Episodic rifting and volcanism at Krafla in north Iceland: Growth of large ground fissures along the plate boundary

The rifting of the plate boundary in north Iceland forms new ground fissures and reactivates old ones. Continuous growth of fissures is observed in the crust above a magma chamber beneath the Krafla caldera as it responds to changes in subterranean magma pressure. Rapid episodes of fissure formation and reactivation are observed in conjunction with magma intrusions at depth along the rift zone that constitutes the plate boundary between the North American and Eurasian plates. The Krafla caldera contains several fissures that open during periods of uplift lasting 100 to 300 days and that close incompletely in subsidence episodes lasting several hours to 3 weeks. The average rate of opening differs from one period to another, ranging from 0.1 to 0.5 mm/d. Fissure closing and average closing rates also vary from one subsidence episode to another, ranging from minimal values of 2.1 mm and 2.3 mm/d to maximal values of 49 mm and 18 mm/d, respectively. During the two intrusion episodes of September 1977 and March 1980 the magma migrated toward the south with an average velocity 0.6 and 0.5 m/s, respectively. In the September 1977 episode some fissures in the rift zone widened with a rate greater than 190 mm/d, whereas during the March episode that probably resulted in a thinner intrusion the maximum widening rate was only 50 mm/d. The data suggest that the magnitude of a subsidence episode is determined at least partly by the available excess magma pressure in the chamber. To explain the extensive growth of open fissures and normal faulting in the rift zone, it is necessary to assume that in addition to the intrusion of the main central dike at depth, many small dikes are intruded at shallow depths (less than 1 km)