What controls the along-strike slopes of volcanic rift zones?

We investigate the dynamics of viscous pressure losses associated with late ral magma transport in volcanic rift zones by performing (I) coupled elasti c-hydrodynamic simulations of downrift magma flow in dikes and (2) analog e xperiments mimicking lateral dike propagation in the presence of an along-r ift topographic slope. It is found that near-source eruptions are likely to be favored by shallow slopes while distant downrift eruptions may be encou raged by steeper slopes, provided that along-rift variations in the tectoni c stress are negligible or uncorrelated on the timescale of multiple dike i ntrusions. This implies the existence of a critical slope to which a volcan ic rift zone would naturally evolve. Such behavior is produced by three-dim ensional (3-D) elastic effects and is controlled by the ratio of the drivin g pressure gradient due to the along-strike topographic slope to the vertic al gradient in the excess magma pressure in the dike. This model may be vie wed as complementary to commonly cited mechanisms that appeal to magma visc osity and the dynamics of freezing of lava flows at the surface to explain the low profiles of basaltic shield volcanoes. Our estimated values of the critical slopes are in general agreement with observations in Hawaiian rift zones, but further development of fully 3-D models is required for more ac curate predictions.