The Plate Tectonic Connection

Abstract

The Earth’s surface has four plate tectonic environments: divergent, convergent, and transform fault plate boundaries, and intra-plate locations. Volcanism is concentrated at plate boundaries or in linear belts or local magma floods, usually of basalt, in intra-plate locations. Andesites occur in all four environments. However, orogenic andesites are associated primarily with convergent plate boundaries, and this association is the significance of the “andesite line” frequently drawn around the Pacific Ocean. This line is a boundary seaward of which no orogenic andesites occur and is attributed to Marshall (1912) and Born (1933). Originally the andesite line was thought coincident with the boundary of the Pacific basin, but it is, instead, the western and northern boundary of the Pacific plate and the eastern boundary of the Juan de Fuca, Cocos, and Nazca plates. Sugisaki (1972) extended its meaning to include all plate boundaries converging at > 2.5 cm/yr.