The spreading axis of the Reykjanes Ridge enters Iceland as a transtensional zone along the Reykjanes Peninsula. The dual character of it expresses itself as 1) transcurrent faults and 2) tensional faults and crater rows, the former trending north-south, the latter northeast-southwest. They have been active alternately at time scales of 500 to 800 years over the past few millenia.
At Mt. Hengill the Iceland segment of the ridge system becomes a pure rift zone. This applies to the northeast branch of the Hengill volcanic system, including the downfaulted rift valley of Lake Þingvallavatn and its continuation towards north. The ground fissures of Þingvellir and their counterparts on the other side of the rift valley dissect the western slope of a 10,000 year old lava shield. The crater is south of Mt. Hrafnabjörg (7 m). The lava margin is clearly expressed in the depth contours of the lake as a diagonal step extending from the outlet of the lake in the southeast to south of Skálabrekka in the nortwest. At the time, the lake was only one third of its present size.
The ground fissures formed as a result of dilatation across the plate boundary concomitant with subsidence of the rift floor. This occurs episodically conjoined with dyke intrusions from the Hengill centre. The last rifting episode occurred in 1789, but the recurrence time is not known. A sunken manmade stone wall at Vatnskot near the centre of the rift shows that the subsidence that took place at the time was 2.5 m. On the west side of the rift, almost all the fault displacement is confined to Almannagjá with a maximum throw of just over 40 m. Faults at Hestvík at the southwest of the lake are a continuation with a total throw of about 400 m, divided among three faults. They cut across the eastern flank of an interglacial lava shield. Judging from the difference in throw, the lava shield may be 10 times older, which means from the last interglacial period.
Kristján Sæmundsson, 2010