It has been known for half a century that P-waves from distant earthquakes suffer a measureable delay on their way to seismic stations in Iceland. The delay takes place in the mantle beneath Iceland and is caused by anomalously high temperature compared to adjacent mantle material. A plume of hot but solid mantle material is hypothesized to rise beneath Iceland. This plume can be imaged in three dimensions by tomographic methods. A network of seismic stations covering the whole country records P- and S-waves that have travelled deep in the mantle. The depth of penetration is limited by the diameter of the network. For deeper penetration it is necessary to expand the seismic network to the ocean bottom around Iceland.
A famous picture from the front page of Nature, where the paper of Wolf et al. was published in 1997. A plume of low-velocity material can be seen to extend down to 3-400 km depth beneath Iceland.
Recent publications of the Institute of Earth Sciences on this topic:
Bjarnason, I.Þ. (2008). An Iceland hotspot saga, Jökull (58), 3-16.
Alfaro, R., Brandsdóttir, B, Rowlands, D.P., White, R.S., Gudmundsson, M.T. (2007). Structure of the Grímsvötn central volcano under the Vatnajökull icecap, Iceland. Geophysical Journal International, 168 (2), 863-876.
Bjarnason, I. Th., C. J. Wolfe, S. C. Solomon (1996). Initial results from the ICEMELT experiment: Body-wave delay times and shear-wave splitting across Iceland. Geophys. Res. Lett., 23, 459-462.
Bjarnason, I. T., P. G. Silver, G. Rümpker, and S. C. Solomon (2002). Shear wave splitting across the Iceland hot spot: Results from the ICEMELT experiment, J. Geophys. Res., 107(B12), 2382, doi:10.1029/2001JB000916.
Wolf, C. J., I. Th. Bjarnason, J. C. VanDecar, S. C. Solomon (1997). Seismic structure of the Iceland mantle plume.Nature, 385, 245-247.