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Chalk
 —  Sedimentary Rock  —
Chalk Image
The Needles, situated on the Isle of Wight, are part of the extensive Southern England Chalk Formation.
Composition
calcite (calcium carbonate)

Chalk (pronounced /ˈtʃɔːk/) is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. It forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. It is common to find chert nodules embedded in chalk. Chalk can also refer to other compounds including magnesium silicate and calcium sulfate.

Chalk is resistant to weathering and slumping compared to 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 porous it can hold a large volume of ground water, providing a natural reservoir that releases water slowly through dry seasons.

Chalk has long been quarried in England, providing building material and marl for fields. In southeast England, Deneholes are a notable example of ancient chalk pits.

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. The Champagne region of France is mostly underlain by chalk deposits, which contain artificial caves used for wine storage.

Contents

Composition

Chalk is composed mostly of calcium carbonate with minor amounts of silt and clay. It is normally formed underwater, commonly on the sea bed, then consolidated and compressed during diagenesis into the form commonly seen today. During diagenesis silica accumulates to form chert nodules within the carbonate rock.

Chalk uses

The traditional uses of chalk have in many cases been replaced by other substances, although the word "chalk" is often still applied to the replacements.

  • Blackboard chalk is a substance used for drawing on rough surfaces, as it readily crumbles leaving particles that stick loosely to these surfaces. Although traditionally composed of natural chalk, modern blackboard chalk is generally made from the mineral gypsum (calcium sulfate), often supplied in sticks of compressed powder about 10 cm long.
  • Sidewalk chalk is similar to blackboard chalk, except that it is formed into larger sticks and often coloured. It is used to draw on sidewalks, streets, and driveways, mostly by children, but also by adult artists.
  • In agriculture chalk is used for raising pH in soils with high acidity. The most common forms are CaCO3 (calcium carbonate) and CaO (calcium oxide).
  • In field sports, including grass tennis courts, powdered chalk was used to mark the boundary lines of the playing field or court. This gives the advantage that, if the ball hits the line, a cloud of chalk or pigment dust can be seen. Nowadays the substance used is mostly titanium dioxide.[1]
  • In gymnastics, rock-climbing, weight-lifting and tug of war, chalk—now usually magnesium carbonate—is applied to the hands to remove perspiration and reduce slipping.
  • Tailor's chalk is traditionally a hard chalk used to make temporary markings on cloth, mainly by tailors. Nowadays it is usually made from talc (magnesium silicate).
  • Toothpaste also commonly contains small amounts of chalk.

See also

References

External links

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1911 encyclopedia

Up to date as of January 14, 2010
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From LoveToKnow 1911

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Simple English

File:MERS (3).JPG
Chalk cliffs

Chalk is a type of limestone. Air and water do not wear chalk away easily, so when chalk is next to the sea, it often makes a large cliff. The "White Cliffs of Dover" in Kent, England are a good example of this. When chalk is near the top of the ground, it often makes chalk hills. Chalk holds water, so these chalk hills have a lot of water in them, and when the weather is very dry, water comes slowly from the chalk. Chalk is mostly formed from the calcareous (CaCO3) skeletons of countless tiny planktonic algae called coccoliths. It was laid down in the Upper Cretaceous period.

Blackboard chalk is not real chalk. It is really gypsum (calcium sulfate), but often people call it "chalk". People use it for drawing on hard things, because it is soft. If this chalk is rubbed on something hard or rough, it will leave a mark. People often write with chalk on a chalkboard or blackboard. A chalk eraser or water can be used to clean the chalk marks away so the board is blank again. Shops usually sell blackboard chalk in sticks that are about 5 cm long.

There are also big pieces of soft chalk for children to write on the ground with, for example on concrete (such as a sidewalk) or on asphalt (such as a driveway). It is usually okay to write on the ground with this kind of chalk, because the rain will wash the chalk away.

Tailors' chalk is not real chalk either. It is really talc (magnesium silicate). Tailors use it to draw on material when they are making clothes.

People in the military say a chalk is when an aeroplane is carrying a special load, especially a group of soldiers in a single aeroplane. "U.S. Army Ranger Chalk Four" was a group that was hurt in the Battle of Mogadishu in Somalia.


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