The elongated low hills between the southern bight of Lake Þingvallavatn and highway 1 east of Hveragerði constitute a swarm of heavily eroded tindars (from subglacial fissure eruptions) related to the Hveragerdi central volcano.The last eruptions occurred in the west of the volcanic complex near the margin of the Hengill volcanic system.
Súlufell (446 m) is a cone shaped mountain remarkable for a large crater, 500 m long and 400 m wide, sunk in its southwestern slope. A 170 m high wall reveals an inner structure of pillow lava overlain by hyaloclastite breccia. The mountain was clearly there when the crater formed. The youngest volcanic member in the succession is a ridge of pillow lava, which extends from the northeast into the crater. The formation of the crater presents a problem, as no debris from the explosion has been found. Basaltic magma may have been involved but only in the presence of water or perhaps carbon dioxide due to an explosive phase change. A geothermal explosion crater is unlikely as there is no evidence of a geothermal system underneath, but it is present about 4 km further south. Silicic magma is rich in gas and might be considered. When such magma erupts, a lava flow or a dome usually results. However, this is not the case here, unless this was a mixed eruption with the pillow lava ridge as the basaltic member. This is not utterly improbable because nearby there is a hyaloclastite ridge of andesitic composition (Stapafell).
Kristján Sæmundsson, 2010