What Is Limestone & How Does It Form?

Limestone often forms in vast structures like this one near Quebec, Canada.

Limestone is a sedimentary rock often used for building. It is mostly composed of calcite and can form in several different ways, either from chemicals secreted by living sea creatures like coral, from the compression of untold millions of tiny, dead, sea organisms or by chemical processes unrelated to animals living or dead.

Ancient Seabeds as Present-Day Limestone

Skeletons of untold millions of ancient sea creatures can become limestone.

Uncountable numbers of sea creatures went into the creation of most ordinary limestone. Far out in the ocean tiny organisms die constantly and float down to the seabed, eventually forming different kinds of limestone, called fossiliferous if tiny creature remains are visible. The calcium carbonate in their shells and skeletons is a large portion of the limestone. A loose, soft limestone formed of poorly joined shells and skeletons is usually called coquina.

Limestone in Chalk Cliffs

The white cliffs at England's Dover are chalk, a soft limestone.

Vast cliffs formed of chalk are actually made of limestone. The white, relatively soft and porous version of limestone is formed of huge numbers of generations of tiny sea creatures. Over eons, they die and drift down to settle on the seabed, where their shells remain behind. Eventually, they collect into large structures which, when exposed to the air, can be weathered away into cliffs like those known as England's White Cliffs of Dover.

Limestone Formed by Sea Coral

Limestone structures can be found along the sea or far inland.

Ocean coral reefs are formed of the skeletons of many generations of corals, each bedding on top of previous ones. Over eons, the reef can build up into vast structures of limestone, which can be found far out at sea, near shorelines, or in some cases, far inland at sites that were once oceans but are now dry land. Contributing to the process is calcium carbonate secreted by algae that live in and among the coral.

Inorganic Formation of Limestone

Limestone in the form of dripstone stalactites on a cavern roof.

Travertine is a form of limestone found in caves. It is formed by a chemical process as calcium carbonate precipitates out of dripping water to form stalactites and stalagmites. It is often called dripstone in a cavern setting. Oolitic limestone is formed by the collection of calcium carbonate around small impurities in shallow seas. As the particles increase in size, they drop to the seabed, gradually collecting over millions of years to form layers of limestone.

References

• Georgia State University; "Limestone"
• Illinois State Museum of Geology Online; "Sedimentary Rocks"
• Missouri Dept. of Natural Resources; "Missouri Limestone"