Introduction

A significant but volumetrically minor amount of dominantly basaltic volcanism occurs within plates as linear chains of volcanoes that grow older in the directions of plate motion (Wilson 1963; Morgan 1971). Examples of this style of volcanism are the Hawaiian–Emperor chain in the Pacific, the Yellowstone–Snake River plain in the western United States, and the Ninetyeast Ridge in the Indian Ocean. These volcanic tracks appear to form over hotspots, which are believed to be the surface manifestations of mantle plumes (Fig. 2.1). As mentioned in Chapter 1, Wilson (1963) suggested that hotspot tracks form as oceanic crust moves over relatively stationary magma sources in the uppermost mantle. Partial melting of plumes, when they intersect the mantle solidus near the base of the lithosphere, leads to large volumes of magma, which are partially erupted or intruded at or near the Earth's surface. In addition to hotspots, two broad mantle upwellings provide the return flow caused by subduction. The mantle upwellings elevate the Earth's surface up to a few hundred meters, and because they elevate the temperature of the uppermost mantle, they also cause minor but widespread melting, giving rise to volcanism and mafic underplating of the crust. Most of the major hotspots on Earth today occur within the mantle upwellings (Fig. 2.1).

It is commonly thought that mantle plumes and mantle upwellings begin life at the D″ thermal boundary layer just above the core–mantle interface.