Flow of lava into fissures in the rift zones of Iceland

In several eruptions of the Krafla volcanic system during the magmatic rifting episode of 1975–1984 it was observed that lava flowed from the surface into preexisting, open fissures of the Krafla fissure swarm. In one instance, during 18 hours of the eruption in July 1980, almost all the erupted lava disappeared into one fissure at a distance of 2 km from the eruptive one. The receiving fissure consequently widened and propagated northwards along the rift. Apparently, dikes were formed that were fed from the surface and propagated horizontally and downwards. In some instances the lava from these dikes reappeared at the surface and was erupted for the second time. The down-flowing lava was not always fully degassed and continued releasing gas as it was filling the receiving fissure. In such cases spattering was observed from the down-flowing lava. In other instances lava accumulated in low spots above fissures that subsequently widened and drained the accumulated lava from below, leaving a “bathtub ring” of solid crust around it. Since these observations at Krafla, several similar cases have been identified in other parts of the rift zones of Iceland, including both the Northern Volcanic Zone and the Reykjanes Peninsula Oblique Rift. Based on petrological data, it is inferred that two prehistoric lavas erupted near the Krafla caldera may have flowed along fissures of the Krafla fissure swarm and erupted for the second time in the Kelduhverfi district, 30 km down–rift. Furthermore, such processes have occurred during the currently ongoing activity on the Reykjanes Peninsula. They may significantly affect the predicted flow paths in lava flow simulations and the design of preventive measures such as lava flow barriers. Although this process has only been documented in Iceland and Hawaii, it is likely to occur in other subaerial magmatic rift zones worldwide.