Eldborg is a beautifully-formed crater rising 60m above the surrounding lava. The spatter cone is the largest crater along a short volcanic rift, 200 m in diameter and 50 m deep. Its last eruption was about 5000-6000 years ago.
According to medieval chronics there was also one around the time of the colonisation of Iceland about 1000 years ago, but today it is known that it came from another of the many craters of the valley Hnappadalur which are part of the volcanic system Ljosufjoll, the mountains of the light. It is easiest to approach Eldborg from Snorrastadir on its southern side, walking 2,5 km through the shrub-covered lava field. It is possible to walk all the way around the crater as well as up it.