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| Titel |
Exchange flow experiments and implications for degassing processes at basaltic volcanoes |
| VerfasserIn |
Frances Beckett, Mike Burton, Heidy Mader, Jeremy Phillips, Margherita Polacci, Alison Rust, Fred Witham |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250049056
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| Zusammenfassung |
| Persistent degassing at basaltic volcanoes can be sustained by an exchange flow of magma
within the conduit, driven by the density difference between degassed crystallised magma at
shallow levels and ascending buoyant gas rich magma. Experiments have been conducted to
investigate buoyancy-driven, low Reynolds number, long time period, exchange flow of
viscous fluids in a vertical pipe (length 1 m diameter 38.4 mm) between two reservoirs.
The viscosity ratio between the two fluids, defined as the viscosity of the more
dense fluid divided by the viscosity of the less dense fluid, was varied from 2 to
1900. Two distinct flow regimes were observed; axisymmetric core annular flow in
which the less viscous fluid occupies a cylindrical core and the denser fluid flows
downwards in an annulus, and side by side flow where both fluids are in contact with the
pipe walls, and a single interface exists between them. The flow regime formed is
dependent on the viscosity ratio between the two fluids. Core annular flows form at
high viscosity ratios, -³ 100 . Side by side flows form at lower viscosity ratios,
-² 100 .
Using data from the literature we define the properties of the ascending and descending
magma as a function of pressure at Stromboli (Aeolian Archipelago). We use MELTS
(Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998) to constrain the composition of the
melt and crystallinity and a gas solubility model to characterise the vesicularity, from which
we calculate the viscosity of the respective magmas and hence viscosity ratio. We find that at
pressures > 200 MPa the flow regime is core annular, but undergoes a transition to side by
side flow at lower pressures as the viscosity of the ascending and descending magma
become more similar. Using knowledge of the persistent, non explosive, gas flux
and non-dimensional scaling relationships determined from the experiments the
radius of the conduit and the volumetric flux of magma at Stromboli are calculated. |
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