Chalk facts for kids
|Calcite (calcium carbonate)|
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.
Ninety million years ago the chalk downland of Northern Europe was ooze at the bottom of a great sea. Protozoans such as foraminifera lived on the marine debris that showered down from the upper layers of the ocean. Their bodies were made of chalk extracted from the rich sea-water. As they died, so a deep layer built up, and slowly it became consolidated into rock. At a later date the sea-bed became dry land, as earth movements thrust it upward.
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 or flint nodules within the carbonate rock.
Chalk has long been quarried in England, providing building material and marl for fields. It is also used to make quicklime and slaked lime, mainly used as lime mortar in buildings. In southeast England, Deneholes are a notable example of ancient chalk pits. Such bell pits may also mark the sites of ancient flint mines, where the prime object was to remove flint nodules for stone tool manufacture. The surface remains at Cissbury are one such example, but perhaps the most famous is the extensive complex at Grimes Graves in Norfolk.
The traditional uses of chalk have in some cases been replaced by other substances, although the word "chalk" is often still applied to the replacements.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 colored. 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.
- 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, to serve as a mild abrasive.
- Polishing chalk is chalk prepared with a carefully controlled grain size, for very fine polishing of metals.
- Chalk is a source of quicklime by thermal decomposition, or slaked lime following quenching with water.
- Builders putty also mainly contains chalk as a filler in linseed oil.
- Woodworking joints may be fitted by chalking one of the mating surfaces. A trial fit will leave a chalk mark on the high spots of the corresponding surface. Chalk transferring to cover the complete surface indicates a good fit.
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.
Images for kids
Former underground chalk mine in Meudon, France
Chalk Facts for Kids. Kiddle Encyclopedia.