Hellisheiði & Nesjavellir (Iceland)
The Hengill volcanic system in SW-Iceland includes a 60-70 km long NE-SW trending fissure swarm and a central volcano. Within the system two high-temperature geothermal fields have been exploited for electric and thermal power production at Hellisheidi on the southern side and Nesjavellir on the northern flank. The tectonic setting of the Hengill volcanic system has been described as a triple junction, formed by the interaction of two transtensional rift zones, the Reykjanes Peninsula and the Western Rift Zone, with the South Icelandic Seismic Zone. The Hengill volcanic system is mainly composed of basaltic hyaloclastite ridges formed during glacial periods, and basalt lava series from interglacial periods. Drill cuttings from the southern part of the Hengill central volcano reveal that pillow basalt and hyaloclastite dominate the strata down to 800-1000 m below sea level, while lava sequences are more prominent at depth. In Nesjavellir, a dominant accumulation of hyaloclastite down to approximately 400 m b.s.l. is present. Below that depth basalt lavas dominate.