Limestone is a sedimentary rock, which means it was formed from small particles of rock or stone that have been compacted by pressure. Sedimentary rock is important because it often contains fossils and gives clues about what type of rock was on the Earth long ago. Just like a tree's rings tell a lot about its environment, layers found in sedimentary rock can tell about important changes in the environment.
Ocean-dwelling organisms such as oysters, clams, mussels and coral use calcium carbonate (CaCO3) found in seawater to create their shells and bones. As these organisms die, their shells and bones are broken down by waves and settle on the ocean floor where they are compacted over millions of years, creating limestone from the sediments and the pressure of the ocean water.
The second way limestone is formed is when water containing particles of calcium carbonate evaporate, leaving behind the sediment deposit. The water pressure compacts the sediment, creating limestone.
The area around the Great Lakes, such as Michigan, Indiana, and Illinois, has a large amount of limestone. Scientists use this along with evidence of fossils to hypothesize that at one time the area was underwater, which created a lot of limestone.
There are a few ways to recognize limestone. First of all, it is a soft stone and when it is scratched with a sharp object, it becomes a white powder. When limestone comes in contact with an acid like vinegar or hydrochloric acid (HCl), the stone will actually bubble and deteriorate and then neutralize the acid.
Limestone is important to making cement, but it is also found in other industries such as sugar refining, glass making, and leather tanning. Crushed limestone underneath roads and underneath railroad tracks. When limestone is heated, it helps manufacture iron and steel as well as alumina and magnesia. Limestone helps clean drinking water and treat sewage. Farmers will often spread fertilizer that contains ground up limestone on their crops. The limestone in the mixture is a source of plant nutrients and neutralizes soil acidity.
Because it is a softer stone, it is easily carved. Limestone appears in many buildings. Indiana limestone, also known as Salem limestone, can be found at the Empire State Building in New York City and on the U.S. Holocaust Memorial and Museum in Washington D.C.