The Grábrók craters probably formed around 3,600 years ago. An alkali-olivine basalt lava flowed out of them and formed the Grábrókarhraun lava field. The scoria craters are the easternmost in the Ljósufjöll volcanic system, which is part of the flank rift zone of the Snæellsnes peninsula. The central volcano of the system with the magma chamber is located in western Hnappadalur and reaches a height of 1,045 meters with the Miðtindur peak.
In 1962, the three Grábrókargígar craters were declared a nature reserve. Moss and heather as well as some birch bushes grow on the lava. In order not to endanger the sensitive vegetation, the marked paths must not be left when walking through the area. The climb to the crater rim of Stóra-Grábrók is easy from the car park. The path consists of wooden planks and steps and can therefore be easily walked even by inexperienced hikers.