The lava field of Hrútagjárdyngja, covers >80 km2, but considerable part of it has been covered by younger lavas. The lava shield reaches the north coast of the Reykjanes Peninsula west of Hafnarfjörður,between Hvaleyraholt and Vatnsleysuvík.
The crater of Hrútagjá is of special interest. The main crater is a shallow depression sitting on top of a lava rise (horst). In the culmination phase of the eruption, a solid crust of a lava around the crater was inflated by magma, resulting in a considerable rise. Later, when the magma escaped through channels, a subsided block of lava was left behind, marked by deep fissures (gjár) on its margins.
Magnús Á. Sigurgeirsson, 2010