The Ţingvellir area is a part of the North Atlantic rift system, almost entirely nested within the Reykjaneshryggur-Langjökull rift system.
It can be described as an area of sea-floor spreading, displaying the close association of crustal rifting and volcanism. Ţingvellir and the Great Rift Valley of Eastern Africa are the only sites on Earth where the effects of two major plates drifting apart can be observed.
Almannagjá is 7.7 km long. Its greatest width is 64 m, and its maximum throw is 30-40 m. It marks the eastern boundary of the North American plate.
Its equivalent across the graben, marking the western boundary of the Eurasian plate is Hrafnagjá. It is 11 km long, 68 m wide and has a maximum throw of 30 m.
The Ţingvellir faults are believed to be the surface expressions of deeply rooted normal faults. The numerous fissures encountered on the valley floor are of similar origin.