rock

(rŏk)
n.

1. Relatively hard, naturally formed mineral or petrified matter; stone.
2. 1. A relatively small piece or fragment of such material.
   2. A relatively large body of such material, as a cliff or peak.
3. A naturally formed aggregate of mineral matter constituting a significant part of the earth's crust.
4. One that is similar to or suggestive of a mass of stone in stability, firmness, or dependability: The family has been his rock during this difficult time.
5. rocks Slang. Money.
6. Slang. A large gem, especially a diamond.
7. Slang. Crack cocaine.
8. 1. A varicolored stick candy.
   2. Rock candy.

idioms:

between a rock and a hard place

1. Confronted with equally unpleasant alternatives and few or no opportunities to evade or circumvent them.

on the rocks

1. In a state of difficulty, destruction, or ruin: Their marriage is on the rocks.
2. Without money; bankrupt: Our accountant says the business is on the rocks.
3. Served over ice cubes: Scotch on the rocks.

[Middle English, from Old North French roque, from Vulgar Latin *rocca.]

rock² (rŏk)

v., rocked, rock·ing, rocks.

v.intr.

1. To move back and forth or from side to side, especially gently or rhythmically.
2. To sway violently, as from a blow or shock. See synonyms at swing.
3. To be washed and panned in a cradle or in a rocker. Used of ores.
4. Music. To play or dance to rock 'n' roll.

v.tr.

1. To move back and forth or from side to side, especially in order to soothe or lull to sleep.
2. To cause to shake or sway violently. See synonyms at agitate.
3. To disturb the mental or emotional equilibrium of; upset: News of the scandal rocked the town.
4. To wash or pan (ore) in a cradle or rocker.
5. In mezzotint engraving, to roughen (a metal plate) with a rocker or roulette.

n.

1. 1. A rocking motion.
   2. The act of rocking.
2. Music. Rock 'n' roll.

idiom:

rock the boat Slang.

1. To disturb the balance or routine of a situation: He has an easygoing managerial style and won't rock the boat unless absolutely necessary.

[Middle English rokken, from Old English roccian.]

rockingly rock'ing·ly adv.

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For more information on rock, visit Britannica.com.

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.

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A relatively common aggregate of mineral grains. Some rocks consist essentially of but one mineral species (monomineralic, such as quartzite, composed of quartz); others consist of two or more minerals (polymineralic, such as granite, composed of quartz, feldspar, and biotite). Rock names are not given for those rare combinations of minerals that constitute ore deposits, such as quartz, pyrite, and gold. In the popular sense rock is considered also to denote a compact substance, one with some coherence; but geologically, friable volcanic ash also is a rock. A genetic classification of rocks is shown below.

Igneous

     Intrusive

          Plutonic (deep)

          Hypabyssal (shallow)

     Extrusive

          Flow

          Pyroclastic (explosive)

     Sedimentary

          Clastic (mechanical or detrital)

          Chemical (crystalline or precipitated)

          Organic (biogenic)

     Metamorphic

          Cataclastic

          Contact metamorphic and pyrometasomatic

          Regional metamorphic (dynamothermal)

     Hybrid

          Metasomatic

          Migmatitic

Exceptions to the requirement that rocks consist of minerals are obsidian, a volcanic rock consisting of glass; and coal, a sedimentary rock which is a mixture of organic compounds. See also Coal; Igneous rocks; Metamorphic rocks; Obsidian; Rock mechanics; Sedimentary rocks; Volcanic glass.

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mint-rock

Traditional English seaside sugar confectionery, made by pulling melted sugar. It is white, with an outer coloured layer (traditionally pink) and an inner ring of coloured sugar that spells out the name of the town where it is sold or of which it is a souvenir.

verb

1. To move vigorously from side to side or up and down: heave, pitch, roll, toss. See repetition.
2. To move to and fro violently: quake, shake, tremble, vibrate. See repetition.
3. To cause to move to and fro violently: agitate, churn, convulse, shake. See calm/agitation, repetition.
4. To impair or destroy the composure of: agitate, bother, discompose, disquiet, distract, disturb, flurry, fluster, perturb, ruffle, shake (up), toss, unsettle, upset. Informal rattle. See calm/agitation.

Idioms beginning with rock:
rock bottom
rocks in one's head, have
rock the boat

See also between a rock and a hard place; on the rocks; steady as a rock.

v

Definition: move back and forth
Antonyms: hold, stabilize

Most people know that stone "arrowheads" were made from a kind of rock called flint, but otherwise have no idea about the relationship of stone as a material to tool manufacture and use. Early hominids were more discerning; they had to be to survive.

Flint   The best material for making a great variety of stone tools, flint is closely related to the semiprecious stones called carnelian, chrysoprase, and jasper--uniform, red, green, or yellow forms of the mineral chalcedony. Large deposits of gray or black chalcedony are called chert; small pieces, called nodules, of gray or black chalcedony found in limestone or chalk are called flint. The harder and more chemically stable flint can easily be picked from its limestone or chalk background.

No one knows what causes flint nodules to appear where they do. Chalcedony is a form of quartz that has tiny crystals and is very dense; hence, it is silicon dioxide, also known to mineralogists as silica, just as quartz is. Limestone and chalk are both calcium carbonate, a different mineral entirely. Veins of silicon dioxide often form as a result of solutions of water containing the silica, especially solutions in superheated water. The solutions travel through limestone or chalk and leave the silica behind when they cool.

Because of its crystal structure, flint breaks in a pattern geologists call a conchoidal fracture; this produces sharp edges but does not propagate throughout a stone, splitting or shattering it. Furthermore, there are no preferential fracture planes, so small pieces of almost any shape can be removed. A nodule broken in two could be the first manufactured stone tool, indistinguishable as a tool today except for microscopic wear patterns that indicate use. The beautifully scalloped surfaces of many later stone tools are one result of the conchoidal fracture pattern.

In the absence of the fine-grained flints, our ancestors often used the best approximations they could find -- quartz (silicon dioxide with a larger crystal structure) and rocks or other materials infused with silica, including petrified wood.

Obsidian   Anyone who has picked up the remains of a shattered glass dish or bottle knows that glass also breaks in a pattern that causes very sharp edges. The break is also a conchoidal fracture, but because glass is more brittle than flint, the glass not only fractures but also easily shatters. A fragment of broken glass can have a very sharp cutting edge that can be used as a tool. The utility of broken glass was not lost on our ancestors. Although manufacture of glass from silicon dioxide (sand that is formed from small particles of quartz) did not start as far as we know until about 1400 bce, implements made from natural glass called obsidian are among the earliest stone tools. Obsidian is a rock produced when granite or rhyolite, quartz combined with feldspar and mica, is melted by a volcano and then cooled very quickly. Some dark forms of obsidian are known as pitchstone. Opal, a glassy form of quartz, was also used for stone tools, but it is less common than obsidian. As with substitutes for flint, volcanic rocks such as lavas or those formed from hot ash flows were sometimes used instead of volcanic glasses.

Other rocks   A third type of stone tool material includes various rocks, such as quartzites or hardened shales, that had been hardened (metamorphosed) by great heat and pressure in the interior of Earth. Quartzites that are metamorphosed sandstones became particularly useful for axe and adz heads after the practice of grinding edges was introduced as part of the Neolithic Revolution. Quartzite axe heads have the advantage of a structure in which cracks do not propagate far, so the tools maintain their integrity even when their edge is chipped. Very early stone bifaces (hand axes) were also made from quartzite, which can be easily flaked to produce a useful but not very attractive tool. As our ancestors became more experienced, they first shifted to the volcanic lavas that could yield smaller, better looking flakes with sharper edges. Eventually, flint came to predominate, even in regions where flint is not a common rock.

Flint and obsidian were mined and traded in the later Stone Age. As population increased and there was less flint to go around, new techniques, such as manufacture of microliths, were developed. Microliths involve flint or obsidian cutting edges embedded in wood, with the bulk of the tool embodying the wooden portion.

Material made of mineral particles bonded together. Rock is a hard, elastic substance which does not significantly soften on immersion in water.

1. Solid natural mineral material, occurring in fragments or large masses and requiring mechanical or explosive techniques for removal.
2. Stone in a mass.
3. A stone of any size.

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A naturally formed solid that is an aggregate of one or more minerals.

The term for an extremely tight player who takes very few risks when playing. When a rock bets or raises one can expect them to have a monster hand.

SoundPoker Says: A rock will only play when they have the best possible hands. Due to the limited number of cards that make up the "best hands", a rock will tend to be very predictable and have a tendency to check and fold often. A tight player who varies up their playing styles with selling and representing techniques will often beat a rock.

See Also: Bet, Drummer, Monster, Raise, Represent, Sell, Tight

Idiot, as in "That rock is really stupid."

1. n. ice cubes.  Can I have a few rocks in my drink, please?
2. n. Xerox Inc. (Securities markets, New York Stock Exchange.)  When she says, "Buy me a thousand rocks at the market," that means she wants one thousand shares of Xerox at whatever the market price is at the moment.
3. n. money; a dollar. (Underworld.)  Twenty rocks for that?
4. n. the testicles. (Usually objectionable.)  I was afraid I'd get kicked in the rocks, so I stayed back.

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sign description: The S-hand taps on the back of the opposite flat hand.

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noun
noun, US

1:
a: A piece of money; a dollar. (1840 —) .

b: orig US A precious stone; a diamond. (1908 —) .
A. Diment He...listened to my vague replies like my advice was worth its weight in sparkling rocks (1968).


2: on the rocks in a destitute situation; (of a marriage) on the point of breaking down; finished. (1889 —) .

3: orig US, usu. pl./pos> An ice-cube or crushed ice served in a drink; on the rocks of spirituous liquor, served with ice. (1946 —) .

4: US, baseball An error, esp. in phr. to pull a rock, to make a mistake. (1939 —) .

5: pl. The testicles; in the coarse phr. to get one's rocks off, to achieve sexual satisfication; to ejaculate; also, to enjoy oneself. (1948 —) .

Previous:	robbo, roaring forties, roar
Next:	rock of ages, rock pile, rock-happy

For a list of words related to rock, see:

• Materials, Formations, and Substances - rock: large mass of stone, concreted mass of stony material, or broken piece of such a mass
• Rocks - rock: concretion of earthly or mineral matter of variable size, shape, and chemical composition
• Styles and Genres - rock: rock-'n'-roll
• Mood and Intent - rock: move oneself rhythmically to and fro
• Drug Names and Forms - rock: Slang. crack cocaine
• Violent Actions - rock: knock backwards with a blow

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See words rhyming with "rock."

  See crossword solutions for the clue Rock.

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
1.
n. - klippe, bjergart, sten

idioms:

• between a rock and a hard place    som en lus mellem to negle
• on the rocks    helt på spanden, (drink) med is
• rock bottom    lavpunkt, det kan ikke blive værre
• rock candy    kandis
• rock climber    bjergbestiger, bjergklatrer
• rock climbing    bjergbestigning, bjergklatring
• rock face    klippevæg
• rock formation    bjergformation, klippeformation
• rock garden    stenhøj, have med mange stenhøje
• rock hard    ryste
• rock music    rockmusik
• rock salt    stensalt
• rock steady    helt rolig, urokkeligt sikker

2.
v. tr. - rokke, vugge, gynge
v. intr. - blive rokket, ryste, vakle
n. - danse til rockmusik

idioms:

• rock the boat    lave rav i den
• rocking chair    gyngestol
• rocking horse    gyngehest

Nederlands (Dutch)
rots, steen, rockmuziek, schommelen, deinen, wiebelen, wiegelen, wiegen, wippen

Français (French)
1.
n. - roche, rocher, (Naut) récif, avec des glaçons (une boisson), pierre, (GB) sucre d'orge, diamant, crack (argot des drogués), couilles (vulg) (npl)

idioms:

• between a rock and a hard place    (pris) entre le marteau et l'enclume
• on the rocks    avec des glaçons, (Naut) sur les récifs, (fig) en pleine débâcle, en faillite, (être) sans le sou
• rock bottom    (être) au plus bas, (toucher) le fond
• rock candy    (US) friandise à base de sucre candi
• rock climber    varappeur
• rock climbing    varappe
• rock face    paroi rocheuse
• rock formation    rocaille, formation rocheuse
• rock garden    rocaille
• rock hard    solide comme le roc
• rock music    musique rock
• rock salt    sel de gemme
• rock steady    extrêmement stable

2.
v. tr. - balancer, bercer, secouer, ébranler (par une révélation)
v. intr. - se balancer, trembler, danser le rock, jouer de la musique rock
n. - musique rock

idioms:

• rock out    balancer
• rock the boat    (fig) faire des vagues
• rocking chair    fauteuil à bascule, rocking chair
• rocking horse    cheval à bascule

Deutsch (German)
1.
n. - Stein, Fels, Felsen, Klippe, Gestein, Fundament, (Slang) Kies, Klunker, Rock

idioms:

• between a rock and a hard place    in einem Dilemma
• on the rocks    mit Eis
• rock bottom    Tiefstand
• rock candy    Kandiszucker
• rock climber    Felskletterer
• rock climbing    Felsklettern
• rock face    Felswand
• rock formation    Felsgruppe
• rock garden    Steingarten
• rock hard    steinhart
• rock music    Rockmusik
• rock salt    Steinsalz
• rock steady    solide

2.
v. - schaukeln, wanken, Rock tanzen, erschüttern
n. - Rock

idioms:

• rock out    Rockmusik sehr laut machen
• rock the boat    für Unruhe sorgen
• rocking chair    Schaukelstuhl
• rocking horse    Schaukelpferd

Ελληνική (Greek)
n. - βράχος, (γεωλ.) πέτρωμα, (Βρετ.) είδος σκληρής καραμέλας, (μουσική) ροκ, (πληθ.) (καθομ.) παγάκια
v. - λικνίζω/-ομαι, κουνώ/-ιέμαι, ταλαντεύω/-ομαι, παίζω ή χορεύω με μουσική ροκ

idioms:

• between a rock and a hard place    σαν τ' αβγό στα δυο λιθάρια, σε οικτρό δίλημμα
• on the rocks    (ποτό) με πάγο χωρίς νερό, υπό διάλυση (για γάμο), σε δύσκολη θέση, (για πλοία) στα βράχια
• rock bottom    κατώτατο(ς), πάτος, κατώτερο σημείο
• rock candy    μεγάλη καραμέλα (σχήματος ραβδιού)
• rock climber    ορειβάτης βράχων
• rock climbing    σκαρφάλωμα σε βράχο, ορειβασία
• rock face    πρόσοψη βράχου
• rock formation    βραχώδης σχηματισμός
• rock garden    βραχόκηπος, ιαπωνικός κήπος
• rock hard    σκληρός σαν γρανίτης
• rock music    μουσική ροκ
• rock salt    ορυκτό αλάτι
• rock steady    σταθερός, βράχος
• rock the boat    (μτφ.) ταρακουνώ συθέμελα το οικοδόμημα, ταράζω τα νερά
• rocking chair    κουνιστή πολυθρόνα
• rocking horse    ξύλινο αλογάκι

Italiano (Italian)
dondolarsi, oscillare, sasso, masso, rock, roccia

idioms:

• between a rock and a hard place    tra l'incudine e il martello
• on the rocks    al verde, in bolletta, in crisi, sull'orlo del fallimento
• rock bottom    fondo
• rock candy    caramelle
• rock climber    scalatore
• rock climbing    scalare
• rock face    parete rocciosa
• rock formation    strato roccioso, formazione rocciosa
• rock garden    giardino roccioso
• rock hard    solido come una roccia
• rock music    musica rock
• rock salt    salgemma
• rock steady    stabile come una roccia
• rock the boat    causare guai
• rocking chair    sedia a dondolo
• rocking horse    cavallino a dondolo

Português (Portuguese)
n. - pedra (f), rocha (f), penhasco (m)
v. - embalar, tremer

idioms:

• between a rock and a hard place    tanto faz como tanto fez (coloq.)
• on the rocks    arruinado, falido
• rock bottom    nível mínimo
• rock candy    açúcar-cande
• rock climber    alpinista (f)
• rock climbing    alpinismo (m)
• rock face    cara de pau (gír.)
• rock formation    formação rochosa
• rock garden    jardim com pedras ornamentais
• rock hard    muito difícil
• rock music    rock (estilo musical)
• rock salt    sal de pedra
• rock steady    realmente confiável/estável
• rock the boat    entornar o caldo
• rocking chair    cadeira de balanço
• rocking horse    carrossel (brinquedo de criança)

Русский (Russian)
качать, укачивать, камень, скала, рок, горная порода, прочный фундамент

idioms:

• between a rock and a hard place    между молотом и наковальней
• on the rocks    со льдом, на грани развала, без копейки, попасть на скалу
• rock bottom    самая низкая точка
• rock candy    леденец
• rock climber    альпинист, скалолаз
• rock climbing    альпинизм
• rock face    скалистая поверхность горы, ничего на лице не показывать
• rock formation    образование скальных пород
• rock garden    альпийский сад
• rock hard    очень твердый, крепчайший
• rock music    рок-музыка
• rock salt    каменная соль
• rock steady    прочнейший, стабильный
• rock the boat    осложнять ситуацию, вносить сумятицу
• rocking chair    кресло-качалка
• rocking horse    деревянная лошадка

Español (Spanish)
1.
n. - piedra, peña, escollo, música rock, roca

idioms:

• between a rock and a hard place    tener que decidir entre dos opciones igualmente malas
• on the rocks    arruinado, quebrado, con cubitos de hielo
• rock bottom    fondo rocoso, fondo, mínimo, bajísimo
• rock candy    azúcar candi
• rock climber    escalador de rocas
• rock climbing    escalada en roca
• rock face    vertiente rocosa
• rock formation    formación rocosa
• rock garden    jardín de rocas
• rock hard    duro como una piedra
• rock music    música rock
• rock salt    sal gema o de grano
• rock steady    firme, seguro

2.
v. tr. - mecer, acunar, sacudir, balancear
v. intr. - mecerse, balancearse, bailar o tocar música rock
n. - piedra, peña, escollo, música rock, roca

idioms:

• rock out    tocar música rock muy fuerte, disfrutar, especialmente bailando música rock
• rock the boat    hacer mover peligrosamente el barco, romper la calma
• rocking chair    mecedora
• rocking horse    caballito de balancín

Svenska (Swedish)
n. - sten, berg, berggrund, (sl) diamant, gungning, rock-musik, skinnknutte, (sl) dollar
v. - vagga, gunga, dansa eller spela rock

中文（简体）(Chinese (Simplified))
1. 岩, 磐石, 岩石, 冰糖, 石块, 石子, 棒棒糖

idioms:

• between a rock and a hard place    左右为难, 进退两难
• on the rocks    触礁, 毁坏, 破产
• rock bottom    最低点
• rock candy    冰糖
• rock climber    攀岩运动者
• rock climbing    攀岩运动
• rock face    风化作用形成的石头表面
• rock formation    岩石形成
• rock garden    岩石庭院, 假山花园
• rock hard    坚硬如岩石的
• rock music    摇滚乐
• rock salt    岩盐
• rock steady    稳若盘石的, 一动不动的, 镇定的
• rock the boat    捣乱
• rocking chair    安乐椅, 摇椅
• rocking horse    摇摆木马

2. 摇, 振动, 摆动, 奏摇滚乐, 跳摇滚舞, 摇动, 使震惊, 使摇晃, 摇滚舞, 摇滚乐

中文（繁體）(Chinese (Traditional))
1.
n. - 岩, 磐石, 岩石, 冰糖, 石塊, 石子, 棒棒糖

idioms:

• between a rock and a hard place    左右為難, 進退兩難
• on the rocks    觸礁, 毀壞, 破產
• rock bottom    最低點
• rock candy    冰糖
• rock climber    攀岩運動者
• rock climbing    攀岩運動
• rock face    風化作用形成的石頭表面
• rock formation    岩石形成
• rock garden    岩石庭院, 假山花園
• rock hard    堅硬如岩石的
• rock music    搖滾樂
• rock salt    岩鹽
• rock steady    穩若盤石的, 一動不動的, 鎮定的
• rock the boat    搗亂
• rocking chair    安樂椅, 搖椅
• rocking horse    搖擺木馬

2.
v. intr. - 搖, 振動, 擺動, 奏搖滾樂, 跳搖滾舞
v. tr. - 搖動, 使震驚, 使搖晃
n. - 搖動, 搖滾舞, 搖滾樂

한국어 (Korean)
1.
n. - 바위, 암초 , 난관

2.
v. tr. - 흔들다, 달래다, 크게 동요 시키다
v. intr. - 흔들리다, 동요하다, 록을 연주하다
n. - 락 뮤직

日本語 (Japanese)
n. - 岩, 岩石, 岩山, 暗礁, 岩礁, 石ころ, 氷砂糖, 揺れ, 動揺, ロックンロール
v. - 揺り動かす, 震動させる, ショックを与える

idioms:

• on the rocks    座礁して, 破産しそうで, 破綻しかけて, オンザロックの
• rock bottom    どん底
• rock candy    氷砂糖
• rock climber    ロッククライマー
• rock climbing    ロッククライミング
• rock face    瘤出
• rock formation    岩層
• rock garden    岩石庭園, 石庭
• rock hard    かちかちの
• rock music    ロックミュージック
• rock salt    岩塩
• rock steady    ロックステディ
• rock the boat    波風を立てる

العربيه (Arabic)
‏(الاسم) صخرة, دعامه, ملجأ (فعل) يهز, يؤرجح‏

עברית (Hebrew)
n. - ‮אבן, אבן יקרה, יהלום, ממתק, סלע, מקור סכנה והרס, כסף, קוקאין במצב מוצק‬
v. tr. - ‮נדנד, נענע, זעזע‬
v. intr. - ‮התנועע, רקד רוקנרול, ניגן מוסיקת רוק‬
n. - ‮מוסיקת רוק‬

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