Helgrindur

The Helgrindur volcanic system on the Snæfellsnes Volcanic Zone has been weakly active in the Holocene, only producing two small basaltic lava flows that are more than 1100 years old. The Helgrindur system consists of a central volcano and fissure swarm. The central volcano is heavily eroded and is marked by a ridge of sub-glacially erupted basaltic hyaloclastite peaks (986 m a.s.l., no glacial cover) but the Holocene products are small basaltic lava flows. The NW-SE trending fissure swarm is 30 km long and its northwest tip extends offshore. Little is known about eruption frequency. Cold carbon dioxide springs are found in a few locations and low temperature geothermal activity occurs locally in the vicinity of the fissure swarm. The central volcano is a ridge composed of several peaks, Helgrindur, with relatively shallow passes in between. The highest peak reaches 986 m a.s.l and there is no glacial cover. The volcanic products form a continuous cover in and around the centre itself but are extensively eroded. Within the fissure swarm the basaltic formations form more and less continuous cover on top of the eroded Neogene (Tertiary) bedrock. Its northwest tip extends offshore. Open fissures or faults that have been active during the Holocene are extremely few and the movements are small. The two Holocene lava flows are relatively small and the craters are low relief spatter cones. Nothing is known about the plumbing system but the geographical change in chemistry of the products seems to point to a local ascent of basaltic magma from depth, rather than lateral flow of magma from the centre along the fissure swarm. The magma appears to ascend along reactivated old faults in the upper crust. Two relatively small basaltic fissure eruptions producing lava flows have occurred during the Holocene but none of historical age. The larger of the two covers 7-8 km2 and extended about 3 km from source. Minor rhyolitic eruptions have occurred, mainly during glacial periods (pre-Holocene), but their age is unknown. Most of the basaltic eruptions have been subglacial. No eruption has taken place during historical times and no lava flows are of Holocene age. Some interglacial lava flows are dated back to the last interglacial period (110,000 years ago). Intensive volcanic activity has occurred during glacial periods and is expressed as hyaloclastite mountains. No known seismic activity. No geothermal activity apart from a few mineral springs. Produced by the central volcano during phreatomagmatic (subglacial) eruptions. No Holocene occurrences. Volumes are not known. Subglacial explosive eruption of rhyolitic magma. The most recent eruption of this type occurred during the late Pleistocene ( 11,500 years ago). Frequency of this scenario is estimated as once every 100,000 years. Due to lack of experience of such events in Iceland little is known about the warning period length but increased seismicity would definitely be a precursory sign. A plinian or subplinian eruption column could reach 10-20 km height and intense tephra fall could last a few hours, depositing tephra with volume of ~0.1 km3. Depending on the wind direction, ash clouds could cause disruption on air traffic routes. Sporadic explosions and effusive activity could follow producing highly viscous lava (~0.2-0.3 km3) flowing relatively a short distance (2-5 km). Helgrindur (also referred to as Lýsuskarð) is a relatively well known system and its surface geology has been mapped. The knowledge of the chemical variation is relatively good. Tephra layers have not been well mapped. Jóhannesson H. 1994. Geological map of Iceland, sheet 2, 1: 250 000 . West-Iceland, second edition. Icelandic Museum of Natural History and Iceland Geodetic Survey, Reykjavík. Jóhannesson H. and Sæmundsson K. 1998. Geological Map of Iceland. 1:500 000 . Tectonics. Icelandic Institute of Natural History, Reykjavík. Sigurdsson H. 1966. Geology of the Setberg area, Snæfellsnes, western Icelan