New Zealand Journal of Geology and Geophysics abstracts
Physical volcanology and emplacement history of the Ben Lomond
rhyolite lava flow, Taupo Volcanic Centre, New Zealand
R. J. STEVENSON*
R. M. BRIGGS
A. P. W. HODDER
Department of Earth Sciences
University of Waikato
Private Bag 3105
Hamilton, New Zealand
*Present address: Bayerisches Geoinstitut, Postfach 10 12 51, D-95440 Bayreuth,
Germany.
Abstract Currently, there are two contrasting models
concerning the state in which rhyolite lava reaches the surface: (1) the
permeable foam model, where the lava reaches the surface as an expanded foam
and collapses into dense obsidian during lava flow; and (2) the traditional
model, where decreased load pressure leads to vesiculation of the upper surface
of the dome.
In order to test these models, textural parameters were documented on outcrops
of the Ben Lomond rhyolite dome, which has two flow lobes comprising an eroded
dome (c. 100 ka old) cut by the Whangamata Fault. A composite stratigraphy
of the upper 60 m of the flow, assembled from three sections (roadcut, airstrip
and fault scarp), comprises, from the flow top: finely vesicular pumice, black
aphyric obsidian, and a spherulitic transition zone above a central crystalline
rhyolite core. An explosion breccia occurs as a pod at a depth of c. 35 m and
cross-cuts the upper two units as an inverted cone-shaped deposit--an infilled
explosion pit.
Eight physical parameters (density and porosity, void aspect ratios,
spherulite size and proportion, microlite size and abundance, and volatile
contents) were measured throughout the exposed thickness of the flow. Results
show that primary vesicles--those that deform pre-existing flow banding--are
most numerous within the finely vesicular pumice layer but are suppressed c.
10-15 m below the surface. Vesicularity and volatile contents are anomalously
high in explosion breccia fabrics. Furthermore, microlites are ubiquitous in
the flow but absent in the explosion breccia. Incomplete degassing of the
ascending magma took place, resulting in volatiles being distributed within the
flow thickness during lava emplacement. The lava flow carapace revesiculated
during extrusion (primary vesiculation), whereas secondary vesiculation
(spherulites and lithophysae) was associated with groundmass crystallisation of
the flow centre, causing localised volatile enrichments responsible for the
formation of explosion breccias. An emplacement model, formulated for the Ben
Lomond dome, shows that vesiculation can occur within different units
throughout emplacement and after flow cessation. Furthermore, textural data
indicate that the permeable foam model is not a viable model for the Ben Lomond
dome.
Keywords Ben Lomond rhyolite dome; Taupo Volcanic Centre;
finely vesicular pumice; obsidian; explosion breccia; spherulites;
lithophysae
New Zealand Journal of Geology and Geophysics, 1994, Vol. 37: 345-358
0028-8306/94/3703-0345 $2.50/0 (c) The Royal Society of New Zealand
1994
PDF file of entire paper: medium quality (1378K); (scanned from paper original: notes about this process)
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