For slow spreading ridges where most of the melt supply is probably highly focussed at disctrete magmatic centers in the middle of the ridge segments, the ratio of total extension to magma-accomodated extension is likely determined by the ability of magma freezing to limit the distance that lateral dikes may travel along the ridge axis. Thus the thermal viability of dikes is likely to be a major factor controlling the observed along-axis variations in crustal morphology. We simulate magma flow in the mid-ocean ridge dikes and calculate how far they are able to propagate given some range of the excess magma pressures at the source. Figure on the right shows evolution of magma flow in a horizontal cross-section through the dike middle and the associated temperature field in one of such simulations. Colors denote dimensionless temperature, dark green being the initial temperature of the host rocks and dark red is initial magma temperature. Interface between solid and liquid corresponds to the boundary between green and yellow (dimensionless freezing temperature of 0.9). x_{N} is a dimensionless propagation distance.