Bob Cammach, age 12, of Monroeville, Pennsylvania, for his question:
What kind of rock is Granite?
Granite is rather a common rock and many people pass it by as though it were nothing worth noticing. But those of us who are interested in the earth have an admiring eye for this sturdy rock. We enjoy its subtle variety of colors and respect the long history of dramatic events that created it.
If you plan to erect a worthy monument to your favorite hero, the best material for the job is granite. This handsome rock is strong and durable, hard and weather resistant. You may select a pepper and salt variety, one with rusty reddish tones, a harmonized blend of grays or even a variety that seems to contain sparkling specks of pixie dust. Then the slab of stone is rough hewn from the quarry, the colors are not at their best. But granite can be polished to a very high gloss. On the finished statue, the colors will look like rich gems and tapestries, frozen under the lustrous surface.
It was not easy to hew your slab of stone from its quarry and it was not easy to carve and polish it. For granite is famous for its hardness. And its hardness results from a series of hardships. The ingredients in granite are the plentiful minerals that make up most of the ordinary stones of the earth's crust. Though samples of granite vary, most of them are about 60 per cent potash feldspar and about 30 per cent quartz. There may be some other blend in the mixture and sometimes a sprinkling of mica.
Feldspar, the main ingredient, is not extra sturdy, though quartz is the hardest of the earth's common minerals. Granite gets its durability from the way in which it was formed. Originally, it was magma, a pool of gases and molten minerals brewed deep in the bowels of the earth. Ages ago, the molten mixture welled upward and perhaps settled in underground cracks and crevices. Granite is classed as an igneous intrusive rock a fire¬ formed rock that intruded between crustal layers below the surface. There it cooled, often in massive beds and the cooling process took ages and ages of time.
In its deep tomb, the long gradual cooling forced the minerals to harden into certain crystal patterns. The assorted lumpy crystals in a sample of granite, as a rule, are about the same size. The sample that has large, coarse crystals most likely formed very deep in the earth. Finer¬ grained samples were formed nearer the surface. Because of its magma origin, we always find beds of granites where the earth was once restless with grow¬ing mountains and fiery volcanoes. Over the patient ages, the buried magma
has been processed gradually into sturdy granite.
The earth sometimes uses another process for changing certain minerals into granites. It occurs when hot magmas and lavas invade and engulf gneisses and feldspars, quartz and various shades and completely alter them from their original forms. The remade metamorphic rock is true granite, often banded with colors of the former ingredients.