Basalt is extrusive. The magma from which it cools breaks through the crust of the earth and erupts on the surface. The volcanoes that make basalt are very common, and tend to form long and persistent zones of rifting in nearly all of the ocean basins. We now believe that these undersea volcanic areas represent huge spreading ridges where the earth’s crust is separating. Basaltic magma is like the blood of the earth – it’s what comes out when the earth’s skin is cut the whole way through. As an eruption ends, the basalt “scab” heals the wound in the crust, and the earth adds some new seafloor crust. Because the magma comes out of the earth (and often into water) it cools very quickly, and the minerals have very little opportunity to grow.