GOAT ROCKS ERUPTIVE PERIODThe Goat Rocks eruptive period began with an explosive eruption that produced pumice of tephra layer T in A.D. 1800 (Yamaguchi, 1983) and ended in 1857. The tephra eruption was followed several decades later by the eruption of an andesite lava flow and extrusion of the Goat Rocks dacite dome. Many eruptions between 1835 and 1857 were witnessed by inhabitants of the surrounding region and were recorded in local newspapers. Such reports commonly included such terms as "smoke," "fire:' and "flame," and on several occasions small amounts of ash were deposited many tens of kilometers from the volcano. The historic record of these eruptions is summarized by Harris (1976), and Majors (1980) noted reports of minor activity at the volcano on three occasions between 1889 and 1921.
DEPOSITSTEPHRA LAYER T The initial eruption of the Goat Rocks eruptive period produced dacite pumice which contains hypersthene, hornblende, and augite, and which has a Si0 2 content of 64 percent (Hoblitt and others, 1980). The tephra was carried mostly northeastward from the volcano by winds and fell at least as far away as northwestern Montana, some 575 kIn from Mount St. Helens (Okazaki and others, 1972). The estimated volume of tephra layer T is on the order of 0.1 km 3 (Crandell and Mullineaux, 1978). No deposits of pyroclastic flows or lahars are known to have been formed at the time this tephra was erupted. Hopson (1971) suggested that the tephra was erupted from the vent at which the Goat Rocks dome was later extruded, but it is also possible that the tephra came from a nearby vent that later was the source of the "floating island" lava flow. "FLOATING ISLAND" LAVA FLOW
A large andesite flow (locality 90) extends northward from a vent on the northwest flank of the volcano that, before 1980, was about 0.5 km southwest of the site of
the Goat Rocks dome (frontispiece). The vent was at an altitude between 2,200 and 2,300 m (C. A. Hopson, unpub. map, 1980). The lava flow is nearly 5 km long and
from 0.2 to 0.7 km wide. The andesite contains hypersthene, augite, and hornblende and has a Si0 2 content of 60.ll percent (Hoblitt and others, 1980). Lawrence
(1941) named this the "floating island" lava flow. The "islands" are lithologically dissimilar deposits which overlie the flow; trees grew on them before the 1980 eruptions, whereas the flow itself was barren. The rubbly nature of the deposits that form the "islands" led Lawrence to suggest that they are masses of till carried downslope on a lava flow that had emerged from a vent beneath a debris-laden glacier. My examination of the islands during the 1970's revealed that they are formed by a mixture of
fine-grained gray and reddish-gray dacite fragments in a sand and silt matrix. Pumice fragments mixed with the lithic material are mineralogically similar to tephra of layer rr. These masses occur in several areas and duplicate the arcuate transverse ridge pattern of the surface of the lava fl
The age of the "floating island" lava flow has long been in question. Verhoogen (1937, p. 273) suggested that it is one of the "latest products of volcanic activity at Mount St. Helens" and labeled the flow "1854" on his geologic sketch map. Lawrence (1954, p. 59)
thought that the flow was erupted before 1838 because of the ages of trees growing on the islands. He noted that tephra erupted about 1800 (layer T) was absent
from the lava flow and that therefore the flow is younger. According to Hoblitt (oral commun., 1979), the magnetic direction of the lava flow is consistent with
an age between 1800 and 1838.
The lava flow bears Rhizocarpon sp. lichens that have diameters of as much as 160 mm, similar to the sizes of lichens on lava flows of Kalama age on the southeast
side of Mount St. Helens. If the age of the "floating island" lava flow is post-1800, as suggested, an unusually rapid growth rate is thereby implied for Rhizocarpon
lichens in this area.
TEPHRA ERUPTED IN 1842
An explosive eruption of Mount St. Helens in 1842 produced lithic ash (Holmes, 1955; Hoblitt and others, 1980) that fell downwind at least as far as The Dalles,
Oreg., 100 km southeast of the volcano. No other products of this eruption have been recognized at the volcano. The ash eruption probably preceded or
accompanied the initial extrusion of the Goat Rocks dome and may have been analogous to repeated eruptions of lithic ash at Mount St. Helens after March 27, 1980, that preceded the initial magmatic eruption on May 18.
GOAT ROCKS DOME
A dome was extruded 600-700 m below the summit on the northwest flank of the volcano during the next recorded eruption of the Goat Rocks eruptive period.
The rock of the dome is a hypersthene-hornblende dacite that contains small amounts of augite. Its Si0 2 content is 63.35 percent (Hoblitt and others, 1980). The
Goat Rocks dome was destroyed during the landslide and eruption of May 18, 1980.
A large convex and digitate fan of rock debris extended downslope from Goat Rocks before the 1980 eruptions (frontispiece). The fan was barren above an altitude of about 1,350 m, but below that altitude it bore a sparse pine forest. In detail, the surface of the fan was a complex of shallow gullies and low bouldery ridges, all trending downslope to the north, as well as mounds a meter or two high and a few tens of meters across that represented the ends of individual flowage lobes. At the downslope margin of the fan, fingers of the fan debris with its sparse forest extended into a mature forest growing on older deposits. The fan was littered with large angular and sub angular blocks of dacite, many of them prismatically jointed. Verhoogen (1937, p. 274)
noted that the blocks were in various stages of disaggregation and concluded that the jointing developed after the blocks came to rest. He inferred that their
source was at Goat Rocks and concluded that the blocks had been carried downslope by a nuee ardente (pyroclastic flow).
Blocks of dacite on the fan were sampled by R. P. Hoblitt (oral commun., 1979) at three localities for the purpose of determining TRM directions, and thereby
possibly proving whether the fan was formed during growth of the dome, or later. The three sites were downslope from the base of Goat Rocks at distances of
about 1.1, 1.8, and 2.7 km, and the vertical distances of each locality below the base of the dome were about 530,730, and 900 m, respectively. The first sample site
was in the middle of a lobe of rock debris 1-1.5 m thick and a few tens of meters wide that had steep blocky margins. Four samples from this deposit all had
random TRM directions; hence, the lobe of debris evidently was formed by a lahar. The second locality was along a downslope-trending bouldery ridge that probably was
formed as a levee along the margin of a pyroclastic flow or lahar. All four samples from this locality were from prismatically jointed blocks. The TRM direction of three
of these paralleled the earth's present magnetic field, and the fourth was random. The third locality was along another downslope-trending ridge of rock debris. Two
samples taken from prismatically jointed blocks at this locality were both oriented. One rock fragment, not prismatically jointed, taken from beneath the ground
surface, had a random TRM direction; the results from a fourth sample were inconclusive.
The results of these determinations show that the fan deposits included both material that was hot, when it was emplaced either by pyroclastic flows or lahars carrying hot material, and debris that was cold. The fan deposits clearly were formed during the growth of the Goat Rocks dome, originating as hot avalanches from the flanks of the dacite mass as it was being extruded. Avalanches that moved across snow probably caused melting and were remobilized as lahars.
The exact age of the Goat Rocks dome and associated fan of debris is not known. The fan deposits were younger than tephra layer T, which was erupted in A.D. 1800. The oldest tree I found on the fan deposits, which was near the northwest margin, started to grow sometime before 1876. The trees on the fan were much smaller and younger than trees growing on adjacent, older surfaces. and clearly represented a first-generation forest on the fan deposits. The largest lichen (Rhizocarpon sp.) I found on dacite blocks that littered the fan surface was near timberline and had a diameter of 63 mm. Most lichens on these blocks, however, were no more than 40 mm in diameter.
Several eyewitness accounts cite some kind of eruptive activity occurring on the northwest or north side of Mount St. Helens during the 1840's and 1850's. Of these accounts, the best documentation is a painting by the Canadian artist Paul Kane, which was based on a sketch he made in March 1847. The painting shows
an eruption column rising above a vent on the northwest side of the volcano in the approximate position of Goat Rocks. The start of dome extrusion has not been
determined. I infer that the eyewitness accounts and Kane's painting record dome extrusion over a period of many years, intermittently accompanied by emission of ash,
steam, and gases which were visible for great distances.
Other accounts of eruptions printed in local newspapers of the mid-1800's place the sites of activity on the south side (1842, 1843-44, 1853) and the northeast side (1850, 1853). Vents associated with these reported eruptions have not been identified.
The last significant eruption of the Goat Rocks period was in 1857, when "volumes of dense smoke and fire" were reported (Frank Balch, quoted by Majors (1980, p.36). A recent study of old records has reported minor eruptions in 1898, 1903, and 1921 (Majors, 1980, p. 36-41). None of these produced deposits that have been recognized, and probably none were accompanied by juvenile magmatic material.
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