Tectonic position

Tectonic settings of Iceland. Click to enlarge

Hengill is one of the highest mountain in the region of ReykjavÝk, Icelandĺs capital. It is the central volcano of the homonymic volcanic zone Hengill, composed of a crater row and a large fissure swarm and located on the western part of the Reykjanes Peninsula, South West Iceland. It is set in the continuation of the Mid-Atlantic ridge (Reykjanes Ridge) at the triple junction of the Reykjanes Peninsula volcanic zone, the Western volcanic zone and the South Iceland Seismic Zone. This volcanic zone has a 100km long NE-SW axis and is 3 to 16 km large. It extends from Selvogur in the south to ┴rmannsfell in the north.

Active central volcanoes on the Reykjanes peninsula, South West Iceland.Click to enlarge



Petrological characteristics  Up

According to Ari Trausti Gu­mundsson.(2001), the Hengill area is mostly composed of tholeiite (54%) and olivine tholeiite (43%), he mentions also the presence of picrite (1%) intermediate rocks (less than 2%) and acidic rocks (less than 1%).



Structure  Up

        Volcanic

Hengill volcano is a hyaloclastite and diorite mountain, 803 m above sea level. Mont Hengill, as it was delimited by Orkustofnun in its GIS, has a 15 km long NW-SE axis and a 25 km long NE-SW axis. In the Hengill-Grensdalur area three other volcanic systems have been identified : the Grensdalur volcano, the Ílkelduhßls area and the Husmuli shield which are considered as either dormant, either extinct. Tomographic studies have shown the presence of a few cubic kilometres low-velocity body, 2-4 km underneath the northern part of Hengill central volcano which would contain partial melt.

The volcano might develop a caldera with time but yet subsidence has affected its shape. The area experiences subsidence due notably to the rifting activity of the area. The total subsidence that may have affected the area has been calculated to be more than 200 m since Hengill has started building up. The youngest part is the main graben that intersects the mount, it is 80 m deep and 5-6 km long. The craters row that was counting the highest summit of the complex before are located in this subsidence area and Skeggi located out of the subsidence area is now the highest summit.

Hengill is at a triple junction, therefore it experiences a rather high seismic activity. Tectonic extension and shear cause seismicity related strike-slip and normal faulting. Rifting events are mainly associated with crustal dike propagation. The first volcano-tectonic episode was described by the vicar of Ůingvellir. Between 1783 and 1789 several events (volcanic and tectonics) occurred in the Hengill triple junction leading to the formation of graben, gashes, uplifts and subsidences. Between 1993 and 1998, another major volcano-tectonic episode took place in the Hengill volcanic zone, it was driven by intrusion at 7 km depth. A quite uniform uplift was observed (2cm/yr) while the seismic records were showing episodic activity. Striking faults as well as normal faults were identified.

Hengill volcanic zone possesses several large and powerful geothermal areas. The Hengill geothermal field is the second largest in Iceland. It is well-know for hiking but also because a power plant was implanted at Nesjavellir in the northeastern perimeter of the Hengill central volcano in 1990 providing first the cities with thermal energy (space heating and bathing) and now since 2005 with electrical energy (120 MW). A new geothermal plant is planned on Hellishei­i, south west of the volcano.



Eruptive history  Up

Geological settings of the Rekjanes Peninsula : holocene lava flows and bedrocks. Click on the map to enlarge it.

The oldest sign of eruptive activity of this volcanic complex are the low highlands in Hverager­i which are 800 000 years old, moreover the considerable amount of palagonite and the 4-5 old dyngjuhraun show that this volcanic zone was active until late in the last part the last ice age. Geothermal activity is still important there.
The presence of stapi, palagonite or pillow lava mounts (e.g. Skar­smřrarfjall) are the sign of the volcanic activity of Hengill during glaciations, while lava flows show its activity during interglacial periods and historical times.
The two latest eruptions occurred about 5000 BP and 1900 BP (radiocarbon dated). An eruption in the Hellisheidi area once thought to have occurred around 1000 AD at the time of a meeting of the Icelandic parliament at Thingvellir is now known to have occurred at a vent about 5 km away in the Brennisteinsfj÷ll volcanic system.



Monitoring  Up

Because of its particular location at the triple junction, Hengill recieves a special attention from the Icelandic Meteorological Office (http://hraun.vedur.is/), 3 GPS are taking measures in the area (HlÝdardalsskˇli, Hverager­i and Ílkelduhßls) recording displacements in addition to meteorological data.
The Orkuveita ReykjavÝkur (http://www.or.is/) which administrates the Nesjavellir power plant, has been organizing extensive researches and drillings in the Hengill area to plan the new power plant. These researches participate to supervision of Hengill volcanic zone.
Finally the University of Iceland (http://www2.norvol.hi.is/) has also been doing some GPS measurement campaign in the Hengill volcanic zone.












Other webpages on this volcano  Up

Icelandic Meteorological Office

Orkuveita Reykjavikur

http://www.volcano.si.edu/



Bibliography : publications on this volcano  Up

Clifton, Amy E. et al., 2002. Surface effects of faulting and deformation resulting from magma accumulation at the Hengill triple junction, SW Iceland, 1994-1998. Journal of Volcanology and Geothermal Research 115, 223-255.

Foulger, G. R.; Toomey, D. R., 1989. Structure and evolution of the Hengill-Grensdalur volcanic complex, Iceland: Geology, geophysics, and seismic tomography. Journal of Geophysical Research, Volume 94, Issue B12, 17511-17522

Gu­mundsson, A. T., 2001. ═slenskar eldst÷­var. Vaka-Helgafell, ReykjavÝk, 320 pp.

Hersir, Gylfi P., Bj÷rnsson, G. and Bj÷rnsson, A., 1990. Eldst÷­var og jar­hiti ß HengilssvŠ­inu : jar­e­lisfrŠ­ileg k÷nnun. Orkustofnun, ReykjavÝk, 93 pp.

Hardardottir, V. ,1983. The petrology of the Hengill volcanic system, southern Iceland Master thesis. Dept. of Geol. Sci., McGill University, Montreal, 260 pp.

═varsson, G., 2004. Hengillinn me­ augum jar­frŠ­ingsins = Hengill through the geologist's eyes. Orkuveita ReykjavÝkur, ReykjavÝk, 44 pp.

Krumbholz, M. et al., 2006. Fluid transport by faults and fractures in the Hengill Volcano, Southwest Iceland. Geophysical Research Abstracts, Vol. 8, 03725.

SŠmundsson, K., 1966. Vulkanismus und Tektonik des Hengill-Gebietes in SŘdwest-Island. ReykjavÝk, 105 pp.

Saemundsson, K.,2006. The 1789 Rifting Event in the Hengill Volcanic System, SW-Iceland. American Geophysical Union

Thordarson,T., Larsen, G., 2007. Volcanism in Iceland in historical time: Volcano types, eruption styles and eruptive history.Journal of Geodynamics 43, 118ľ152.

Vogfjord, K S, Hjaltadottir, S and Slunga, R () Sub-surface Fault Mapping with Microearthquakes Reveals Volcano-Tectonic Interaction in the Hengill Region, Iceland



Maps and pictures of Hengill  Up

Tectonics of Iceland

Active central volcanoes on the Reykjanes peninsula, South West Iceland

Volcanic zones, geothermal fields and tectonism in Hengill zone.

Geological settings of the Reykjanes Peninsula, South West Iceland : Holocene lava flows and bedrocks

Earthquakes in the Hengill volcanic area, SW-Iceland

Tomographic images of the Hengill-Grensdalur area

Pictures of Hengill area on ═slandsmyndasafn (Iceland Image Library) Mats Wibe Lund

Pictures of Hengill taken by Helgi Jˇhann Hauksson