rock1 / räk/ • n. 1. the solid mineral material forming part of the surface of the earth and other similar planets, exposed on the surface or underlying the soil or oceans. ∎ a mass of such material projecting above the earth's surface or out of the sea: there are dangerous rocks around the island. ∎ Geol. any natural material, hard or soft (e.g., clay), having a distinctive mineral composition. ∎ (the Rock) Gibraltar. ∎ (the Rock) informal name for Newfoundland1 . 2. a large piece of such material that has become detached from a cliff or mountain; a boulder: the stream flowed through a jumble of rocks. ∎ a stone of any size, esp. one small enough to be picked up and used as a projectile. ∎ Brit. a kind of hard confectionery in the form of cylindrical peppermint-flavored sticks. ∎ inf. a precious stone, esp. a diamond. ∎ inf. a small piece of crack cocaine. ∎ (rocks) vulgar slang testicles. 3. used in similes and metaphors to refer to someone or something that is extremely strong, reliable, or hard: imagining himself as the last rock of civilization being swept over by a wave of barbarism. ∎ (usu. rocks) (esp. with allusion to shipwrecks) a source of danger or destruction: the new system is heading for the rocks. 4. (rocks) inf., dated money. PHRASES: between a rock and a hard place inf. in a situation where one is faced with two equally difficult alternatives. get one's rocks off vulgar slang have an orgasm. ∎ obtain pleasure or satisfaction. on the rocks inf. 1. (of a relationship or enterprise) experiencing difficulties and likely to fail. 2. (of a drink) served undiluted and with ice cubes. DERIVATIVES: rock·less adj. rock·like / -ˌlīk/ adj. rock2 • v. 1. [tr.] cause (someone or something) to move gently to and fro or from side to side: she rocked the baby in her arms. ∎ [intr.] move in such a way: the vase rocked back and forth on its base | [as adj.] (rocking) the rocking movement of the boat. ∎ (with reference to a building or region) shake or cause to shake or vibrate, esp. because of an impact, earthquake, or explosion: [tr.] a terrorist blast rocked a Tube station | [intr.] the building began to rock on its foundations. ∎ cause great shock or distress to (someone or something), esp. so as to weaken or destabilize them or it: diplomatic upheavals that rocked the British Empire. 2. [intr.] inf. dance to or play rock music. ∎ fig. (of a place) have an atmosphere of excitement or much social activity: the new town really rocks | [as adj.] (rocking) a rocking resort. • n. 1. rock music: [as adj.] a rock star. ∎ rock and roll. 2. [in sing.] a gentle movement to and fro or from side to side: she placed the baby in the cradle and gave it a rock. PHRASES: rock the boat see boat.PHRASAL VERBS: rock out inf. perform rock music loudly and vigorously. ∎ enjoy oneself in an enthusiastic and uninhibited way, esp. by dancing to rock music.
To the geologist, the term rock means a naturally occurring aggregate of minerals that may include some organic solids (e.g., fossils ) and/or glass . Rocks are generally subdivided into three large classes: igneous, sedimentary, and metamorphic. These classes relate to common origin, or genesis. Igneous rocks form from cooling liquid rock or related volcanic eruptive processes. Sedimentary rocks form from compaction and cementation of sediments. Metamorphic rocks develop due to solid-state, chemical and physical changes in pre-existing rock because of elevated temperature , pressure, or chemically active fluids.
With igneous rocks, the aggregate of minerals comprising these rocks forms upon cooling and crystallization of liquid rock. As crystals form in the liquid rock, they become interconnected to one another like jigsaw puzzle pieces. After total crystallization of the liquid, a hard, dense igneous rock is the result. Also, some volcanic lavas, when extruded on the surface and cooled instantaneously, will form a natural glass. Glass is a mass of disordered atoms, which are frozen in place due to sudden cooling, and is not a crystalline material like a mineral. Glass composes part of many extrusive igneous rocks (e.g., lava flows) and pyroclastic igneous rocks. Alternatively, some igneous rocks are formed from volcanic processes, such as violent volcanic eruption. Violent eruptions eject molten, partially molten, and non-molten igneous rock, which then falls in the vicinity of the eruption. The fallen material may solidify into a hard mass, called pyroclastic igneous rock. The texture of igneous rocks (defined as the size of crystals in the rock) is strongly related to cooling rate of the original liquid. Rapid cooling of liquid rock promotes formation of small crystals, usually too small to see with the unaided eye. Rocks with this cooling history are called fine-textured igneous rocks. Slow cooling (which usually occurs deep underground) promotes formation of large crystals. Rocks with this cooling history are referred to as coarse-textured igneous rocks.
The mineral composition of igneous rocks falls roughly into four groups: silicic , intermediate, mafic , and ultramafic. These groups are distinguished by the amount of silica (SiO4), iron (Fe), and magnesium (Mg) in the constituent minerals. Mineral composition of liquid rock is related to place of origin within the body of the earth. Generally speaking, liquids from greater depths within the earth contain more Fe and Mg and less SiO4 than those from shallow depths.
In sedimentary rocks, the type of sediment that is compacted and cemented together determines the rock's main characteristics. Sedimentary rocks composed of sediment that has been broken into pieces (i.e., clastic sediment), such as gravel, sand , silt, and clay , are clastic sedimentary rocks (e.g., conglomerate, sandstone , siltstone, and shale, respectively). Sedimentary rocks composed of sediment that is chemically derived (i.e., chemical sediment), such as dissolved elements like calcium (Ca), sodium (Na), iron (Fe), and silicon (Si), are chemical sedimentary rocks. Examples of chemical sedimentary rocks are limestone (composed of calcium carbonate), rock salt (composed of sodium chloride), rock gypsum (composed of calcium sulfate), ironstones (composed of iron oxides), and chert (composed of hydrated silica). Biochemical sedimentary rocks are a special kind of chemical sedimentary rock wherein the constituent particles were formed by organisms (typically as organic hard parts, such as shells), which then became sedimentary particles. Examples of this special kind of sedimentary rock include chalk, fossiliferous limestone, and coquina. Sedimentary rocks are formed from sediment in two stages: compaction and cementation. Compaction occurs when sediments pile up to sufficient thickness that overlying mass squeezes out water and closes much open space . Cementation occurs when water flowing through the
compacted sediment deposits mineral crystals upon particles thus binding them together. The main cement minerals are calcite (CaCO3), hematite (Fe2O3), and quartz (SiO2).
With metamorphic rocks, the nature of the pre-existing rock (protolith) determines in large part the characteristics of the ultimate metamorphic rock . Regardless of protolith, however, almost all metamorphic rocks are harder and more dense than their protoliths. A protolith with flat or elongate mineral crystals (e.g., micas or amphiboles) will yield a metamorphic rock with preferentially aligned minerals (due to directed pressure). Such metamorphic rocks are called foliated metamorphic rocks (e.g., slate and schist ). Non-foliated metamorphic rocks (e.g., marble and quartzite) come from protoliths that have mainly equidimensional mineral crystals (e.g., calcite and quartz, respectively). For example, a protolith shale will yield a foliated metamorphic rock, and a protolith limestone will yield marble, a non-foliated metamorphic rock. Metamorphic rocks possess distinctive grades or levels of metamorphic change from minimal to a maximum near total melting . Low-grade metamorphic rocks generally have fine-textured crystals and low-temperature indicator minerals like the mica chlorite. High-grade metamorphic rocks generally have coarse-textured crystals and very distinctive foliation, plus high-temperature indicator minerals like the silicate mineral staurolite.
Rock is a brittle natural solid found mainly in the outer reaches of Earth's crust and upper mantle. Material that would be brittle rock at such shallow depths becomes to one degree or another rather plastic within the body of the earth. The term "rock" is not generally applied to such non-brittle internal Earth materials. Therefore, rock is a concept related to the outer shell of the earth. The term rock may also be properly applied to brittle natural solids found on the surfaces of other planets and satellites in our solar system . Meteorites are rock. Naturally occurring ice (e.g., brittle water ice in a glacier, H2O) is also a rock, although we do not normally think of ice this way.
Rock has been an important natural resource for people from early in human evolution . Rocks' properties are the key to their specific usefulness, now as in the past. Hard, dense rocks that could be chipped into implements and weapons were among the first useful possessions of people. Fine-textured and glassy rocks were particularly handy for these applications. Later on, rock as building stone and pavement material became very important, and this continues today in our modern world. All of Earth's natural mineral wealth, fossil energy resources, and most groundwater are contained within rocks of the earth's crust.
See also Lithification; Metamorphism
A rock in biblical contexts may also be a source of sustenance (with allusion to Numbers 20:11, in which water issued from the rock struck by the staff of Moses), and a shelter, as in Isaiah 32:2, ‘the shadow of a great rock in a weary land’.
A rock (especially with the notion of one on which a ship may be wrecked) can also be taken as a sign of danger, as in rocks ahead.
between a rock and a hard place in a situation where one is faced with two equally difficult alternatives.
on the rocks (of a relationship or enterprise) experiencing difficulties and likely to fail.
Rock of Ages symbolizing the foundation of Christian belief; the phrase is now probably best-known from the hymn ‘Rock of Ages, cleft for me’ (1773), by the English clergyman Augustus Toplady (1740–78).