Lateral magma flow, caldera collapse, and a mechanism of large eruptions in Iceland

Abstract

Lateral magma flow from shallow magma chambers, and associated caldera collapse, has frequently been invoked as a mechanism of fissure eruptions in Iceland. This mechanism is examined from the point of view of fluid dynamics and rock mechanics. It is found that if the magma from a shallow chamber is to flow laterally rather than vertically towards the surface, the density of the roof of the chamber must be much less than the density of the magma, and also less than the density of the crust along the pathway of the resulting dike. It is concluded that formation or reopening of caldera fractures has no effect on the magmatic pressure in the chamber and thus does not lead to magma being squeezed out of it. It is also found that most shallow magma chambers in Iceland are too small to give rise to large fissure eruptions of the order of several km 3. A different mechanism is proposed, of which the salient features are the following. Large eruptions are fed by partially molten magma reservoirs, with an average magma fraction of 0.25, located at the boundary of the crust and upper mantle, at depths of 8–10 km beneath the volcanic zones. If the magmatic pressure in a reservoir increases, the crustal layers above may bend slightly and thereby develop high compressive stresses. For a while, the reservoir may thus contain magma with excess pressure much greater than the tensile strength of the crust. Subsequently, when an eruption occurs, the eruptive volume may be abnormally large compared with the reservoir volume. It is estimated that the ratio of reservoir volume to eruptive volume may then be as little as about 200, compared with an ordinary ratio of about 5000. This mechanism is applied to the Lakagigar eruption of 1783, which lasted about 8 months and produced some 12 km 3 of lava. It is concluded that, if this eruption was fed by the Grimsvötn reservoir, only the uppermost few km of that reservoir supplied magma during the eruption.