Careful study of a series of intrusions and eruptions at Krafla has added substantially to our understanding of processes at rifting plate boundaries. During more than a dozen deflation events between 1975 and 1979, most of the magma that left the shallow reservoirs beneath Krafla formed dikes in the fissure zone extending north and south of the caldera, with only minor amounts reaching the surface in associated eruptions. The character of Krafla's activity changed in 1980, and the largest eruptions since the rifting episode began in 1975 were associated with events in July and October 1980 and January— February 1981.
Krafla Caldera, Mývatn Area, Iceland (65.73°N, 16.6°W). All times are GMT. After more than 9 months without an intrusive event or eruption at Krafla, instruments recorded the simultaneous onset of deflation and harmonic tremor at 0036 on November 18, followed by the start of a fissure eruption at 0152. Between 0400 and 0500, geologists flew over the active fissure and observed vigorous lava fountaining, feeding flows that had advanced as much as 5 km to the west. Rates of lava extrusion along the fissure varied but were probably the most voluminous seen since activity began in December 1975. Extrusion occurred along the entire fissure, which extended from near the center of the caldera about 8 km to the north (the October 1980 fissure vents were in almost the same location but stopped about 1 km north of the southern end of the November 1981 fissure). Strong northerly winds blew some scoria onto the nearby power station, but no damage occurred. Discrete earthquakes initially accompanied harmonic tremor but stopped after a few hours. By 1000, lava extrusion had weakened considerably and was confined to three 1-km-long segments of the fissure. Inflation resumed November 22, but minor eruptive activity continued. Lava extrusion stopped early November 23, but late that afternoon occasional minor spattering resumed. Initial reconnaissance mapping indicates that lava flows covered 16–20 km2 and that the longest flow traveled roughly 6 km from the fissure.