| Chalk | |
|---|---|
| — Sedimentary Rock — | |
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| 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 |
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.
The traditional uses of chalk have in many cases been replaced by other substances, although the word "chalk" is often still applied to the replacements.
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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.
   | Chalk cliffs in the North of Ireland - Landscapes - White Rocks |
 | The Chalk in the southern North Sea subsurface - Allard Sytse van der Molen : Sedimentary development, seismic stratigraphy and burial compaction of the Chalk Group in the Netherlands North Sea area |
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