An important method for imaging large-scale structures in Earth's interior is seismic tomography, by which the area under consideration is "illuminated" from all sides with seismic waves from earthquakes from as many different directions as possible; these waves are recorded with a network of seismometers.
Studies using the major and trace-element compositions of Icelandic volcanics [REF] showed that the source of present-day volcanism was about 100 °C greater than that of the source of mid-ocean ridge basalts.