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| Titel |
Brittleness and shear thinning: the explosive-effusive transition of lava dome experimentally investigated. |
| VerfasserIn |
B. Cordonnier, K. U. Hess, O. Spieler, L. Caricchi, Y. Lavallée, D. B. Dingwell |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250029290
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| Zusammenfassung |
| The dome building eruptions generate repeated dome failure and pyroclastic flows. They vary
in character and behavior from effusive domes to brittle pyroclastic events. The deformation
of highly crystallized dome lavas is key to understanding their rheology and to fixing their
failure onset. In this study we investigate the stress and strain-rate dependence on several
glasses and Mt Unzen dome lavas. Their rheology has been determined for temperatures from
900 to 1010Ë C and stresses from 2 to 60 MPa in uniaxial compression. Additionally,
tensile tests were performed on several volcanic products to track down the porosity
effect.
This survey aims to distinguish the Non-Newtonian effects affecting magmatic melts also
known as indicators of the brittle field. Towards our experiments we observed three
majors viscosity decrease types. Two were dependant and typical of the solid fraction
(IAVD & DAVD). The first is instantaneous and on the whole recovered during
stress release. The second is time-dependent and non-recoverable. The third and last
effect observed is attributed to the melt fraction and its self heating under stress
(VHE).
The IAVD is typical of that observed in previous experiments on crystal-bearing melts.
On crystal free melts, this viscosity decrease is observed at much lower magnitude. We infer
that the crystal phase responds elastically to the stress applied and relaxes once the load
is withdrawn. The DAVD appears more complex and this regime depends on the
stress (and/or strain-rate) history. We distinguish four different domains: Newtonian,
non-Newtonian, crack propagation and failure domains. Each of these domains expresses
itself as a different regime of viscosity decrease. Due to stress localization, cracking
appears in crystal-bearing melts (intra-phenocryst and/or the in the melt matrix)
earlier than in crystal-free melts. For low stresses, the apparent viscosity is higher
for crystal-bearing melts (as predicted by Einstein-Roscoe equations). However,
while the stress (or strain rate) increases, the apparent viscosity is decreasing to
that of the crystal-free melt and could be even lower if viscous heating effects are
involved.
Through this survey we distinguished apparent viscosity variations linked to the crystal
network and/or the glass. Most of them are becoming relevant close the relaxation time scale
of the material and may initiate the material failure.
More important, the crystalline phase is commonly believed to increase the
viscosity according to the Einstein-Roscoe equations. Indeed, those equations are
confirmed here for low stresses and strain rates. However, more importantly, the
presence of the crystalline phases results in an apparent viscosity that becomes
strongly stress and strain-rate dependent. Einstein-Roscoe overestimates this apparent
viscosity by several orders of magnitude. This study demonstrates the dominance of
non-Newtonian rheology in understanding the extrusion of dome lavas at Mt Unzen. |
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