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Titel |
The effect of porosity and crystallinity on magma rheology at Unzen volcano, Japan |
VerfasserIn |
Rebecca Coats, Yan Lavallée, Jackie E. Kendrick, Paul A. Wallace, James D. Ashworth, Adrian Hornby, Takahiro Miwa |
Konferenz |
EGU General Assembly 2016
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 18 (2016) |
Datensatznummer |
250121696
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Publikation (Nr.) |
EGU/EGU2016-512.pdf |
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Zusammenfassung |
Eruptions of lava domes are some of the most unpredictable and dangerous volcanic
phenomena, and their frequent occurrence highlights the priority of improving our
understanding of the mechanisms at play during magma ascent. For example, between 1991
and 1995 Mt. Unzen sustained an extensive period of dome growth. During this time
approximately 2.1x108 m3 dense rock equivalent of magma was emplaced at effusion rates of
0.1-4x105 m3d−1. Repeated partial collapses of the dome generated frequent pyroclastic
density currents which caused several fatalities and damage to populated areas near
Shimabara City.
One of the keys to grasping the switch between effusive to explosive behaviour, as well as
the onset of a dome collapse event lies within understanding the rheological behaviour of
upwelling magma. Although the conditions which lead to failure of magma are now
increasingly studied, we lack an understanding of the influence of mechanisms at play during
magma deformation.
Strain rate and temperature play primary roles in the transition of silicate melts from
viscous bodies to elastic solids; this regime change is known as the viscous-brittle transition
and is bound by the glass transition of the interstitial melt, which signals the beginning of
either fracture or flow. Crystals and pores present in the system can alter the position of the
viscous-brittle transition.
Laboratory experiments which aim to simulate realistic volcanic conditions permit the
controlled study of volcanic processes, and help determine the behaviour of multi-phase
magmatic suspensions. For magma with high crystal contents (>40%) rheology is strongly
influenced by the crystalline phase and is strain rate dependent. Here we present the results of
high-temperature deformation experiments on variably porous (9-33%), crystal-rich (>50%)
dacite lavas from Unzen volcano, Japan.
Uniaxial compression tests were carried out on dacitic samples from Unzen at room
temperature (∼20˚ C) and high temperature (900˚ C), with strain rates of 10−1, 10−3,
10−5s−1. For high temperature experiments we also performed dynamic, stepped strain-rate
experiments. We discuss how the strain rate influences the rheological response of the
magma, noting a decrease in viscosity, with increasing strain rate, which evidences
non-Newtonian, shear-thinning behaviour. All experiments with a brittle responses
show a nonlinear decrease in failure strength with porosity. In addition, the data
clearly show that magma at 900˚ C is stronger than its room temperature rock
counterpart.
We conclude that the strength of multi-phase magmas is strongly influenced by porosity
across all temperatures, while crystal load is more important at temperatures above the glass
transition, influencing the rheological properties and producing shear thinning behaviour.
However, we also note that magmas are stronger at higher deformation rates, which has
important implications for linking hazard to eruption rate, and we note that the same magma
is stronger at high temperature, suggesting that in dome building volcanoes it may be the
cooling, residual lava that is at higher risk of collapse than freshly erupted lava. |
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