Andesite Magmas, Ejecta, Eruptions, and Volcanoes

Studies of the tectonic significance, petrography, and chemical composition of andesites have been more common than detailed investigations of the physical properties of andesite magma, the volcanologic characteristics of its eruption, or the geologic history of its eruptive sites. Indeed, these features sometimes are regarded as of little significance for studies of andesite genesis, as opposed to near-surface phenomenology. However, as will be shown in this chapter, the volcano-logic and geologic contexts of andesite seem to favor its origin by low-pressure differentiation of basalt, and the physical properties of andesite magma probably control these differentiation processes.