Skálafell was built up in several eruptions in a narrow fissure system. The top crater is a circular rampart formed from lava spatter around a small lava lake. The lava escaped underground through tunnels, which is evident from the pit craters on the slope. The entrance of one of the tunnels can be seen in the crater. The top crater formed in the last eruption about 3000 years ago whereas the oldest lavas of the fissure system are over 8000 years old. The latter are exposed in a fault wall east of Skálafell and in the coastal cliff to the southeast. The fault throws are up to 20 m downward in the oldest lavas but barely visible in the youngest flow. These faults bound a 5 km broad rift zone on the southeast side. The corresponding faults opposite are those of Kinn with a bridge across one of them, symbolizing the plate boundary. The north-south trending fissures on the lava plain northeast of Skálafell are horizontal shear fractures consisting of side stepping segments with push-ups at the offsets.
Kristján Sæmundsson, 2010