In Krýsuvík, there is a group of maar type explosion craters, probably over 6000 years old. They line up along three north-south trending rows and one trending N50°E. The largest measure about 300 m across and are lake-filled. The oldest and westermost, of which Gestsstaðavatn is the largest, erupted mainly scoria. The others, two of which pass through Grænavatn, erupted rock debris, including blocks, from the near surface strata, and lava spatter in case of the easternmost row.
The main crater in the western part of Grænavatn is a hydrothermal explosion crater. It erupted only allogenic material, which forms about a 10 m thick layer above the lake level along the south of the crater rim, and spread over the surroundings with diminishing block size with distance. It was followed by a smaller crater forming a recess in the east of the lake. This one produced a sheet of welded lava spatter, which is thickest along the eastern crater rim and contains an abundance of gabbroic xenoliths.
The age difference between the Krýsuvík craters is probably small. The trend of the crater rows is the same as that of earthquake fractures of the Reykjanes Peninsula, where the eruptive and extensional fissures generally trend N35-40°E. In Krýsuvík, postglacial faults of that trend are absent. A pecularity of the Krýsuvík craters is the very small fraction of magmatic material erupted, the richness in gabbroic xenoliths, and the short crater rows of about 0.5 km only. This suggests an eruptive phase where a semi-solidified magma batch residing in the roots of the Krýsuvík volcanic centre was mobilised, apparently during a seismic episode.
Kristján Sæmundsson, 2010