The 1789 Rifting Event in the Hengill Volcanic System, SW-Iceland

A volcano-tectonic episode in South Iceland in the 18th century was initiated by rifting and eruptions along the 30 km long Laki fissure within the Eastern Volcanic Zone (EVZ) and an eruption at the northern Reykjanes Reykjanes Ridge in 1783, forming the island of Nýey. Severe earthquakes within the South Iceland Seismic Zone (SISZ) followed in 1784. The sequence ended in 1789, with a rifting event in the Hengill volcanic system, located at the junction of the Western Volcanic Zone (WVZ) with the SISZ. The Hengill system consists of a central volcano, Mt. Hengill, which is transected by a 60 km long SW-NE striking rift zone. The rift north of Mt. Hengill forms a 6 km wide graben, partly filled with the lake Thingvallavatn. The central volcano is marked by high volcanic production, occurrences of acid rocks and a high temperature geothermal field. Rifting events within the Hengill system are considered to be mainly associated with crustal dike propagation as dikes have only breached the surface in an eruption four times during the Holocene. Extension rate is signified by tension gashes of about 70 m aggregate width across the rift zone in a 10,000 year old lava and by a maximum throw of about 40 m at Thingvellir near its western margin. In 1789, settlements only existed at the distal ends of the Hengill rift zone, at Selvogur and Thingvellir. A fairly detailed contemporary description of the rifting event exists, written by the vicar of Thingvellir. He states that the rifting 1789 began in early June and lasted 10 days with considerable earthquake activity. The central part of the Thingvellir graben subsided and the lake transgressed beyond its former shore while the margins of the graben were elevated so that water wells ran dry. Trails across the main boundary faults of Almannagjá and Hrafnagjá became impassable for horses. Old surface fissures widened and new formed. Fault movement was also observed southwest of lake Thingvallavatn, where a fault subsidence was reported being equal to a person in height, obviously considered rather spectacular for its size. There is mentioning of new hot springs having formed south of Mt. Hengill and a collapse of houses at the southwest end of the rift zone in Selvogur. A peat bog at the northeastern shore of the lake overlain by lake mud at 1.4-1.5 m depth indicates the amount of subsidence during the rifting event. A man-made rockwall submerged near the north shore of lake Thingvallavatn lies at a depth of 2.6 m, of which about 0,2 m are probably due to latent creep spanning the last 2 centuries. As there is no mentioning of subsequent events within the Hengill system, the 1789 rifting is considered a singular event consisting of dike propagation from the Hengill central volcano into both segments of the rift zone.