Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate: CaCO3). 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 primary source of the calcite in limestone is most commonly marine organisms. These organisms secrete shells that settle out of the water column and are deposited on ocean floors as pelagic ooze or alternatively is conglomerated in a coral reef (see lysocline for information on calcite dissolution). Secondary calcite may also be deposited by supersaturated meteoric waters (groundwater that precipitates the material in caves). This produces speleothems such as stalagmites and stalactites. Another form taken by calcite is that of oolites (oolitic limestone) which can be recognised by its granular appearance. Limestone makes up about 10% of the total volume of all sedimentary rocks.
Pure limestones are white or almost white. Because of impurities, such as clay, sand, organic remains, iron oxide and other materials, many limestones exhibit different colors, especially on weathered surfaces. 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. Folk and Dunham classifications are used to describe limestones more precisely.
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 where evaporation of the water leaves a solution that is supersaturated with chemical constituents of calcite. 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.
Limestone is partially soluble, especially in acid, and therefore forms many erosional landforms. These include limestone pavements, pot holes, cenotes, caves and gorges. Such erosion landscapes are known as karsts. Limestone is less resistant than most igneous rocks, but more resistant than most other sedimentary rocks. Limestone is therefore usually associated with hills and downland 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 and the Isle of Wight, England; on Fårö near the Swedish island of Gotland, the Niagara Escarpment in Canada/USA, Notch Peak in Utah, and the Ha Long Bay National Park in Vietnam.
In Belgium, Holland, and France there are 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 (Belgium/Netherlands) .
Uses of limestone
Limestone is especially popular in architecture, and many landmarks around the world, especially in North America and Europe, are made primarily of the material. So many buildings in Kingston, Ontario, Canada were constructed from it, that it was nicknamed the 'Limestone City'. Limestone is readily available and relatively easy to cut into blocks or more elaborate carving. It is also long-lasting and stands up well to exposure. However, it is a very heavy material, making it impractical for tall buildings. It is also quite expensive.
Limestone was most popular in the early 20th and late 19th centuries. Train stations, banks and other structures from that era are normally made of limestone. Limestone is used as a facade on some skyscrapers, but only in thin plates for covering rather than solid blocks. In the United States, Indiana, most notably the Bloomington area, has long been a source of high quality quarried limestone, called Indiana limestone. Many famous buildings in London are built from Portland limestone.
Limestone was also a very popular building block in the middle ages in the areas where it occurred since it is hard, durable, and commonly occurs in easily accessible surface exposures. Many medieval churches and castles in Europe are made of limestone. Beer stone was a popular kind of limestone for medieval buildings in southern England.
Limestone and marble are very reactive to acid solutions, making acid rain a significant problem. Many limestone statues and building surfaces have suffered severe damage due to acid rain.
Other uses include:
- the manufacture of quicklime (calcium oxide)and slaked lime (calcium hydroxide)
- cement and mortar
- pulverized limestone is used as a soil conditioner to neutralize acid soil conditions
- crushed for use as aggregate - the solid base for many roads
- geological formations of limestone are among the best petroleum reservoirs
- as a reagent in desulfurizations
- glass making