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| Titel |
Rheological controls on the eruption style and size of historical eruptions from Mt. Ruapehu, New Zealand |
| VerfasserIn |
Geoff Kilgour, Heidy Mader |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250090439
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| Publikation (Nr.) |
 EGU/EGU2014-4677.pdf |
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| Zusammenfassung |
| The physical state of a magma controls a range of igneous processes, from crystal
and bubble growth, the ability to exsolve gas, drive convection, and eventually
whether the magma erupts or stalls. During magma ascent, crystallisation and bubble
expansion significantly alter the physical state of a magma. Therefore, a quantitative
assessment of magma rheology through time could help to explain the style, duration and
size of eruptions. The aim of this work is to determine the rheology of magma
from storage to eruption using existing constitutive equations. An assessment of the
changing rheological properties of magma could provide insights into the generation of
seismicity, deformation, and gas efflux; all signals that are regularly monitored at active
volcanoes.
Regular eruptions have occurred at Mt. Ruapehu, New Zealand since 1830, yet all have
been small volume (< 0.001 km3). This work combines melt inclusion volatile contents with
geochemistry to track the physical state of the magma from its source region (300
MPa) to eruption. In doing so, we hoped to explain the size and style of Ruapehu
eruptions.
The rheology of Ruapehu magma has been determined using a combination of
thermodynamic models and rheological calculations. We used a thermodynamic model
(MELTS) to determine the composition of three, representative Ruapehu magmas from 300
MPa to ~ 30 MPa. The outputs of the model agreed with experimental data and
provided the crystal and bubble (assuming no gas loss) content, along with the melt
content and composition. We calculated the melt viscosity, and the relative effect of
bubbles and crystals, to quantify the rheology of the magma during ascent (under
assumed equilibrium conditions). The crystal content at which a yield strength would
develop, and therefore the point at which a magma would likely stall, has been
constrained to ~ 0.3, which is marginally higher than the crystal content of erupted
scoria.
Historical eruptions from Ruapehu were H2O-undersaturated and as a consequence,
crystallisation and bubble growth were suppressed until the magma reached saturation, at ~
100 to 50 MPa. From this work, we suggest that small-volume magmas (< 0.001 km3) at
Ruapehu are more likely to erupt compared to magmas of a similar composition that are
H2O-saturated. This partly explains the propensity for small-volume phreatic and
phreatomagmatic eruptions at Ruapehu (in the presence of Crater Lake). |
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