Grímsnes is a relatively small volcanic system located SE of Thingvallavatn lake. It is displaced about 15-20 km to the east of the other en-echelon group of volcanic fields of the Western Volcanic Zone extending across the Reykjanes Peninsula. Its lava flows cover 54 km2 and were erupted from a group of 11 fissures that produced a series of NE-SW-trending crater rows. The eruptions of its basaltic lavas were considered to have been restricted to a relatively short interval between about 6,500 and 5,500 years ago, but radiocarbon dating showed that some of the lava flows were emplaced between about 7,400 and 8,900 years ago.