magma molten substance beneath earth's surface They succeeded in closing off the third of five underground conduits formed beneath the surface crust and through which red-hot magma has been flowing . Not all magmas have the same composition , and magmas with different compositions melt at different temperatures . If the magma pressure is great enough , it will force its way right up the dyke to the surface , spilling out to form lava flows . Detailed geochemical profiling of the lava sequences has revealed a complex history of eruption from magma chambers which , although linked in the same geotectonic setting , had individual development paths within the crust .