 |
| Titel |
Gas slug rise in open versus plugged basaltic conduits: the transition from Strombolian to sustained volcanic eruptions |
| VerfasserIn |
Ed Llewellin, Simon Mathias, Elisabetta Del Bello, Steve Lane, Jacopo Taddeucci |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040693
|
|
|
|
|
|
| Zusammenfassung |
| Strombolian eruptions are characterized by low-energy explosive activity, which may repeat,
at fairly regular intervals, for long periods of time. Explosions at the vent are thought to result
from the arrival of discrete slugs of magmatic gas that have risen through the magmatic
plumbing system. We develop a one dimensional, analytical model of gas slug rise in a
volcanic conduit which we use to investigate the controls on the dynamics of Strombolian
eruptions.
We consider a partially-filled, cylindrical conduit containing degassed magma, that is
initially in magma-static equilibrium with a constant pressure magma reservoir
at depth. We introduce a slug of gas at the base of the conduit and consider the
temporal evolution of the pressure distribution in the conduit, and the motion of the
magma above and below the slug, as the slug rises, decompresses, and expands
isothermally. We validate our model against published data for gas slugs rising
and decompressing in a vertical pipe [James et al., 2008, Geological Society of
London, Special Publications 307, 147-167] by imposing the condition of zero
magma-flux at the base of the conduit, and constant pressure at the top; we find
excellent agreement. If we impose the more geologically-sound condition of constant
pressure at the base of the conduit, we can consider two scenarios of volcanological
relevance:
1) Vent plugged with cooler, more-viscous magma. In this case, as the slug rises and
expands beneath the plug, it pushes the degassed magma below it down the conduit,
consequently, magma re-enters the chamber from the base of the conduit. The slug reaches
the viscous plug at the top of the conduit with a significant over-pressure; this may be
sufficient to disrupt the plug, causing a Strombolian explosion, or the gas may percolate
away. Fresh magma then moves up from the chamber, into the conduit to restore
magma-static equilibrium.
2) Open vent. As the slug rises and expands, the volume of magma held in the falling-film
that encircles the slug increases. Since this film is supported viscously by the conduit walls, it
does not contribute to the magma-static pressure at the base of the conduit which, therefore,
decreases, causing fresh magma to flow upwards.
Fresh magma entering the base of the conduit may cause a new slug to form, either
through decompression and exsolution, or by tapping a gas-rich layer at an asperity, or at
the chamber roof. In either case, scenario 1 (plugged vent) will lead to periodic,
Strombolian eruption, whilst scenario 2 (open vent) will lead to sustained eruption
at higher discharge rate. A noteworthy feature of the model is that the conduit is
recharged with fresh magma following slug rise, even if no magma is erupted at the
surface.
We apply our model to the case of Stromboli, and compare model output with
monitoring observations. We find that the most important parameters controlling
eruption style and vigour are magma viscosity, mass of gas in the slug, and the ratio
of slug radius to conduit radius. Our findings potentially impact the dangerous
transition from ‘touristic’ Strombolian activity to more-hazardous paroxysmal activity. |
|
|
|
|
|
|