Assorted prang hygieia dustless chalk provides 5% pure calcium carbonate and free of foreign materials that can cause scratching, streaking and pitting on board surfaces. Assorted colors includes two each red, blue, green, yellow, purple and orange. Chalk definition is - a soft white, gray, or buff limestone composed chiefly of the shells of foraminifers. How to use chalk in a sentence. Chalk App Earn restaurant rewards by picking your favorite NFL, MLB, NCAA Football, NCAA Basketball, and NHL teams. Games on the Chalk app include Pick ‘Em, Streaks, Weekly Bankroll, Fantasy, and Sports Trivia, and rewards can be redeemed for food and beverages. Grab your friends for a fun night out and share the full Chalk experience.

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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[edit]

Chalk is a type of sedimentary rock of a carbonate type.

Formation[edit]

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[edit]

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[edit]

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[edit]

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]}

Uses[edit]

Most people first encounter chalk in school where it refers to blackboard chalk, which was originally made of mineral chalk, since it readily crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be readily erased. Blackboard chalk manufacturers now may use mineral chalk, other mineral sources of calcium carbonate, or the mineral gypsum (calcium sulfate). While gypsum-based blackboard chalk is the lowest cost to produce, and thus widely used in the developing world, use of carbonate-based chalk produces larger particles and thus less dust, and it is marketed as 'dustless chalk'.^[5][3]

Coloured chalks, pastel chalks, and sidewalk chalk (shaped into larger sticks and often coloured), used to draw on sidewalks, streets, and driveways, are primarily made of gypsum rather than calcium carbonate chalk.^[6]

Chalk is a source of quicklime by thermal decomposition, or slaked lime following quenching of quicklime with water.^[7] 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.

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. Builder's putty also mainly contains chalk as a filler in linseed oil.

Chalk may be used for its properties as a base. 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). Small doses of chalk can also be used as an antacid. Additionally, the small particles of chalk make it a substance ideal for cleaning and polishing. For example, toothpaste commonly contains small amounts of chalk, which serves as a mild abrasive. Polishing chalk is chalk prepared with a carefully controlled grain size, for very fine polishing of metals.^[8] Chalk can also be used as fingerprint powder.

Previous uses[edit]

Several traditional uses of chalk have been replaced by other substances, although the word 'chalk' is often still applied to the usual replacements. Tailor's chalk is traditionally a hard chalk used to make temporary markings on cloth, mainly by tailors. It is now usually made of talc (magnesium silicate).

Chalk was traditionally used in recreation. In field sports, such as tennis played on grass, powdered chalk was used to mark the boundary lines of the playing field or court. If a ball hits the line, a cloud of chalk or pigment dust will be visible. In recent years, powdered chalk has been replaced with titanium dioxide.^[9] In gymnastics, rock-climbing, weightlifting and tug of war, chalk — now usually magnesium carbonate — is applied to the hands and feet to remove perspiration and reduce slipping.

Chalk may also be used as a house construction material instead of brick or wattle and daub: quarried chalk was cut into blocks and used as ashlar, or loose chalk was rammed into blocks and laid in mortar.^[10][11] There are still houses standing which have been constructed using chalk as the main building material. Most are pre-Victorian though a few are more recent.^[12]

See also[edit]

• Blackboard – Reusable writing surface
• Chalk line – Tool for marking straight lines
• Chalking the door – Christian tradition of blessing one's home
• Clunch – A traditional building material of chalky limestone rock
• Flint – Cryptocrystalline form of the mineral quartz
• French chalk – A hydrated magnesium phyllosilicate mineral
• Hill figure – Large visual representation created by cutting into a steep hillside to reveal the underlying geology
• Pastel – Art medium consisting of powdered pigment in the form of a stick
• List of types of limestone – Limestone deposits listed by location

References[edit]

1. ^'Introducing the Chalk'. Chalk East. Archived from the original on 3 June 2012.
2. ^ ^ab'Chalk'. Craven & Pendle Geological Society. Archived from the original on 20 June 2009.
3. ^ ^abcKing, Hobart M. 'Chalk: A biological limestone formed from shell debris'. Geology.com. Retrieved 1 March 2021.
4. ^Huxley, T. H. 1868. On a piece of chalk. Macmillan's Magazine 'Archived copy'. Archived from the original on 18 July 2011. Retrieved 11 June 2015.CS1 maint: archived copy as title (link)
5. ^Thakker, M., Shukla, P. and Shah, D.O., 2015. Surface and colloidal properties of chalks: A novel approach using surfactants to convert normal chalks into dustless chalks. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 480, pp.236-244. DOI: 10.1016/j.colsurfa.2015.01.054
6. ^'How chalk is made - material, making, used, processing, procedure, product, industry'. www.madehow.com. Archived from the original on 3 November 2017.
7. ^Blount, Bertram (1990). Chemistry for Engineers and Manufacturers: Chemistry of manufacturing processes. University of Wisconsin – Madison.
8. ^Information on polishing powdersArchived 2011-11-04 at Wikiwix, from the 1879 book 'The Workshop Companion'
9. ^'Archived copy'(PDF). Archived(PDF) from the original on 29 October 2013. Retrieved 24 October 2013.CS1 maint: archived copy as title (link)
10. ^Walker, Peter; et al. (2005). Rammed earth: design and construction guidelines. Bracknell, England: Building Research Establishment. p. 5. ISBN9781860817342.
11. ^Whitaker, William (1872). Memoirs of the Geological Survey of Great Britain. 4. London: Longmans, Green. p. 389. OCLC2531996.
12. ^Easton, David (1996). The Rammed Earth House. White River Junction, VT: Chelsea Green Publishing. p. 15. ISBN9780930031794.

Further reading[edit]

• Gordon, Helen (23 February 2021). 'Rock of ages: How chalk made England'. The Guardian. Adapted from Notes From Deep Time: A Journey Through Our Past and Future Worlds by Helen Gordon.
• 'Landscapes'. White Rocks. The 'White Rocks' is the name given to cliffs to the east of Portrush in County Antrim, Northern Ireland.

Home » Rocks » Sedimentary Rocks » Chalk

Limestone Chalk: A fine-grained, light-colored limestone chalk formed from the calcium carbonate skeletal remains of tiny marine organisms.

What Is Chalk?

Chalk is a variety of limestone composed mainly of calcium carbonate derived from the shells of tiny marine animals known as foraminifera and from the calcareous remains of marine algae known as coccoliths. Chalk is usually white or light gray in color. It is extremely porous, permeable, soft and friable.

Benthic Foraminifera: Scanning electron microscope views of six different benthic foraminifera. Clockwise from top left: Elphidium incertum, Elphidium excavatum clavatum, Trochammina squamata, Buccella frigida, Eggerella advena, and Ammonia beccarii. The calcium carbonate shells from organisms like these can accumulate to form chalk. Images by the United States Geological Survey.

How Does Chalk Form?

Chalk forms from a fine-grained marine sediment known as ooze. When foraminifera, marine algae, or other organisms living on the bottom or in the waters above die, their remains sink to the bottom and accumulate as ooze. If most of the accumulating organic debris consists of calcium carbonate, then chalk will be the type of rock that forms from the ooze. However, if the accumulating organic debris comes from diatoms and radiolarians, the ooze will consist mainly of silica, and the rock type that forms will be diatomite.

Extensive deposits of chalk are found in many parts of the world. They often form in deep water where clastic sediments from streams and beach action do not dominate the sedimentation. They can also form in epeiric seas on continental crust and on the continental shelf during periods of high sea level.

Chalk is widely known among the people of western Europe and a few other parts of the world because it is a bright white rock that can form vertical cliffs along shorelines. The chalk cliffs are eroded at water level by wave action, and as the base of the cliff is undercut, collapses occur when the undercutting reaches a vertical joint or other plane of weakness.

The spectacular cliffs on both sides of the English Channel are composed of chalk. They are known as the 'White Cliffs of Dover' on the United Kingdom side of the Channel and the Cap Blanc-Nez along the coast of France. The English Channel Tunnel, nicknamed 'The Chunnel', that connects England and France was bored through the West Melbury Marly Chalk, a thick and extensive chalk unit that underlies the area.

Chalk Cliffs: Things like fossils and flint can often be found at chalk cliffs. As the soft chalk weathers away, flint nodules fall to the beach below. Image of chalk cliffs along the Baltic Sea, photo copyright iStockphoto / hsvrs.

Cretaceous: A Time of Chalk

Much chalk was deposited during the Cretaceous Period of geologic time. It was a time of global high sea levels that began at the end of the Jurassic Period about 145 million years ago and the beginning of the Paleogene Period about 66 million years ago. During the Cretaceous, warm waters of epeiric seas, seas that flooded continental crust during sea level highs, existed in many parts of the world.

Warm waters of the epeiric seas facilitated chalk deposition because calcium carbonate is more soluble in cold water rather than warm water, and because organisms that produce calcium carbonate skeletal debris will more actively produce in warm water. More chalk formed during the Cretaceous Period than in any other period in geologic history. The Cretaceous received its name after the Latin word creta, which means 'chalk'.

Coarse Chalk: A specimen of chalk with a coarse grain size from the Cretaceous-age Kristianstad Basin collected at a gravel pit near the community of Luneburg, northern Germany. This specimen is from the geological collection of the City Museum of Berlin, and the image is used under a Creative Commons license. Click to enlarge.

Identifying Chalk

The keys to identifying chalk are its hardness, its fossil content, and its acid reaction. At a glance, diatomite and gypsum rock have a similar appearance. An examination with a hand lens will often reveal the fossil content, separating it from gypsum. Its reaction with dilute (5%) hydrochloric acid will separate it from both gypsum and diatomite.

The acid reaction will surprise you if you are used to testing other types of limestone and have never tested chalk. When you apply a drop of acid, capillary action pulls it deep into pore spaces of the specimen. There, the enormous surface area of calcium carbonate that contacts the drop of acid usually produces a spectacular effervescence. Instead of holding the specimen in your hand during the test, place it on surface that will not be damaged by the acid, with a couple paper towels beneath it. You don’t want to have the specimen in your hand and be startled by the effervescence.

Sidewalk Chalk

Oil and Gas Production from Chalk: Map showing the location of oil and gas production in the Austin Chalk of Texas, Louisiana, Arkansas and Mississippi. Fields are shown in yellow, well locations are shown in green and red. Image by the United States Geological Survey. [1] Click to enlarge.

Porosity and Permeability of Chalk

At a microscopic level, there can be a lot of space between the fossil particles that make up chalk. Land underlain by chalk directly below the soil is often well drained. In these areas, water that infiltrates into the soil encounters the top of the chalk and easily flows into the chalk's pore spaces. It then flows downward to the water table and then follows the direction of groundwater flow to a stream or another body of surface water. In some areas, people drill water wells into subsurface chalk layers for residential, commercial, and community water supplies.

In areas where oil and natural gas form in the subsurface, the pore spaces of chalk can serve as a reservoir. Many oil and gas fields are located where subsurface chalk units serve as reservoirs. The Austin Chalk is a subsurface rock unit beneath parts of Texas, Arkansas, Louisiana and Mississippi. It yields oil and natural gas from both conventional and continuous reservoirs. [1]

Blackboards and Chalk

Small pieces of chalk have been used by students for over 1000 years for writing on small slates and large classroom panels known as 'blackboards'. It is an inexpensive and erasable writing material and the most widely known use of chalk. Much of the early blackboard writing was done with pieces of natural chalk or natural gypsum.

Today pieces of natural chalk and natural gypsum have been replaced by sticks manufactured from natural chalk; sticks manufactured using other sources of calcium carbonate; or sticks manufactured using natural gypsum. Gypsum chalk is the softest and writes smoothest; however, it produces more dust than calcium carbonate chalk. Calcium carbonate chalk is harder, requires more pressure to produce wide marks, and makes less dust. It is sometimes marketed as 'dustless chalk' but that description is not quite true. Even though most chalk today is not made from mineral chalk, people still use the name 'chalk' for this familiar writing material.

Chalk School Grades