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My current project at University of South Florida (USF) involves field mapping, grain size analyses, and modeling of the 2450 BP eruption of Pululagua Volcano (Ecuador), which deposited a large volume of tephra in the Inter-Andean Valley, and greatly affected people living there at the time (the Cotocolao tribe).
Here are some results I presented in two recent scientific meetings, the American Geophysical Union Fall Meeting in San Francisco, California (December 2005) and the Cities on Volcanoes 4 in Quito, Ecuador (January 2006).
Introduction
Pululagua Volcano is part of the active Andean Volcanic Front of Ecuador (Fig. 1-A) and is located at 15 km north of Quito (Fig. 1-B). The explosive activity leading to the formation of an irregularly shaped caldera (Fig. 1-B) occurred as a series of volcanic episodes during which a total estimated 5-6 km3 (DRE) of hornblende-bearing dacitic magma was erupted. The general stratigraphy of the Pululagua products (Figure 1-C), as well as circular isopach (Fig. 2) and isopleth maps, were presented by Papale & Rosi (1993).
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Stratigraphy and grain size distribution
This study focused on the basal part of the products of Pululagua eruption, eruptive unit U1 (Fig. 1-C), with particular attention to the Basal Fall (BF) layer. We intend to use mainly the climactic phase of the eruption to validate the sedimentation code, TEPHRA. As a result, we differentiated more sub-units (Figs.3-5): (i) a basal grey ash (BGA), resulting from the opening of the vent (Papale and Rosi, 1993); (ii) a first fallout deposit (BF1), overlain by (iii) the main (climactic phase) fallout layer of the plinian eruption: BF2; (iv) BF3 fallout deposit and at the top of the sequence, (v) the White Ash (WA) fallout deposit. Our three stratigraphic sections are located along a south-east axis and are displayed on Figure 2. Note that the proximal section (Fig.3) shows at least two surge sequences interlayered within the BF deposits and are a consequence of Plinian column instabilities and collapses.
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The three samples PL19, PL24 and PL40 (location shown in Figure 2) have been sieved down to 4.0Φ to get standard grain size distribution, presented in Figure 6. As expected, the median Φ decrease and the sorting improve (increase in the slope of the cumulative curve) with distance to the vent. The very bad sorting of PL40 may be due to the influence of ballistics (4km from the inferred vent) or due to the sedimentation from the plume margin. |
Modelling
The goal of modelling is to use the isomass data to estimate conditions during the climactic 2450 BP eruption of Pululagua. We use the TEPHRA sedimentation model and nonlinear inversion techniques to search for best-fit eruption parameters, such as volume and eruption column height. Input and output parameters of the model are shown in Tables 1 and 2. The calculated tephra accumulation based on optimal parameters from the inversion is plotted against the measured tephra accumulation for each sample location in Fig. 12-A. The model underestimates tephra accumulation in proximal areas (the same pattern has been observed for Cerro Negro, Connor and Connor, 2005), whereas calculated accumulation is in good agreement with field data for medial and distal areas. The calculated isomass map (Fig. 12-B) shows the form of the deposit, not based on interpolation of sparse data, but rather based on a physical model of the eruption.
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The maximum column height predicted by the model is 35 km and is in good agreement with the maximum value calculated by Papale and Rosi (1993) of 36 km. The total volume of erupted material predicted by the model is 0.19 km3. This volume is significantly less than the 0.75 km3 proposed by Papale and Rosi (1993) for the entire BF layer, but is in good agreement with the 0.158 km3 volume calculated using Pyle’s (1989) exponential decay model for our field data (Fig. 13), assuming circular ispoachs.
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Conclusions
• Calculated tephra accumulation with the TEPHRA code shows good agreement with field data, even if proximal tephra accumulation is slightly underestimated.
• Computed eruption column matches maximum plume height calculated by Papale and Rosi (1993). Computed total volume erupted matches total volume calculation based on the field data using the exponential decay model (Pyle, 1989).
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