Start & Images     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

Extrusions of steep-sided domical masses of viscous silicic lava commonly follow discharge of more fluid gas-rich magma as pyroclastic material. At least half a dozen dacitic domes have formed at Mount St. Helens in the past few thousand years, including the large summit dome that capped the volcano prior to the 1980 eruptions; accordingly, the appearance of a lava dome within the crater formed on May 18 was anticipated. Detailed observations were made on rates, shapes, and compositions of successive phases of growth of the ensuing composite dacitic dome that grew later during 1980. Many prehistoric domes, at Mount St. Helens and at other similar volcanoes, probably evolved by similarly complex growth patterns that generally could not, however, be deduced from the resulting final deposit.

Rise of magma to a shallow level beneath the new crater was indicated in early June by increased SO₂ gas emission and by a central warm area detected by airborne radiometers. The first dome, of dacitic composition similar to associated pyroclastic material, grew for at least 7 days following the June 12explosive activity (Moore, Lipman, and others), eventually reaching about 365 m in diameter and 45 m in height. The central part of this dome was blown out during the July eruption, and the resulting inner crater was largely filled by a second smaller dome that grew for about a day following explosive eruptions on August 7. A somewhat more mafic dome of silicic andesite, intermediate in size between the June and August domes, formed at the same site in about half a day following the Oct. 16-18 pyroclastic eruptions. Between December 27 and January 3, two dacitic dome lobes were erupted from flanks of the October dome without significant premonitory pyroclastic activity. All the 1980 dome units were subcircular in plan and developed apical sags or collapse pits during late stages of growth, due either to lateral spreading, degassing, or to withdrawal of magma from below. Further growth of the composite dome of 1980 was anticipated, and beginning on February 12, 1981, another extrusion occurred from the crest of the October dome.

Physical, electric, and thermomagnetic properties of samples of the June 1980 dome, determined under laboratory conditions, suggest atypically low oxygen and sulfur activities in the magma, probably due to effective degassing prior to extrusion of the dome (Olhoeft and others). The thermal energy of the June dome, calculated from laboratory determinations of physical properties, was used to calibrate energy estimates for other phases of the 1980 eruptions (Friedman, Olhoeft, and others). The total energy yield of the eruptions through October 1980 is estimated at 1.33 x 10²⁵ ergs, similar in yield to the eruption of Mauna Loa (Hawaii) in 1950 but only half as great as the directed-blast eruption of Bezymianny (Kamchatka) in 1956.

Five episodes of explosive magmatic activity have occurred between May 25, 1980, and early January 1981, within the May 18 crater beneath the pre-May 18 summit of the volcano. After explosions waned in three of these episodes (June 12, August 7, and October 16-18), a dacite or andesite lava dome rose within the small explosion crater. In late December and early January 1981, extrusion of lava tripled the volume of the preexisting October dome.

The dacitic June dome grew for at least 7 days following the June 12 explosive activity, and by June 28-29 was dormant and possibly slightly collapsing. It grew to 365 m in diameter and 45 m in height, with a volume of about 4.7 x 10⁶ m³. The dome was partly destroyed and covered with tephra during the July 22 eruption.

The dacitic to andesitic August dome grew for slightly more than a day following the August 7 explosive activity. It was emplaced within the July 22 crater to a height of about 60 m, with a surface diameter of about 120 m and a volume of 0.7 X 10⁶ m³.

The andesitic October dome grew rapidly for only about one-half day following the October 16-18 explosive activity that blasted out the August dome and probably part of the remaining June dome. For several days following this rapid growth, the dome sagged at the summit and continued to spread outward. At the end of October the dome was about 225 m in diameter and 37 m high and had a volume of about 1.5 X 10⁶ m³.

The December composite dome grew by enlargement of the October dome from about December 27, 1980, to January 2-4, 1981. The eruption added two subcircular dacite lobes to the October dome by slow extrusion of lava beneath a solidified carapace. The southeast lobe is about 200 m in diameter and 90 m high, and the northwest lobe is about 75 m in diameter and 40 m high. The resulting composite dome has a volume of about 5 X 10⁶ m³, of which about 3.5 X 10⁶ m³ was erupted during the end-of-the-year event.
 

Following the great eruption of May 18, five episodes of moderate explosive activity occurred in 1980: May 25, June 12, July 22, August 7, and October 16-18. After explosions waned in three of these episodes (June, August, and October), stiff, viscous dacite or andesite lava domes rose within and, in two cases, overflowed the inner explosion crater. Beginning in late December, nonexplosive extrusion enlarged the October dome. All of the domes appear to have been fed from the same conduit, which lies 1 km beneath the previous summit area of the mountain and beneath the center of the pre-May 18 explosion crater. The domes are compared in table 58.

Detailed maps of the domes have not yet been made from aerial photographs. In this report the domes are shown in a series of oblique physiographic drawings (fig. 307), based in part on the topographic map made from July 1 aerial photographs. Details of the domes are based on vertical and oblique photographs and on reports from observers. Calculated volumes of the domes include only the visible part and do not consider that part in buried craters or within the feeding conduit.
 

Michael Holtsclaw, Terry Leighley, Donald Peterson, and Peter Rowley made important observations of dome growth.
 

The first dome of the 1980 eruptions grew in the crater following the explosive eruption during the night of June 12. Weather conditions prevented direct observations until the morning of June 15, and at this time the dome was a symmetrical circular mass, tan to light gray, with an estimated diameter of 200 m and height of 30-40 m.

Several features suggest that an intrusive dome may have been close to the surface before and during the June 12 eruption. On June 11, a crescent-shaped lake about 300-400 m long and 50-75 m wide occupied much of the south side of the crater floor (fig. 307A). This lake partly surrounded a tephra-covered mound centered where the dome later emerged. This mound probably resulted from an early subsurface rise of the June dome, although possibly it reflected an earlier cryptodome emplaced at the end of the May 25 activity. The character of the pyroclastic flows erupted on June 12 also suggests the presence of a cryptodome at shallow depth. The deposits of June 12 contain abundant blocks of dense gray dacite as much as 2 m in diameter, which contrast with the highly vesicular pumiceous ash flows erupted on May 18. The June 12 deposits are similar, in some ways, to block-and-ash flows emplaced as a result of dome collapse and perhaps originated, in part, from fragmentation of the solidified margin of a growing, subsurface dome. The small pyroclastic flows of May 25 were transitional in fragment size and vesicularity between those of May 18 and June 12, and were probably representative of the evolving shallow magma body that eventually reached the surface as a dome. All subsequent ash flows contain such relatively dense, breadcrusted blocks, which were presumably derived from preexisting domes or cryptodomes.

Observations made during the latter half of June revealed a slow growth in the height and diameter of the dome. The cracked, breadcrust-like surface remained similar in appearance but became somewhat grayer in color, because of increased ash cover, and local accumulations of yellowish sulfur and iron hydroxides appeared. Eruptions of tephra occurred locally from areas of gas emissions at the margin of the dome in late June. Between June 15 and 28, the crater walls and floor were devoid of running or standing water, but by June 28 a small pond had formed on the southwest side of the crater. Aerial photographs taken July 1 indicate that the dome was about 365 m in diameter, 45 m high, and precisely centered beneath the previous phreatic crater that had formed at the summit of the volcano between March 27 and May 18 (fig. 307B).

Measurements of dome size were started on June 15, using helicopter altimetry, and on June 19, using theodolite-angle measurements; but observations were repeatedly hampered by limited visibility due to clouds, fume, and wind-blown ash. The helicopter altimetry observations of changes in dome height were based on the lowest helicopter elevation at which the top of the dome could be observed across the top of a tephra rampart 200 m north of the dome summit. While measurements were taken, the helicopter hovered over a landmark 2 km north of the rampart until the dome crest came into view above the rampart. Although the helicopter altimeter readings are considered accurate only to about 15 m, the geometry of the line of sight permitted recognition of changes in dome height greater than 1.5-2 m. Repeated helicopter measurements indicated upward growth of the dome at 2-3 m per day from June 15 through the late afternoon of June 19. No observations were made June 20-27 because of poor weather, but measurements of June 28-29 indicated no further dome growth, and possibly a slight subsidence.

Theodolite measurements were possible on June 19, 29, 30, and July 1, from a ridge 8.5 km north of the dome. They indicate that the dome was dormant and possibly collapsed about 2.5 ±1 m between June 19 and July 1. These measurements suggest that the dome grew actively for at least 7 days (June 12-19), after which extrusion stopped and the dome collapsed slightly under its own weight. The volume of the dome at the end of June was about 4.7 x 10⁶ m³. It is composed of a relatively dense plagioclase-hypersthene-hornblende-augite dacite that is chemically similar (63.3-64.0 percent SiO₂) to pumice erupted June 12 (Lipman, Norton, and others, this volume).
 

The explosive eruption of July 22 formed a new elongate crater (250 X 600 m), removed more than half of the June dome, and covered the remainder of the June dome with 5-10 m of tephra (fig. 307C). This crater exposed cross sections of the June dome in its east, north, and west walls. Deep vertical joints in the dome remained incandescent for more than a month. The south part of the crater extended, as a narrow trench, several hundred meters beyond the southern limit of the dome. The reason for this departure from the circular shape of other post-May 18 craters and domes is not known.

During the afternoon and night of August 7, relatively small explosive eruptions from the deep, northern part of the July crater ejected ash and fed a small pumiceous pyroclastic flow that swept north, part way to Spirit Lake. A dome began to rise in the vent crater during the morning of August 8. By the end of the day, the dome had filled the lower half of the 90-m-deep crater. The dome rose another 20 m by the morning of August 9, as determined by helicopter altimetry, and during that day reached to within 30 m vertically of the July crater rim. The height of the dome remained stable from August 10 through the morning of August 15, but helicopter measurements on August 17 indicated subsidence of 6-7 m, which may have occurred during a small eruption on the afternoon of August 15. A large cavern, about 10-20 m in diameter and depth, developed in the west base of the dome during the August 15 event. Dull incandescence was visible in this cavern and in deep summit cracks through August and much of September. The August dome was about 120 m in diameter (fig. 307D) and had a volume of about 0.7 x 10⁶ m³. No samples were collected in place from this dome; however, blocks apparently blown from it are generally similar, though slightly more mafic (63.0 percent SiO₂) than samples from the June dome.
 

A series of explosive eruptions that fed several pyroclastic flows began the evening of October 16, blowing out the small August dome completely, as well as most of the surrounding remnants of the June dome. The final eruption at 1428, October 18, threw a plume of pyroclastic material to a height of 6 km, leaving a shallow saucer-shaped depression about 250 m in diameter at the site of the former crater. As visibility improved at about 1520, a new dome, about 5 m high and 25 m across, was visible in this depression. An hour later the dome was about 10 m high and 40 m wide, and at 1830 it was 20-25 m high and 50-70 m wide. As the dome grew, slabs spalled off and rolled down the flanks, revealing the bright incandescent interior. Small trickles of orange-red, viscous lava oozed from several points on the west and east sides; the eastern streams fed a small pond in the moat between the dome and crater wall. Orange flame flickered from several holes in the side of the dome.

At 0925 on October 19, the dome covered most of the crater floor and was subcircular in plan, with a diameter of about 185 m. The height of the dome was about 50 m, as determined by helicopter altimeter and confirmed later in the morning by theodolite measurements from a ridge 8.5 km to the north. The surface of the dome was convex upward and was cut by deep cracks with incandescent walls. Several spires reached a few meters above the general surface. The sides of the dome were nearly vertical and frequently sloughed off, creating small talus cones (breches d'ecroulement). Noisy emission of gas occurred from various fumaroles around the base of the dome; several of these remained active until the December activity.

Theodolite measurements initiated at 1030 on October 19, documented changes in shape and dimensions of the dome over the next several days....