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Referência Bibliográfica

PIMENTEL, A., SPARKS, R.S.J., PACHECO, J.M. (2007) - Emplacement dynamics of felsic lava flows from Terceira Island, Azores: a morphological analysis. IUGG XXIV General Assembly, Perugia, Itália, 02 - 13 de Julho (Poster).

Resumo

A physical model was developed to analyze the emplacement dynamics of thick felsic lava flows on Terceira Island (Azores). The model follows a simplified deterministic approach to describe, by a system of equations, the effects of the main governing factors during the lava emplacement.

We considered that lava is fed from an overpressurized magma chamber through a cylindrical conduit, in a laminar regime. The initial effusion rate is given by the Poiseuille flow law for magma with Newtonian rheology. In this simplified case, lava discharge rate at the vent follows an exponential decrease with time.

It was assumed that, during emplacement, the erupted volume of lava is related to flow parameters (such as flow width, length and thickness) by a shape factor. This assumption was also used to correlate the final volume of the lava flow with the actual lava dimensions, at present time. Based on experimental studies, it was considered that the width of a lava flow is a function of the initial effusion rate and the underlying slope.

The model also takes into account the effect of the complex rheology of lava, considering the apparent viscosity of the lava, which depends of the initial viscosity and follows a power-law to describe its variation with time and distance.

Finally, the Jefferys equation was used to incorporate the influence of the speed of the flow-front in the model as function of the physical properties of the lava and the underlying slope.

The physical approach presented here was used to model the emplacement of recent (< 20 ka) felsic lavas from Santa Bárbara and Pico Alto volcanoes on Terceira Island (Azores). Lava flows from both volcanoes are very similar in terms of chemical composition (65 - 70 wt% SiO_{2}), average crystal content (~ 12 vol%), and even in morphology. They reach a maximum of 2800 m in length, with widths ranging from 110 to 900 m and thickness of 15 to 70 m. The calculated volumes range from about 3x105 to 108 m^{3}.

Model solutions show a good correlation with the length range observed on the felsic lava flows, demonstrating that the proposed physical model is able to reproduce the actual dimensions of these thick flows. Modeling the emplacement dynamics of felsic lavas from Terceira also allowed to obtain a better understanding of the dominant eruptive conditions throughout their extrusion (e.g. initial lava viscosity and effusion rates).

This generic model can also be applied to other volcanic areas or even to lava flows with different compositions.

Observações

Anexos