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TitlePage   Preface   Contents    

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
Chemical Compositions, pg. 233-250
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



PHREATIC-EXPLOSION PITS

Phreatic activity of two types occurred either continuously or sporadically for several weeks or months after deposition of most pyroclastic flows. The first type created fumaroles, which occur in all pyroclastic-flow deposits and which are the most common means of venting air, steam, and other gases from the cooling deposit. They form small (as much as 2 m) circular vents either scattered in an irregular pattern or along straight lines. Less commonly, fumaroles are concentrated in areas as large as several hundred square meters. Some fumaroles were rooted in underlying lateral-blast deposits.

The second and most spectacular type of phreatic activity produced phreatic-explosion pits. These craters occur in pyroclastic-flow deposits and debris-avalanche deposits of May 18 and rarely in pyroclastic-flow deposits of June 12. Most pits are circular in plan, 5-100 m in diameter and 1-20 m deep. The largest is the big phreatic-explosion pit, which is 0.7 km long (east-west) and 0.3 km wide (north-south). It originally was about 38 m deep, but was partly filled by pyroclastic-flow deposits of June 12. Crater walls in debris-avalanche material are steep and craters are shaped like inverted cones. Except for pits near Spirit Lake that contain water, pit walls in pyroclastic material have gentle slopes because the deposits have such low strengths that they continually sloughed, thus enlarging the crater for distances of as much as 50 m and locally forming flat pit floors.

Phreatic-explosion pits started to form on May 18 when hot pyroclastic flows and debris flows covered water in streams, ponds, and springs; the water flashed to steam, and the upward-directed steam explosions reamed holes in the overlying deposits. Some large pits, including the big phreatic-explosion pit, resulted from migrating phreatic vents and from coalescence of multiple pits. All large phreatic-explosion pits are on the north edge of the pumice plain, along the south side of Spirit Lake, and along or near the former North Fork Toutle River west of Spirit Lake. Most of the reaming of the pits occurred within several days after the May 18 and June 12 eruptions. Some that formed in pyroclastic-flow or debris-avalanche deposits on May 18 were partly filled by pyroclastic-flow deposits that date to later in the day. Pits as much as several meters in diameter also formed during the winter of 1980-1981 in deposits of June 12 on the floor of the big phreatic-explosion pit after rainwater pooled there (D. A. Swanson, oral commun., 1981).

The phreatic explosions deposited beds of ejected debris as much as 4 m thick as aprons around the pits. Bedding in these deposits consist of planar and sand-wave forms, including dunes. Dunes, which occur only around some large pits within pyroclastic-flow deposits, are as much as 3 m long, 1.5 m wide, and 1 m high. The dunes, rudely oriented with crests parallel to the edge of the pit, are of progressively smaller amplitude and wavelength with greater distance away from the pit. Trenches dug through many dunes demonstrate that most or all are anti-dunes; that is, most deposition was on the stoss side. Depositional mechanisms consist of surge for sand-wave and some planar beds, and of air fall for other planar beds. Bomb sags and steep dips on stoss sides of dunes suggest that in many places the beds were wet and cohesive when formed.
 

EROSION BY PYROCLASTIC FLOWS

The northern flank of Mount St. Helens, for a width of more than 4 km, and the amphitheater floor contain north-trending grooves and channels due in large part to erosion by pyroclastic flows. They are incised in older 1980 pyroclastic flows and in pre-1980 volcanic rocks, which consist largely of moderately well consolidated breccias and much subordinate lava flows and tuff. Grooves are several meters wide, 0.2-2 m deep, and 20-100 m long. Channels are considerably larger, as much as 50 m wide, 40 m deep, and more than 1 km long. The most spectacular channel, which forms the stairsteps, has been the main path for pyroclastic flows down the flank of the volcano. A second major channel, about 0.5 km to the east, also has a steplike floor and also has been a major path for pyroclastic flows. Both these channels, as well as some smaller channels, largely coincide with stream channels that existed before the eruption of May 18. Pyroclastic flows thus were partly directed down preexisting ravines and enlarged them. Gently sloping surfaces, such as the treads of the stairsteps and much of the amphitheater floor, are covered by pyroclastic-flow deposits. With each major eruption, a new pyroclastic flow flushed out most earlier deposits and left a thin cover of new deposits. Erosion dominated over deposition on the volcano flank, whereas deposition dominated on the pumice plain. The presence of channels and grooves in bedrock and the presence of remnants of pyroclastic-flow deposits of different ages demonstrates that pyroclastic flows of May 18 poured northward over the entire northern flank of the volcano, whereas those of June 12 and July 22 were restricted to an area less than 2 km wide. Those of August 7 were confined to an area about 0.5 km wide, and those of May 25 and October 16-18 were funnelled largely down the stairsteps.

Prior to the eruption of August 7, we drove vertical rows of 16-cm-long iron nails flush with the bedrock (breccia) wall in the stairsteps as high as 10 m above the floor. After passage of pyroclastic flows, the upper part of the stairsteps showed erosion of 4 cm or more, irrespective of height above the floor; thus, erosion was not confined to low parts of the stairsteps. All nails from the stations near the base of the stairsteps were removed by the flow and the configuration of the channel differed from that before the eruption. The parts of other channels low on the volcano flank also are deeper and wider with respect to the pre-May 18 stream valleys than those parts higher on the flank. Erosion was greatest where the pyroclastic flows traveled at their greatest speed near the base of the steep flank; there the flows abraded and scoured most parts of the bedrock channel to depths measured in meters. Furthermore, preliminary topographic contours suggest that pyroclastic flows of May 18 may have gouged out the base of the stairsteps more than 35 m below the pre-May 18 surface. During the eruption of October 16-18, the walls of the stairsteps were further modified by local removal of more than several meters of bedrock. For example, a nearly vertical, 10-m-high bedrock (breccia) cliff between the second and third step down from the top of the stairsteps became a gently sloping, 3-m-wide gully.
 

ERUPTIVE PROCESSES

The eruptive mechanisms of the 1980 pyroclastic flows on the northern flank of Mount St. Helens appear to differ somewhat from those reported for many other historic pyroclastic flows, where collapse of Plinian columns or avalanches of debris from domes or spines has been observed to be the main source. Specifically, most of the Mount St. Helens pyroclastic flows formed when bulbous masses of inflated ash, lapilli, and blocks erupted a few hundred meters or less above the inner crater and then collapsed and spilled over the constraining rampart. Such rapid accumulation took place during or before pulses of maximum upward acceleration (that is, gas thrust) of the Plinian column. After material spilled over the rampart, it moved relatively slowly down the gently sloping floor of the amphitheater, then accelerated (to as much as 100 km/hr; Hoblitt, 1980) as it plunged down the northern flank of the volcano. Dense, cauliflower-shaped ash clouds evolved from and rose hundreds of meters above the ground-hugging pyroclastic flows.

The eruptions of July 22, August 7, and October 17-18 (table 42) were the best observed, and they clearly show this avalanche-type emplacement of pyroclastic flows, commonly preceded by fountains of pyroclastic material erupting from the vent (fig. 293; Hoblitt, 1980). The larger pyroclastic flows of May 18, which more closely resemble preserved prehistoric welded ash-flow tuffs in their volume and grain size, could have had different emplacement mechanisms, but the size of the eruptive clouds and bad weather during the eruption made visual observations difficult. Several pyroclastic flows that were seen on the northern flank, however, suggest the same mechanisms as were documented for the pyroclastic flows of the July 22, August 7, and October 17 eruptions (D. A. Swanson and J. G. Rosenbaum, oral communs., 1980). Rosenbaum likened one of the few visible eruptions in the afternoon of May 18 to a "pot boiling over." Further evidence that most 1980 pyroclastic flows on the north resulted from upwelling of material is that no erosion of the rampart or inner amphitheater was seen after any major eruption. Such erosion might be expected if column collapse were significant, for then the pyroclastic flows would be expected to have a high velocity within the amphitheater, something not observed so far. R. L. Christiansen (oral commun., 1980), however, concluded that column collapse contributed significantly to, if not produced, a pyroclastic flow that he saw form between about 1630 and 1930 on May 18.

In contrast to the mechanism that led to the pyroclastic flows on the north, the small pyroclastic flows emplaced on the western, southern, and eastern flanks (fig. 289) of Mount St. Helens were formed by Plinian column collapse of the St. Vincent type (Hay, 1959), in which part of the edge of the Plinian column lost upward momentum and gravitationally collapsed onto the outer slope of the volcano (Rooth, 1980). Most of these pyroclastic flows took place on May 18, but at least one occurred on May 25. Deposits on the amphitheater walls near the inner crater from the morning eruption of October 17, and from the 1235 eruption of October 18 were also caused by column collapse.
 

CHANGES WITH TIME

About 85 percent of the volume of the 1980 pyroclastic-flow deposits was erupted on May 18. Since then, with the exception of the small deposits of May 25, each successive eruption has produced a smaller volume pyroclastic-flow deposit. The eruption of December 27-28 produced only a large dome and small amounts of air-fall ash. The interval between successive eruptions has also increased, with the exception of the relatively short interval between the eruptions of July 22 and August 7. The block and lapilli to ash ratios are generally higher in successively younger deposits, and petrographic and chemical data from pumice (Kuntz and others, this volume; Lipman, Norton, and others, this volume) suggest a subtle trend toward slightly more mafic magma with time. The maximum measured postemplacement temperature also has been greater for successively younger deposits (Banks and Hoblitt, this volume).

These temporal changes suggest that, unless a new body of magma is emplaced under Mount St. Helens, the amount of magma in the chamber that has a composition likely capable of producing pyroclastic flows is being rapidly depleted. If this is true, there likely will continue to be progressively longer intervals between future eruptions, and these eruptions will produce either domes or hotter pyroclastic-flow deposits that contain relatively more lapilli and block pumice and that have increasingly mafic compositions. Geologic history indicates (Crandell and others, 1975, 1979; Crandell and Mullineaux, 1978; Hoblitt and others, 1980) that this trend may terminate by eruption of andesitic lava flows and perhaps also by dacite domes.
 



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