Geothermal mapping in the Hrómundartindur area, SW-Iceland

Abstract

A geological map is the foundation on which a geothermal exploration programme needs to be based. All other data need to be interpreted in view of the observed or known geological features. During geothermal exploration, geological maps should emphasize young igneous rocks that could act as heat sources at depths, the distribution and nature of fractures and faults, and the distribution and nature of hydrothermal alteration. A geothermal map of the Hrómundartindur area, SW-Iceland is presented. Hrómundartindur is part of the Hengill active central volcanic complex. The aim of the study was to familiarize the author with geothermal mapping in young volcanic terrain, including tectonic structures, hydrology and hydrothermal manifestations. Field observations were interpreted in view of the geothermal manifestations mapped. The most prominent manifestations occur in the northwest part of the field area, apparently connected to the volcanic fissures of two volcanic formations, the older Hrómundartindur eruptive fissure of late Pleistocene age and the much younger Tjarnarhnúkur volcanic vent/fissure of early Holocene age. The widespread hydrothermal manifestations range from slightly altered host rocks (soil, scree, Hrómundartindur hyaloclastite) to host rocks completely transformed to hydrothermal clays. These manifestations are active (hot) and extinct (cold), the extinct ones bearing witness to former activity and indicated separately on the geothermal map. Isotherms were mapped within the warm (>15°C) and hot (>50°C) fields, within which many hot springs, mud pits, steam vents and warm springs occur.