rock

The formation of sedimentary rock by diagenesis. Sand and other sediment grains are compacted and cemented together.

Faults are caused by the movement of rock layers, producing such features as block mountains and rift valleys. A normal fault is caused by a tension or stretching force acting in the rock layers. A reverse fault is caused by compression forces. Faults can continue to move for thousands or millions of years.

The folding of rock strata occurs where compression causes them to buckle. Over time, folding can assume highly complicated forms, as can sometimes be seen in the rock layers of cliff faces or deep cuttings in the rock. Folding contributed to the formation of great mountain chains such as the Himalayas.

Gabbro through a polarized microscope. Gabbro is an igneous rock which forms in major plutonic intrusions.

Constituent of the Earth's crust composed of minerals or materials of organic origin that have consolidated into hard masses. There are three basic types of rock: igneous, sedimentary, and metamorphic. Rocks are composed of a combination (or aggregate) of minerals, and the property of a rock will depend on its components. Where deposits of economically valuable minerals occur they are termed ores. As a result of weathering, rock breaks down into very small particles that combine with organic materials from plants and animals to form soil. In geology the term ‘rock’ can also include unconsolidated materials such as sand, mud, clay, and peat.

The change of one type of rock to another is called the rock cycle.

Rock studies The study of the Earth's crust and its composition fall under a number of interrelated sciences (generally known as geology), each with its own specialists. Among these are geologists, who identify and survey rock formations and determine when and how they were formed, petrologists, who identify and classify the rocks themselves, and mineralogists, who study the mineral contents of the rocks. Palaeontologists study the fossil remains of plants and animals found in rocks.

Applications of rock studies Data from these studies and surveys enable scientists to trace the history of the Earth and learn about the kind of life that existed here millions of years ago. The data are also used in locating and mapping deposits of fossil fuels such as coal, oil, and natural gas, and valuable mineral-containing ores providing metals such as aluminium, iron, lead, and tin, and radioactive elements such as radium and uranium. These deposits may lie close to the Earth's surface or deep underground, often under oceans. In some regions, entire mountains are composed of deposits of iron or copper ores, while in other regions rocks may contain valuable non-metallic minerals such as borax and graphite, or precious gems such as diamonds and emeralds.

Rock as construction material and building stone In addition to the mining and extraction of fuels, metals, minerals, and gems, rocks provide useful building and construction materials. Rock is mined through quarrying, and cut into blocks or slabs as building stone, or crushed or broken for other uses in construction work. For instance, cement is made from limestone and, in addition to its use as a bonding material, it can be added to crushed stone, sand, and water to produce strong, durable concrete, which has many applications, such as the construction of roads, runways, and dams.

Among the most widely used building stones are granite, limestone, sandstone, marble, and slate. Granite provides one of the strongest building stones and is resistant to weather, but its hardness makes it difficult to cut and handle. Limestone is a hard and lasting stone that is easily cut and shaped and is widely used for public buildings. The colour and texture of the stone can vary with location; for instance, Portland stone from the Jurassic rocks of Dorset is white, even-textured and durable, while Bath stone is an oolitic limestone that is honey-coloured and more porous. Sandstone varies in colour and texture; like limestone, it is relatively easy to quarry and work and is used for similar purposes. Marble is a classic stone, worked by both builders and sculptors. Pure marble is white, streaked with veins of black, grey, green, pink, red, and yellow. Slate is fine-grained rock that can be split easily into thin slabs and used as tiles for roofing and flooring. Its colour varies from black to green and red.

Rock identification Rocks can often be identified by their location and appearance. For example, sedimentary rocks lie in stratified, or layered, formations and may contain fossils; many have markings such as old mud cracks or ripple marks caused by waves. Except for volcanic glass, all igneous rocks are solid and crystalline. Some appear dense, with microscopic crystals, and others have larger, easily seen crystals. They occur in volcanic areas, and in intrusive formations that geologists call batholiths, laccoliths, sills, dikes, and stocks. Many metamorphic rocks have characteristic bands, and are easily split into sheets or slabs. Rock formations and strata are often apparent in the cliffs that line a seashore, or where rivers have gouged out deep channels to form gorges and canyons. They are also revealed when roads are cut through hillsides, or by excavations for quarrying and mining. Rock and fossil collecting has been a popular activity since the 19th century and such sites can provide a treasure trove of finds for the collector.

Rock

River in Wisconsin and Illinois; length 460 km/286 mi. It rises west of Fond du Lac in southeast Wisconsin, and flows south and southwest through Rockford and Rock Falls to the Mississippi River near Rock Island in northwest Illinois. Its valley is an important industrial area and has small hydropower plants.