Donald A. Swanson

Magma supply rate at Kilauea volcano, 1952-1971

The three longest Kilauea eruptions since 1952 produced lava at an overall constant rate of about 9 x 106 cubic meters per month (vesicle-free). This is considered to represent the rate of magma supply from a deep source, probably the mantle, because little or no summit deformation indicating high-level storage accompanied any of the three eruptions.

Predicting Eruptions at Mount St. Helens, June 1980 Through December 1982

Thirteen eruptions of Mount St. Helens between June 1980 and December 1982 were predicted tens of minutes to, more generally, a few hours in advance. The last seven of these eruptions, starting with that of mid-April 1981, were predicted between 3 days and 3 weeks in advance. Precursory seismicity, deformation of the crater floor and the lava dome, and, to a lesser extent, gas emissions provided telltale evidence of forthcoming eruptions. The newly developed capability for prediction reduced risk to life and property and influenced land-use decisions.

Potassium-Argon Ages of Lavas from the Hawi and Pololu Volcanic Series, Kohala Volcano, Hawaii

Kohala is regarded as the oldest major shield volcano on the island of Hawaii. Potassium-argon ages of nine lava flows from the predominantly tholeiitic Pololu Volcanic Series of Kohala Volcano range from about 0.33 m.y. o t about 0.45 m.y. These results suggest that the subaerial part of the shield was built over a period on the order of 0.1 m.y. in the late Pleistocene, and that much of the island of Hawaii is younger than 0.4 m.y. Mugeante lava flows from the younger Hawi Volcanic Series are between 0.06 and 0.25 m.y. old. These ages confirm that the hiatus between the basaltic shield-building phase of volcanism and the veneer of differentiated alkali lava on Kohala is less than 0.2 m.y. It is inferred that the Lualualei stand of the sea began less than 0.4 m.y. ago and ended before about 0.15 m.y. ago.

The complex filling of alae crater, Kilauea Volcano, Hawaii

Since February 1969 Alae Crater, a 165-m-deep pit crater on the east rift of Kilauea Volcano, has been completely filled with about 18 million m3 of lava. The filling was episodic and complex. It involved 13 major periods of addition of lava to the crater, including spectacular lava falls as high as 100 m, and three major periods of draining of lava from the crater. Alae was nearly filled by August 3, 1969, largely drained during a violent ground-cracking event on August 4, 1969, and then filled to the low point on its rim on October 10, 1969. From August 1970 to May 1971, the crater acted as a reservoir for lava that entered through subsurface tubes leading from the vent fissure 150 m away. Another tube system drained the crater and carried lava as far as the sea, 11 km to the south. Much of the lava entered Alae by invading the lava lake beneath its crust and buoying the crust upward. This process, together with the overall complexity of the filling, results in a highly complicated lava lake that would doubtless be misinterpreted if found in the fossil record.

Triassic blueschist from northern California and north-central Oregon

Four samples of blueschist from the eastern Klamath Mountains near Yreka, northern California, and from north-central Oregon near Mitchell have radiometric ages of approximately 220 m.y. (Middle Triassic). In the Klamath Mountains, there are two types of blueschist occurrence: (1) tectonic blocks of mafic composition in a phyllitic quartzite and siliceous phyllite terrane, which has also undergone blueschist-facies meta-morphism and lies between a belt of serpentinite and an upper Paleozoic or lower Mesozoic greenstone-chert terrane, and (2) discontinuous layers and tectonic blocks in phyllitic rocks of the greenschist metamorphic facies, which occur beneath a thrust plate of lower Paleozoic sedimentary rocks. In north-central Oregon, blueschist blocks occur in strongly sheared upper Paleozoic metasedimentary and metavolcanic rocks overlain by Cretaceous sedimentary rocks. The blueschist blocks in the Klamath Mountains and north-central Oregon are inferred to have formed during subduction that accompanied widespread Middle Triassic tectonism in the western Cordillera.

Regional correlation of Grande Ronde Basalt flows, Columbia River Basalt Group, Washington, Oregon, and Idaho

The tholeiitic flood basalts of the Columbia River Basalt Group of middle and late Miocene age cover more than 200,000 km2 in Washington, Oregon, and Idaho. The most voluminous formation of the Group, the Grande Ronde Basalt, erupted for 2 m.y. from north-northwest-trending fissure systems concentrated in southeast Washington and adjacent Oregon and Idaho. Four magnetostratigraphic units (designated R1, N1, R2, and N2 from oldest to youngest) are recognized on the basis of polarity in the Grande Ronde and provide the broad stratigraphic framework for the formation. In this study, major-element chemistry and relative stratigraphic position within the polarity intervals are used to identify and correlate individual flows and sequences of flows within the Grande Ronde Basalt on a regional scale. Systematic examination of more than 350 analyses from 47 stratigraphic sections show that most flows fall into one of five major chemical groupings, which are distinguished primarily by small but significant variations in MgO, TiO2, and P2O5 content. In addition, four minor chemical types local to the eastern part of the province have been identified. Feeder dikes of each chemical type have also been located. Flows or packets of flows of each chemical type can be correlated between field sections to define specific chemical-stratigraphic subunits. These subunits consist of several flows collectively 30–150 m thick. Subunits of most chemical types are repeated at irregular intervals throughout the formation; no progressive chemical trend occurs within the Grande Ronde. Many of the chemical-stratigraphic subunits extend to the margins of the province, although most are confined to the source region in eastern Washington. Although the total number of subunits is less in the west away from the fissure systems, the total thicknesses of the N2 and R2 magnetostratigraphic units are each as thick or thicker than the corresponding units in eastern Washington. The greatest thicknesses occur in the central part of the province within the Pasco basin. The distribution of basalt relative to the location of vents, as well as the relative east-west thicknesses, suggests that basalt flowed hundreds of kilometres westward during the most voluminous Grande Ronde eruptions, ponding against the irregular margin of the Cascade Range and being diverted through the ancestral Columbia Gorge toward the Washington-Oregon coast. Between these huge sheetflood events, smaller eruptions blanketed areas within the source region, and ongoing regional subsidence created a shallow westward-draining basin in the center of the province.

Explosive eruptions triggered by rockfalls at Kīlauea volcano, Hawai‘i

Ongoing eruptive activity at Kīlauea volcano’s (Hawai‘i) summit has been controlled in part by the evolution of its vent from a 35-m-diameter opening into a collapse crater 150 m across. Geologic observations, in particular from a network of webcams, have provided an unprecedented look at collapse crater development, lava lake dynamics, and shallow outgassing processes. These observations show unequivocally that the hundreds of transient outgassing bursts and weak explosive eruptions that have punctuated the vent’s otherwise nearly steady-state behavior, and that are associated with composite seismic events, were triggered by rockfalls from the vent walls onto the top of the lava column. While the process by which rockfalls drive the explosive bursts is not fully understood, we believe that it is initiated by the generation of a rebound splash, or Worthington jet, which then undergoes fragmentation. The external triggering of low-energy outgassing events by rockfalls represents a new class of small transient explosive eruptions.

Forecasts and predictions of eruptive activity at Mount St. Helens, USA: 1975–1984

Abstract Public statements about volcanic activity at Mount St. Helens include factual statements, forecasts, and predictions. A factual statement describes current conditions but does not anticipate future events. A forecast is a comparatively imprecise statement of the time, place, and nature of expected activity. A prediction is a comparatively precise statement of the time, place, and ideally, the nature and size of impending activity. A prediction usually covers a shorter time period than a forecast and is generally based dominantly on interpretations and measurements of ongoing processes and secondarily on a projection of past history. The three types of statements grade from one to another, and distinctions are sometimes arbitrary. Forecasts and predictions at Mount St. Helens became increasingly precise from 1975 to 1982. Stratigraphic studies led to a long-range forecast in 1975 of renewed eruptive activity at Mount St. Helens, possibly before the end of the century. On the basis of seismic, geodetic and geologic data, general forecasts for a landslide and eruption were issued in April 1980, before the catastrophic blast and landslide on 18 May 1980. All extrusions except two from June 1980 to the end of 1984 were predicted on the basis of integrated geophysical, geochemical, and geologic monitoring. The two extrusions that were not predicted were preceded by explosions that removed a substantial part of the dome, reducing confining pressure and essentially short-circuiting the normal precursors.

Deformation Monitoring at Mount St. Helens in 1981 and 1982

For several weeks before each eruption of Mount St. Helens in 1981 and 1982, viscous magma rising in the feeder conduit inflated the lava dome and shoved the crater floor laterally against the immobile crater walls, producing ground cracks and thrust faults. The rates of deformation accelerated before eruptions, and thus it was possible to predict eruptions 3 to 19 days in advance. Lack of deformation outside the crater showed that intrusion of magma during 1981 and 1982 was not voluminous.

Yakima Basalt of the Tieton River Area, South-Central Washington

Up to 1700 feet of the upper Miocene-lower Pliocene Yakima Basalt of the Columbia River Group underlie much of the eastern flank of the Cascade Range in the Tieton River area, Yakima County, Washington. Local prebasalt relief was more than 1700 feet, so thicknesses of each of the 15 exposed flows vary widely. Single flows can be traced for many miles, and terminate only against local topographic highs. The flows show typical colonnade-entablature jointing, and commonly overlie thin pillow-palagonite complexes. The basalts have Yakima-type chemistry. Plagioclase varies chiefly from An65 (microphenocrysts) to An45 (microlites). Clinopyroxenes range from Ca-rich pigeonite to augite, with subcalcic augite most abundant. Complex continuous zoning with respect to optic angles occurs between all the clinopyroxene phases. Olivine is sparse, and phenocrysts of all minerals are rare. Plots of 40 modal analyses indicate that plagioclase and pyroxene began crystallizing at about the same time, and crystallized at the same rate until the flows were at least 75 per cent crystalline. Individual flows differ in their plagioclase/pyroxene ratios and, to a lesser degree, in other microscopic characteristics. Therefore they can be correlated between the measured sections. The stratigraphy thus defined indicates that the youngest flow in the area is slightly older than the Vantage Sandstone Member (about 13.5 m.y.), a prominent sedimentary interbed farther east on the Columbia River Plateau. Flow directions show that the basalts advanced into the area from the east and southeast. Sedimentary interbeds between some of the flows contain a heavy mineral suite indicative of a northern or northeastern provenance, and crossbedding measurements suggest westward and southwestward current directions. The regional paleoslope, therefore, sloped westward or southwestward; and the basalts probably extended somewhat beyond the present Cascade Crest before being dammed by the ancient Western Cascades. Floods of pyrogenic hornblende in the interbeds and overlapping K/A ages suggest sporadic explosive activity of Tatoosh-type plutons in the Cascades contemporaneous with Yakima extrusions. The basalts are warped into five nearly west-trending folds and an eastward-sloping homocline. The homocline is related directly to Cascade uplift, which may have begun at about the time that Yakima-type flows ceased flooding the area.