Grjótagjá marks the western margin of a fault zone where fissures have extended and faulted after the lava from Jarðbaðshólar formed some 2700 years ago. However, in the previous 5000 year period, the faulting activity was confined further to the west. There the Stóragjá fissure was formed in an 8000 year old lava of which a small portion has been preserved. One can see that the Jarðbaðshólar lava lies undisturbed across each end of the fissure. The warm water in these fissures flows to the south, where it mixes with cold groundwater and eventually reaches Lake Mývatn. During the Krafla fires, the water temperature in Stóragjá, which had been 26°C, became comfortable for bathing, whilst the water in Grjótagjá, originally 42 to 42.5°C, became too hot. The water has since gradually cooled towards the original temperature.