Gjástykki forms a large part of the Krafla fissure swarm north of Mt. Hrútafjöll. It comprises a graben 2-5 km wide and 40-50 m deep in which several fissures and faults are found. Gjástykki is located in the lava shields of Gjástykkisbunga and Stóravíti, which formed at the end of the ice age. Younger lavas originating in Éthólar about 2700 years ago, now mostly buried, and lavas from the Krafla fires occupy the southern part of the Gjástykki graben. The faults and fissures are up to 5 m in width. During the Éthólar and Krafla eruptions lava flowed down into them. The Éthólar lava, by then degassed, eventually reappeared out of a fissure south of Kelduhverfi (Skinnstakkahraun). The fissures and faults have been formed in several rifting episodes. When studied 70 years ago, the combined gap of fissures in Gjástykki was estimated to be at least 60 m. During the Krafla fires the extension in Gjástykki was measured at 6-8 m, increasing from north to south.