Molten Rocks

Igneous Plutons

As blocks of continental crust rode across one another, some rocks became so hot that they melted. Molten rock at the Earths surface erupts to form either volcanoes or quiet lava flows. When molten rock remains deep below ground, it cools and crystallizes to form bodies of rock that are called igneous plutons.

Plutons are scattered throughout the Southern Appalachians like plums in a pudding. Some plutons are now exposed at the land surface due to erosion of overlying rock; they weather to form unusual, smooth-sided domes like Looking Glass Rock, south of Asheville, N.C. The plutons are composed of granite and similar rocks. People use granite that has a uniform texture and few fractures, such as the Mount Airy granite, in buildings, bridges, statues, and monuments. The next time you visit a cemetery, you may see granite that formed millions of years ago far below the land surface.

Some molten granitic rock cools very slowly and forms coarse-grained veins called pegmatites. These have been the source of high-purity minerals, such as feldspar, quartz, and mica, and gemstones, such as emerald and beryl. The main uses of feldspar are in glass, pottery, and ceramics.

Quartz has many uses, including as gemstones and in high-quality optical lenses. Native Americans used mica for ornaments, and now it is used as an insulator in electronic and electrical equipment. Ultra-high purity quartz mined and processed in Spruce Pine, North Carolina is vital to the computer industry worldwide.

Looking Glass Rock