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| Titel |
Insights into the sub-plinian eruption of a basaltic monogenetic volcano using different analytical methods |
| VerfasserIn |
Simone Jordan, Jean-Luc Le Pennec, Lucia Gurioli, Olivier Roche |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250101240
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| Publikation (Nr.) |
 EGU/EGU2015-346.pdf |
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| Zusammenfassung |
| The basaltic monogenetic La Vache and Lassolas cone complex in the Chaîne
des Puys, France, erupted about 8600 years ago in a sub-plinian event, producing
simultaneously a >25km2 widespread tephra fall deposit, two scoria cones and ca.
15km long lava flow. By detailed sampling and studying of the tephra deposit we
were able to determine variations within the 8m thick, mostly massive proximal
sections. Shape analysis of the coarse ash fraction was performed using the G3
morpho-grainsizer at the Laboratoire Magmas et Volcans. This instrument has the
great advantage to permit analysis of the shape parameters of up to 3000 particles,
depending on the grain size, in a relative short time frame of half an hour. With this new
technique it is now possible to analyse a large number of samples with a statistically
significant number of particles. We present here the results of the analysis of two
different particles size classes (250-300μm and 1000-1180μm). The results indicate
variations in the shape parameters that correlate with variations in the grain size
throughout the eruption sequence that were identified using grain size analysis.
Detailed componentry analysis reveal that these variations in the shape parameters may
be caused by changing amounts of three different clast types: sub-rounded, lithic
granitic clasts, spherical, non-vesicular juvenile clasts, and ragged, vesicular juvenile
clasts. The two different juvenile clast types raise questions about their formation
processes. For further characterisation of the juvenile clasts density measurements were
performed on the larger size fraction, displaying densities of 0.7-2.2 g/cm3 for
the spherical, non-vesicular clasts and densities of 0.5-1.9 gr/cm3 for the ragged,
vesicular clasts. Apart from the density values around 2.2 g/cm3of some of the
spherical juvenile clasts that are considered to be the result of large phenocryst cores
inside the spherical clasts, both juvenile clast types are similar in their range of
densities. The observed differences in the vesicularity of the clasts imply that the
non-vesicular spherical clasts are micro-vesicular. However, further textural analysis are
needed to answer the question of the formation process of these clasts. This study
shows that shape analysis in combination with detailed tephra studies can reveal
subtle variations in massive deposits and can give insights into eruption dynamics. |
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