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Limestone is a sedimentary rock, made up mostly of the mineral calcite, a form of calcium carbonate, CaCO3.
The calcium carbonate is originally produced by living organisms. Later, some of it goes into solution in sea water. Limestone rocks include mainly organic remains and precipitated carbonate.
Limestone makes up about 10% of the total volume of all sedimentary rocks.
Limestones are mostly made of calcium carbonate. It is originally made by small photosynthetic protists like certain bacteria, coccolithophores, and algae. Then the carbonate gets used by various animals, like bivalves and the polyps which make coral reefs. They build their shells or framework out of CaCO3 crystals.
Carbonates are rocks formed from these sources, plus any CaCO3 which is precipitated out of solution. Some carbonate dissolves in the water. It gets precipitated when the physical condition of the water changes. For example, if the concentration of CaCO3 is high, then a temperature drop, or a pH (acidity) change, can cause precipitation.
Florida Bay, the shallow lagoon behind the Florida Keys, is a model for the conditions which produced limestones on the continental shelf. Of course, all the species were somewhat different then but, as an ecosystem, it is similar.
In the Florida Bay a type of flowering plant called sea grass is in abundance. Also in abundance is white, milky water full of calcium carbonate. This is mainly formed by Melobesia, a small red alga which lives on the sea grass.p151 Finally, the organisms produce huge amounts of slime, which is a polysaccharide. There is good reason to think the slime is produced by microorganisms to control the way their carbonate skeletons are formed.p139 As a side-effect the slime limits the amount of carbonate which goes into solution.p155
Florida Bay is a flat platform which has laid down a huge amount of carbonate rock.
Many limestones contain fossils, and some are made up almost entirely of skeletal material. Chalk is ~99% coccoliths; Solnhofen limestone is famous for its animal fossils; and there are many shelly limestones from different periods. Limestone is not found much before the Cambrian period, because shelled animals had not yet evolved.
When limestone is heated to a high temperature, and squeezed at the base of a mountain, it becomes the metamorphic rock marble. Marbles are fairly dense, relatively easy to cut, or be shaped into blocks, and some forms are white in colour.
Limestone is hard enough to resist weathering but usually soft enough to be worked (cut and carved) by stonemasons. It is used for building material. Limestone, particularly Portland limestone, became popular in the 19th and 20th centuries for building things such as churches, banks and houses. However, acid rain does slowly erode limestone.
Limestone is used to make glass and cement. It is also used to make iron in a blast furnace, to remove impurities from the iron ore.
Limestone quarry at Cedar Creek, Virginia, USA
Cutting limestone blocks at a quarry in Gozo, Malta
La Zaplaz formations in the Piatra Craiului Mountains, Romania.
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The Cudgel of Hercules, a tall limestone rock (Pieskowa Skała Castle in the background)
The Megalithic Temples of Malta such as Ħaġar Qim are built entirely of limestone. They are among the oldest free-standing structures in existence.
The Great Pyramid of Giza, one of the Seven Wonders of the Ancient World has an outside cover made entirely from limestone.
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Riley County Courthouse built of limestone in Manhattan, Kansas, USA
A limestone plate with a negative map of Moosburg in Bavaria is prepared for a lithography print.
A stratigraphic section of Ordovician limestone exposed in central Tennessee, U.S. The less-resistant and thinner beds are composed of shale. The vertical lines are drill holes for explosives used during road construction.
Photo and etched section of a sample of fossiliferous limestone from the Kope Formation (Upper Ordovician) near Cincinnati, Ohio, U.S.
A concretionary nodular (septarian) limestone at Jinshitan Coastal National Geopark, Dalian, China.
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