Understanding Rocks: Composition, Types & Uses

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Introduction to Rock

Rock, the solid material that makes up the earth's crust. Soil, sand, and other loose natural materials are not usually considered to be rock, although they are derived principally from rocks. Rock used for construction is commonly called stone.

Virtually all rocks are composed of one or more minerals, which are natural substances that have characteristic properties and somewhat specific chemical compositions. The largest single group of minerals in rocks of the earth's crust are the silicates, which consist of silicon, oxygen, and one or more metals. The metals found most frequently in silicates are aluminum, sodium, magnesium, calcium, and potassium.

The scientific study of rocks and their relationship to one another—and to the earth as a whole—is called geology. A geologist or other scientist who specializes in the study of rocks themselves is a petrologist; an expert in minerals is a mineralogist.

Rocks are extremely varied in their appearance and general properties. Petrologists often identify and classify rocks on the basis of three main characteristics: texture, structure, and composition. Texture refers to the shape, size, and appearance of the individual particles in rock. Smooth-textured rocks may have small, flat particles. Coarse-textured rocks may have a variety of large angular particles.

Structure refers to the overall arrangement and appearance of rock in its natural setting. For example, some rock formations occur as massive blocks or columns, while others have a pronounced layered arrangement.

Composition refers to the minerals contained in the rock. By determining the kinds and abundances of minerals in a sample of rock, a scientist can often identify the rock. The color of a rock sample is often a clue to its mineral composition; red and brown rocks, for example, may owe their color to the presence of iron-rich minerals. Density (weight of a given volume) is another aid in identifying composition; relatively dense rock may contain minerals that are rich in heavy metals such as lead.

In addition, the rock's original location, especially in relation to nearby rocks that are already known, is of help in identification and classification.

Interesting facts about rocks Balanced Rock, in the Garden of the Gods near Colorado Springs, Colorado, is an enormous block of sandstone delicately balanced on a small base. Bendable rock. Most rocks cannot be bent or squeezed out of shape. But thin slabs of itacolumite, a rare kind of sandstone found in India and North Carolina, can be bent by hand because of their crystalline structure. Eight elements make up more than 98 percent of all the rocks in the world. These elements are found in about the following 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). Floating rock. Pumice is a rock that floats on water. It was once volcanic lava filled with gases. When the gases escaped, they left millions of tiny holes that filled with air. Rock of Gibraltar is a huge block of limestone near the southern tip of the mainland of Europe.

How Rocks Are Formed

There are three major classes of rocks, grouped according to the way in which they are formed: igneous, sedimentary, and metamorphic.

Igneous Rocks

are formed by the cooling of molten mixtures of minerals, called magmas, which are found far below the earth's crust. When magmas cool below the surface, the rocks formed are called intrusive. When they cool above the surface, the rocks formed are called extrusive.

Igneous rocksRockColorStructure Basalt Dark greenish-gray to blackDense, microscopic crystals, often form columns Gabbro Greenish-gray to blackCoarse crystals Granite White to gray, pink to redTightly arranged medium-to-coarse crystals Obsidian Black, sometimes with brown streaksGlassy, no crystals, breaks with a shell-like fracture Peridotite Greenish-grayCoarse crystals. Pumice Grayish-whiteLight, glassy, frothy, fine pores, floats on waterIntrusive Rock

Granite is the best-known intrusive igneous rock. It is composed mainly of feldspar and quartz, mixed with mica or hornblende. This firm, enduring rock is widely used for buildings and monuments. Other intrusive rocks include pegmatite, granite porphyry, diorite, gabbro, syenite, and peridotite.

Large masses of intrusive rock below the surface of the earth are called batholiths. Some cooled magma rises vertically to form projections called dikes. Some dikes have been uncovered by erosion and extend above the earth's surface. Another form of cooled magma, sills, consists of horizontal layers of igneous rock between layers of other kinds of rock.

Extrusive Rock

he most common form of this rock is lava from volcanoes. Basalt, a heavy, dark-colored rock, is a form of cooled lava. Other rocks formed by cooled lava include obsidian (sometimes called natural glass), pumice, rhyolite, and andesite. Metal ores are often found near masses of igneous rock. These ores were originally produced by the cooling of magma that had a metallic content, or by the chemical interaction of other rocks with cooling magma.

Sedimentary Rocks

are formed of loose materials—mud, sand, pebbles, and bits of organic matter—moved from one place to another. These materials are called sediment. Streams deposit vast amounts of sediment in the oceans. It is then moved about by waves and currents and left in smooth layers known as strata. Much sediment is also deposited on land by winds and ice. After millions of years, the sediment solidifies into rock because of chemical action and the pressure caused by overlying layers of material. About three-fourths of the earth's surface is covered with sedimentary rock.

Sandstone, a common sedimentary rock, is formed mainly of quartz particles cemented together by silica, calcium carbonate, or iron oxide. Shales are clays that have hardened into rock. Limestone is formed from calcium carbonate deposits made up largely of the microscopic remains of various sea organisms that produce shells. Conglomerate is sedimentary rock that is composed of gravel and other small, usually rounded rock fragments, often of quartz or limestone. These are held together by much finer-grained material that fills the spaces between the fragments. Breccia is similar to conglomerate, but its fragments are less rounded.

Sedimentary rocksRockColorStructure Breccia Gray to black, tan to redAngular pieces of rock, held together by natural cement Coal Shiny to dull blackBrittle, in seams or layers Flint Dark gray, black, brownHard, glassy, breaks with a sharp edge Limestone White, gray, and buff to black and redDense, forms thick beds and cliffs. May contain fossils Sandstone White, gray, yellow, redFine or coarse grains cemented together in beds Shale Yellow, red, gray, green, blackDense, fine particles, soft, splits easily, smells like clayMetamorphic Rocks

are rocks formed underground by the altering of sedimentary or igneous rocks. Such altering can be caused by heat, pressure, chemical action, or movement of the earth's crust, either separately or in combination. Slate, marble, quartzite, gneiss, and schist are all classed as metamorphic rocks.

Many metamorphic rocks display foliation, a banding or layering similar to that of sedimentary rocks. Foliation is particularly characteristic of gneiss and schist, and can also be seen in many examples of slate.

Metamorphic rocksRockColorStructure Amphibolite Light green to blackFine-to-coarse grains, hard, often sparkles Gneiss Gray and pink to black and redMedium to coarse crystals arranged in bands Marble Many colors, often mixedMedium to coarse crystals, may be banded Quartzite White, gray, pink, buffMassive, hard, often glassy Schist White, gray, red, green, blackFlaky particles, finely banded, feels slippery, often sparkles with mica Slate Black, red, green, purpleFine grains, dense, splits into thin, smooth slabs

Rock Collecting

Many people collect rocks as a hobby. Some gather them for color, such as agate with its bands of many hues; others collect specimens for odd or beautiful shapes; some for imprints of fossils. Some gather historic rocks, such as stones from battlefields or Indian mounds. Many rock collectors are also interested in lapidary work—the cutting and polishing of rocks into decorative objects such as book ends and costume jewelry. Some hobbyists engrave rocks.

For people who want to do their own collecting, every part of the country offers specimens. Mountains, seashore, river banks, woods, and lava plains are especially abundant in varied rocks. Many people simply pick up those on the surface. Others carry rock hammers, picks, nippers, and Geiger counters. Hobbyists can buy rocks from specialty stores; scientific supply houses offer mineral and rock collections for study in schools.

In the United States and Canada there are a number of local clubs for collectors. Many groups organize field trips. Most clubs are associated with a regional organization. Some 750 local and 6 regional clubs form the American Federation of Mineralogical Societies, founded in 1947. The federation, whose headquarters are in Oklahoma City, Oklahoma, publishes a newsletter. The Mineralogical Society of America, Washington, D.C., is a professional organization. It was founded in 1919.