HEAT-FLOW AND GEOTHERMAL PROCESSES IN ICELAND

Heat flow values, derived from temperature measurements in shallow bor eholes in Iceland, vary substantially across the country. The near-sur face temperature gradients range from almost 0 to 500-degrees-C/km. Th e thermal conductivity of water-saturated rocks varies from 1.6 to 2.0 W/m-degrees-C. The temperature gradient in Iceland is mainly dependen t on four factors: (1) the regional heat flow through the crust, (2) h ydrothermal activity, (3) the permeability of the rock, and (4) residu al heat in extinct volcanic centers. As Iceland is mainly made of basa ltic material the radiogenic heat production is almost negligible. The thermal conductivity is, on the other hand, mainly influenced by the porosity of the rock; it increases as the porosity decreases. Iceland is made of sequences of flood basalts that formed within the volcanic rift zone-a continuation of the axis of the Mid-Atlantic ridge-and sub sequently drifted sideways. Fresh basaltic lava is usually highly poro us (30%) and fractured, and heat is mainly transported by convection. Therefore, a very low or even no temperature gradient is observed at s hallow levels within the volcanic rift zone. As the basalt becomes bur ied the pores close due to lithostatic pressure and formation of secon dary minerals. Below 500-1000 m depth in an uneroded lava pile, the he at is mainly transported by conduction. In the lowlands and valleys of Iceland outside the volcanic rift zone, 1000-1500 m of the original l ava pile has been eroded, leaving thermal conduction as the most impor tant heat transport mechanism. The regional temperature gradient has b een measured in drillholes in dense and poorly permeable rocks away fr om the geothermal fields. The results show that the temperature gradie nt varies from 50 to 150-degrees-C/km. The highest values are found cl ose to the volcanic rift zone and the gradient decreases with distance from the spreading axis. This result is mainly based on numerous shal low boreholes (60-500 m) but in some cases the results have been confi rmed by 1000-2000 m deep boreholes. By extrapolating the temperature g radient down and assuming a slight increase in the thermal conductivit y with depth, partially molten material can be expected at 10-30 km de pth. Geothermal reservoirs are quite common in Iceland. They are prima rily convective systems associated with young tectonic fractures, carr ying heat from several kilometers depth towards the surface. Within th e volcanic rift zone the heat sources seem to be hot intrusions; away from it, the heat is mined from the underlying crust. The highest valu es of the near-surface temperature gradient are found above the geothe rmal systems. Drilling of 30-60 m deep boreholes is therefore a powerf ul tool for geothermal prospecting outside the volcanic rift zone. In the deeper parts of the geothermal systems, a gentle temperature gradi ent is observed and the temperature is lower than would be expected fr om the regional temperature gradient. This is due to geothermal convec tion which has removed heat from the deeper parts of the geothermal sy stems. Convective geothermal systems must have a downflow part, where cold water is flowing down into the deeper parts of the geothermal sys tems along fractures. Such downflow areas have been observed and appea r as areas with an anomalously low temperature gradient.