Cooling of tephra during fallout from eruption columns

Thomas, Rick; Sparks, R. S. J.

doi:10.1007/bf00569939

Cited by 100 publications

(56 citation statements)

References 29 publications

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“…Vesicle shapes formed during ascent within the conduit may be modified after fragmentation by bubble growth (e.g., Thomas and Sparks 1992;Kaminsky and Jaupart 1997) and shape relaxation due to capillary forces (e.g., Klug and Cashman 1996). However, for silicic and for microlite-rich basaltic magmas, post-fragmentation bubble growth should be of limited extent, due to permeable outgassing (e.g., Rust and Cashmann 2011;Gonnermann and Houghton 2012).…”

Section: Resultsmentioning

confidence: 99%

“…Moreover, the characteristic time scale for shape relaxation, τ relaxation , would need to be much shorter than the characteristic quenching time, τ quenching . To evaluate the effect of post-fragmentation shape relaxation, we estimate τ quenching ∼ 100 s for centimeter-size pyroclasts following the approach of Thomas and Sparks (1992). For Vulcanian, Plinian, ultraplinian, and basaltic Plinian eruptions, τ relaxation = ηR/σ ∼ 10 3 to 10 5 s τ quenching (assuming that the viscosity in the case of the basaltic Plinian magmas accounts for the effect of microlites).…”

Section: Resultsmentioning

confidence: 99%

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Relating vesicle shapes in pyroclasts to eruption styles

et al. 2013

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Vesicles in pyroclasts provide a direct record of conduit conditions during explosive volcanic eruptions. Although their numbers and sizes are used routinely to infer aspects of eruption dynamics, vesicle shape remains an underutilized parameter. We have quantified vesicle shapes in pyroclasts from fall deposits of seven explosive eruptions of different styles, using the dimensionless shape factor , a measure of the degree of complexity of the bounding surface of an object. For each of the seven eruptions, we have also estimated the capillary number, Ca, from the magma expansion velocity through coupled diffusive bubble growth and conduit flow modeling. We find that is smaller for eruptions with Ca 1 than for eruptions with Ca 1. Consistent with previous studies, we interpret these results as an expression of the relative importance of structural changes during magma decompression and bubble growth, such as coalescence and shape relaxation of bubbles by capillary stresses. Among the samples analyzed, Strombolian and Hawaiian fire-fountain eruptions have Ca 1, in contrast to Vulcanian, Plinian, and ultraplinian eruptions. Interestingly, the basaltic Plinian eruptions of Tarawera volcano, New Zealand in 1886 and Mt. Etna, Italy in 122 BC, for which the cause of intense explosive activity has

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Relating vesicle shapes in pyroclasts to eruption styles

et al. 2013

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“…After expelling up to 0.5 km 3 of dacite (~0.25 km 3 DRE) and 0.35 km 3 of andesite (~0.17 km 3 DRE), the eruptive regime dramatically increased, three orders of magnitude in volume, and generated deposits that completely covered the northeast part of Umnak Island. Most likely, the pyroclastic current deposits are responsible for the charred vegetation buried underneath the fall deposits because the fall deposits were not hot enough to char vegetation when they were deposited (Thomas and Sparks 1992). The pyroclastic deposits, however, were not hot enough to cause welding or scoria oxidation, which suggests that their temperature was between 200 and 600 °C (Riehle 1973).…”

Section: Eruption Dynamicsmentioning

confidence: 99%

Physical volcanology of the 2,050 bp caldera-forming eruption of Okmok volcano, Alaska

Burgisser

2005

Bull Volcanol

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In the Aleutian volcanic chain (USA), the 2050 ± 50 BP collapse of Okmok caldera generated pyroclasts that spread over 1000 km 2 on Umnak Island. After expelling up to 0.25 km 3 DRE of rhyodacitic Plinian air fall and 0.35 km 3 DRE of andesitic phreatomagmatic tephra, the caldera collapsed and produced the 29 km 3 DRE Okmok II scoria deposit, which is composed of valley-ponding, poorly sorted, massive facies and over-bank, stratified facies with planar and cross bedding. Geological and sedimentological data suggest that a single density current produced the Okmok II deposits by segregating into a highly concentrated base and an overriding dilute cloud.The dense base deposited massive facies, whereas the dilute cloud sedimented preferentially on hills as stratified deposits. The pyroclastic current spread around Okmok in an axisymmetric fashion, encountering topographic barriers on the southwest, and reaching Unalaska Island across an 8-km strait on the east, and reaching the shoreline of Umnak in the other directions. The kinematic model by Burgisser and Bergantz (2002, Earth Planet. Sci. Lett. 202:405-418) was used to show how decoupling of the pyroclastic current was triggered by both sea entrance and interaction with the topography. In the former case, the dense part of the current and the lithics transported by the dilute cloud went underwater. In the latter case, topographical barriers noticeably decelerated both parts of the decoupled current and favored sedimentation by partial or complete blocking.The resulting unloading of the dilute current drastically reduced the runout distance by triggering an early buoyant lift-off.

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“…In order to achieve this the Roza fountains needed to be high (>> 504 1 km) and sedimentation from the fountains needed to be enhanced by fallout of coarse, hot 505 pyroclasts from the lower parts of associated convecting plumes of potentially Subplinian to Plinian 506 intensity (e.g., Thomas and Sparks, 1992 linked to bubble rise and coalescence, degassing processes and melt rheology driven by microlite 535 crystallisation (Houghton and Gonnermann, 2008). An in-depth discussion of the parameters 536 controlling more vigorously explosive phases of the Roza eruption is beyond the scope of this paper 537 and will be dealt with in a future publication.…”

Section: Fountain and Eruption Dynamics 476mentioning

confidence: 99%