Abstract A study was made of 903 fractures, most of which are of postglacial age, on the Reykjanes Peninsula. Some 120 fractures, belonging to the Vogar fissure swarm, he completely within the same postglacial lava flow and were studied in great detail. For the Vogar fractures, 75% are pure tension fractures and 25% are normal faults. The average length of the Vogar fractures is 611 m, the average width is 0.6 m, the average spacing (measured perpendicular to the average strike, N54E, of fractures) is 456 m, the estimated average depth is of the order of several hundreds of meters, and the average throw of the faults is 2.3 m. All major fractures on the Peninsula appear to have formed by coalescence of smaller fractures as a result of absolute tensile stress. The fractures group into four fissure swarms, each being a part of a volcanic system which, it is proposed, is fed by an elongated partially molten magma reservoir located at a depth of about 8 km beneath the peninsula. It is suggested that a slight magmatic pressure increase in these reservoirs, and resulting uplift and tensile stress at the surface, is the main cause of postglacial fracture formation.