Energy Partitioning in the Phreatomagmatic Basaltic Eruption of Grímsvötn in 2004

The energy of an explosive eruption is transferred to the surroundings in a variety of ways, but for plinian magmatic eruptions it is commonly assumed that most of the energy is lost to the atmosphere through the eruption plume. For a phreatomagmatic eruption some energy is also lost to a surrounding water body, by heating and boiling, and in the case of a partly subglacial eruption, a substantial part may be lost to ice melting. However, detailed studies of energy partitioning have been lacking. The eruption of Grímsvötn in November 2004 lasted for six days and produced only basaltic tephra. About half of the erupted material was deposited at the eruption site, within a 500-700 m wide and 150-200 m deep ice cauldron. The remaining half formed a well defined tephra fan towards north and northeast. Through repeated pre- and post-eruption surveying of glacier geometry the volume of ice melted in the eruption could be measured. Detailed measurements of key parameters such as volume and mass of erupted material were carried out and a total deposit grain size distribution could be determined on the basis of extensive sieving of both proximal and distal parts of the deposit. The heat capacity of the tephra and energy used for generation of new surface (fragmentation energy) was determined through laboratory measurements. On the basis of these data, the total mass of erupted material was determined as about 6 x 1010 kg, and the total thermal energy of the eruption was found to be 7 x 1016 J. Moreover, the partitioning of the total energy into energy lost to: a) ice melting, b) meltwater heating, c) residual heat in volcanic pile, d) kinetic energy and e) total fragmentation energy could be determined. The results indicate that about a third of the energy was used for ice melting, that about a tenth was expended for meltwater heating and remained as heat in the crater, and that energy expended for fragmenting the magma amounted to a few percent. The remaining energy, about half of the total, was available for driving the sustained 6-10 km high eruption plume during the 33 hour long main phase of the eruption.