Fig. 2. Ground water flow in a tension fracture in the rift zone of Southwest Iceland. The opening (aperture) of the tension fracture, located in a Holocene pahoehoe lava flow, exceeds 10 m and it forms a part of the Thingvellir Fissure Swarm. The lava flows of the Thingvellir Swarm have a very high fracture-related permeability and ground water yield.