Fragmentation of magma during Plinian volcanic eruptions |
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| Authors: | James E Gardner Richard M E Thomas Claude Jaupart Steve Tait |
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| Institution: | (1) Laboratoire de Dynamic des Systèmes Géologiques, Institut de Physique du Globe, 4 place Jussieu, F-75252 Paris cédex 05, France, FR |
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| Abstract: | The ratio of the volume of vesicles (gas) to that of glass (liquid) in pumice clasts (V
G
/V
L
) reflects the degassing and dynamic history experienced by a magma during an explosive eruption. V
G
/V
L
in pumices from a large number of Plinian eruption deposits is shown here to vary by two orders of magnitude, even between
pumices at a given level in a deposit. These variations in V
G
/V
L
do not correlate with crystallinity or initial water content of the magmas or their eruptive intensities, despite large ranges
in these variables. Gas volume ratios of pumices do, however, vary systematically with magma viscosity estimated at the point
of fragmentation, and we infer that pumices do not quench at the level of fragmentation but undergo some post-fragmentary
evolution. On the timescale of Plinian eruptions, pumices with viscosities <109 Pa s can expand after fragmentation, as long as their bubbles retain gas, at a rate inversely proportional to their viscosity.
Once the bubbles connect to form a permeable network and lose their gas, expansion halts and pumices with viscosities <105 Pa s can collapse under the action of surface tension. Textural evidence from bubble sizes and shapes in pumices indicates
that both expansion and collapse have taken place. The magnitudes of expansion and collapse, therefore, depend critically
on the timing of bubble connectivity relative to the final moment of quenching. We propose that bubbles in different pumices
become connected at different times throughout the time span between fragmentation and quenching. After accounting for these
effects, we derive new information on the fragmentation process from two characteristics of pumices. The most important is
a relatively constant minimum value of V
G
/V
L
of ∼1.78 (64 vol.% vesicularity) in all samples with viscosities >105 Pa s. This value is independent of magma composition and thus reflects a property of the eruptive mechanism. The other characteristic
is that highly expanded pumices (>85 vol.% vesicularities) are common, which argues against overpressure in bubbles as a mechanism
for fragmenting magma. We suggest that magma fragments when it reaches a vesicularity of ∼64 vol.%, but only if sheared sufficiently
strongly. The intensity of shear varies as a function of velocity in the conduit, which is related to overpressure in the
chamber, so that changes in overpressure with time are important in controlling the common progression from explosive to effusive
activity at volcanoes.
Received: 19 April 1995 / Accepted: 3 April 1996 |
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| Keywords: | Magma fragmentation Pumice vesicularity Volcanic eruptions Bubble connectivity |
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