 1 ft. The first material to fall "was cold and was like mud" and seemed to fall vertically. During the heavy ash fall, the witness experienced feelings of peacefulness and serenity and had to make a conscious effort to stay awake. After 45 min to 1 hr, the ash fall had diminished so that he could see the ground with the aid of a flashlight. Hours later ash in the area ranged from 6 in. to knee deep in drifts.
22N. --Eight to ten minutes after the witnesses first observed the eruption cloud, ash began falling in small clumps "like snowflakes." The ash fall increased so that within minutes the darkness was similar to that in a cave. The intense ash fall continued for a period of 1/2 to 1 hr. Although the ash made noise while falling, it felt dry, and when examined afterwards it felt like talc. Within hours after the ash fell it was about 2 in. deep. The next morning it had compacted to 1 1/2 in. at the same locality.
20N. --The first material to fall was sand-sized ash, which felt moist and was not warm. As the ash fall intensified, it became extremely dark and, based on the noises that the ash made as it fell through the trees, the particle size increased. These larger particles fell for about 45 min and then stopped quite quickly. Ash continued to fall until about 1000, when it got light very quickly: "It was just like you opened your eyes." The depth of ash was about 1 in. Ash collected by the witnesses from a pickup truck 2 1/2 mi to the northwest was examined by one of the authors and consisted entirely of ash with numerous accretionary lapilli.
25N.--Before the ash fall began, the witnesses drove to site 27N where they remained for the duration of the ash fall. As the eruption cloud moved overhead, "pea-sized ash" started to fall. This material was damp, and fell with a substantial northward component of motion. After 15 s, material that seemed to contain rocks pelted the car for 20-30 s; then fine ash fell vertically. The ash fall became very dense so that it was like a "giant sifter was over you." Except for a 3-s period of light after about 20 min, the complete darkness remained until 1100.
27N.--The first material to fall was a little muddy rain. This was followed by 20-40 "pings" of solid particles hitting the vehicle during a 30-s span. It then became totally dark as ash began to fall heavily. This continued for about 2 hr with a 10-min interval of very limited visibility occurring after about 40 min.
29N.--The witness was driving to the north when warm mudballs began to fall. These balls would flatten out but not spatter; "When they hit the windshield of the pickup, they'd be about the size of a golfball." He drove into dense falls of these mudballs four or five times. After a short time the mudballs began to be replaced by dry ash.
23N.--The witness drove north to Riffe Lake (about 4 mi) before any material began to fall. The first material consisted of mudballs "the size of 50-cent pieces," which splattered all over the car and covered the windshield. They were quite wet and "felt a little warm." The mudballs began falling roughly 15 min after the start of the eruption and fell for 2 or 3 min. As the mudballs tapered off they were replaced by dry ash.
17NEa.--The witness had driven 2 or 3 mi to the northeast. Seven or eight minutes after the eruption's start, rocks began falling from the part of the cloud that had passed overhead. He collected two "golfball-sized" rocks (determined by one of the authors to be dacite) from the fall. The rock fall continued for roughly 30 s and was then replaced by material that splattered on the windshield. As he drove, this material was replaced by "mud drops," which would flatten but not splatter. The maximum diameter of a mudball observed after flattening was about 3/4 in. Gradually the mud fall abated and the ash fall became heavier. The ash was finer than sand. For a short time he encountered a second fall of mud drops, similar to but heavier than the first. This was quickly replaced by intense ash fall. Soon the ash fall became so intense that he could no longer see to drive. The ash fall eventually abated very gradually.
17NEb.--The witnesses left their original observation site roughly 2 min after the landslide began and had driven about 5 mi when the upper part of the cloud passed over them. The first material to fall was rock. Individual pieces ranged up to 1/2 in. in diameter, but most were pea sized. These rocks put small dents in the car's roof. The rocks were followed by large clods, 5-6 in. in diameter, that would "explode" upon impact. The clods were replaced after a few minutes by a warm mudfall, that gradually decreased over 15-20 min and was replaced by dry vertically falling ash. It was several hours before the ash fall abated.
27NE.--After leaving his vantage point, the witness drove south to USFS road 125. Fairly dry pellets began falling about the time he turned north onto USFS road 125. These did not stick to his vehicle. A short time later ash began falling. The ash seemed to be associated with a good deal more water in that it stuck to the windshield and could not be removed by the wipers. Wet ash had to be scraped repeatedly from the windshield for about half an hour, after which the ash became drier.
50Ea.-- At roughly 0900, the front of the eruption cloud passed overhead, and shortly thereafter ash began to fall. Mixed with the ash were charred "wood, cones, and some branches up to 12-16 inches long." Also noted in the falling material were a few pieces of pumice. The ash fall lasted about 30 min and left a deposit 1/4 - 3/8 in. thick.
50Eb.-- About 0900, the eruption cloud enveloped the area. The first material to fall consisted of B-B-sized pellets. About 5 min later branches and evergreen cones began falling. The branches were as much as 16 in. long and 3/4 in. in diameter and so numerous that at any one time nearly 100 were in the field of view. These items were singed and hot when they landed. About 10 min after the ash fall began, the B-B-like pellets were replaced by larger disc-shaped pieces up to 3/4 in. in diameter. Branches and ash continued to fall for about 80 min; then the ash fall ceased. By 1045 it was clear at 8,000 ft and above. However, below 5,500 ft ash continued to fall until after 1600.
50Ec.-- At about 0850, chunks of light-gray pumice began falling, making a pinging noise in the trees. Most of the pumice was the size of pea gravel, but occasional pieces reached 3/4 in. in diameter. The pumice fall lasted about 5 min. At 0900 the sky grew very dark and B-B-sized pellets began falling. The pellets splattered on impact. Coarse sand-sized ash, finer than the first B-B-sized material, continued to fall while the witness drove to Randle.
75N.--At 0915 (by the witness' watch), "twigs from evergreen
trees coated with ash" began to fall. These twigs were as much as 3/4 or
1 in. in length and were charred. The start of the ash fall was very gradual,
but when the group reached their camp about 1 hr after the start of the
eruption, a layer of fine ash coated the tent. The ash was finer than sand
but caused a gritty sound on one's teeth. The ash fall remained quite light
so that it was barely visible in the air. However, it did accumulate slowly
on the ground so that several hours later, it had attained a thickness
of 3/16 - 1/4 in.
Several minutes elapsed between the initial eruptive events and the development of a vertical eruptive column. However, once the column began to develop it apparently grew very quickly, rising from about 7.5 to 18 km in roughly 4 min, and then mushrooming at about that altitude to a diameter of 55 km in another 4 min (40W). The column maintained its mushroom-shaped top for about 15 min, after which the top was blown in an easterly direction by high-altitude winds (75N). The rapid growth of the column and mushroom-shaped top and the rather short existence of the top may indicate that the early vertical eruption was most energetic during the first few minutes.
Mudflows, at least in the South Fork Toutle River valley, began very shortly after the eruption. Only about 10 min elapsed between the start of the eruption and the maximum flood down the South Fork Toutle, 10 km west of the mountain. In this part of the valley, the flood moved extremely fast, bursting into the air as it hit obstructions (8W). Further down valley, near the confluence of the North Fork and South Fork Toutle River, the mudflow had slowed considerably but still moved at a velocity perhaps as great as 40 km/hr (40NW).
High horizontal velocities were not associated with airborne material beyond the area devastated by the directed blast, nor with material that fell at points within that area, except for a short time immediately after the blast cloud arrival. At many localities, coarse lithic fragments preceded or accompanied the first minute or so of ash fall. Much of the early material was damp or wet and fell as large mudballs that may have been aggregates of accretionary lapilli. In general, the particle size and moisture content decreased with time.
Little evidence exists for the presence of large quantities of magmatic gases. When asked about odors, many eyewitnesses said only that they thought that they smelled "sulfur" during and after the dense ash fall and compared this odor to that of a match. Although several individuals experienced trouble with breathing, most attributed this difficulty to having their mouths and noses clogged with ash. Two individuals (8W, 20NW) attributed breathing difficulties to a lack of oxygen. Two other descriptions might be interpreted to indicate a lack of oxygen or exposure to a noxious gas: one of the individuals mentioned being cold, nauseous, and sleepy (2lNa); the other reported sleepiness accompanied by a feeling of serenity (18N). However, these symptoms may be the individuals' reactions to shock.
The material that fell on and near Mount Adams (50E) and on Mount Rainier (75N) was probably carried to great altitude by the vertical column and then by high-altitude winds to the northeast. Wind direction from Mount St. Helens on May 18 was not directly toward either Mount Adams or Mount Rainier. The short duration of the ash fall at Mount Adams might be explained if only the edge of the "mushroom top" passed over the area. The remnants of evergreen trees that fell on Mount Adams and Mount Rainier were certainly from the forest destroyed by the directed blast. Somehow these fragments became incorporated in the early stages of the vertical eruption and were carried to great heights and distances.
The early stages of the vertical eruption also contained pumice, which
was the first material to fall at one locality (50Ec), and which was also
noted on Mount Adams (50Ea). Closer to the mountain, both groups to the
east of Mount St. Helens (15E, 15ENE) also observed falling pumice. The
widespread presence of pumice in the air-fall material suggests that the
eruption was magmatic at a very early stage.
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