Dynamics of the Ljotipollur Phreatomagmatic Eruption, S. Central Iceland

The Ljotipollur explosion crater formed during the 1480 AD Veidivotn eruption, when a basaltic dike propagated >100 km SW from Bardabunga volcano. The crater measures 1.6x0.8 km, and is elongate along the NE-SW trend of the fissure. Extending to the SW of Ljotipollur, the eruptive fissure is flanked by spatter ramparts and is the source of a lava flow, evidently the result of purely magmatic activity. In contrast, the eruptive products at Ljotipollur consist of proximal welded airfall tuffs, spatter, and rheomorphic lavas, medial and distal deposits of tephra and pillow fragments, and a field of large lithic blocks. Extensive ejecta accumulation on the SE rim generated basaltic pyroclastic flows, exposed as a succession of ash lenses in the cliffs east of the crater. The block field is primarily distributed to the NW, covering an area of 1.2x0.8 m, and consists predominantly of angular blocks of agglutinate up to 3.3 m in size, with a minor proportion of smaller, sub-rounded hyaloclastite blocks. We interpret this eruption to have started with a purely magmatic phase leading to construction of spatter ramparts along the eruptive fissure, similar to those immediately SW of the explosion crater. At some point the dike intersected an aquifer or hydrothermal system and the resulting access to water led to a transition to a phreatomagmatic phase. This was sufficiently powerful to destroy and eject the spatter ramparts, generate pyroclastic flows, and core through tens to >100 meters of preexisting hyaloclastite, subglacial pillows, and river gravels. We have modeled the dynamics of the phreatomagmatic explosions to derive vent conditions from the distribution of ejecta in the block field. By addressing the explosive expansion of material out of the vent and computing the trajectories of ejected blocks, we can obtain the initial pressure and water mass required to produce the observed deposit. We will present the results of our modeling and discuss the implications for water supply. There is abundant evidence for explosive magma-water interactions elsewhere along this highly active rift system. Eruptions along this dike swarm occur every ~80 years, so developing an understanding of the formation of these deep explosion craters, and the hazards associated with them, is essential.