Subaqueous, basaltic lava dome and carapace breccia on King George Island, South Shetland Islands, Antarctica |
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| Authors: | J L Smellie I L Millar D C Rex P J Butterworth |
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| Institution: | (1) British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK Fax: +1223 62616 e-mail: J.Smellie@bas.ac.uk, GB;(2) NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK, GB;(3) Department of Earth Sciences, University of Leeds LS2 9JT, UK, GB;(4) Arco British Ltd., Cordon Square, Cross Lanes, Guildford GU1 1UE, UK, GB |
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| Abstract: | On King George Island during latest Oligocene/earliest Miocene time, submarine eruptions resulted in the emplacement of a
small (ca. 500 m estimated original diameter) basalt lava dome at Low Head. The dome contains a central mass of columnar rock
enveloped by fractured basalt and basalt breccia. The breccia is crystalline and is a joint-block deposit (lithic orthobreccia)
interpreted as an unusually thick dome carapace breccia cogenetic with the columnar rock. It was formed in situ by a combination
of intense dilation, fracturing and shattering caused by natural hydrofracturing during initial dome effusion and subsequent
endogenous emplacement of further basalt melt, now preserved as the columnar rock. Muddy matrix with dispersed hyaloclastite
and microfossils fills fractures and diffuse patches in part of the fractured basalt and breccia lithofacies. The sparse glass-rich
clasts formed by cooling-contraction granulation during interaction between chilled basalt crust and surrounding water. Together
with muddy sediment, they were injected into the dome by hydrofracturing, local steam fluidisation and likely explosive bulk
interaction. The basalt lava was highly crystallised and degassed prior to extrusion. Together with a low effusion temperature
and rapid convective heat loss in a submarine setting, these properties significantly affected the magma rheology (increased
the viscosity and shear strength) and influenced the final dome-like form of the extrusion. Conversely, high heat retention
was favoured by the degassed state of the magma (minimal undercooling), a thick breccia carapace and viscous shear heating,
which helped to sustain magmatic (eruption) temperatures and enhanced the mobility of the flow.
Received: 1 August 1996 / Accepted: 15 September 1997 |
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| Keywords: | Key words Lava dome Endogenous Basaltic Submarine Rheology Antarctica |
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