There are three craters on Þeistareykjabunga, two of which are eruptive and one is a pit-crater. Stóravíti is an eruptive crater in one of the largest lava shields in Iceland. It is about 140 m deep with a maximum diameter of 600 m. The shield eruption began 11-12,000 years ago and may have continued for decades, during which time the lava spread over an area of about 525 km2. Lava ponds are often formed in the craters of lava shields. Such lava, which may be spongy from gas bubbles or degassed, may breach the crater rim and flow out. By the end of the Stóravíti eruption the magma column underneath the crater fell down to groundwater level resulting in an explosive steam eruption, when rocks were hurled up to 2 km east of the crater. The pit-crater of Litlavíti may have acted as magma drainage for Stóravíti at the end of the eruption. Langavíti is the second of the two eruptive craters on Þeistareykjabunga, located 2 km north of Stóravíti, forming its highest point. The lava from Langavíti, which extends into the area of habit-ation in the west of Kelduhverfi, is easily recognizable by its abundance of pyroxene, olivine and plagioclase phenocrysts.