Hverfell is the southernmost crater on a 20 km long eruptive fissure that extends intermittently north to Éthólar in the south of Gjástykki. It was formed in phreatic explosions about 2700 years ago. Such explosions occur when water meets magma, causing violent outbursts of steam and fragmentation of magma into ash. At the time of eruption, Lake Mývatn extended to where Hverfell is now located. The area is highly fractured, and water from the lake may have entered fissures and reached the eruptive vent of Hverfell. Ash was spread both to the north and south of the crater and one can find examples in the north where it has been baked into tuff due to the effects of geothermal heat. It lies on the 2900 year old acidic tephra layer H3 from Mt. Hekla. Ash from Hverfell is evident in the soil as far as Kelduhverfi in the north. The crater rim measures as much as 150 m in height and is just over 1 km in diameter. Towards the end of the eruption a slope failure occured from the southern side of Hverfell. After that a small mound of ash formed on the crater rim in the middle of the scar. The scar can still be seen, although the slide mass itself has been buried under lava.