The Rock

Nature's Strength

Rocks

The solid material that makes up the earth's crust. The entire solid surface of the earth is made up of rocks of various types. Soil, sand, and other loose materials are not usually considered to be rock, although they are derived principally from rocks.

Virtually all rocks are composed of one or more minerals, which are natural substances that have characteristic properties and more-or-less specific chemical compositions. The largest single group of minerals in the rocks of the earth's crust are silicates, which consist of silicon, oxygen, and one or more of the metals found most frequently in silicates which 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 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. Structure refers to the overall arrangment and appearance of rock in its natural setting. 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.

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 - 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.

Intrusive - Granite is the best-known intrusive igneous rock and is composed mainly of feldspar and quartz, mixed with mica or hornblende. 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,

Extrusive - The most common form of this rock is lava from volcanos. Basalt, a heavy, dark-colored rock, is a formed 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 colling of magma that had a metallic content, or by the chemical interaction of other rocks with cooling magma.

IGNEOUS ROCK

Four samples of rock formed underground by heating and cooling: upper left: grantite; upper right: obsidian, a noncrystalline rock fromed from lava; lower left: granite porphyry, with large crystals of feldspar; lower right: felsite, an extremely fine-grained igneous rock.

Sedimentary - Formed by 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 overlaying layers of materials. About three-fourths of the earth's crust is covered by 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 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 ROCK

These uplifted and folded strata in the Rocky Mountains of Montana originated as layers of sediment at the bottom of an ancient sea.

Metamorphic - 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 seperately or in a combination. Slate, marble, quartzite, gneiss, and schist are all classed as metamorphic rock. 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 ROCK

This outcrop of banded gneiss in Massachusettes shows the layered, or foliated, appearance that is typical of some metamorphic rocks. The gneiss was formed when underground heat and pressure seperated and aligned the light and dark minerals in a block of granite. The area shown in the photo is about seven feet (about 2 meters) from top to bottom.

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