Ronald Scott Taylor, age 11, of High Point, N.C., for his question:
DO ROCKS GROW?
Rocks do not grow but through the centuries they can change their, appearances and sometimes even change their mineral composition.
Three are three main types of rocks: igneous rocks, sedimentary rocks and metamorphic rocks.
Rock is the hard, solid part of the earth's crust. In many parts of the world the rock is covered by a layer of soil which is actually nothing more than tiny bits of rock mixed with organic materials from plants and animals.
Igneous rocks form when the molten or melted rock material deep within the earth, called magma, cools and solidifies.
Sedimentary rock is made up of materials that once were part of older rocks or plants and animals. They were deposited millions of years ago as layers of loose material.
Metamorphic rock is the rock that has changed its appearance and sometimes its mineral composition. These changes can be caused by hot magma, pressure and heat from mountain building movements in the earth or the chemical action of liquids and gases.
All kinds of rock, including igneous and sedimentary, have gone through such metamorphism to produce metamorphic rocks.
Granite, as an example, is an igneous rock that contains quartz, feldspar and mica. Metamorphism causes feldspar and quartz crystals to form layers between which mica crystals often lie in wavy bands. This new rock is called gneiss.
Metamorphism recrystalizes the calcite in limestone to form marble. The quartz grains in sandstone become more tightly packed to form quartzite.
Soft shales and clays harden to form slate, a rock that easily splits into smooth slabs. Felsites and impure sandstones, limestones and shales change into schists that glisten with mica and other minerals such as hornblende, chlorite and garnet.
Pumice is a rock that can actually float on water. It was once volcanic lava filled with gases. When the gases escaped, they left millions of tiny holes that filled with air.
More than 98 percent of all the rocks are made of eight elements. These elements are found in these percentages: oxygen, 46.5; silicon, 27.6; aluminum, 8.0; iron, 5.0; calcium, 3.6; sodium, 2.8; potassium, 2.6; and magnesium, 2.0.
Most rocks cannot be bent out of shape, as you know. But a thin slab of itacolumite, a rare type of sandstone found in parts of North Carolina, can be bent by hand because of its crystalline structure.
Geologists can trace the history of the earth by studying rocks.
Most rocks are aggregates, or combinations, of one or more minerals. Some rocks seem to be dense and massive and have no mineral grains. But if you examine a very thin slice of such rock under a microscope, you will see grains of minerals.