ABSTRACT
The awesome power and beauty of volcanic eruptions epitomise the global magma/igneous rock system. Historical records from many cultures contain descriptions of volcanoes and volcanic eruptions. The latter are seismically and volcanically active zones of plate convergence where oceanic lithosphere is being subducted. The crust and the 10 to 50 km thick uppermost layer of the mantle form a single, relatively strong unit called the lithosphere. The mantle makes up 84 per cent of the volume and 67 per cent of the mass of the Earth. On Earth the cyc1e of plate growth, suboceanic cooling, subduction and reheating accounts for most of the heat lost by the mantle. Continental drift, and the more comprehensive plate tectonics paradigm, were both conceived in the twentieth century. Continental rifts and paleorifts are typically linear zones where the entire thickness of the lithosphere has been ruptured under extension.
