Dale Gerhardt., age 12, of Mohawk, New York, for his question:
How are rocks formed?
The story of the rocks dates back to t1‑B birth of the earth. In its youth, we are taldp our wor ld was a chaos of blazing gases. All the chemical elements from which our cool solid world is made were there in gaseous form. Some four or five million million years ago, the blazing ball began to cool. The gases, or most of them, reached their freezing points and became solid rocks. The minerals from which rocks are made reach their freezing points and become solid far above normal temperatures.
The 90 odd elements from which our world is made tend to combine with one another and form countless different substances. In the earth’s crust, only eight of them are present in great abundance. Oxygen and silica make up almost 75% of the rocks and another 24% is composed from aluminum, iron, calcium, sodium, potassium and magnesium. The remaining 17 is made from about 83 elements.
Various factors in the earth's activity caused the elements to combine and freeze as they did. There were seething volcanoes, shifting seas, the constant attack of the weather and the movements of the earths crust itself and these factors are still at work making, changing and remaking the countless different rocks of the earth s crust. Each chunk of rock has its own individual history, but for convenience we class these countless life stories in three main groups.
The igneous rocks are those formed by fire, by volcanic heat. Their histories begin deep below the surface where temperatures are high enough to melt rock into pools of buried magma. Active volcanoes erupt this material to the surface as molten lava which cools to become solid rock. Granite, pumice, obsidian and feldspar are some of the igneous rocks. Basalt, which forms massive layers under much of the earth's surface, is an igneous, fire‑formed rock.
Sedimentary rocks are formed by water. The weather is constantly dissolving, and washing away the mountain slopes. In the sea, a constant rain of particles settles to form sediments on the floor. Later, these sedimentary layers may lift above the water and become hard dry beds of sandstone or limestone. Sandstone is a layer of sand and gravel cemented together. Limestone is formed from the shells of countless little sea dwellers.
Metamorphic rocks have suffered still more change so that often the original form is lost. Buried under terrific pressure and heat, the sedimentary rock limestone becomes precious marble. Sedimentary shales may become metamorphic slate. Volcanic heat too may metamorphize a rock. Igneous granite, when percolated with the heat and gases from a new lava flow may become a mottled rock called gneiss ‑ pronounced nice.