The crater Eldborg (The Fire Castle) is situated on a short SW-NE fissure, which probably was most volcanically active 5000-8000 years ago. It is the most symmetric and beautiful of the scoria craters of the country and rises about 60 metres above the surroundings (100 m above sea level). It is oblong in shape, 200 m long and 50 m deep.

The Book of Settlements describes an event involving an old and a blind man, Thorir, who came out of his farm one evening for fresh air. Then suddenly he saw a very large and a savage looking man coming sailing on a steel boat. He walked up to the gate of the milking pen of the farm Hrip and dug a hole with his iron stick. Then he left.

During the night, an eruption started exactly where he had dug, and a large crater was created where the farm stood.

The older Eldborg lava field is of the Pa hoe hoe type with an area of 32 kmē and the younger one, much smaller, is of the AA type. Both lava fields are basaltic and relatively well vegetated. The best way to approach the crater is from the parking area to its south. The walk takes about half an hour each way and people are kindly asked to stick to the trodden and marked paths.