Chalk is a soft, white, porous, sedimentary carbonate rock, a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of sub-microscopic plankton which had fallen to the sea floor. Chalk is common throughout Western Europe, where deposits underlie parts of France, and steep cliffs are often seen where they meet the sea in places such as the Dover cliffs on the Kent coast of the English Channel.
Chalk is mined for use in industry, such as for quicklime, bricks and builder's putty, and in agriculture, for raising pH in soils with high acidity. It is also used for "blackboard chalk" for writing and drawing on various types of surfaces, although these can also be manufactured from other carbonate-based minerals, or gypsum.
Composition
Chalk is a type of sedimentary rock of a carbonate type.
Formation
Chalk was formed in the Cretaceous, between 99 and 65 million years ago.[1] Ninety million years ago what is now the chalk downland of Northern Europe was ooze accumulating at the bottom of a great sea. Chalk was one of the earliest rocks, made up of microscopic particles to be studied under the microscope, when it was found to be composed almost entirely of coccoliths. Their shells were made of calcite extracted from the rich seawater. As they died, a substantial layer gradually built up over millions of years and, through the weight of overlying sediments, eventually became consolidated into rock. Later earth movements related to the formation of the European Alps raised these former sea-floor deposits above sea level.[citation needed]
It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells of single-celled marine animals known as foraminifera, microscopic fragments of larger ostracods and bivalves,[2] and from the calcareous remains (coccoliths) shed from a type of single-celled algae called coccolithophores.[3] The foraminifera fall to the sea floor and are then consolidated and compressed during diagenesis into chalk rock.[citation needed]
Chemical composition
Chalk is a type of limestone, composed of the mineral calcite. Calcite is an ionic salt called calcium carbonate or CaCO3.[citation needed]
The chemical composition of chalk is calcium carbonate, with minor amounts of silt and clay.[4] While it is similar in appearance to both gypsum and diatomite, chalk is identifiable by its hardness, fossil content, and its reaction to acid (it produces effervescence on contact).[3]
Geology and geographic distribution
Chalk as seen in Cretaceous deposits of Western Europe is unusual among sedimentary limestones in the thickness of the beds. Most cliffs of chalk have very few obvious bedding planes, unlike most thick sequences of limestone such as the Carboniferous Limestone or the Jurassic oolitic limestones. This may indicate very stable conditions over tens of millions of years.[citation needed]
Flint (a type of chert) is very common as bands parallel to the bedding or as nodules in seams, or linings to fractures, embedded in chalk. It is probably derived from sponge spicules[2] or other siliceous organisms as water is expelled upwards during compaction. Flint is often deposited around larger fossils such as Echinoidea which may be silicified (i.e. replaced molecule by molecule by flint).[citation needed]
Chalk has greater resistance to weathering and slumping than the clays with which it is usually associated, thus forming tall, steep cliffs where chalk ridges meet the sea. Chalk hills, known as chalk downland, usually form where bands of chalk reach the surface at an angle, so forming a scarp slope. Because chalk is well jointed it can hold a large volume of ground water, providing a natural reservoir that releases water slowly through dry seasons.[citation needed]
The Chalk Group is a European stratigraphic unit deposited during the late Cretaceous Period. It forms the famous White Cliffs of Dover in Kent, England, as well as their counterparts of the Cap Blanc Nez on the other side of the Dover Strait. The Champagne region of France is mostly underlain by chalk deposits, which contain artificial caves used for wine storage. Some of the highest chalk cliffs in the world occur at Jasmund National Park in Germany and at Møns Klint in Denmark – both once formed a single island.[citation needed]
Mining
Chalk is mined from chalk deposits both above ground and underground. Chalk mining boomed during the Industrial Revolution, due to the need for chalk products such as quicklime and bricks. Some abandoned chalk mines remain tourist destinations due to their massive expanse and natural beauty.[citation needed]
| This article uses material from the Wikipedia article Metasyntactic variable, which is released under the Creative Commons Attribution-ShareAlike 3.0 Unported License. |