Limestone is a common type of sedimentary rock that is made from calcite and is used for a variety of different applications. It can be used in building applications, flooring, cement production, concrete filler, steel production, counter tops, blackboard chalk, and a variety of others applications. Limestone is typically formed in one of two ways.
The most common area that limestone forms in is shallow, clear, warm marine waters. This kind of limestone is an organic sedimentary rock due to how it forms. The formation of limestone in these areas is a result of the many organisms in the ocean that create calcium carbonate (calcite) shells and skeletons. Marine life such as brachiopods, crinoids, coral, clams, bryozoa, algae, and many other organisms are responsible for this kind of formation. Animals producing these shells and skeletons only do so in shallow water as they require sunlight, turbulence, and a food source to construct their shell.
These animals typically live on the bottom of the sea and after they die, the shells and skeletons are left over. Over time the shells will accumulated and begin covering each other. Reefs are also a large source of limestone as they are made from calcite as well. Other materials can sometimes mix in with the calcite causing impurities like silt or sand which will result in slightly different limestone.
Currently there are many locations where limestone is forming. The majority are found in shallow water that is near the equator. Areas of the Indian Ocean, Gulf of Mexico, Pacific Islands, Caribbean Sea and the Persian Gulf all have limestone-forming environments.
Though less abundant than the biological sedimentary limestone, chemical sedimentary limestone may also form in ocean and fresh water. This form of limestone results from the precipitation of calcium carbonate out of the water and it will accumulate on surfaces at the bottom of the water body.
The other method of limestone formation is through evaporation. This typically occurs in cave formations such as stalagmites and stalactites. Water seeping into the cave from fractures, breaks, or porous material in the cave ceiling will drip down from the roof of the cave to the cave floor. The water may evaporate before it falls to the cave floor or after it falls to the cave floor but in either case, deposits of calcium carbonate that may have been dissolved in the water will be left behind.
Over time, the evaporation of the water and the deposition of the calcium carbonate will accumulate to form many of the icicle shaped cave formations which are common in some caves. Water evaporating and depositing before it touches the ground will start forming stalactites whereas water dropping to the ground and then evaporating will form stalagmites. These formations are a chemical sedimentary rock which is commonly referred to as travertine but is still limestone when calcium carbonate is present in the water and is left behind after evaporation. Formation of limestone may also occur as chemical sedimentary rock at the shore of a lake or hot spring due to evaporation.