Heat flow and geothermal processes in Iceland

Abstract

Heat flow values, derived from temperature measurements in shallow boreholes in Iceland, vary substantially across the country. The near-surface temperature gradients range from almost 0 to 500°C/km. The thermal conductivity of water-saturated rocks varies from 1.6 to 2.0 W/m°C. The temperature gradient in Iceland is mainly dependent on four factors: 1. (1) the regional heat flow through the crust, 2. (2) hydrothermal activity, 3. (3) the permeability of the rock, and (4) residual heat in extinct volcanic centers. As Iceland is mainly made of basaltic 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 subsequently drifted sideways. Fresh basaltic lava is usually highly porous (30%) and fractured, and heat is mainly transported by convection. Therefore, a very low or even no temperature gradient is observed at shallow levels within the volcanic rift zone. As the basalt becomes buried the pores close due to lithostatic pressure and formation of secondary minerals. Below 500–1000 m depth in an uneroded lava pile, the heat is mainly transported by conduction. In the lowlands and valleys of Iceland outside the volcanic rift zone, 1000–1500 m of the original lava pile has been eroded, leaving thermal conduction as the most important heat transport mechanism. The regional temperature gradient has been measured in drillholes in dense and poorly permeable rocks away from the geothermal fields. The results show that the temperature gradient varies from 50 to 150°C/km. The highest values are found close to the volcanic rift zone and the gradient decreases with distance from the spreading axis. This result is mainly based on numerous shallow boreholes (60–500 m) but in some cases the results have been confirmed by 1000–2000 m deep boreholes. By extrapolating the temperature gradient down and assuming a slight increase in the thermal conductivity with depth, partially molten material can be expected at 10–30 km depth. Geothermal reservoirs are quite common in Iceland. They are primarily convective systems associated with young tectonic fractures, carrying heat from several kilometers depth towards the surface. Within the volcanic rift zone the heat sources seem to be hot intrusions; away from it, the heat is mined from the underlying crust. The highest values of the near-surface temperature gradient are found above the geothermal systems. Drilling of 30–60 m deep boreholes is therefore a powerful tool for geothermal prospecting outside the volcanic rift zone. In the deeper parts of the geothermal systems, a gentle temperature gradient is observed and the temperature is lower than would be expected from the regional temperature gradient. This is due to geothermal convection which has removed heat from the deeper parts of the geothermal systems. Convective geothermal systems must have a downflow part, where cold water is flowing down into the deeper parts of the geothermal systems along fractures. Such downflow areas have been observed and appear as areas with an anomalously low temperature gradient.