Limestone
Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate, CaCO3). It makes up about 10 percent of the total volume of all sedimentary rocks. Limestones often contain variable amounts of silica in the form of chert or flint, as well as varying amounts of clay, silt, and sand as disseminations, nodules, or layers within the rock.
The calcite in limestone is produced mainly by marine organisms, many of which secrete shells that settle out of the water column and are deposited on ocean floors. Moreover, coral reefs are made from the calcium carbonate skeletons of coral-building organisms. Secondary calcite may be precipitated from groundwater and deposited in caves, producing formations such as stalagmites and stalactites.
Limestone can be found in many varieties, depending on its mineral composition and physical structure. When composed of calcium carbonate alone, it is white or nearly white. Other colors are produced by the presence of minor constituents such as clay, sand, organic remains, iron oxide, and other materials. In addition, limestone may be crystalline, clastic, granular, or massive, depending on the method of formation. Crystals of calcite, quartz, dolomite, or barite may line small cavities in the rock.
Travertine is a banded, compact variety of limestone formed along streams, particularly where there are waterfalls, and around hot or cold springs. Calcium carbonate is deposited upon evaporation of the water. Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells. During regional metamorphism that occurs during the mountain-building process (orogeny), limestone recrystallizes into marble.
Limestone containing bivalve fossils.
Various fossils collected from a limestone quarry in North Carolina, U.S.A. Clockwise from upper left: echinoids, bryozoans, and brachiopods.
A water-cooled limestone saw.
Limestone landscapes
Limestone is partially soluble, especially in acidic water, and therefore forms many landforms produced by erosion. These include limestone pavements (horizontal surfaces of exposed limestone), caves, gorges, potholes, and cenotes (limestone sinkholes filled with freshwater). Such erosion landscapes, usually marked by underground drainages, are known as "karsts."
Limestone is less resistant to erosion than most igneous rocks, but more resistant than most other sedimentary rocks. Limestone is therefore usually associated with hills and occurs in regions with other sedimentary rocks, typically clays.
Bands of limestone emerge from the Earth's surface in often spectacular rocky outcrops and islands. Examples include the Burren in Co. Clare, Ireland; the Verdon Gorge in France; Malham Cove in North Yorkshire, England; on Fårö, near the Swedish island of Gotland; the Niagara Escarpment bordering Canada and the United States, Notch Peak in Utah, and the Ha Long Bay National Park in Vietnam. In addition, Belgium and Holland have several huge quarries, with a total gallery length of more than a hundred kilometers. An example of a hill with a lot of quarries is Mount Saint Peter (in the Belgium-Netherlands region) [1].
Chalk
Chalk is a soft, white, porous form of limestone. It is relatively resistant to erosion and slumping, compared to the clays with which it is usually associated. Consequently, it forms tall, steep cliffs where chalk ridges meet the sea. Chalk hills, known as chalk downland, usually form where bands of chalk reach the surface at an angle.
Chalk is formed in shallow waters by the gradual accumulation of the calcite mineral remains of microorganisms, over millions of years. Chalk beds commonly have embedded flint nodules.
Because chalk is porous, chalk downland usually holds a large body of groundwater, providing a natural reservoir that releases water slowly through dry seasons. The River Somme is an example of such water release from chalk.
Chalk has been quarried from prehistory, providing building material and marl for fields. In southeast England, deneholes are a notable example of ancient chalk pits.
The Chalk Formation is a European stratigraphic unit in the upper Cretaceous period. This includes the famous White cliffs of Dover of Kent in England, which are formed entirely of chalk deposits. The Champagne region of France is mostly on chalk formations, with the famous caves being carved out beneath the hills.
Uses
- Blackboard chalk is a substance used for drawing on rough surfaces, as it readily crumbles leaving particles that stick loosely to these surfaces. Blackboard chalk, often supplied in sticks about 5 cm long, is currently made from the mineral gypsum (calcium sulfate) rather than calcium carbonate.
- In Lawn Tennis, when grass courts are used, chalk is used to mark the boundaries. This gives the advantage that, if the ball hits the line, a cloud of chalk dust can be seen.
- In gymnastics, rock-climbing, bouldering and weight-lifting, chalk - usually in the form of magnesium carbonate - is applied to the hands to prevent slippage.
- Tailor's chalk traditionally is hard chalk used to make temporary markings on cloth, for instance by tailors. Nowadays it is usually made from talc (magnesium silicate).
Uses of limestone
Limestone is readily available and relatively easy to cut into blocks or carve elaborately. It is also long-lasting and stands up well to exposure. It is therefore popular in architecture and has been used to make many landmarks around the world, especially in Europe and North America. In the United States, Indiana has long been noted as a source of high-quality, quarried building limestone, while many famous buildings in London are built from Portland limestone.
Many medieval churches and castles in Europe are made of limestone. In the late nineteenth and early twentieth centuries, many train stations, banks, and other structures were made of limestone. Kingston, Ontario, Canada, has so many buildings constructed from limestone that the place was nicknamed "Limestone City." In recent years, thin limestone plates have been used as a facade on some skyscrapers.
On the other hand, limestone is a very heavy material, making it impractical for tall buildings. It is also quite expensive. A more significant problem is that limestone and marble are very reactive to acidic solutions, such as acid rain. Many limestone statues and building surfaces have suffered severe damage from acid rain.
Today, limestone is used in the manufacture of cement, mortar, quicklime (calcium oxide), and slaked lime (calcium hydroxide). In addition, crushed limestone is useful in constructing the solid base for many roads, and pulverized limestone is used as a soil conditioner to neutralize acidic soil conditions. Geological formations of limestone are among the best petroleum reservoirs.
See also
- Rock (geology)
- Marble
- Mineral
- Quarry
External links
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