Chalk is a soft, white,
porous
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
,
sedimentary
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
carbonate rock
Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3), and dolomite rock (also known as dolosto ...
. It is a form of
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
composed of the mineral
calcite and originally formed deep under the sea by the compression of microscopic
plankton
Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...
that had settled to the
sea floor. Chalk is common throughout
Western Europe
Western Europe is the western region of Europe. The region's countries and territories vary depending on context.
The concept of "the West" appeared in Europe in juxtaposition to "the East" and originally applied to the ancient Mediterranean ...
, where deposits underlie parts of France, and steep cliffs are often seen where they meet the sea in places such as the
Dover cliffs
The White Cliffs of Dover is the region of English coastline facing the Strait of Dover and France. The cliff face, which reaches a height of , owes its striking appearance to its composition of chalk accented by streaks of black flint, depos ...
on the
Kent
Kent is a county in South East England and one of the home counties. It borders Greater London to the north-west, Surrey to the west and East Sussex to the south-west, and Essex to the north across the estuary of the River Thames; it faces ...
coast of the
English Channel
The English Channel, "The Sleeve"; nrf, la Maunche, "The Sleeve" (Cotentinais) or ( Jèrriais), (Guernésiais), "The Channel"; br, Mor Breizh, "Sea of Brittany"; cy, Môr Udd, "Lord's Sea"; kw, Mor Bretannek, "British Sea"; nl, Het Kana ...
.
Chalk is mined for use in industry, such as for
quicklime,
bricks and builder's
putty, and in
agriculture
Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people t ...
, for raising
pH in soils with high
acidity
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a ...
. 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
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywal ...
.
Description
![Seale Chalk Pit 02](https://upload.wikimedia.org/wikipedia/commons/9/91/Seale_Chalk_Pit_02.JPG)
Chalk is a fine-textured, earthy type of
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
distinguished by its light color, softness, and high porosity.
It is composed mostly of tiny fragments of the
calcite shells or skeletons of
plankton
Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...
, such as
foraminifera
Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...
or
coccolithophore
Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the king ...
s.
These fragments mostly take the form of calcite plates ranging from 0.5 to 4 microns in size, though about 10% to 25% of a typical chalk is composed of fragments that are 10 to 100 microns in size. The larger fragments include intact plankton skeletons and skeletal fragments of larger organisms, such as
molluscs,
echinoderm
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...
s, or
bryozoans
Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a l ...
.
Chalk is typically almost pure calcite, , with just 2% to 4% of other minerals. These are usually
quartz
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...
and
clay minerals, though collophane (cryptocrystalline
apatite, a
phosphate
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid .
The phosphate or orthophosphate ion is derived from phosph ...
mineral) is also sometimes present, as nodules or as small pellets interpreted as fecal pellets. In some chalk beds, the calcite has been converted to
dolomite Dolomite may refer to:
*Dolomite (mineral), a carbonate mineral
*Dolomite (rock), also known as dolostone, a sedimentary carbonate rock
*Dolomite, Alabama, United States, an unincorporated community
*Dolomite, California, United States, an unincor ...
, , and in a few cases the dolomitized chalk has been dedolomitized back to calcite.
Chalk is highly porous, with typical values of porosity ranging from 35 to 47 per cent.
While it is similar in appearance to both
gypsum
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywal ...
and
diatomite
Diatomaceous earth (), diatomite (), or kieselgur/kieselguhr is a naturally occurring, soft, siliceous sedimentary rock that can be crumbled into a fine white to off-white powder. It has a particle size ranging from more than 3 μm to le ...
, chalk is identifiable by its hardness,
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
content, and its reaction to
acid (it produces
effervescence
Effervescence is the escape of gas from an aqueous solution and the foaming or fizzing that results from that release. The word effervescence is derived from the Latin verb ''fervere'' (to boil), preceded by the adverb ''ex''. It has the same li ...
on contact).
[
]
Formation
Chalk was formed in the Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
Period, between 99 and 65 million years ago. It was deposited on extensive continental shelves at depths between , during a time of nonseasonal (likely arid) climate that reduced the amount of erosion from nearby exposed rock. The lack of nearby erosion explains the high purity of chalk. The coccolithophores, foraminifera, and other microscopic organisms from which the chalk came mostly form low-magnesium calcite skeletons, so the sediments were already in the form of highly stable low-magnesium calcite when deposited. This is in contrast with most other limestones, which formed from high-magnesium calcite or aragonite that rapidly converted to the more stable low-magnesium calcite after deposition. Because this conversion process is responsible for the early cementing of limestone, chalk lacked early cementing, which partially accounts for its high porosity. Chalk is also the only form of limestone that commonly shows signs of compaction.
Flint
Flint, occasionally flintstone, is a sedimentary cryptocrystalline form of the mineral quartz, categorized as the variety of chert that occurs in chalk or marly limestone. Flint was widely used historically to make stone tools and sta ...
(a type of chert
Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a ...
) is very common as bands parallel to the bedding
Bedding, also known as bedclothes or bed linen, is the materials laid above the mattress of a bed for hygiene, warmth, protection of the mattress, and decorative effect. Bedding is the removable and washable portion of a human sleeping environm ...
or as nodules in seams
Seam may refer to:
Science and technology
* Seam (geology), a stratum of coal or mineral that is economically viable; a bed or a distinct layer of vein of rock in other layers of rock
* Seam (metallurgy), a metalworking process the joins the en ...
, or linings to fractures
Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
, embedded in chalk. It is probably derived from sponge spicule
Spicules are structural elements found in most sponges. The meshing of many spicules serves as the sponge's skeleton and thus it provides structural support and potentially defense against predators.
Sponge spicules are made of calcium carbon ...
s[ or other ]siliceous
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
organisms as water is expelled upwards during compaction. Flint is often deposited around larger fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s such as Echinoidea
Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells ( tests) ...
which may be silicified
In geology, petrifaction or petrification () is the process by which organic material becomes a fossil through the replacement of the original material and the filling of the original pore spaces with minerals. Petrified wood typifies this proce ...
(i.e. replaced molecule by molecule by flint).
Geology and geographic distribution
Chalk is so common in Cretaceous marine beds that the Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
Period
Period may refer to:
Common uses
* Era, a length or span of time
* Full stop (or period), a punctuation mark
Arts, entertainment, and media
* Period (music), a concept in musical composition
* Periodic sentence (or rhetorical period), a concept ...
was named for these deposits. The name Cretaceous was derived from Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
''creta'', meaning ''chalk''.
The Chalk Group is a Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located entirel ...
an stratigraphic unit deposited during the late Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
Period. It forms the famous White Cliffs of Dover
The White Cliffs of Dover is the region of English coastline facing the Strait of Dover and France. The cliff face, which reaches a height of , owes its striking appearance to its composition of chalk accented by streaks of black flint, depos ...
in Kent
Kent is a county in South East England and one of the home counties. It borders Greater London to the north-west, Surrey to the west and East Sussex to the south-west, and Essex to the north across the estuary of the River Thames; it faces ...
, England, as well as their counterparts of the Cap Blanc Nez
Cap Blanc-Nez (french: kap blɑ̃ ne, literally "Cape White Nose" in English; from Dutch ''Blankenesse'', white headland) is a cape on the Côte d'Opale, in the Pas-de-Calais ''département'', in northern France, culminating at 134 m. The clif ...
on the other side of the Dover Strait
The Strait of Dover or Dover Strait (french: Pas de Calais - ''Strait of Calais''), is the strait at the narrowest part of the English Channel, marking the boundary between the Channel and the North Sea, separating Great Britain from continent ...
. The Champagne region
The wine region within the historical province of Champagne in the northeast of France is best known for the production of champagne, the sparkling white wine that bears the region's name. EU law and the laws of most countries reserve the term " ...
of France is mostly underlain by chalk deposits, which contain artificial caves used for wine storage
Wine is an alcoholic drink typically made from fermented grapes. Yeast consumes the sugar in the grapes and converts it to ethanol and carbon dioxide, releasing heat in the process. Different varieties of grapes and strains of yeasts are m ...
. Some of the highest chalk cliffs in the world occur at Jasmund National Park
The Jasmund National Park (German: ''Nationalpark Jasmund'') is a nature reserve on the Jasmund peninsula, in the northeast of Rügen island in Mecklenburg-Vorpommern, Germany. It is famous for containing the largest chalk cliffs in Germany, the ...
in Germany
Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...
and at Møns Klint
Møns Klint is a 6 km stretch of limestone and chalk cliffs along the eastern coast of the Danish island of Møn in the Baltic Sea. Some of the cliffs fall a sheer 120 m to the sea below. The highest cliff is , which is 128 m above sea le ...
in Denmark
)
, song = ( en, "King Christian stood by the lofty mast")
, song_type = National and royal anthem
, image_map = EU-Denmark.svg
, map_caption =
, subdivision_type = Sovereign state
, subdivision_name = Kingdom of Denmark
, establish ...
.
Chalk deposits are also found in Cretaceous beds on other continents, such as the Austin Chalk
The Austin Chalk is an upper Cretaceous geologic formation in the Gulf Coast region of the United States. It is named after type section outcrops near Austin, Texas. The formation is made up of chalk and marl.
Fossils
The putative galloanseran ...
, Selma Group
The Selma Group is a geological formation in North America, within the U.S. states of Alabama, Mississippi, and Tennessee. The strata date from the Santonian to the Maastrichtian stages of the Late Cretaceous. The group is composed of, in ascend ...
, and Niobrara Formation
The Niobrara Formation , also called the Niobrara Chalk, is a geologic formation in North America that was deposited between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian stages of the Late Cretaceous. It is com ...
s of the North American interior. Chalk is also found in western Egypt (Khoman Formation) and western Australia ( Miria Formation).
Mining
Chalk is mined from chalk deposits both above ground and underground
Underground most commonly refers to:
* Subterranea (geography), the regions beneath the surface of the Earth
Underground may also refer to:
Places
* The Underground (Boston), a music club in the Allston neighborhood of Boston
* The Underground ...
. Chalk mining boomed during the Industrial Revolution
The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe, and the United States, that occurred during the period from around 1760 to about 1820–1840. This transition included going f ...
, due to the need for chalk products such as quicklime and bricks.
Uses
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
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywal ...
(calcium sulfate
Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris ...
). 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".[
Coloured chalks, pastel chalks, and ]sidewalk chalk
Sidewalk chalk is typically large and thick sticks of chalk (calcium sulfate, gypsum, rather than calcium carbonate, rock chalk) that come in multiple colors and are mostly used for drawing on pavement or concrete sidewalks, frequently four sq ...
(shaped into larger sticks and often coloured), used to draw on sidewalks, streets, and driveway
A driveway (also called ''drive'' in UK English) is a type of private road for local access to one or a small group of structures, and is owned and maintained by an individual or group.
Driveways rarely have traffic lights, but some that bear ...
s, are primarily made of gypsum rather than calcium carbonate chalk.
Glazing putty mainly contains chalk as a filler in linseed oil.
Chalk and other forms of limestone may be used for their properties as a base. Chalk is a source of quicklime by thermal decomposition
Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is re ...
, or slaked lime following quenching
In materials science, quenching is the rapid cooling of a workpiece in water, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as pha ...
of quicklime with water. In agriculture
Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people t ...
, chalk is used for raising pH in soils with high acidity
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a ...
. Small doses of chalk can also be used as an antacid
An antacid is a substance which neutralizes stomach acidity and is used to relieve heartburn, indigestion or an upset stomach. Some antacids have been used in the treatment of constipation and diarrhea. Marketed antacids contain salts of alu ...
. 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
An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away by friction. While finishing a material often means polishing it to gain a smooth, reflec ...
. Polishing chalk is chalk prepared with a carefully controlled grain size, for very fine polishing of metals.
Chalk beds form important petroleum reservoirs in the North Sea
The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea, epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the ...
and along the Gulf Coast
The Gulf Coast of the United States, also known as the Gulf South, is the coast, coastline along the Southern United States where they meet the Gulf of Mexico. The list of U.S. states and territories by coastline, coastal states that have a shor ...
of North America.
Previous uses
In southeast England, denehole
A denehole (alternatively dene hole or dene-hole) is an underground structure consisting of a number of small chalk caves entered by a vertical shaft. The name is given to certain caves or excavations in England, which have been popularly suppose ...
s are a notable example of ancient chalk pits. Such bell pits may also mark the sites of ancient flint
Flint, occasionally flintstone, is a sedimentary cryptocrystalline form of the mineral quartz, categorized as the variety of chert that occurs in chalk or marly limestone. Flint was widely used historically to make stone tools and sta ...
mines, where the prime object was to remove flint nodules for stone tool manufacture. The surface remains at Cissbury
Cissbury Ring is an biological Site of Special Scientific Interest north of Worthing in West Sussex. It is owned by the National Trust and is designated a Scheduled monument for its Neolithic flint mine and Iron Age hillfort.
Cissbury R ...
are one such example, but perhaps the most famous is the extensive complex at Grimes Graves
Grime's Graves is a large Neolithic flint mining complex in Norfolk, England. It lies north east from Brandon, Suffolk in the East of England. It was worked between 2600 and 2300 BC, although production may have continued well into ...
in Norfolk
Norfolk () is a ceremonial and non-metropolitan county in East Anglia in England. It borders Lincolnshire to the north-west, Cambridgeshire to the west and south-west, and Suffolk to the south. Its northern and eastern boundaries are the No ...
.
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 (also known as French chalk) is traditionally a hard chalk used to make temporary markings on cloth, mainly by tailor
A tailor is a person who makes or alters clothing, particularly in men's clothing. The Oxford English Dictionary dates the term to the thirteenth century.
History
Although clothing construction goes back to prehistory, there is evidence of ...
s. It is now usually made of talc
Talc, or talcum, is a clay mineral, composed of hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thickening agent a ...
(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
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolub ...
. In gymnastics, rock-climbing, weightlifting and tug of war
Tug of war (also known as tug o' war, tug war, rope war, rope pulling, or tugging war) is a sport that pits two teams against each other in a test of strength: teams pull on opposite ends of a rope, with the goal being to bring the rope a certa ...
, 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
Wattle and daub is a composite building method used for making walls and buildings, in which a woven lattice of wooden strips called wattle is daubed with a sticky material usually made of some combination of wet soil, clay, sand, animal dung a ...
: quarried chalk was cut into blocks and used as ashlar, or loose chalk was rammed into blocks and laid in mortar. 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.
A mixture of chalk and mercury can be used as fingerprint powder
Fingerprint powders are fine powders used, in conjunction with fingerprint brushes, by crime scene investigators and other law enforcement personnel to search for and enhance latent/invisible fingerprints that can be used to determine identificati ...
. However, because of the toxicity of the mercury, the use of such mixtures for fingerprinting was abandoned in 1967.
See also
*
*
*
*
*
*
*
*
*
*
References
Further reading
*
*{{cite web , title=Landscapes , website=White Rocks , url=http://www.geographyinaction.co.uk/Landscapes/Landscapes_WhiteRocks.html , access-date=1 December 2005 , archive-date=7 June 2009 , archive-url=https://web.archive.org/web/20090607085758/http://www.geographyinaction.co.uk/Landscapes/Landscapes_WhiteRocks.html , url-status=dead The "White Rocks" is the name given to cliffs to the east of Portrush in County Antrim, Northern Ireland.
Sewing equipment