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3.6 Magma Dynamics and Volcanic Processes

Igneous rocks contain many clues to their evolutionary history, such as xenoliths, phenocrysts, and vesicles. The morphology of volcanic deposits and the size and shape of the contained fragments can yield information on the mechanism of eruption. Glass in volcanic flows can be used to extract cooling histories for a range of different compositions. The projects described in this section combine field and laboratory studies in an integrated approach to understanding the transport, contamination, eruption and cooling behavior of magmas.

Exotic materials carried to the Earth's surface by mantle derived magmas give clues to the composition of the mantle and crust though which the magma passed prior to eruption. Melting of hydrous minerals, such as amphibole, contained in mantle xenoliths can be used to obtain information on the pressure and temperature history of the xenoliths during transport and more importantly allows an estimation of the time between entrainment of the xenoliths and eruption.

In many magmatic systems, there is chemical disequilibrium between the magma and its surroundings. This disequilibrium is the driving force for mineral dissolution which may lead to changes in the composition of the magma. The rate at which material dissolves in a silicate melt depends on its composition as well as the composition of the dissolving crystals and the geometry of the crystal - melt system. Experiments that model the dissolution process show that the geometry of the crystal - melt system is of fundamental importance in controlling the rate of dissolution and consequently the degree of contamination of the magma.

When magma approaches the Earth's surface, decompression leads to the exsolution of volatiles. In the case of highly viscous silica-rich magmas, this can lead to extremely hazardous explosive eruptions. Several ongoing projects are examining the mechanisms by which bubbles form in magmas during decompression, the role of crystals during bubble formation and magma fragmentation and the speed at which magma fragments. These are important to our understanding of the hazards posed by volcanic eruptions.

Another parameter that is required for volcanic hazard assessment is melt viscosity. There is already a predictive model for the viscosity of calc-alkaline lavas. However this model is not applicable to alkaline lavas such as those erupted from the volcanoes in Southern Italy. An on-going project is working toward a predictive model for alkaline lavas that will be used in further developing eruption models.

Once magma has been erupted, it begins to cool. The rate at which lava cools will affect its ability to flow and therefore may control the degree of hazard posed by flows of different composition. Two studies of glass-bearing lava flows using relaxational geospeedometry show how the composition and mode of eruption of the lava affect the cooling rate.

Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Deutschland
Tel: +49-(0) 921 55 3700 / 3766, Fax: +49-(0) 921 55 3769, E-mail: bayerisches.geoinstitut(at)