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Description: The control of magmatic physical properties on the range and volumetric distribution of eruptions has been investigated for the active volcanic zones of Iceland. Magmatic density and viscosity both exert control over observed erupted volumes. The largest volume of erupted material sits at a density and viscosity minimum corresponding to the composition of basalts at the arrival of plagioclase on the liquidus. These basalts are buoyant with respect to the upper crust. Almost 70% of the erupted volume in Iceland's Northern Volcanic Zone (NVZ) comprises basalts that lie close to this minimum. However, small volume eruptions with densities greater than those of the upper crust are found in Iceland. Amongst these eruptions are the picrites, and it is likely that their eruption is facilitated by the generation of overpressure in magma chambers in the lower crust and uppermost mantle. This conclusion is in agreement with petrological constraints on the depth of crystallisation under Iceland.