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Náttúrufræðingurinn - 1988, Side 51

Náttúrufræðingurinn - 1988, Side 51
SUMMARY Geothermal activity in the tidal zone and below sea level around Iceland by Jón Benjamínsson Marine Research Institute Skúlagata 4 IS-101 REYKJAVÍK Iceland Geothermal activity in the form of hot springs, steaming ground and gas vents is common in Iceland but the geothermal ac- tivity under the sea around Iceland has not been accounted for. Geothermal gra- dients in Iceland tend to be low in the ol- dest rocks in Eastern and Western Iceland but increasing towards the volcanic zones in the central region. The geothermal wa- ter, originally rainwater, sea water or a mixture of these two, percolates through the bedrock. It is warmed by the under- lying large volcanic heat source and rises through faults and cavities in the bedrock because of the lowering of its specific gravity. This paper reports 53 locations where geothermal activity is found or suspected to be present below sea level or in the tid- al zone. Most of these sites are in the Western Fjord region. No submarine ac- tivity has been proven to be present off the east- and south coasts. Twelve of the sites cannot be seen directly but are in- ferred from indirect evidence. The deep- est signs of geothermal activity are found at Hörgárgrunn in Eyjafjörður at 97 m b.s.l. and on the Kolbeinsey Ridge at 90 m b.s.l. The highest temperature mea- sured, 100°C, was in Öxarfjörður. There, as on the Kolbeinsey Ridge, the geother- mal area is inferred to represent a high temperature system. The others reported are low temperature systems. One geo- thermal site near Reykjavík seems to have sunk to a greater depth over the years. This finding is in agreement with the ob- served trend in sea level changes in the Reykjavík area. 169

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