Influence of Magma Properties on the Dynamics of Explosive Eruptions

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Collaborative studies in physical volcanology and experimental petrology on products of explosive eruptions of El Chichon (1982) and Vesuvius (79 AD) have elucidated the dependence of eruption processes on magma properties. Variation in mass eruption rate during the 79 A.D. Vesuvius eruption produced complex compositional gradients in the erupted products, as two contrasting magmas were mixed during withdrawal from the chamber. The extent of mixing is closely linked to mass eruption rate, as proposed on theoretical grounds for compositionally-stratified reservoirs, and demonstrates that geochemical gradients in volcanic deposits can only be interpreted in conjunction with information about eruption dynamics. Modelling of the Vesuvius and El Chichon sequences using petrologically-inferred intensive parameters has shown that the evolution of eruptive processes is consistent with current models of eruption column behavior and that differences between events at the two centers can be attributed to contrasting volatile contents, crystallinity and viscosity of their respective magmas. New techniques in physical volcanology have now provided the means to calculate the intensity (mass eruption rate) of recent and ancient explosive eruptions. Analysis of the available global data base on intensity and magnitude (total volume of erupted magma) of explosive eruptions reveals a close correlation, suggesting a dependence of intensity on magnitude and in turn, on reservoir size. Eruptions with intensities in excess of 2x10-8 kg/s are invariably associated with major pyroclastic flow generation and caldera collapse. A correlation also exists between the runout distance of pyroclastic flows and the intensity of the preceding plinian phase. The predictive power of these observations is of great significance to volcanic hazards studies. This project will continue work in physical volcanology and experimental petrology and apply the results of previous studies to an investigation of intensity, magnitude, and magmatic composition of ancient explosive eruptions from the Cascade Range of the U. S. The study should contribute to an understanding of eruptive processes, magma evolution and hazards assessment of this region.