Geology topics

Research about Mt. St. Helens

Source-linked reports with geographic coverage including Mt. St. Helens.

8 recordsLinked to original sources

USGS2019

Shear velocity structure from ambient noise and teleseismic surface wave tomography in the Cascades around Mount St. Helens

Mount St. Helens (MSH) lies in the forearc of the Cascades where conditions should be too cold for volcanism. To better understand thermal conditions and magma pathways beneath MSH, data from a dense broadband array are used to produce high‐resolution tomographic images of the crust and upper mantle. Rayleigh‐wave phase‐velocity maps and three‐dimensional images of shear velocity ( Vs ), generated from ambient noise and earthquake surface waves, show that west of MSH the middle‐lower crust is anomalously fast (3.95 ± 0.1 km/s), overlying an anomalously slow uppermost mantle (4.0–4.2 km/s). This combination renders the forearc Moho weak to invisible, with crustal velocity variations being a primary cause; fast crust is necessary to explain the absent Moho. Comparison with predicted rock velocities indicates that the fast crust likely consists of gabbros and basalts of the Siletzia terrane, an accreted oceanic plateau. East of MSH where magmatism is abundant, middle‐lower crust Vs is low (3.45–3.6 km/s), consistent with hot and potentially partly molten crust of more intermediate to felsic composition. This crust overlies mantle with more typical wave speeds, producing a strong Moho. The sharp boundary in crust and mantle Vs within a few kilometers of the MSH edifice correlates with a sharp boundary from low heat flow in the forearc to high arc heat flow and demonstrates that the crustal terrane boundary here couples with thermal structure to focus lateral melt transport from the lower crust westward to arc volcanoes.

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USGS2017

Voluminous arc dacites as amphibole reaction-boundary liquids

Dacites dominate the large-volume, explosive eruptions in magmatic arcs, and compositionally similar granodiorites and tonalites constitute the bulk of convergent margin batholiths. Shallow, pre-eruptive storage conditions are well known for many dacitic arc magmas through melt inclusions, Fe–Ti oxides, and experiments, but their potential origins deeper in the crust are not well determined. Accordingly, we report experimental results identifying the P–T–H 2 O conditions under which hydrous dacitic liquid may segregate from hornblende (hbl)-gabbroic sources either during crystallization–differentiation or partial melting. Two compositions were investigated: (1) MSH–Yn−1 dacite (SiO 2 : 65 wt%) from Mount St. Helens’ voluminous Yn tephra and (2) MSH–Yn−1 + 10% cpx to force saturation with cpx and map a portion of the cpx + melt = hbl peritectic reaction boundary. H 2 O-undersaturated (3, 6, and 9 wt% H 2 O) piston cylinder experiments were conducted at pressures, temperatures, and fO 2 appropriate for the middle to lower arc crust (400, 700, and 900 MPa, 825–1100 °C, and the Re–ReO 2 buffer ≈ Ni–NiO + 2). Results for MSH–Yn−1 indicate near-liquidus equilibrium with a cpx-free hbl-gabbro residue (hbl, plg, magnetite, ± opx, and ilmeno-hematite) with 6–7 wt% dissolved H 2 O, 925 °C, and 700–900 MPa. Opx disappears down-temperature consistent with the reaction opx + melt = hbl. Cpx-added phase relations are similar in that once ~10% cpx crystallizes, multiple saturation is attained with cpx, hbl, and plg, +/− opx, at 6–7 wt% dissolved H 2 O, 940 °C, and 700–900 MPa. Plg–hbl–cpx saturated liquids diverge from plg–hbl–opx saturated liquids, consistent with the MSH–Yn−1 dacite marking a liquid composition along a peritectic distributary reaction boundary where hbl appears down-temperature as opx + cpx are consumed. The abundance of saturating phases along this distributary peritectic (liquid + hbl + opx + cpx + plg + oxides) reduces the variance, so liquids are restricted to dacite–granodiorite–tonalite compositions. Higher-K dacites than the Yn would also saturate with biotite, further limiting their compositional diversity. Theoretical evaluation of the energetics of peritectic melting of pargasitic amphiboles indicates that melting and crystallization of amphibole occur abruptly, proximal to amphibole’s high-temperature stability limit, which causes the system to dwell thermally under the conditions that produce dacitic compositions. This process may account for the compositional homogeneity of dacites, granodiorites, and tonalites in arc settings, but their relative mobility compared to rhyolitic/granitic liquids likely accounts for their greater abundance.

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USGS2012

An aeromagnetic study of Mount St. Helens

Aeromagnetic data from surveys flown by the U.S. Geological Survey over Mount St. Helens, Washington, before and after the climactic May 18, 1980, eruption were used to determine the bulk magnetic properties of the volcano and to delineate a buried source. We assumed that most of the edifice of preeruption and posteruption Mount St. Helens has a magnetization direction near the present earth's field of 69° inclination and 20° declination and calculated its intensity as 4.1 A/m. After subtraction of magnetic anomalies due to topogaphy magnetized with this direction and intensity, the preeruption and posteruption surveys revealed nearly identical residual magnetic highs and lows, indicative that their sources were not altered or removed by the May 1980 eruption. The residual highs were explained by a 200-m-deep source lying mostly within the edifice of Mount St. Helens. The source could be terrain that predates Mount St. Helens, such as a buried ridge or a cone or a valley filled with lava. We calculated the magnetization of the material removed by the May 1980 eruption and found its intensity to be 4.2 A/m in a direction near the present earth's field, similar to that assumed for the volcano as a whole. This similar result confirmed the validity of the magnetization vector assumed for the entire edifice.

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USGS2012

Evidence from gravity data for an intrusive complex beneath Mount St. Helens

On the basis of gravity data, aided by aeromagnetic, magnetotelluric, side-looking radar, and geologic information, we tentatively identify a large, shallow intrusive complex beneath Mount St. Helens. The complex is roughly 5–6 km thick and has apparently intruded a buried and compressed Mesozoic forearc sedimentary sequence. The lateral extent of the intrusive complex is uncertain, and we give three alternative models varying from about 18 by 10 km to as much as 18 by 22 km. The western boundary of the inferred Mount St. Helens intrusion abuts several of a number of Tertiary and younger plutons that crop out in the area. The Mount St. Helens intrusion apparently is adjacent to sedimentary rock or shallow volcanic rock along most of the remainder of its boundary. A ringlike drainage pattern around Mount St. Helens suggests subsidence caused by removing magma from deep beneath Mount St. Helens and adding this mass to the Mount St. Helens edifice and its underlying intrusion. Our analysis indicates that the average bulk density of the volcanic rock comprising Mount St. Helens is about 2.15 g/cm 3 .

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USGS2000

Was the 18 May 1980 lateral blast at Mt St Helens the product of two explosions?

The 18 May 1980 lateral blast at Mt St Helens has been interpreted as the product of a single explosion by some stratigraphers and as two closely spaced explosions by others. The stratigraphic evidence that bears on this question is inconclusive; strata change dramatically over short distances and this complexity provides wide latitude for interpretation. Some independent non-stratigraphic evidence, however, suggests that the blast was the product of two explosions or clusters of explosions. The independent evidence comes from eyewitness accounts and photographs, from satellite sensors, and from seismic records. This paper reviews the pertinent evidence, offers a new interpretation, and concludes that the blast was indeed the product of two explosions or clusters of explosions.

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USGS1992

Correlation between atmospheric precipitation and recent explosions at Mount St. Helens, Washington

Scientists attribute the recent small explosion-like seismic signals at Mount St. Helens to either the geyser-like flashing of superheated groundwater to steam or the release of magmatic gas from the cooling magma system, or both. The contribution of magmaic gas in these events is not currently known. If meteoric water from rain or melting snow is the source, however, we might expect these events to occur most frequently during the rainy season, perhaps even during or immediately following individual storms.

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USGS1992

Small explosions interrupt 3-year quiescence at Mount St. Helens, Washington

On December 11, 1989, geologists working in the crater at Mount St. Helens discovered two thin layers of ash separated by fresh snow-clear evidence that at least two small explosions had occurred recently. The explosions were neither seen nor heard, but on December 7 scientists suspected that a small ash-producing explosion had occurred when seismometers near the volcano recorded a long explosion-like signal, and titlt and displacement meters showed minor deformation of the dome. There were no other large seismic signals to account for the second ash layer, which was most likely associated with one of several smaller signals in early December. The December ash-producing explosions were the first eruptive activity at Mount St. Helens since October 1986. There have been at least five more ash-producing explosions since December 1989, all without recognized seismic or other geophysical precursors. The ash from these explosions appears to be pulverized pieces of dacite dome. The absence of glass shards in the ash suggests that no new magmatic material was ejected. Several of the explosions were accompanied by snow and rock avalanches, pyroclastic flows, ballistic showers, and debris flows. These ash-producing explosions are part of a series of at least 28 explosion-like seismic events that began on August 24, 989. Seismic signals from these events resemble those associated with confirmed ash-producing explosions in April-May 1986. Yet not all of the 1989-1991 events produced ash plumes. Excellent visual observations during four of the events indicated that neither a steam nor ash plume was generated. There is little information about the other events because they occurred when the mountain was not visible, nor was there physical evidence of ashfall or surface changes when scientists visited the crater days to weeks alter. Considerable deformation of the north side of the dome occurred during the series of explosion-like seismic events. Sections of the dome slumped northward and two new vents were formed. However, monitoring the changes associated with individual events was often impossible because several key electronic-distance-meter (EDM) targets and tiltmeters were destroyed by the series of events.

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USGS1990

Of parachuting spiders and meat-eating beetles

Scientists have been amazed at the rapid recovery of life that followed the volcanic eruption at Mount St. Helens 10 years ago, but to the uninitiated, the place is still a stark and awesome wasteland. Here and there on the rolling hills of gray volcanic ash around the volcano are clumps of invading fireweed and another pioneer called pearly everlasting. Alder and Willow saplings grow sporadically along the otherwise barren stream courses. Plugs of grass sprout in piles of elk droppings. And deer mic and pocket gophers, the survivialists of the animal world, also have managed to gain foothold.

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