Vonarskard is located in the centre of Iceland and within the Northern Rift Zone. In the region are three active central volcanoes. To the west is the Tungnafellsjokull central volcano which is slightly eroded. It has not erupted in Postglacial time (i.e. last 9000 years). In the centre is the Vonarskard central volcano with its 8 km wide caldera and active high-temperature field. The Vonarskard volcano is clearly a slightly younger structure than Tungnafellsjokull volcano. Subglacial volcanics from the younger Bardarbunga volcano cover the eastern part of the Vonarskard volcano (Fridleifsson et al. 2005).
The oldest rock formations which can be related to the Vonarskard volcano may be 300.000-500.000 years old and are exposed at the northern caldera rim. These older formations consists of interglacial lava flows intercalated with subglacial hyaloclastites and tillites. The caldera is about 8 km in diameter but only the western half of it is exposed at the surface. The caldera escarpment is well exposed on the northern rim and inward dipping slabs are seen on the lower eastern slopes of Tungnafellsjokull volcano. The southern rim is marked by arched subglacially erupted rhyolite extrusions.
Continued volcanic activity has filled in the caldera. The presently exposed caldera filling consists mainly of subglacial basaltic hyaloclastites and pillow lavas. The rocks are generally aphyric in character contrasting the Bardarbunga system where nearly all formations are highly plagioclase porphyritic and easily distinguished. Some of the youngest pillow lavas within the caldera are of special interest because they resemble pillow lavas of fast spreading ridges of the ocean floor (Fridleifsson et al. 2005).