Grænavatn

In Krýsuvík there is a cluster of explosion craters. All of them are probably more than 6,000 years old; the best known are Grænavatn and Gestsstaðavatn. Within the cluster there are at least four crater rows: three trending north–south and one northeast–southwest. The westernmost and oldest row produced mainly scoria (Gestsstaðavatn), while the others consist primarily of fragmented bedrock mixed with large blocks (Grænavatn), along with scoria from the easternmost eruption (the eastern bay of Grænavatn).

The youngest features are two crater rows that pass through Grænavatn. The main crater in the older of these is located on the western side. There, eruptions ejected rock fragments and pulverized bedrock from the substrate. This material forms a layer at least 10 meters thick on the southern crater rim. Ejecta from this eruption spread across the surrounding area and over nearby craters, with block size decreasing with distance. A spatter lava, rich in gabbro xenoliths, originates from the younger crater row on the eastern side of Grænavatn. The uppermost part of the lava consists of poorly consolidated loose scoria. Fragments of the underlying bedrock are also present there.

The age difference between the Grænavatn craters is probably small. Augun, small craters with ponds on both sides of the road, lie along a 300-meter-long crater row trending N50°E. This row is divided into two parts that do not align. The explosion craters follow the orientation of earthquake fractures. The volume of lava produced during the eruptions was very small, but they were accompanied by large quantities of gabbro xenoliths, and the eruption fissures were short. It is therefore most likely that the eruptions occurred during a seismic episode that mobilized a crystallizing magma body at the roots of the central volcano of the Krýsuvík volcanic system. Postglacial extensional fractures are not observed in the immediate area.

Source: See here
Image: Helga