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| Titel |
Study of the August 2006 tephra fall deposit of the Tungurahua volcano, Ecuador : Insights into the eruption dynamics |
| VerfasserIn |
Julia Eychenne, Jean-Luc Le Pennec, Liliana Troncoso, Patricio Ramon, Claude Robin |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250035736
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| Zusammenfassung |
| Tungurahua, one of Ecuador’s most active volcanoes, started a new eruptive period in 1999.
This activity continues today, with phases of fluctuating magnitude (VEI 1 to 3) and intensity;
it has led to several evacuations of the population. The paroxysmal phase occurred in August
2006 and resulted in a volcanic column which rose 15 km above the crater, associated with
pyroclastic flows and surges which slid down the southern, western and northern flanks of the
volcano.
Over much of the dispersal area (7 to 70 km from the vent) the thickness of the fallout
deposits was measured at 67 localities, and 22 samples were also collected. The thickness
varies from 10 to 0.2 cm. Grain size analyses were carried out from -5 Ï (31.5 mm) to 10 Ï (1
μm), using sieving method up to 63 μm (4 Ï) and laser diffraction method down to 63 μm
(with a Malvern Mastersizer instrument). Maximum scoria sizes were calculating at the 22
sampling locations, measuring the 3 principal axes of the 5 largest clasts. Pyroclastic flow and
surge deposits were also sampled, and their relations with the fall deposit were studied in the
field.
Median grain sizes vary from -0.5 Ï to 3.0 Ï in the studied area. Sorting is unusually poor
for a tephra deposit with values from 1.8 to 4.3. All the samples show a bimodal grainsize
distribution, with one mode coarser than 2 Ï and an other mode finer than 4 Ï. Sorting and
median diameter of the coarse subpopulation correlate with thickness of the deposit
and distance from the vent, whereas in the case of the fine subpopulation these
parameters are constant. A deposit volume of about 40 million m3 was calculated
using an exponential or a power-law thinning rate. After Carey and Sparks’ model
(1986) the eruptive column height was estimated. A column height Ht of 15 km was
obtained, which fits the satellites’ data. The eruption is a VEI 3 subplinian phase (Pyle,
1989).
Multimodal grain size distributions suggest different settling behaviors during the
eruption. The coarse grain size population probably reflects the normal, weight-dominated
settling of clasts from the volcanic plume. Two processes could explain the significant fine
ash enrichment. The first one is rain flushing (precipitation of fine ash to the ground with
rain), as at some localities the eruption coincided with rain falls. The second one is a
contamination by fine-grained ash elutriated from nearby pyroclastic flows and surges. In
order to better understand these complex grain size distributions, grain size and componentry
analyses of pyroclastic surges samples are still ongoing, whose results will be compared with
those from the tephra fall deposit. |
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