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Titel |
Cyclicity in slug-driven basaltic eruptions: insights from large-scale analogue experiments |
VerfasserIn |
Ed Llewellin, Elisabetta Del Bello, Steve Lane, Antonio Capponi, Simon Mathias, Jacopo Taddeucci |
Konferenz |
EGU General Assembly 2013
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 15 (2013) |
Datensatznummer |
250074630
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Zusammenfassung |
Basaltic eruptions commonly exhibit cyclical or pulsatory behaviour. Strombolian eruptions
are typically discrete and rhythmic, with return interval of minutes-to-hours; lava
fountains may fluctuate over similar, or shorter, timescales. In both cases the cyclicity
results from the separation of gas from the magma, and its localization into discrete
gas slugs or gas-rich packets. We report analogue experiments which model the
fluid dynamics of slug-driven basaltic eruptions. Experiments were conducted in
liquid-filled vertical pipes at a range of scales, from 0.02 to 0.2 m in diameter, and 2 to
13 m in height, allowing us to investigate Reynolds numbers 10-3 < Re < 105,
encompassing the natural range for volcanoes. The dynamics of both discrete gas
slugs (Taylor bubbles) and continuous sluggy flow were quantified. A significant
novelty of this study is that we explore the role played by the boundary conditions
at the top and bottom of the conduit, which may be either closed (zero flux) or
held at constant pressure. This setup allows us to mimic plugged or open vent, and
the influence of a magma chamber. Our study combines the direct observation of
in-conduit fluid dynamic processes with measured pressure variations inside the
conduit.
Our results demonstrate that, when discrete gas slugs are injected, plugging the vent has a
strong influence on the development of overpressure in the system, and on the potential for
the system to manifest cyclic behaviour. When gas is injected continuously, with constant
pressure boundaries at the top and bottom of the conduit, the system spontaneously
self-organizes into rhythmic sluggy flow when the injection rate exceeds a critical value. In
both cases we find that the capacity of the system to sustain well-formed, discrete slugs
depends strongly on the Reynolds number of the flow. Well-formed Taylor bubbles – which
have a smooth hemispherical cap, occupy the width of the conduit, and ascend relatively
slowly – only form when the liquid column is stagnant, or is flowing in the laminar regime.
Slugs that rise through turbulent liquid are poorly-formed and, at the extreme, ascend
as highly-turbulent gas-rich packets; these occupy less of the conduit width and
therefore ascend more rapidly than true Taylor bubbles. A major factor influencing the
turbulence in the liquid is the separation between slugs; when slugs are closely
spaced, the wake of one slug tends to disrupt the next. At the onset of eruption,
therefore, the tendency is for poorly-formed, fast-moving slugs to catch up with the
well-formed, slow-moving lead slug, increasing the potential for an impulsive,
explosive onset. During sustained eruption, lower gas fluxes lead to lower-frequency,
higher amplitude fluctuations in eruption rates, whilst higher gas fluxes lead to
higher-frequency, lower amplitude fluctuations. The spectrum of natural activity, from
discrete Strombolian eruptions to sustained, pulsatory lava fountaining can, therefore, be
characterized as a change in the behaviour of the separated gas phase in the conduit. |
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