GEOTHERMAL MANIFESTATIONS AND STRUCTURES IN INNSTIDALUR AND MIDDALUR, SW- ICELAND

Geothermal activity is considerable in the Innstidalur and Middalur valleys of the Hengill central volcano. Detailed geological, structural and geothermal mapping was carried out, presented by maps in the scale of 1:15,000. This included mapping of 165 springs, by measuring their temperature, pH and conductivity, and estimating flow rate. A description is given of hydrothermal manifestations in the area. The main geothermal activity on the surface is in the southeast part, in Middalur valley, and in the northwestern part of the area. The geothermal activity relates to a volcanic fissure and faults trending close to NE-SW. By activity, the geothermal manifestations can be divided into three main areas. In the northwestern part, geothermal activity is characterized by boiling water, mud pools and steaming ground, with some sulphur fumaroles, altered grey to brownish rock. In the central part of the field, extinct alteration is evidenced by brownish grey altered hyaloclastites with smectitic clays and calcium carbonates as main alteration minerals. In the third area, in Middalur valley, it is characterized by hot ground, mud pools, steam vents and some travertine deposits. Cold water springs in the area are located at an altitude of 350-500 m a.s.l., above the geothermal alteration zone. In some cases, the cold water has passed through heated or steaming ground and has been heated up. The proposed geothermal model of the area shows the geothermal activity associated with active fissures and faults.