Science Fair Project Encyclopedia
Chalk is a soft, white, porous form of limestone composed of the mineral calcium carbonate. It is relatively resistant to erosion and slumping compared to the clays that it is usually associated with, and so forms 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.
Because chalk is porous, chalk downland usually holds a large water table, providing a natural reservoir that releases water slowly through dry seasons.
Blackboard chalk is a substance used for drawing on rough surfaces, as it readily crumbles leaving particles that stick loosely to these surfaces. Blackboard chalk, often supplied in sticks about 5 cm long, is not actually made from the mineral chalk but from gypsum (calcium sulfate). Similarly, the "chalk" used by tailors is usually made from talc (magnesium silicate).
In military terminology, a chalk is a specific aircraft load, especially a group of airborne soldiers which deploy from a single aircraft. U.S. Army Ranger Chalk Four was the group which took the initial casualties in the Battle of Mogadishu in Somalia.
- Chalk Formation
- Southern England Chalk Formation
- List of minerals
- Mohs scale of mineral hardness
- Sidewalk chalk
- Agricultural lime
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