"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. Basalt is commonly very fine grained, and it is nearly impossible to see individual minerals without magnification."