Can you describe the difference in the rates of cooling in hypabyssal and plutonic rocks?
Plutonic rocks form deep in the crust. The insulating effect of the crust, and the high temperatures at these depths make the intrusive rock cool very slowly. This results in coarse to very coarse crystals, euhedral and in some cases very coarse porphyritic textures. A large granitic pluton may take more than 1 million years to cool. A hypabyssal rock is one in which igneous rocks have intruded to shallow crustal levels. The limited crustal insulation allows the rocks to cool relatively rapidly, probably in time spans of years to hundreds of years, depending upon the size of the body and its depth. The rate of cooling allows the rock to form fine to medium grained crystals, the earlier formed crystals being phenocrysts with porphyritic textures, and a groundmass which is much finer and with fewer porphyritic crystals.
