7.09 - Hot Spots and Melting Anomalies

Abstract

Volcanic hot spots are regions of intraplate volcanism or excess volcanism at plate margins that are not adequately explained by the plate tectonic paradigm. Hot spots have been associated with topographic swells, distinct radiogenic isotope characteristics, flood basalt events, and age-progressive volcanism, consistent with the movement of plates over relatively fixed point sources of magma. We classify at least 13 chains as having long-lived (>50My) age progressive volcanism, at least eight chains with short-lived (<20My) age progression, and a number of hot spots with complex or no age progression. Broad topographic swells commonly surround hot-spot volcanoes; they tend to subside with age and are generally absent around volcanoes older than ∼50–70Ma. The large Pacific chains appear to be formed by hot spots that are nearly stationary with respect to each other, but prior to ∼50Ma, could have moved with respect to the geomagnetic poles and the Atlantic and Indian hot spots. The oldest volcanism of some, but not all, hot-spot tracks is in the form of voluminous eruptions characterizing large igneous provinces. The connection between volcano chains and large igneous provinces is most clear for cases in which flood volcanism starts on or near continental margins, but the connection is weak or non-existent for other flood basalt provinces. The Sr–Pb–He isotope geochemistry of oceanic islands, with few exceptions, is significantly more diverse than, and distinguishably different from that of mid-oceanic ridges. Seismic studies of some prominent hot spots reveal anomalously low seismic wave speeds in the upper mantle and unusually thin mantle transition zones, which together suggest elevated mantle temperatures. Studies of the dynamical origin of hot spots are complemented by laboratory and theoretical modeling. Melt can be generated by variations in temperature, composition, and mantle upwelling. While there is a variety of petrologic and geophysical evidence for the magmatism at hot spots such as Hawaii, Galápagos, and Iceland being caused by large increases in mantle potential temperature, significant amounts of water or more mafic lithologies could allow for a lower temperature increase. Predictions of basic fluid dynamics and observations of topographic swells provide strong arguments for swells originating from buoyant upwellings below many hot-spot regions. These upwelling features are expected to be abundant in the convecting mantle but their forms, evolution, and depths of origin are likely to be much more diverse than originally conceived. Causes for hot spots that may originate above the lower mantle remain poorly understood. Unraveling the dynamics of both shallow and deep sources will require considering the interplay among factors such as thermal buoyancy, compositional buoyancy, variable fusability due to lithologic heterogeneity, and strongly varying rheology in the Earth’s mantle.