Solidification of basaltic magma during flow in a dike

Basaltic magmas at temperatures of 1100 degrees to 1200 degrees C commonly ascend in dikes through the Earth9s upper crust, where temperatures are considerably lower. As heat exchange between an invading magma and wall rocks commences, temperatures near dike margins are well below solidus temperatures, and a layer of cooled, solidified magma is thus present between the wall rocks and the uncooled magma flowing in the center of the dike. The rate of growth (or decay) of this layer is controlled by dike thickness, magma flow rate, distance from the source region, temperature difference between magma and host rocks, and temperature dependence of magma viscosity. In agreement with available data obtained during observation of volcanic fissure eruptions and from study of ancient dikes, we calculate that magmas flowing distances more than a few kilometers from an upper crustal source region and at initial velocities of 1 m/s in dikes that are 2 m thick solidify within a few hours. Temperatures at the contact between magma and wall rocks may remain well below that of the initial, uncooled magma for the duration of heat transfer. The short duration of heat transfer and small temperature increase in the wall rocks explains the lack of contact metamorphic effects near the contact of many dikes. Magmas leaving the source region at different times become juxtaposed. As the solidified layer grows inward from the dike walls, this juxtaposition is preserved and may account for the compositional and textural zonation of some dikes.