Josie Morissette, age 11, of Stratford, Conn., for her question:
HOW DO ROCKS GET THEIR SIZE AND COLOR?
Rocks are made up of the very earth itself. They provide us with building material that is hard and lasting.
Rocks are made of minerals. They contain metal ores and other useful substances. Some sections of the country have more rocks than others.
Geologists put rocks into three groups: igneous, sedimentary and metamorphic rocks. Included in the igneous group are granite, basalt and pumice; clay, sandstone, shale and limestone in the sedimentary group; marble and slate in the metamorphic classification.
Unlike a plant or animal, a single specimen of rock can be composed of several different kinds of rock. A rock sample might very easily be part marble and part slate. For this reason a mere chip or fragment can seldom be clearly identified.
Igneous rocks were once hot and fluid. If the molten rock cooled slowly within the earth, the minerals contained in it had time to form crystals. The more slowly it cooled, the larger the crystals. On the surface cooling, such as a slow of lava from a volcano, produces glassy stone.
Sedimentary rocks are composed of weathered fragments or sediments of igneous rocks and older sedimentary rocks. They may be loose, as clay, sand or gravel, or they may be firm and compact, as when these sediments are compressed by the weight of overlaying deposits and impregnated with solutions of cementing minerals such as lime, silica or iron. Shale and sandstone are compacted, cemented forms of clay and sand.
Metamorphic rocks are those that have been altered by tremendous pressures and heat. They differ from igneous rocks, whose crystals are mixed, by having the crystals of each mineral more or less lined up in bands or layers. They dif=er from sedimentary rocks, which may also have a layered appearance, being much harder and crystalline.
All rocks exposed to the earth's surface are gradually eroded by the action of heat, cold, rain, snow and ice. This eroding process is called weathering. X
In weathering, a rock's surface is expanded by heat and contracted by cold. However, it does not expand or contract as a unit because various minerals composing it differ in their degree of expansion and contraction. So the rock cracks. And water seeps into it. In time, fragments of rocks are broken off and later become smaller fragments, eventually small enough for rains to carry off to streams and rivers.