Grabrok belongs to the volcanic system of Ljosufjoll. It reaches from Berserker lava field in the North of the Snaefellsnes peninsula, close to Stykkisholmur, over the actual Ljósufjöll and Hnappadalur all the way to the Grabrok craters. It extends over a distance of 90 km and is one of the longest in Iceland. The last eruption took place about thousand years ago at Hnappadalur.