ON THE VARIATIONS OF FLOW-RATE IN NONEXPLOSIVE LAVA ERUPTIONS

A physical model is developed to determine factors which influence the dynamics of non-explosive lava eruptions. In the model, magma rises in a laminar regime from an overpressured chamber surrounded by elastic rock and erupts at the surface, where the accumulation of lava may alter the vent pressure. In a flow rate versus volume diagram, an eruption follows a path which is sensitive to relative changes in flow variables such as viscosity, conduit dimensions and thickness of lava over the vent. The path does not depend on the unknown elastic properties of the country rock or the chamber dimensions. With appropriate volcanological data, an observed flow rate versus volume path can be interpreted. The approach is applied to three well-documented eruptions. In each case the eruption rate was partially influenced by decreasing chamber pressure. The 1988-1990 eruption of Lonquimay (Chile) exhibits an early phase of conduit shrinkage which lasted about 100 days, probably due to magma solidification against country rock. The 1979 eruption of La Soufriere de Saint Vincent (West Indies) was initially affected by lava dome growth and later passed through a phase possibly caused by constriction of the conduit. The 1943-1952 eruption of Paricutin (Mexico) was affected by changes of lava level in the cinder cone and magma properties. In these three eruptions, the pre-eruptive chamber overpressures were similar (10-20 MPa) even though the erupted volumes differed by as much as two orders of magnitude.