Being able to picture Earth features far beneath the surface is what makes seismic tomography such an exciting tool, even though it is in its infancy. It shows variations in the velocity of P and S waves in 3-D. Regions of fast waves are likely be cooler than those in which wave speeds are relatively slow. The detail depends on the spacing between seismic recorders and the distribution of natural seismic events, whose interactions produce tomographic data. Despite being rarely affected by seismicity themselves, the British Isles have a remarkably dense network of seismic stations that was developed for research. Given arrival times at the different stations by waves from earthquakes that occurred over a wide range of epicentral angles from the British Isles, it becomes possible to probe in detail what lies beneath. Exploiting the potential to the full, a group of British and US geophysicists has shown that the ‘British’ mantle is far from boring (Arrowsmith, S.J. et al. 2005. Seismic imaging of a hot upwelling beneath the British Isles. Geology, v. 33, p. 345-348).
Down to a depth of 600 km, Britain is underlain by a series of significantly slow and fast mantle ‘blobs’. The seismically slow, probably warm mantle zones seem to follow large features last active during Early Palaeogene magmatism that affected the Hebrides and Northern Ireland, and roughly parallel the 60 Ma dyke swarms that radiate from these centres. They also correlate with regions of anomalously high gravity. It seems highly likely that both features are long-lived relics of a spur of the still active Iceland plume that is intimately associated with spreading on the Mid-Atlantic Ridge. The warm zones also underlie those parts of the British Isles that were most affected by uplift and erosion during the Cenozoic: as much as 3 km in the case of the Irish Sea. Such areas also focused extension at the time of the magmatism, and they are still most affected by minor seismicity.
Estimates of the magnitude of the temperature anomaly associated with the slowing of P-waves are as much as 200 °C above ambient mantle temperature; sufficient to be associated with partial melts. That Britain might once more have active volcanoes is highly unlikely, and the anomalies are probable parts of the Iceland plume system that became trapped beneath zones of crustal thinning. Their loss of heat is sufficiently slow for them to have bolstered areas of uplift and erosion for tens of million years. There is even a chance that some form of convection might yet be going on.