The lava pile penetrated by the Iceland Research Drilling Project (IRDP) hole formed in an axial rift zone that remained active for at least the last 13 m.y. The stratigraphic sequence is formed of subaerial volcanics, and it is likely that volcanism was subaerial for several million years or even tens of millions of years prior to the formation of the IRDP sequence. The volcanics in eastern Iceland are tholeiitic, like all Tertiary sequences investigated in Iceland to date, and the lava extrusion rates in eastern Iceland are reported to be similar to those of Tertiary in northern and western Iceland, respectively. The IRDP hole was sited in a dyke swarm extending from the Breiddalur central volcano in the south. The IRDP hole was sited in a regional thermal anomaly with a gradient of about 80¿C/km that was found by a series of 100-m thermal gradient wells in eastern Iceland. Aquifiers of 48¿C were encountered at about 600-m depth, which the thermal gradient lowered considerably. This suggests that the regional thermal anomaly is caused by the flow of warm water at relatively shallow depths.