Grænavatn (63° 53′ 04.6″N 22° 03′ 17.2″W) is famous for its green color. The green color is primarly related to minerals of silica and sulphur, which enter the lake from a geothermal spring. Grænavatn is an explosion crater, classified as a maar. The crater is elongated with a diameter of 360 m in east-west direction compared to a diameter of 300 m in north-south direction. The eruptive materials, mostly tephra, of the Grænavatn contain a unusally high content of gabbro xenoliths, which originate from thick intrusions, sills, at a shallow depth beneath Grænavatn (Guðmundsson, 2017).
Guðmundsson, Á. (2017). The glorious geology of Iceland’s Golden Circle. Springer. 334 pp.