| Hengill | |
| Elevation M: | 803 |
| Elevation Ref: | [1] [2] |
| Listing: | List of volcanoes in Iceland |
| Location: | Iceland |
| Map: | Iceland |
| Relief: | 1 |
| Map Size: | 200 |
| Label Position: | right |
| Coordinates: | 64.0856°N -21.3133°W |
| Type: | Shield volcano, fissure vents |
| Age: | 200,000 years ago |
| Last Eruption: | 150 CE ± 75 years |
| Easiest Route: | Hike |
Hengill (pronounced as /is/) is a volcanic table mountain situated in the south-west of Iceland, to the south of Þingvellir.
The volcano is still active, as evidenced by its numerous hot springs and fumaroles, but the last eruption occurred approximately 2,000 years ago, before the settlement of Iceland.
The mountain is south of Lake Thingvallavatn (Þingvallavatn in Icelandic), south-west of the Nesjavellir geothermal area, south of Þingvellir National Park and is the most prominent part of a volcanic region that extends to the coast.[1] South of the mountain towards the coast is the Hellisheiði geothermal area to its south-west, and the town of Hveragerði on Iceland's main ringroad, Route 1 about east of Reykjavík.[1] At the coast is the port of Þorlákshöfn.[1]
The mountain is part of the long Hengill volcanic system, which is part of the Reykjanes volcanic belt, and is a hyaloclastite massif of tuyas and tindars. The dominant lava in the Hengill volcanic system is tholeiitic basalt, but andesite and about rhyolite occur within the central mountain. The largest lava shield covers about . Four small lava shields are picrite basalt and one lava shield contains about of olivine tholeiite. It is located at the intersection with two other tectonic volcanic systems, the West volcanic zone of Iceland and South Iceland seismic zone, and is co-located to the plate boundary between the North American, Eurasian and Hreppar plates. This intersection is called the Hengill triple junction. Hveragerdi to the south is a volcano in the volcanic system that became extinct 300,000 years ago, but still has significant seismic activity. It can be regarded as a predecessor volcano. This seismic activity is significant not only because of human impact but also because the earthquakes are grouped along faults striking north–south or east–west, but not north to north-east, as might be supposed based on the surface geological structure and that the rift zone is deep as locally. The earthquake distribution has been interpreted to have defined the heat source of the Hengill volcano.
Extending to the north-east from the central volcano is a fissure swarm with a number of tindars associated with an asymmetrical graben. Lake Thingvallavatn, occupies the proximal graben, and has at its southern end lava flows from the mountain and has hosted explosive eruptions historically. Amongst the faults on the north-west side of the graben is the Almannagjá chasm which extends from the northern shore of Lake Thingvallavatn.
Towards the south-east from the mountain towards the sea is a lava covered plateau whose height is about with scattered higher tindars and tuyas.
During the Holocene volcanic eruptions in the zone have been predominantly effusive and basaltic with minor tephra production. Erupted lava volumes have ranged from .
| Date | Event | Comment | |
|---|---|---|---|
| 2008 CE | No loss of life but 50% property damage in Ölfus region | ||
| 1789 CE | Subsurface dyke intrusion | Dyke injection later identified | |
| 1000 CE | Adjacent volcanic system eruption | Fissure west of Hengill, in the Brennisteinsfjöll volcanic system. This eruption was at the time of a meeting of the Icelandic parliament at Thingvellir. | |
| 150±75 CE | Eruption | Along a fissure length of and about lava erupted. Included phreatomagmatic eruption from lake Thingvallavatn. | |
| 80±75 BCE | Eruption | ||
| 1730±50 BCE | Eruption | ||
| ~3250 BCE | Eruption | ||
| ~3750 BCE | Eruption | Along a fissure length of and about lava erupted | |
| ~5000 BCE | Eruption | ||
| ~5550±500 BCE | Eruption | Not well dated | |
| ~7100 BCE | Eruption | ||
| ~7300 BCE | Eruption | ||
| ~7550 BCE | Eruption | ||
| ~8200 BCE | Eruption | ||
| ~8250 BCE | Eruption | ||
| ~8350 BCE | Eruption |
The official view of the volcanic hazard is that it is confined to a radius of and is related to lava flows, plugging of geothermal boreholes by lava, volcano-tectonic faulting and volcanic gas pollution.[3]
There was either hydrothermal fluid or magma increase at a depth of approximately within the inferred brittle-ductile transition zone of the area centred at Ölkelduháls, between Hengill and Hrómundartindur to its east during 2017 and 2018. This volume had had been contracting between 2006–2017, and was about north-west from an area of uplift between 1993 and 1999.
There are three associated geothermal areas related to the high amount of fissuring in the Hengill volcanicsystem producing high water subsurface permeability. These areas include Iceland’s second largest geothermal field which now is an important source of energy for the south of the country. The three areas are:
The area with its mountains and hot springs is well suited for hiking and there are a lot of hiking trails.The town of Hveragerði with its multitude of hot springs is also part of the Hengill area.
Some folk tales are connected to the region.[4] For example, in a folk tale collected and published by Jón Árnason in 1862, a young farmer is said to have killed the sleeping troll woman Jóra in Icelandic pronounced as /ˈjouːra/ while she lay in wait for innocent wanderers or horsemen on the trail over Dyrafjöll in Icelandic pronounced as /ˈtɪːraˌfjœtl̥/ north of Hengill.[5] In another folk tale, according to some people, a woman's body found in the area was believed to be that of Halla, the wife of the famous Icelandic outlaw Fjalla-Eyvindur.