ROCKS

CONCEPT

It might come as a surprise to learn that geologists regularly use an unscientific-sounding term, rocks. Yet as is almost always the case with a word used both in everyday language and within the realm of a scientific discipline, the meanings are not the same. For one thing, rock and stone are not interchangeable, as they are in ordinary discussion. The second of these two terms is used only occasionally, primarily as a suffix in the names of various rocks, such as limestone or sandstone. On the other hand, a rock is an aggregate of minerals or organic material. Rocks are of three different types: igneous, formed by crystallization of molten minerals, as in a volcano; sedimentary, usually formed by deposition, compaction, or cementation of weathered rock; and metamorphic, formed by alteration of preexisting rock.

HOW IT WORKS

An Introduction to Rocks

To expand somewhat on the definition of rock, the term may be said to describe an aggregate of minerals or organic material, which may or may not appear in consolidated form. Consolidation, which we will explore further within the context of sedimentary rock, is a process whereby materials become compacted, or experience an increase in density. It is likely that the image that comes to mind when the word rock is mentioned is that of a consolidated one, but it is important to remember that the term also can apply to loose particles.

The role of organic material in forming rocks also belongs primarily within the context of sedimentary, as opposed to igneous or meta-morphic, rocks. There are, indeed, a handful of rocks that include organic material, an example being coal, but the vast majority are purely inorganic in origin. The inorganic materials that make up rocks are minerals, discussed in the next section. Rocks and minerals of economic value are called ores, which are examined in greater depth elsewhere, within the context of Economic Geology.

Minerals Defined

The definition of a mineral includes four components: it must appear in nature and therefore not be artificial, it must be inorganic in origin, it must have a definite chemical composition, and it must have a crystalline internal structure. The first of these stipulations clearly indicates that there is no such thing as a man-made mineral; as for the other three parts of the definition, they deserve a bit of clarification.

At one time, the term organic, even within the realm of chemistry, referred to all living or formerly living things, their parts, and substances that come from them. Today, however, chemists use the word to describe any compound that contains carbon and hydrogen, thus excluding carbonates (which are a type of mineral) and oxides such as carbon dioxide or carbon monoxide.

NONVARYING COMPOSITION.

The third stipulation, that a mineral must be of nonvarying composition, limits minerals almost exclusively to elements and compounds—that is, either to substances that cannot be chemically broken down to yield simpler substances or to substances formed by the chemical bonding of elements. The chemical bonding of elements is a process quite different from mixing, and a compound is not to be confused with a mixture, whose composition is highly variable.

Another way of putting this is to say that all minerals must have a definite chemical formula, which is not possible with a mixture such as dirt or glass. The Minerals essay, which the reader is encouraged to consult for further information, makes reference to certain alloys, or mixtures of metals, that are classified as minerals. These alloys, however, are exceptional and fit certain specific characteristics of interest to mineralogists. The vast majority of the more than 3,700 known varieties of mineral constitute either a single element or a single compound.

CRYSTALLINE STRUCTURE.

The fact that a mineral must have a crystalline structure implies that it must be a solid, since all crystalline substances are solids. A solid, of course, is a type of matter whose particles, in contrast to those of a gas or liquid, maintain an orderly and definite arrangement and resist attempts at compression. Thus, petroleum cannot be a mineral, nor is "mineral spirits," a liquid paint thinner made from petroleum (and further disqualified by the fact that it is artificial in origin).

Crystalline solids are those in which the constituent parts are arranged in a simple, definite geometric pattern that is repeated in all directions. These solids are contrasted with amorphous solids, such as clay. Metals are crystalline in structure; indeed, several metallic elements that appear on Earth in pure form (for example, gold, copper, and silver) also are classified as minerals.

Identifying Minerals

The type of crystal that appears in a mineral is one of several characteristics that make it possible for a mineralogist to identify an unidentified mineral. Although, as noted earlier, there are nearly 4,000 known varieties of mineral, there are just six crystal systems, or geometric shapes formed by crystals. Crystallographers, or mineralogists concerned with the study of crystal structures, are able to identify the crystal system by studying a good, well-formed specimen of a mineral, observing the faces of the crystal and the angles at which they meet.

Other characteristics by which minerals can be studied and identified visually are color, streak, and luster. The first of these features is not particularly reliable, because impurities in the mineral may greatly affect its hue. Therefore, mineralogists are much more likely to rely on streak, or the color of the powder produced when one mineral is scratched by a harder one. Luster, the appearance of a mineral when light reflects off its surface, is described by such terms as vitreous (glassy), dull, or metallic.

HARDNESS.

Minerals also can be identified according to what might be called tactile properties, or characteristics best discerned through the sense of touch. One of the most important among such properties is hardness, defined as the ability of one mineral to scratch another. Hardness is measured by the Mohs scale, introduced in 1812 by the German mineralogist Friedrich Mohs (1773-1839).

The scale rates minerals from 1 to 10, with 1 being equivalent to the hardness of talc, a mineral so soft that it is used for making talcum powder. A 2 on the Mohs scale is the hardness of gypsum, which is still so soft that it can be scratched by a human fingernail. Above a 5 on the scale, roughly equal to the hardness of a pocketknife or glass, are potassium feldspar (6), quartz (7), topaz (8), corundum (9), and diamond (10).

OTHER PROPERTIES.

Other tactile parameters are cleavage, the planes across which the mineral breaks, and fracture, the tendency to break along something other than a flat surface. Minerals also can be evaluated by their density (ratio of mass to volume) or specific gravity (ratio between the mineral's density and that of water). Density and specific-gravity measures are particularly important for extremely dense materials, such as lead or gold.

In addition to these specifics, others may be used for identifying some kinds of minerals. Magnetite and a few other minerals, for instance, are magnetic, while minerals containing uranium and other elements with a high atomic number may be radioactive, or subject to the spontaneous emission of high-energy particles. Still others are fluorescent, meaning that they glow when viewed under ultraviolet light, or phosphorescent, meaning that they continue to glow after being exposed to visible light for a short period of time.

Mineral Groups

Minerals are classified into eight basic groups:

• Class 1: Native elements
• Class 2: Sulfides
• Class 3: Oxides and hydroxides
• Class 4: Halides
• Class 5: Carbonates, nitrates, borates, iodates
• Class 6: Sulfates, chromates, molybdates, tungstates
• Class 7: Phosphates, arsenates, vanadates
• Class 8: Silicates

The first group, native elements, includes metallic elements that appear in pure form somewhere on Earth; certain metallic alloys, alluded to earlier; and native nonmetals, semi-metals, and minerals with metallic and nonmetallic elements. Sulfides include the most important ores of copper, lead, and silver, while halides are typically soft and transparent minerals containing at least one element from the halogens family: fluorine, chlorine, iodine, and bromine. (The most well known halide, table salt, is a good example of an unconsolidated mineral.)

Oxides are noncomplex minerals that contain either oxygen or hydroxide (OH). Included in the oxide class are such well-known materials as magnetite and corundum, widely used in industry. Other nonsilicates (a term that stresses the importance of silicates among mineral classes) include carbonates, or carbon-based minerals, as well as phosphates and sulfates. The latter are distinguished from sulfides by virtue of the fact that they include a complex anion (a negatively charged atom or group of atoms) in which an atom of sulfur, chromium, tungsten, selenium, tellurium, or molybdenum (or a combination of these) is attached to four oxygen atoms.

There are two other somewhat questionable classes of nonsilicate that might be included in a listing of minerals—organics and mineraloids. Though they have organic components, organics—for example, amber—originated in a geologic and not a biological setting. Mineraloids, among them, opal and obsidian, are not minerals because they lack the necessary crystalline structure, but they can be listed under the more loosely defined heading of "rocks."

SILICATES.

Only a few abundant or important minerals are nonsilicates, for example, the iron oxides hematite, magnetite, and goethite; the carbonates calcite and dolomite; the sulfides pyrite, sphalerite, galena, and chalcopyrite; and the sulfate gypsum. The vast majority of minerals, including the most abundant ones, belong to a single class, that of silicates, which accounts for 30% of all minerals. As their name implies, they are built around the element silicon, which bonds to four oxygen atoms to form what are called silica tetrahedra.

Silicon, which lies just below carbon on the periodic table of elements, is noted, like carbon, for its ability to form long strings of atoms. Carbon-hydrogen formations, or hydrocarbons, are the foundation of organic chemistry, while formations of oxygen and silicon—the two most abundant elements on Earth—provide the basis for a vast array of geologic materials. There is silica, for instance, better known as sand, which consists of silicon bonded to two carbon atoms.

Then there are the silicates, which are grouped according to structure into six subclasses. Among these subclasses, discussed in the Minerals essay, are smaller groupings that include a number of well-known mineral types: garnet, zircon, kaolinite, talc, mica, and the two most abundant minerals on Earth, feldspar and quartz. The name feldspar comes from the Swedish words feld ("field") and spar ("mineral"), because Swedish miners tended to come across the same rocks that Swedish farmers found themselves extracting from their fields.

REAL-LIFE APPLICATIONS

Rocks and Human Existence

Rocks are all around us, especially in our building materials but also in everything from jewelry to chalk. Then, of course, there are the rocks that exist in nature, whether in our backyards or in some more dramatic setting, such as a national park or along a rugged coastline. Indeed, humans have a long history of involvement with rocks—a history that goes far back to the aptly named Stone Age.

The latter term refers to a period in which the most sophisticated human tools were those made of rock—that is, before the development of the first important alloy used in making tools, bronze. The Bronze Age began in the Near East in about 3300 b.c. and lasted until about 1200 b.c., when the development of iron-making technology introduced still more advanced varieties of tools.

These dates apply to the Near East, specifically to such areas as Mesopotamia and Egypt, which took the lead in ancient technology, followed much later by China and the Indus Valley civilization of what is now Pakistan. The rest of the world was even slower in adopting the use of metal: for instance, the civilizations of the Americas did not enter the Bronze Age for almost 4,000 years, in about a.d. 1100. Nor did they ever develop iron tools before the arrival of the Europeans in about 1500.

THE STONE AGE.

In any case, the Stone Age, which practically began with the species Homo sapiens itself, was unquestionably the longest of the three ages. The Stone Age is divided into two periods: Paleolithic and Neolithic, sometimes called Old and New Stone Age, respectively. (There was also a middle phase, called the Mesolithic, but this term is not used as widely as Paleolithic or Neolithic.) Throughout much of this time, humans lived in rock caves and used rock tools, including arrowheads for killing animals and (relatively late in prehistory) flint for creating fire.

The Paleolithic, characterized by the use of crude tools chipped from pieces of stone, began sometime between 2.5 and 1.8 million years ago and lasted until last ice age ended (and the present Holocene epoch began), about 10,000 years ago. The Neolithic period that followed saw enormous advances in technology, so many advances that historians speak of a "Neolithic Revolution" that included the development of much more sophisticated, polished tools. The mining of gold, copper, and various other ores began long before the development of the first alloys (bronze is formed by the mixture of copper and tin). Yet even after humans discovered metals, they continued to use stone tools.

THE PYRAMIDS AND OTHER STONE STRUCTURES.

Indeed, the great pyramids of Egypt, built during the period from about 2600-2400 b.c., were constructed primarily with the use of stone rather than metal tools. The structures themselves, of course, also reflect the tight connection between humans and rocks. Built of limestone, the pyramids are still standing some 4,500 years later, even as structures of clay and mud built at about the same time in Mesopotamia (a region poor in stone resources) have long since dwindled to dust.

Incidentally, the great pyramids once had surfaces of polished limestone, such that they gleamed in the desert sun. Centuries later, Arab invaders in the seventh century a.d. stripped this limestone facing to use it in other structures, and the only part of the facing that remains today is high atop the pyramid of Khafre. For this reason, Khafre's pyramid is slightly taller than the structure known as the Great Pyramid, that of Cheops, or Khufu, which was originally the largest pyramid.

The centuries that have followed the building of those great structures likewise are defined, at least in part, by their buildings of stone. The Bible is full of references to stones, whether those used in building Solomon's temple or the precious gemstones said to form the gates of the New Jerusalem described in the Book of Revelation. Greece and Rome, too, are known for their structures of stone, ranging from marble (lime-stone that has undergone metamorphism) to unconsolidated stones in early forms of concrete, pioneered by the Egyptians.

Still later, medieval Europe built its cathedrals and castles of stone, though it should be noted that the idea of the castle came from the Middle East, where the absence of lumber for fortresses caused Syrian castle builders to make use of abundant sandstone instead. Other societies left behind their own great stone monuments: the Great Wall of China, Angkor Wat in southeast Asia, the pyramids of Central America and Machu Picchu in South America, the great cliffside dwellings of what is now the southwestern United States, and the stone churches of medieval Ethiopia.

Certainly there were civilizations that created great structures of wood, but these structures were simply not as durable. The oldest wood building, a Buddhist temple at Horyuji in Japan, dates back only to a.d. 607, which, of course, is quite impressive for a wooden structure. But it hardly compares to what may well be the oldest known human structure, a windbreak discovered by the paleobiologist Mary Leakey (1913-1996) in Tanzania in 1960. Consisting of a group of lava blocks that form a rough circle, it is believed to be 1.75 million years old.

Mineralogy and Petrology

Not surprisingly, mineralogy is concerned with minerals—their physical properties, chemical makeup, crystalline structures, occurrence, distribution, and physical origins. Researchers whose work focuses on the physical origins of minerals study data and draw on the principles of physics and chemistry to develop hypotheses regarding the ways minerals form. Other mineralogical studies may involve the identification of a newly discovered mineral or the synthesis of mineral-like materials for industrial purposes.

The study of rocks is called petrology, from a Greek root meaning "rock." (Hence also the words petroleum and petrify. ) Its areas of interest with regard to rocks are much the same as those of mineralogy as they relate to minerals: physical properties, distribution, and origins. It includes two major subdisciplines, experimental petrology, or the synthesis of rocks in a laboratory as a means of learning the conditions under which rocks are formed in the natural world, and petrography, or the study of rocks observed in thin sections through a petrographic microscope, which uses polarized light.

Owing to the fact that most rocks contain minerals, petrology draws on and overlaps with mineralogical studies to a great extent. At the same time, it goes beyond mineralogy, inasmuch as it is concerned with materials that contain organic substances, which are most likely to appear within the realm of sedimentary rock. Petrologists also are concerned with the other two principal types of rock, igneous and metamorphic.

Igneous Rocks

Igneous rock is rock formed by the crystallization of molten materials. It most commonly is associated with volcanoes, though, in fact, it comes into play in the context of numerous plate tectonic processes, such as seafloor spreading (see Plate Tectonics). The molten rock that becomes igneous rock is known as magma when it is below the surface of the earth and lava when it is at or near the earth's surface. Its most notable characteristic is its interlocking crystals. For the most part, igneous rocks do not have a layered texture.

When igneous rocks form deep within the Earth, they are likely to have large crystals, an indication of the fact that a longer period of time elapsed while the magma was cooling. On the other hand, volcanic rocks and others that form at or near Earth's surface are apt to have very small crystals. Obsidian (which, as we have noted, is not truly a mineral owing to its lack of crystals) is formed when hot lava comes into contact with water; as a result, it cools so quickly that crystals never have time to develop. Sometimes called volcanic glass, it once was used by prehistoric peoples as a cutting tool.

CLASSIFYING AND IDENTIFYING IGNEOUS ROCKS.

Igneous rocks can be classified in several ways, referring to the means by which they were formed, the size of their crystals, and their mineral content. Extrusive igneous rocks, ejected by volcanoes to crystallize at or near Earth's surface, have small crystals, whereas intrusive igneous rocks, which cooled slowly beneath the surface, have larger crystals. Sometimes the terms plutonic and volcanic, which roughly correspond to intrusive and extrusive, respectively, are used.

Igneous rocks made of fragments from volcanic explosions are known as pyroclastic, or "fire-broken," rocks. Those that consist of dense, dark materials are known as mafic igneous rocks. On the other hand, those made of lightly colored, less-dense minerals, such as quartz, mica, and feldspar, are called felsic igneous rocks. Among the most well known varieties of igneous rock is granite, an intrusive, felsic rock that includes quartz, feldspar, mica, and amphibole in its makeup. Also notable is basalt, which is mafic and extrusive.

Sedimentary Rocks

Earlier, we touched on the subject of consolidation, which can be explained in more depth within the context of sedimentary rock. Consolidation is the compacting of loose materials by any number of processes, including recrystallization and cementation. The first of these processes is the formation of new mineral grains as a result of changes in temperature, pressure, or other factors. In cementation, particles of sediment (material deposited at or near Earth's surface from several sources, most notably preexisting rock) are cemented together, usually with mud.

Compaction, recrystallization, and other processes, such as dehydration (which also may contribute to compaction), are collectively known as diagenesis. The latter term refers to all the changes experienced by a sediment sample under conditions of low temperature and low pressure following deposition. If the temperature and pressure increase, diagenesis may turn into metamorphism, discussed later in the context of metamorphic rock.

FORMATION OF SEDIMENTARY ROCKS.

Sedimentary rock is formed by the deposition, compaction, and cementation of rock that has experienced weathering (breakdown of rock due to physical, chemical, or biological processes) or as a result of chemical precipitation. The latter term refers not to "precipitation" in terms of weather but to the formation of a solid from a liquid, by chemical rather than physical means. (The freezing of water, a physical process, is not an example of precipitation.)

Sedimentary rock usually forms at or near the surface of the earth, as the erosive action of wind, water, ice, gravity, or a combination of these forces moves sediment. Yet this formation also may occur when chemicals precipitate from seawater or when organic material, such as plant debris or animal shells, accumulate. Evaporation of saltwater, for instance, produces gypsum, a mineral noted for its lack of thermal conductivity; hence its use in drywall, the material that covers walls in most modern homes. (Ancient peoples made alabaster, a fine-grained ornamental stone, from gypsum.)

CLASSIFICATION AND SIZES.

Sedimentary rock is classified with reference to the size of the particles from which the rock is made as well as the origin of those particles. Clastic rock comes from fragments of preexisting rock (whether igneous, sedimentary, or metamorphic) and organic matter, while nonclastic sedimentary rock is formed either by precipitation or by organic means. Examples include gypsum, salts, and other rocks formed by precipitation of saltwater as well as those created from organic material or organic activity—coal, for example.

Ranging in size from fine clay (less than 0.00015 in., or 0.004 mm) to boulders (defined as any rock larger than 10 in., or 0.254 m), sedimentary rock bears a record of the environment in which the original sediments were deposited. This record lies in the sediment itself. For example, rocks containing conglomerate, material ranging in size from clay to boulders (including the intermediate categories of silt, sand, gravel, pebbles, and cobble), come from sediment that was deposited rapidly as the result of slides or slumps. (Slides and slumps are discussed in Mass Wasting.)

Sedimentary rocks are of particular interest to paleontologists, stratigraphers, and others working in the field of historical geology, because they are the only kinds of rock in which fossils are preserved. The pressure and temperature levels that produce igneous and metamorphic rock would destroy the organic remnants that produce fossils; on the other hand, sedimentary rock—created by much less destructive processes—permits the formation of fossils. Thus, the study of these formations has contributed greatly to geologists' understanding of the distant past. (See the essays Historical Geology, Stratigraphy, and Paleontology. For more about sedimentary rock, see Sediment and Sedimentation.)

Metamorphic Rocks

Metamorphic rock is formed through the alteration of preexisting rock as a result of changes in temperature, pressure, or the activity of fluids (usually gas or water). These changes in temperature must be extreme (figures are given later), such that the preexisting rock—whether igneous, sedimentary, or metamorphic—is no longer stable.

Often formed in mountain environments, metamorphic rocks include such well-known varieties as marble, slate, and gneiss—metamorphosed forms of limestone, shale, and granite, respectively. Also notable is schist, composed of various minerals, such as talc, mica, and muscovite. There is not always a one-to-one correspondence between precursor rocks and metamorphic ones: increasing temperature and pressure can turn shale progressively into slate, phyllite, schist, and gneiss.

The presence of mica in a rock—or of other minerals, including amphibole, staurolite, and garnet—is a sign that the rock might be metamorphic. These minerals, typical of metamorphic rocks, are known as metamorphic facies. Also indicative of metamorphism are layers in the rock, more or less parallel lines along which minerals are laid as a result of the high pressures applied to the rock in its formation. Metamorphism, the process whereby metamorphic rock is created, also may produce characteristic formations, such as an alignment of elongate crystals or the separation of minerals into layers.

METAMORPHISM.

Given the conditions described for metamorphism, one might conclude that in terms of violence, drama, and stress, it is a process somewhere between sedimentation and the formation of igneous rock. That, in fact, is precisely the case: the temperature and pressure conditions necessary for metamorphism lie between those of diagenesis, on the one hand, and the extreme conditions necessary for the production of igneous rock, on the other hand. Specifically, metamorphism occurs at temperatures between 392°F (200°C) and 1,472°F (800°C) and under levels of pressure between 1,000 and 10,000 bars. (A bar is slightly less than the standard atmospheric pressure at sea level. The latter, equal to 14.7 lb. per square inch, or 101,325 Pa, is equal to 1.01325 bars.)

There are several types of metamorphism: regional, contact, dynamic, and hydrothermal. Regional metamorphism results from a major tectonic event or events, producing widespread changes in rocks. Contact or thermal metamorphism results from contact between igneous intrusions and cooler rocks above them, which recrystallize as a result of heating. Dynamic metamorphism takes place in the high-pressure conditions along faults. Finally, hydrothermal metamorphism ensues from contact with fluids heated by igneous rock. Reacting with minerals in the surrounding rock, the fluids produce different minerals, which, in turn, yield metamorphic rocks.

TYPES OF METAMORPHIC ROCKS.

Metamorphic rocks that contain elongate or platy minerals, such as mica and amphibole, are called foliated rocks. These rocks have a layered texture, which may manifest as the almost perfect arrangement of materials in slate or as the alternating patterns of light and dark found in some other varieties of rock. Metamorphic rocks without visible layers are referred to as unfoliated rocks. As a foliated metamorphic rock, slate is particularly good for splitting into thin layers—hence one of its most important applications is in making shingles for roofing. By contrast, marble, which is unfoliated, is valued precisely for its lack of tendency to split.

Petrologists attempting to determine exactly which rocks or combinations of rocks metamorphosed to produce a particular sample often face a challenge. Many metamorphic rocks are stubborn about giving up their secrets; on the other hand, it is possible to match up precursor rocks with certain varieties. For example, as noted earlier, marble comes from limestone, while gneiss usually (but not always) comes from granite. Quartzite is metamorphosed sandstone. Nonetheless, it is not as easy to trace the history of a metamorphic rock as it is to say that a raisin was once a grape or that a pickle was once a cucumber.

Where to Find Rocks

In general, one might find igneous rocks such as basalt in any place known for volcanic activity either in the recent or distant past. This would include such well-known areas of volcanism as Hawaii, the Philippines, and Italy, but also places where volcanic activity occurred in the distant past. (See, for instance, the discussion in the essay titled "Paleontology" regarding possible volcanic activity in what is now the continental United States at the conclusion of the Triassic period.)

The best place for metamorphic rock would be in areas of mountain-building and powerful tectonic activity, as for instance in the Himalayas or the Alps of central Europe. Sedimentary rock is basically everywhere, but a good place to find large samples of it would include areas with large oil deposits, which are always found in sedimentary rock.

Closer to home, a wide array of sedimentary rocks can be located in the plains and lowlands of the United States, particularly in the West and Midwest, where large samples are exposed. Igneous and metamorphic rocks can be found, predictably, in regions where mountains provide evidence of past tectonic activity: New England, the Appalachians, and the various mountain ranges of the western United States such as the Rockies, Cascades, and Sierra Nevada.

The Rock Cycle

Given what we have seen about the characteristics of the three rock varieties—igneous, sedimentary, and metamorphic—it should be clear that there is no such thing as a rock that simply is what it is, without any possibility of changing. Rocks, in fact, are constantly changing, as is Earth itself. This process whereby rocks continually change from one type to another—typically through melting, metamorphism, uplift, weathering, burial, or other processes—is known as the rock cycle.

The rock cycle can go something like this: Exposed to surface conditions such as wind and the activity of water, rocks experience weathering. The result is the formation of sediments that are eventually compacted to make sedimentary rocks. As the latter are buried deeper and deeper beneath greater amounts of sediment, the pressure and temperature builds. This process ultimately can result in the creation of metamorphic rock. On the other hand, the rock may undergo such extreme conditions of temperature that it recrystallizes to form igneous rock. Whatever the variety—igneous, sedimentary, or metamorphic—the rock likely will be in a position eventually to experience erosion, in which case the rock cycle begins all over again.

WHERE TO LEARN MORE

Atlas of Igneous and Metamorphic Rocks, Minerals, and Textures (Web site). <http://www.geosci.unc.edu/Petunia/IgMetAtlas/mainmenu.html>.

Bishop, A. C., Alan Robert Woolley, and William Roger Hamilton. Cambridge Guide to Minerals, Rocks, and Fossils. New York: Cambridge University Press, 1999.

Busbey, Arthur Bresnahan. Rocks and Fossils. Alexandria, VA: Time-Life Books, 1996.

Discovery Channel Rocks and Minerals: An Explore Your World Handbook. New York: Discovery Books, 1999.

"The Essential Guide to Rocks." BBC Education (Web site). <http://www.bbc.co.uk/education/rocks/>.

"Igneous, Sedimentary, and Metamorphic Rock Info. "University of British Columbia (Web site). <http://www.science.ubc.ca/~geol202/petrology/rock.html>.

RocksForKids.com (Web site). <http://www.rocksforkids.com/>.

Rocks and Minerals (Web site). <http://www.fi.edu/tfi/units/rocks/rocks.html>.

Symes, R. F., Colin Keates, and Andreas Einsiedel. Rocks and Minerals. New York: Dorling Kindersley, 2000.

Vernon, R.H. Beneath Our Feet: The Rocks of Planet Earth. New York: Cambridge University Press, 2000.

KEY TERMS

ALLOY:

A mixture of two or more metals.

CEMENTATION:

A process of consolidation whereby particles of sediment are cemented together, usually with mud.

COMPOUND:

A substance made up of atoms of more than one element, chemically bonded to one another.

CONGLOMERATE:

Unconsolidatedrock material containing rocks ranging in size from very small clay (less than 0.00015 in., or 0.004 mm) to boulders (defined as any rock larger than 10 in., or 0.254 m). Sedimentary rock often appears in the form of conglomerate.

CONSOLIDATION:

A process whereby materials become compacted, or experience an increase in density. This takes place through several processes, including recrystallization and cementation.

CRYSTALLINE SOLID:

A type of solid in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions.

DEPOSITION:

The process wherebysediment is laid down on the Earth's surface.

DIAGENESIS:

A term referring to all the changes experienced by a sediment sample under conditions of low temperature and low pressure followingdeposition. Higher temperature and pressure conditions may lead to metamorphism.

ELEMENT:

A substance made up of only one kind of atom. Unlike compounds, elements cannot be broken chemically into other substances.

EROSION:

The movement of soil and rock as the result of forces produced bywater, wind, glaciers, gravity, and other influences. In most cases, a fluid medium, such as air or water, is involved.

IGNEOUS ROCK:

One of the three principal types of rock, along with sedimentary and metamorphic rock. Igneous rock is formed by the crystallization of molten materials, for instance, in a volcano or other setting where plate tectonicprocesses take place.

LAVA:

Molten rock at or near the surface of the earth that becomes igneousrock. Below the surface, lava is known as magma.

MAGMA:

Molten rock beneath the surface of the earth that becomes igneousrock. Once it is at or near the surface, magma is known as lava.

MINERAL:

A naturally occurring, typically inorganic substance with a specific chemical composition and a crystalline structure.

MINERALOGY:

An area of geology devoted to the study of minerals. Mineralogy includes several subdisciplines, such ascrystallography, the study of crystal formations within minerals.

MIXTURE:

A substance with a variable composition, meaning that it is composed of molecules or atoms of differing types in varying proportions.

ORE:

A rock or mineral possessing economic value.

ORGANIC:

At one time, chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon and hydrogen, thus excluding carbonates (which are minerals), and oxides such as carbon dioxide.

PETROLOGY:

An area of geology devoted to the study of rocks, including their physical properties, distribution, and origins.

PRECIPITATION:

In the context of chemistry, precipitation refers to the formation of a solid from a liquid.

RECRYSTALLIZATION:

The formation of new mineral grains as a result of changes in temperature, pressure, or other factors.

ROCK:

An aggregate of minerals or organic matter, which may be consolidated or unconsolidated.

ROCK CYCLE:

The ongoing process whereby rocks continually change from one type to another, typically through melting, metamorphism, uplift, weathering, burial, or other processes.

SAND:

A term that can have several meanings. The sand at a beach could be a variety of unconsolidated materials, though most likely it is silica (SiO₂). Sand is also a term used for a size of rock ranging from very fine to very coarse.

SEDIMENT:

Material deposited at or near Earth's surface from a number of sources, most notably preexisting rock.

SEDIMENTARY ROCK:

One of the three major types of rock, along with igneous and metamorphic rock. Sedimentary rock usually is formed by the deposition, compaction, and cementation of rock that has experienced weathering. It also may be formed as a result of chemical precipitation.

UNCONSOLIDATED ROCK:

Rock that appears in the form of loose particles, such as sand.

UPLIFT:

A process whereby the surface of Earth rises, owing to either a decrease in downward force or an increase in upward force.

WEATHERING:

The breakdown of rocks and minerals at or near the surface of Earth as the result of physical, chemical, or biological processes.

Rocks

Types of rocks

The rock cycle

Current research

Resources

Geologists define rocks as aggregates of minerals, which are naturally occurring, inorganic substances with specific chemical compositions and structures. A rock can consist of many crystals of one mineral or combinations of many minerals. Several exceptions, such as coal and obsidian, are not composed of minerals but are considered to be rocks. Common uses for rocks include building materials, roofs, sculpture, jewelry, tombstones, chalk. Coal is mined and burned for heat. Many metals are derived from rocks known as ores. Oil and natural gas are also found in rocks.

Prehistoric humans used rocks as early as 2,000,000 BC. Flint and other hard rocks were important raw materials for crafting arrowheads and other tools. By 500,000 BC, rock caves and structures made from stones had become important forms of shelter for early man. During that time, early man had learned to use fire, a development that allowed humans to cook food and greatly expand their geographical range. Eventually, probably no later than 5000 BC, humans realized that metals such as gold and copper could be derived from rocks. Many ancient monuments were crafted from stone, including the pyramids of Egypt, built from limestone around 2500 BC, and the buildings of Chichen Itza in Mexico, also of limestone, were built around AD 450.

Since at least the 1500s, scientists have studied minerals and mining, fundamental aspects of the study of rocks. Georgius Agricola (the Latin name for Georg Bauer) published De Re Metallica (Concerning Metallic Things) in 1556. By 1785, the British geologist James Hutton published Theory of the Earth, in which he discussed his observations of rocks in Great Britain and his conclusion that Earth is much older than previous scientists had estimated.

Types of rocks

Geologists, scientists who study Earth and rocks, distinguish three main groups of rocks: igneous rocks, sedimentary rocks, and metamorphic rocks. These distinctions are made on the basis of the types of minerals in the rock, the shapes of individual mineral grains, and the overall texture of the rock, all of which indicate the environment, pressure, and temperature in which the rock formed.

Igneous rocks

Igneous rocks form when molten rock, known as magma (if below the surface of Earth) or lava (at the surface of Earth), solidifies. The minerals in the rock crystallize or grow together so that the individual crystals lock together. Igneous rocks make up much of the oceanic and continental crust, as well as most of the rock deeper in the Earth.

Igneous rocks can be identified by the interlocking appearance of the crystals in them. Typical igneous rocks do not have a layered texture, but exceptions exist. For example, in large bodies of igneous rock, relatively dense crystals that form early can sink to the bottom of the magma, and less dense layers of crystals that form later can accumulate on top. Igneous rocks can form deep within Earth or at the surface of Earth in volcanoes. In general, igneous rocks that form deep within Earth have large crystals that indicate a longer period of time during which the magma cools. Igneous rocks that form at or near the surface of Earth, such as volcanic igneous rocks, cool quickly and contain smaller crystals that are difficult to see without magnification. Obsidian, sometimes called volcanic glass, cools so quickly that no crystals form. Nevertheless, obsidian is considered to be an igneous rock.

Igneous rocks are classified on the basis of their mineral content and the size of the crystals in the rock. Extrusive igneous rocks have small crystals and crystallize at or near Earth’s surface. Intrusive igneous rocks cool slowly below Earth’s surface and have larger crystals. Rocks made up of dense, dark-colored minerals such as olivine, pyroxene, amphibole, and plagioclase are called mafic igneous rocks. Lighter-colored, less dense minerals, including quartz, mica, and feldspar, make up felsic igneous rocks.

Common igneous rocks include the felsic igneous rocks granite and rhyolite, and the mafic igneous rocks gabbro and basalt. Granite is an intrusive igneous rock that includes large crystals of the minerals quartz, feldspar, mica, and amphibole that form deep within the Earth. Rhyolite includes the same minerals, but forms as extrusive igneous rock near the surface of Earth or in volcanoes and cools quickly from magma or lava, so its crystals are difficult to observe with the naked eye. Similarly, gabbro is more coarse-grained than basalt and forms deeper in Earth, but both rocks include the minerals pyroxene, feldspar, and olivine.

Excellent exposures of igneous rocks occur in the volcanoes of Hawaii, volcanic rocks of Yellowstone National Park (located in Wyoming, Idaho, and Montana), in Lassen Volcanic National Park and Yosemite National Park (both in California), and in the crater of Mount St. Helens (Washington).

Sedimentary rocks

Sedimentary rocks are those made of grains of preexisting rocks or organic material that, in most cases, have been eroded, deposited, compacted, and cemented together. They typically form at the surface of Earth as sediment moves as a result of the action of wind, water, ice, gravity, or a combination of these. Sedimentary rocks also form as chemicals precipitate from seawater, or through accumulation of organic material such as plant debris or animal shells. Common sedimentary rocks include shale, sandstone, limestone, and conglomerate. Sedimentary rocks typically have a layered appearance because most sediment is deposited in horizontal layers and buried beneath later deposits over long periods of time. Sediments deposited rapidly, however, tend to be poorly layered if layers are present at all.

Sedimentary rocks form in many different environments at the surface of Earth. Eolian, or wind blown, sediments can accumulate in deserts. Rivers carry sediments and deposit them along their banks or into lakes or oceans. Glaciers deposit sediments that were picked up as the glacier expands and moves; glacial deposits are well exposed in the northern United States. Sediments can travel in currents below sea level to the deepest parts of the ocean floor. Secretion of calcium carbonate shells by reef-building organisms produce large quantities of limestone. Evaporation of seawater has resulted in the formation of widespread layers of salt and gypsum. Swamps rich in plants can produce coal if organic material accumulates and is buried before aerobic bacteria can destroy the dead plants.

Sedimentary rocks are classified on the basis of the sizes of the particles in the rock and the composition of the rock. Clastic sedimentary rocks comprise fragments of preexisting rocks and minerals. Chemical precipitates are sedimentary rocks that form by precipitation of minerals from seawater, salt lakes, or mineral-rich springs. Organic sedimentary rocks form from organic matter or organic activity, such as coal and limestone made by reef-building organisms like coral. Grain sizes in sedimentary rocks range from fine clay and silt to sand to boulders.

The sediment in a sedimentary rock reflects its environment of deposition. For example, wind-blown sand grains commonly display evidence of abrasion of their surfaces as a result of colliding with other grains. Sediments transported long distances tend to decrease in size and are more rounded than sediment deposited near their precursor rocks because of wearing against other sediments or rocks. Large or heavy sediments tend to settle out of water or wind if the energy of the water or wind is insufficient to carry the sediments. Sediments deposited rapidly as a result of landslides or slumps tend to include a larger range of sediment sizes, from large boulders to pebbles to sand grains and flakes of clay. Such rocks are called conglomerate. Along beaches, the rhythmic activity of waves moving sediment back and forth produces sandstones in which the grains are well rounded and of similar size. Glaciers pick up and carry a wide variety of sediments and often scratch or scrape the rocks over which they travel.

Sedimentary rocks are the only rocks in which fossils can be preserved because at the elevated temperatures and pressures in which igneous and high grade metamorphic rocks form, fossils and organic remnants are destroyed. The presence of fossils and the types of fossil organisms in a rock provide clues about the environment and age of sedimentary rocks. For example, fossils of human beings are not present in rocks older than approximately two million years because humans did not exist before then. Similarly, dinosaur fossils do not occur in rocks younger than about 65 million years because dinosaurs became extinct at that time. Fish fossils in sedimentary rock indicate that the sediments that make up the rock were deposited in a lake, river, or marine environment. By establishing the environment of the fossils in a rock, scientists learn more about the conditions under which the rock formed.

Spectacular exposures of sedimentary rocks include the Grand Canyon (Arizona), the eolian sandstones of Zion National Park (Utah), the limestone of Carlsbad National Park (New Mexico), and glacial features of Voyageurs National Park (Minnesota).

Metamorphic rocks

Metamorphic rocks are named for the process of metamorphism, or change, that affects rocks. The changes that form metamorphic rocks usually include increases in the temperature (generally to at least 392°F [200°C]) and the pressure of a precursor rock, which can be igneous, sedimentary, or metamorphic, to a degree that the minerals in the rock are no longer stable. The rock might change in mineral content or appearance, or both, although the general chemical composition remains unchanged. Clues to identifying metamorphic rocks include the presence of minerals such as mica, amphibole, staurolite, and garnet, and layers in which minerals are aligned as a result of pressure applied to the rock. Common metamorphic rocks include slate, schist, and gneiss. Metamorphic rocks commonly occur in mountains, such as the Appalachian Mountains, parts of California, and the ancient, eroded metamorphic rocks in the Llano Uplift of central Texas.

Metamorphic rocks are classified according to their constituent minerals and texture. Foliated metamorphic rocks are those that have a layered texture. In foliated metamorphic rocks, elongate or platy minerals such as mica and amphibole become aligned as a result of pressure on the rock. Foliation can range from alternating layers of light and dark minerals typical of gneiss to the seemingly perfect alignment of platy minerals in slate. Some metamorphic rocks are unfoliated and have a massive texture devoid of layers. Mineralogy of metamorphic rocks reflects the mineral content of the precursor rock and the pressure and temperature at which metamorphism occurs.

As sediments undergo metamorphism, the layers of sediment can be folded or become more pronounced as pressure on the rock increases. Elongate or platy minerals in the rock tend to become aligned in the same direction. For example, when shale metamorphoses to slate, it becomes easier to split the well-aligned layers of the slate into thin, flat sheets. This property of slate makes it an attractive roofing material. Marble-metamorphosed limestone-typically does not have the pronounced layers of slate, but is used for flooring and sculptures.

Metamorphism of igneous rocks can cause the different minerals in the rocks to separate into layers. When granite metamorphoses into gneiss, layers of light-colored minerals and dark-colored minerals form. As with sedimentary rocks, elongate or platy minerals become well-aligned as pressure on the rock increases.

It is possible for metamorphic rocks to metamorphose into other metamorphic rocks. In some regions, especially areas where mountain building is taking place, it is not unusual for several episodes of metamorphism to affect rocks. It can be difficult to unravel the effects of each episode of metamorphism.

The rock cycle

The rock cycle illustrates how the three main rock types can change from one type to another. As rocks exposed at the surface weather, they form sediments that can be deposited to form sedimentary rocks. As sedimentary rocks are buried beneath more sediment, they are subjected to increases in both pressure and temperature, which can result in metamorphism and the formation of metamorphic rock. If the temperature of metamorphism is extremely high, the rock might melt completely and later recrystallize as an

KEY TERMS

Cementation— Process through which minerals are glued together, usually as a result of precipitation of solids from solutions in sediments. Calcite, quartz, and clay minerals such as chlorite are common cement-forming minerals in sedimentary rocks.

Compaction— Reduction of volume of material. Sediments typically compact following burial beneath newer sediments.

Igneous rock— Rock formed by solidification of molten minerals.

Lava— Molten rock that occurs at the surface of Earth, usually through volcanic eruptions. Lava solidifies into igneous rock when it cools.

Magma— Molten rock found below the surface of Earth. It can crystallize, or solidify, to form igneous rock.

Metamorphic rock— Rock formed by alteration of preexisting rock through changes in temperature, pressure, or activity of fluids.

Mineral— A naturally occurring, inorganic substance with a definite chemical composition and structure.

Rock— A naturally occurring solid mixture of minerals.

Rock cycle— The processes through which rocks change from one type to another, typically through melting, metamorphism, uplift, weathering, burial, or other processes.

Sedimentary rock— Rock formed by deposition, compaction, and cementation of weathered rock or organic material, or by chemical precipitation. Salt and gypsum form from evaporation and precipitation processes.

Uplift— An episode in the history of a region when tectonic forces lift the region’s crust to a higher elevation.

Weathering— Biological, chemical, and mechanical attack on rock which breaks it up and alters it at or near the surface of Earth.

igneous rock. Igneous, sedimentary, and metamorphic rocks can erode and later form sedimentary rock. Rocks can move through the rock cycle along other paths, but uplift or burial, weathering, and changes in temperature and pressure are the primary causes of changes in rocks from one type to another.

Current research

Geologists who study rocks attempt to answer a variety of questions: What do rocks and the ratios of stable to unstable isotopes within rocks tell us about the age of Earth, the times at which Earth’s tectonic plates collided to produce mountains, and global warming? At what times were glaciers present on different continents? Where might we expect to have earthquakes and volcanic eruptions? What types of fossils occur in rocks and how do the fossils differ among rocks from all over the world? In which rocks might we find safe supplies of water, hydrocarbons, and mineral resources such as copper, diamonds, graphite, and aluminum? Although these problems are not often easy to solve, rocks supply important information about them.

Geologists examine rocks in various settings. Some go out to places where rocks are exposed at the surface of Earth in order to map occurrences and to collect samples of rocks for further study in the laboratory. Others work in the laboratory examining thin slices of rock under microscopes, determining the structure and chemical composition of individual crystals within a rock, determining the ratios of different isotopes of atoms within a crystal or rock, or examining the fossils in rocks. Geologists who work in different areas of Earth try to compare the rocks and fossils they find in order to determine how Earth has changed through time. For example, the eastern coast of South America and the western coast of Africa share many common rocks and fossils, suggesting that these areas might have been closer in the past.

Geologists also pay close attention to ongoing phenomena: large, destructive earthquakes in California and Japan; a surge in the Bering Glacier of Alaska, the largest glacier in North America; and volcanic activity in Mexico, West Indies, Indonesia, Papua New Guinea, and Italy. Scientists are actively involved in the search for safe locations to dispose of some of our dangerous wastes. Our understanding of Earth’s processes are also helping unravel questions about other planets and astronomical bodies in our solar system. In addition, studies of how and where rocks form continue.

See also Geology; Metal; Ore.

Resources

BOOKS

Blatt, H., R. Tracy, and B. Owens. Petrology: Igneous, Sedimentary, and Metamorphic. New York: Freeman, 2005.

Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, New Jersey: Prentice Hall, 2004.

Gretchen M. Gillis

Rocks

Geologists define rocks as aggregates of minerals . Minerals are naturally occurring, inorganic substances with specific chemical compositions and structures. A rock can consist of many crystals of one mineral, or combinations of many minerals. Several exceptions, such as coal and obsidian, are not composed of minerals but are considered to be rocks. Common uses for rocks include building materials, roofs, sculpture, jewelry, tombstones, chalk, and coal for heat. Many metals are derived from rocks known as ores. Oil and natural gas are also found in rocks.

Prehistoric humans used rocks as early as 2,000,000 b.c. Flint and other hard rocks were important raw materials for crafting arrowheads and other tools. By 500,000 b.c., rock caves and structures made from stones had become important forms of shelter for early man. During that time, early man had learned to use fire, a development that allowed humans to cook food and greatly expand their geographical range. Eventually, probably no later than 5000 b.c., humans realized that metals such as gold and copper could be derived from rocks. Many ancient monuments were crafted from stone, including the pyramids of Egypt, built from limestone around 2500 b.c., and the buildings of Chichen Itza in Mexico, also of limestone, built around a.d. 450.

Since at least the 1500s, scientists have studied minerals and mining , fundamental aspects of the study of rocks. Georgius Agricola (the Latin name for Georg Bauer) published De Re Metallica (Concerning Metallic Things) in 1556. By 1785, the British geologist James Hutton published Theory of the Earth, in which he discussed his observations of rocks in Great Britain and his conclusion that Earth is much older than previous scientists had estimated.

Types of rocks

Geologists, scientists who study the earth and rocks, distinguish three main groups of rocks: igneous rocks , sedimentary rocks, and metamorphic rocks. These distinctions are made on the basis of the types of minerals in the rock, the shapes of individual mineral grains, and the overall texture of the rock, all of which indicate the environment, pressure , and temperature in which the rock formed.

Igneous rocks

Igneous rocks form when molten rock, known as magma (if below the surface of the Earth) or lava (at the surface of the Earth), solidifies. The minerals in the rock crystallize or grow together so that the individual crystals lock together. Igneous rocks and magma make up much of the oceanic and continental crust, as well as most of the rock deeper in the Earth.

Igneous rocks can be identified by the interlocking appearance of the crystals in them. Typical igneous rocks do not have a layered texture, but exceptions exist. For example, in large bodies of igneous rock, relatively dense crystals that form early can sink to the bottom of the magma, and less dense layers of crystals that form later can accumulate on top. Igneous rocks can form deep within the Earth or at the surface of the Earth in volcanoes. In general, igneous rocks that form deep within the Earth have large crystals that indicate a longer period of time during which the magma cools. Igneous rocks that form at or near the surface of the Earth, such as volcanic igneous rocks, cool quickly and contain smaller crystals that are difficult to see without magnification. Obsidian, sometimes called volcanic glass , cools so quickly that no crystals form. Nevertheless, obsidian is considered to be an igneous rock.

Igneous rocks are classified on the basis of their mineral content and the size of the crystals in the rock. Extrusive igneous rocks have small crystals and crystallize at or near the Earth's surface. Intrusive igneous rocks cool slowly below the Earth's surface and have larger crystals. Rocks made up of dense, dark-colored minerals such as olivine, pyroxene, amphibole, and plagioclase are called mafic igneous rocks. Lighter-colored, less dense minerals, including quartz, mica, and feldspar, make up felsic igneous rocks.

Common igneous rocks include the felsic igneous rocks granite and rhyolite, and the mafic igneous rocks gabbro and basalt. Granite is an intrusive igneous rock that includes large crystals of the minerals quartz, feldspar, mica, and amphibole that form deep within the Earth. Rhyolite includes the same minerals, but forms as extrusive igneous rock near the surface of the Earth or in volcanoes and cools quickly from magma or lava, so its crystals are difficult to observe with the naked eye . Similarly, gabbro is more coarse-grained than basalt and forms deeper in the Earth, but both rocks include the minerals pyroxene, feldspar, and olivine.

Fabulous exposures of igneous rocks occur in the volcanoes of Hawaii, volcanic rocks of Yellowstone National Park (located in Wyoming, Idaho, and Montana), and in Lassen Volcanic National Park and Yosemite National Park (both in California).

Sedimentary rocks

Sedimentary rocks are those made of grains of preexisting rocks or organic material that, in most cases, have been eroded, deposited, compacted, and cemented together. They typically form at the surface of the Earth as sediment moves as a result of the action of wind , water , ice , gravity, or a combination of these. Sedimentary rocks also form as chemicals precipitate from seawater, or through accumulation of organic material such as plant debris or animal shells. Common sedimentary rocks include shale, sandstone, limestone, and conglomerate. Sedimentary rocks typically have a layered appearance because most sediments are deposited in horizontal layers and are buried beneath later deposits of sediments over long periods of time. Sediments deposited rapidly, however, tend to be poorly layered if layers are present at all.

Sedimentary rocks form in many different environments at the surface of the Earth. Eolian, or wind blown, sediments can accumulate in deserts. Rivers carry sediments and deposit them along their banks or into lakes or oceans. Glaciers form unusual deposits of a wide variety of sediments that they pick up as the glacier expands and moves; glacial deposits are well exposed in the northern United States. Sediments can travel in currents below sea level to the deepest parts of the ocean floor. Secretion of calcium carbonate shells by reef-building organisms produce large quantities of limestone. Evaporation of seawater has resulted in the formation of widespread layers of salt and gypsum. Swamps rich in plants can produce coal if organic material accumulates and is buried before aerobic bacteria can destroy the dead plants.

Sedimentary rocks are classified on the basis of the sizes of the particles in the rock and the composition of the rock. Clastic sedimentary rocks comprise fragments of preexisting rocks and minerals. Chemical precipitates are sedimentary rocks that form by precipitation of minerals from seawater, salt lakes, or mineral-rich springs. Organic sedimentary rocks formed from organic matter or organic activity, such as coal and limestone made by reef-building organisms like coral. Grain sizes in sedimentary rocks range from fine clay and silt to sand to boulders.

The sediment in a sedimentary rock reflects its environment of deposition. For example, wind-blown sand grains commonly display evidence of abrasion of their surfaces as a result of colliding with other grains. Sediments transported long distances tend to decrease in size and are more rounded than sediment deposited near their precursor rocks because of wearing against other sediments or rocks. Large or heavy sediments tend to settle out of water or wind if the energy of the water or wind is insufficient to carry the sediments. Sediments deposited rapidly as a result of slides or slumps tend to include a larger range of sediment sizes, from large boulders to pebbles to sand grains and flakes of clay. Such rocks are called conglomerate. Along beaches, the rhythmic activity of waves moving sediment back and forth produces sandstones in which the grains are well rounded and of similar size. Glaciers pick up and carry a wide variety of sediments and often scratch or scrape the rocks over which they travel.

Sedimentary rocks are the only rocks in which fossils can be preserved because at the elevated temperatures and pressures in which igneous and metamorphic rocks form, fossils and organic remnants are destroyed. The presence of fossils and the types of fossil organisms in a rock provide clues about the environment and age of sedimentary rocks. For example, fossils of human beings are not present in rocks older than approximately two million years because humans did not exist before then. Similarly, dinosaur fossils do not occur in rocks younger than about 65 million years because dinosaurs became extinct at that time. Fish fossils in sedimentary rock indicate that the sediments that make up the rock were deposited in a lake , river, or marine environment. By establishing the environment of the fossils in a rock, scientists learn more about the conditions under which the rock formed.

Spectacular exposures of sedimentary rocks include the Grand Canyon (Arizona), the eolian sandstones of Zion National Park (Utah), the limestones of Carlsbad National Park (New Mexico), and glacial features of Voyageurs National Park (Minnesota).

Metamorphic rocks

Metamorphic rocks are named for the process of metamorphism , or change, that affects rocks. The changes that form metamorphic rocks usually include increases in the temperature (generally to at least 392°F [200°C]) and the pressure of a precursor rock, which can be igneous, sedimentary, or metamorphic, to a degree that the minerals in the rock are no longer stable. The rock might change in mineral content or appearance, or both. Clues to identifying metamorphic rocks include the presence of minerals such as mica, amphibole, staurolite, and garnet, and layers in which minerals are aligned as a result of pressure applied to the rock. Common metamorphic rocks include slate, schist, and gneiss. Metamorphic rocks commonly occur in mountains, such as the Appalachian Mountains, parts of California, and the ancient, eroded metamorphic rocks in the Llano Uplift of central Texas.

Metamorphic rocks are classified according to their constituent minerals and texture. Foliated metamorphic rocks are those that have a layered texture. In foliated metamorphic rocks, elongate or platy minerals such as mica and amphibole become aligned as a result of pressure on the rock. Foliation can range from alternating layers of light and dark minerals typical of gneiss to the seemingly perfect alignment of platy minerals in slate. Some metamorphic rocks are unfoliated and have a massive texture devoid of layers. Mineralogy of metamorphic rocks reflects the mineral content of the precursor rock and the pressure and temperature at which metamorphism occurs.

As sediments undergo metamorphism, the layers of sediment can be folded or become more pronounced as pressure on the rock increases. Elongate or platy minerals in the rock tend to become aligned in the same direction. For example, when shale metamorphoses to slate, it becomes easier to split the well-aligned layers of the slate into thin, flat sheets. This property of slate makes it an attractive roofing material. Marble-metamorphosed limestone-typically does not have the pronounced layers of slate, but is used for flooring and sculptures.

Metamorphism of igneous rocks can cause the different minerals in the rocks to separate into layers. When granite metamorphoses into gneiss, layers of light-colored minerals and dark-colored minerals form. As with sedimentary rocks, elongate or platy minerals become well-aligned as pressure on the rock increases.

It is possible for metamorphic rocks to metamorphose into other metamorphic rocks. In some regions, especially areas where mountain building is taking place, it is not unusual for several episodes of metamorphism to affect rocks. It can be difficult to unravel the effects of each episode of metamorphism.

The rock cycle

The rock cycle is a depiction of how the three main rock types can change from one type to another. As rocks exposed at the surface weather , they form sediments that can be deposited to form sedimentary rocks. As sedimentary rocks are buried beneath more sediment, they are subjected to increases in both pressure and temperature, which can result in metamorphism and the formation of metamorphic rock . If the temperature of metamorphism is extremely high, the rock might melt completely and later recrystallize as an igneous rock. Igneous, sedimentary, and metamorphic rocks can erode and later form sedimentary rock. Rocks can move through the rock cycle along other paths, but uplift or burial, weathering , and changes in temperature and pressure are the primary causes of changes in rocks from one type to another.

Current research

Scientists who study rocks attempt to answer a wide variety of questions: What do rocks and the ratios of stable to unstable isotopes within rocks tell us about the age of the Earth, the times at which the Earth's tectonic plates collided to produce mountains, and global warming ? At what times were glaciers present on different continents? Where might we expect to have earthquakes and volcanic eruptions? What types of fossils occur in rocks and how do the fossils differ among rocks from all over the world? In which rocks might we find safe supplies of water, hydrocarbons, and mineral resources such as copper, diamonds, graphite, and aluminum ? Although these problems are not often easy to solve, rocks supply important information about them.

Scientists examine rocks in various settings. Some scientists go out to places where rocks are exposed at the surface of the Earth in order to map occurrences and to collect samples of rocks for further study in the laboratory. Others work exclusively in the laboratory examining thin slices of rock under microscopes, determining the structure and chemical composition of individual crystals within a rock, determining the ratios of different isotopes of atoms within a crystal or rock, or examining the fossils in rocks. Scientists who work in different areas of Earth try to compare the rocks and fossils they find in order to determine how the Earth has changed through time. For example, the eastern coast of South America and the western coast of Africa share many common rocks and fossils, suggesting that these areas might have been closer in the past.

Scientists also pay close attention to several significant ongoing phenomena: large, destructive earthquakes in California and Japan; a surge in the Bering Glacier of Alaska, the largest glacier in North America ; and volcanic activity in Mexico, West Indies, Indonesia, Papua New Guinea, and Italy. Scientists are actively involved in the search for safe locations to dispose of some of our dangerous wastes. Our understanding of Earth's processes are also helping unravel questions about other planets and astronomical bodies in our solar system . In addition, studies of how and where rocks form continue.

See also Geology; Metal; Ore.

Resources

books

Chesterman, Charles W. National Audubon Society Field Guide to North American Rocks and Minerals. New York: Alfred A. Knopf, 2001.

Monroe, James S. Physical Geology: Exploring the Earth. Pacific Grove, CA: Brooks/Cole-Thomson Learning, 2001.

Oxlade, Chris. Rock. Chicago: Heinemann Library, 2002.

other

The Standard Deviants. The Rockin' World of Geology, Parts 1 and 2 (Videorecording) Falls Church, VA: Cerebellum Corporation, 1997.

Gretchen M. Gillis

KEY TERMS

Cementation

—Process through which minerals are glued together, usually as a result of precipitation of solids from solutions in sediments. Calcite, quartz, and clay minerals such as chlorite are common cement-forming minerals in sedimentary rocks.

Compaction

—Reduction of volume of material. Sediments typically compact following burial beneath newer sediments.

Igneous rock

—Rock formed by solidification of molten minerals.

Lava

—Molten rock that occurs at the surface of the Earth, usually through volcanic eruptions. Lava solidifies into igneous rock when it cools.

Magma

—Molten rock found below the surface of the Earth. It can crystallize, or solidify, to form igneous rock.

Metamorphic rock

—Rock formed by alteration of preexisting rock through changes in temperature, pressure, or activity of fluids.

Mineral

—A naturally occurring, inorganic substance with a definite chemical composition and structure.

Rock

—A naturally occurring solid mixture of minerals.

Rock cycle

—The processes through which rocks change from one type to another, typically through melting, metamorphism, uplift, weathering, burial, or other processes.

Sedimentary rock

—Rock formed by deposition, compaction, and cementation of weathered rock or organic material, or by chemical precipitation. Salt and gypsum form from evaporation and precipitation processes.

Uplift

—An episode in the history of a region when tectonic forces lift the region's crust to a higher elevation.

Weathering

—Biological, chemical, and mechanical attack on rock which breaks it up and alters it at or near the surface of the Earth.

Rocks

Rocks are composed of minerals, which are natural inorganic (nonliving) substances with specific chemical compositions and structures. A rock may consist of many crystals of one mineral or combinations of many minerals. Hundreds of different kinds of minerals make up hundreds of different kinds of rocks. Geologists, scientists who study Earth and rocks, divide rocks into three main groups: igneous rocks, sedimentary rocks, and metamorphic rocks. These distinctions are made on the basis of the types of minerals in the rock, the shapes of individual mineral grains, and the overall texture of the rock. All of these properties indicate the environment, pressure, and temperature in which the rock formed.

Igneous rock

The first rocks on Earth were igneous rocks. Igneous rocks are formed by the cooling and hardening of molten material called magma. The word igneous comes from the Latin word ignis, meaning "fire." There are two types of igneous rocks: intrusive and extrusive. Intrusive igneous rocks form within Earth's crust: the molten material rises, filling any available crevices in the crust, and eventually hardens. These rocks are not visible until Earth above them has eroded away. A good example of intrusive igneous rock is granite. Extrusive igneous rocks form when the magma pours out onto Earth's surface or erupts at Earth's surface from a volcano. Once on the surface (where it is called lava), it begins to cool and the minerals in the rock crystallize or grow together so that the individual crystals lock together. Extrusive rocks are also called volcanic rocks. Basalt, formed from hardened lava, is the most common extrusive rock. Obsidian, a black, glassy rock, is also an extrusive rock.

Words to Know

Igneous rock: Rock formed from the cooling and hardening of magma.

Lava: Molten rock that occurs at the surface of Earth, usually through volcanic eruptions.

Magma: Molten rock found below the surface of Earth.

Metamorphic rock: Rock formed by transformation of preexisting rock through changes in temperature and pressure.

Mineral: A naturally occurring, inorganic substance with a definite chemical composition and structure.

Rock cycle: Processes through which rocks change from one type to another, typically through melting, metamorphism, uplift, weathering, burial, or other processes.

Sedimentary rock: Rock formed from compressed and solidified layers of organic or inorganic matter.

Weathering: Natural process that breaks down rocks and minerals at Earth's surface into simpler materials (sediment) by physical (mechanical) or chemical means.

Essentially, Earth's continents are slabs of granite sitting on top of molten rock. The crustal plates of Earth are continually shifting, being torn open by faults and altered by earthquakes and volcanoes. New igneous material is continually added to the crust, while old crust falls back into Earth, sometimes deep enough to be remelted. Igneous rocks are the source of many important minerals, metals, and building materials.

Sedimentary rock

Sedimentary rocks are those produced by the accumulation of sediments. These may be fine rock particles or fragments, skeletons of microscopic organisms, or minerals leached from rocks. Rock fragments and leached minerals are created through weathering, a natural process that breaks down rocks and minerals at Earth's surface into simpler materials by physical (mechanical) or chemical means.

Wind, water, ice, gravity, temperature changes, or a combination of these are all physical actions that break down preexisting rocks. Chemical weathering represents a second stage of rock disintegration in which small pieces of rock produced by physical weathering are then further

broken apart by chemical processes. Acid reactions are a common form of chemical weathering, and the most common such reactions occur when carbon dioxide and sulfur dioxide in the air react with water to form weak carbonic and sulfuric acid. Both of these acids have the ability to attack many kinds of rocks, changing them into other forms. For example, when carbonic acid reacts with limestone, it produces calcium bicarbonate.

The sediments created by weathering and the decay of organisms are then transported and deposited by wind, water, or ice. Over long periods of time, layer upon layer of sediments are deposited on top of each other and their own weight causes them to compress and harden into sedimentary rock. The horizontal layers of sedimentary rock are called strata. Common sedimentary rocks include shale, sandstone, and limestone.

Sedimentary rocks are the only rocks in which fossils can be preserved. The elevated temperatures and pressures needed to form both igneous and metamorphic rocks destroy fossils and organic remnants. The presence of fossils and the types of fossil organisms in a rock provide clues about the environment and age of sedimentary rocks. For example, fish fossils in sedimentary rock indicate that the sediments that make up the rock were deposited in a lake, river, or marine environment. By establishing the environment of the fossils in a rock, scientists learn more about the conditions under which the rock formed.

Metamorphic rock

Metamorphic rock is rock that has changed from one type of rock into another. The word metamorphic comes from Greek and means "of changing form." Metamorphic rock is produced from igneous, sedimentary, or even other metamorphic rocks. Most of Earth's crust is made up of metamorphic rock. Igneous and sedimentary rocks become metamorphic rock as a result of intense heat from magma and pressure from burial within Earth. Although the rock undergoes extreme heat and a great deal of pressure, it does not melt. If the rock melted, it would become igneous and not metamorphic rock. Instead, the heat and pressure combine to change the mineral makeup of the rock. Essentially, metamorphic rocks are made of the same minerals as the original rock, but the various minerals have been rearranged to make a new rock.

There are two basic types of metamorphic rock: regional and thermal. Regional metamorphic rock, found mainly in mountainous regions, is formed by pressure. Different amounts of pressure produce different types of rock. The greater the pressure, the more drastic the change (also, the deeper the rock the higher the temperature, which adds to the potential for diverse changes). For example, a pile of mud can turn into shale (a fine-grained sedimentary rock) with relatively low pressure, about 3 miles (5 kilometers) down into Earth. With more pressure and some heat, shale can transform into slate and mica. Carried even deeper, slate transforms into schist (pronounced shist) and then gneiss (pronounced nice).

Thermal metamorphic rock, also called contact metamorphic rock, is formed by considerable pressure and, more important, intense heat. When molten rock pushes up into Earth's crust, the incredible pressure behind it forces the molten rock into any empty space. The accompanying intense heat causes the surrounding rock to completely recrystallize, forming a new rock. An example of this type of thermal metamorphic rock is marble, which is actually limestone whose calcite has recrystallized. Sandstone made mostly of quartz fragments recrystallizes into quartzite. Thermal metamorphic rocks are not as common or plentiful as regional metamorphic rocks.

The rock cycle

The rock cycle depicts how the three main rock types can change from one type to another. All rocks exposed at Earth's surface undergo weathering, forming sediments that can be deposited to form sedimentary rocks. As sedimentary rocks are buried beneath more sediment, they are subjected to increases in both pressure and temperature, which can result in metamorphism and the formation of metamorphic rock. If the temperature of metamorphism is extremely high, the rock might melt completely and later recrystallize as an igneous rock. Rocks can move through the rock cycle along other paths, but uplift or burial, weathering, and changes in temperature and pressure are the primary causes of changes in rocks from one type to another.

[See also Coal; Minerals ]

bog iron ore An impure, limonitic deposit (see LIMONITE), usually porous, and probably formed as a result of bacterial action in swampy conditions. It is a low-grade iron ore which was used extensively in early iron smelting.