Episodic magma flow in narrow channels through the lower crust in Iceland: geodetic evidence

Magma flow through the ~10-35 km thick lower crust in Iceland is highly episodic. Half a century of crustal deformation studies in Iceland has provided indirect information on a number of magma channels in the lower crust. Geodetic modelling of volume change associated with magma accumulation in the upper crust, and extrusion rates for eruptions directly fed from the lower crust or the mantle, reveal magma flow through lower crust at rates of 0.5-50 m3/s or less (except at the Hekla and Bárðarbunga volcanoes where it is has been higher in recent eruptions). Considering the viscosity of basaltic magma flowing in such channels (~ 10-100 Pa s), driving pressure related to density difference between magma and host rock along the channel length (of about 300 kg/m3) and buoyancy in an underlying magma lens counterbalanced by viscous drag, suggests the cross-sectional area of these channels are 5000 m3/s. We suggest that the lack of stored tectonic stress in the ductile lower crust limits the possibilities of formation of extensive dikes there.