Bimodal Density Distribution of Cryptodome Dacite from the 1980 Eruption of Mount St. Helens, Washington

Hoblitt, Richard P.; Harmon, Russell S.

doi:10.1007/bf00302002

Cited by 111 publications

(124 citation statements)

References 34 publications

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“…During and after the blast explosion, much of this fragmented cryptodome magma was able to actively expand, generating the coarse bread-crust bombs that are common in the blast deposits of both BZ and MSH. The material from the expanded internal parts of these bombs is virtually identical with the material of the low- Belousov (1996), Hoblitt and Harmon (1993), and unpublished data of authors density clasts from the blast deposits. The outer dense crust of the bombs represents the chilled material of the internal, partly molten parts of the magma bodies; the initial vesiculation of the outer parts of the bombs was arrested by rapid cooling when the bombs were hurled through the air.…”

Section: Components Of the Depositssupporting

confidence: 67%

Directed blasts and blast-generated pyroclastic density currents: a comparison of the Bezymianny 1956, Mount St Helens 1980, and Soufrière Hills, Montserrat 1997 eruptions and deposits

2007

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We compare eruptive dynamics, effects and deposits of the Bezymianny 1956 (BZ), Mount St Helens 1980 (MSH), and Soufrière Hills volcano, Montserrat 1997 (SHV) eruptions, the key events of which included powerful directed blasts. Each blast subsequently generated a high-energy stratified pyroclastic density current (PDC) with a high speed at onset. The blasts were triggered by rapid unloading of an extruding or intruding shallow magma body (lava dome and/or cryptodome) of andesitic or dacitic composition. The unloading was caused by sector failures of the volcanic edifices, with respective volumes for BZ, MSH, and SHV c. 0.5, 2.5, and 0.05 km 3 . The blasts devastated approximately elliptical areas, axial directions of which coincided with the directions of sector failures. We separate the transient directed blast phenomenon into three main parts, the burst phase, the collapse phase, and the PDC phase. In the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into the atmosphere, with much of the expansion having an initially lateral component. The erupted material fails to mix with sufficient air to form a buoyant column, but in the collapse phase, falls beyond the source as an inclined fountain, and thereafter generates a PDC moving parallel to the ground surface. It is possible for the burst phase to comprise an overpressured jet, which requires injection of momentum from an orifice; however some exploding sources may have different geometry and a jet is not necessarily formed. A major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be attributed to shock waves within an overpressured jet, or alternatively to dynamic pressures and shocks within the energetic collapse and PDC phases. Internal shock structures related to unsteady flow and compressibility effects can occur in each phase. We withhold judgment about published shock models as a primary explanation for the damage sustained at MSH until modern 3D numerical modeling is accomplished, but argue that much of the damage observed in directed blasts can be reasonably interpreted to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified PDC. This view is reinforced by recent modeling cited for SHV. In distal and peripheral regions, solids concentration, maximum particle size, current speed, and dynamic pressure are diminished, resulting in lesser damage and enhanced influence by local topography on the PDC. Despite the different scales of the blasts (devastated areas were respectively 500, 600, and >10 km 2 for BZ, MSH, and SHV), and some complexity involving retrogressive slide blocks and clusters of explosions, their pyroclastic deposits demonstrate strong similarity. Juvenile material composes >50% of the deposits, implying for the blasts a dominantly magmatic mechanism although hydrothermal explosions also occurred. The character of the magma fragmented by explosions (highly viscous, phen...

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Section: Components Of the Depositssupporting

confidence: 67%

Directed blasts and blast-generated pyroclastic density currents: a comparison of the Bezymianny 1956, Mount St Helens 1980, and Soufrière Hills, Montserrat 1997 eruptions and deposits

2007

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“…Surge (n=19 determined by the methods of Houghton and Wilson (1989) and Hoblitt and Harmon (1993). Percent vesicularity is defined by:…”

Section: Pyroclastic Surge Depositmentioning

confidence: 99%

Pyroclastic surges and flows from the 8-10 May 1997 explosive eruption of Bezymianny volcano, Kamchatka, Russia

Belousov

Voight

Belousova

et al. 2002

Bulletin of Volcanology

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The 8-10 May 1997 eruption of Bezymianny volcano began with extrusion of a crystallized plug from the vent in the upper part of the dome. Progressive gravitational collapses of the plug caused decompression of highly crystalline magma in the upper conduit, leading at 13:12 local time on 9 May to a powerful, vertical Vulcanian explosion. The dense pyroclastic mixture collapsed in boil-over style to generate a pyroclastic surge which was focused toward the southeast by the steepwalled, 1956 horseshoe-shaped crater. This surge, with a temperature <200°C, covered an elliptical area >30 km 2 with deposits as much as 30 cm thick and extending 7 km from the vent. The surge deposits comprised massive to vaguely laminated, gravelly sand (Md -1.2 to 3.7φ; sorting 1.2 to 3φ) of poorly vesiculated andesite (mean density 1.82 g cm -3 ; vesicularity 30 vol%; SiO 2 content ~58.0 wt%). The deposits, with a volume of 5-15×10 6 m 3 , became finer grained and better sorted with distance; the maximal diameter of juvenile clasts decreased from 46 to 4 cm. The transport and deposition of the surge over a snowy landscape generated extensive lahars which traveled >30 km. Immediately following the surge, semi-vesiculated block-and-ash flows were emplaced as far as 4.7 km from the vent. Over time the juvenile lava in clasts of these flows became progressively less crystallized, apparently more silicic (59.0 to 59.9 wt% SiO 2 ) and more vesiculated (density 1.64 to 1.12 g cm -3 ; vesicularity 37 to 57 vol%). At this stage the eruption showed transitional behavior, with mass divided between collapsing fountain and buoyant column. The youngest pumice-and-ash flows were accompanied by a sustained sub-Plinian eruption column ~14 km high, from which platy fallout clasts were deposited (~59.7% SiO 2 ; density 1.09 g cm -3 ; vesicularity 58 vol%). The explosive activity lasted about 37 min and produced a total of ~0.026 km 3 dense rock equivalent of magma, with an average discharge of ~1.2×10 4 m 3 s -1 . A lava flow 200 m long terminated the eruption. The evolutionary succession of different eruptive styles during the explosive eruption was caused by vertical gradients in crystallization and volatile content of the conduit magma, which produced significant changes in the properties of the erupting mixture.

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“…Only after deposition could the deposits char wood because of the more prolonged exposure and additional heating on account of the high temperature of inner parts of large fragments. (Hoblitt and Harmon 1993). The stratigraphy of MSH blast deposits has been described in many papers (Hoblitt et al 1981;Moore and Sisson 1981;Waitt 1981;Fisher et al 1987;Fisher 1990;Druitt 1992), but for comparison herein reliance is placed on two of the most recent papers (Fisher 1990;Druitt 1992).…”

Section: Directed Blast Impactmentioning

confidence: 99%

Deposits of the 30 March 1956 directed blast at Bezymianny volcano, Kamchatka, Russia

Belousov

1996

Bulletin of Volcanology

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Bimodal Density Distribution of Cryptodome Dacite from the 1980 Eruption of Mount St. Helens, Washington

Cited by 111 publications

References 34 publications

Directed blasts and blast-generated pyroclastic density currents: a comparison of the Bezymianny 1956, Mount St Helens 1980, and Soufrière Hills, Montserrat 1997 eruptions and deposits

Directed blasts and blast-generated pyroclastic density currents: a comparison of the Bezymianny 1956, Mount St Helens 1980, and Soufrière Hills, Montserrat 1997 eruptions and deposits

Pyroclastic surges and flows from the 8-10 May 1997 explosive eruption of Bezymianny volcano, Kamchatka, Russia

Deposits of the 30 March 1956 directed blast at Bezymianny volcano, Kamchatka, Russia

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