Chalk

This article is about the sedimentary rock. For other uses, see Chalk (disambiguation).

Chalk

Sedimentary rock
Beachy Head is a part of the extensive Southern England Chalk Formation.
Composition
Calcite (calcium carbonate)

Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled 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.

Description

"Nitzana Chalk curves" situated at Western Negev, Israel, are chalk deposits formed in the Mesozoic era's Tethys Ocean

Open chalk pit, Seale, Surrey, UK

Chalk is a fine-textured, earthy type of limestone distinguished by its light colour, softness, and high porosity. ^{[1] [2]} It is composed mostly of tiny fragments of the calcite shells or skeletons of plankton, such as foraminifera or coccolithophores. ^[1] 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, echinoderms, or bryozoans. ^{[3] [4] [5]}

Chalk is typically almost pure calcite, CaCO₃, with just 2% to 4% of other minerals. These are usually quartz and clay minerals, though collophane (cryptocrystalline apatite, a phosphate 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, CaMg(CO₃)₂, and in a few cases the dolomitized chalk has been dedolomitized back to calcite. ^[3]

Chalk is highly porous, with typical values of porosity ranging from 35 to 47 per cent. ^[3] 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). ^[5]

Formation

In Western Europe, chalk was formed in the Late Cretaceous Epoch and the early Palaeocene Epoch (between 100 and 61 million years ago). ^{[6] [7]} It was deposited on extensive continental shelves at depths between 100 and 600 metres (330 and 1,970 ft), 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, resulting in the early cementation of such limestones. In chalk, absence of this calcium carbonate conversion process prevented early cementation, which partially accounts for chalk's high porosity. ^[3] Chalk is also the only form of limestone that commonly shows signs of compaction. ^[8]

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 ^[4] 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). ^[9]

Geology and geographic distribution

Chalk from the White Cliffs of Dover, England

Chalk is so common in Cretaceous marine beds that the Cretaceous Period was named for these deposits. The name Cretaceous was derived from Latin creta, meaning chalk. ^[10] Some deposits of chalk were formed after the Cretaceous. ^[11]

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. ^[3] Some of the highest chalk cliffs in the world occur at Jasmund National Park in Germany and at Møns Klint in Denmark. ^[12]

Chalk deposits are also found in Cretaceous beds on other continents, such as the Austin Chalk, ^[13] Selma Group, ^[14] and Niobrara Formations of the North American interior. ^[15] Chalk is also found in western Egypt (Khoman Formation) ^[16] and western Australia (Miria Formation). ^[17]

Chalk of Oligocene to Neogene age has been found in drill cores of rock under the Pacific Ocean at Stewart Arch in the Solomon Islands. ^[18]

There are layers of chalk, containing Globorotalia , in the Nicosia Formation of Cyprus, which formed during the Pliocene. ^[11]

Mining

Main article: Chalk mining

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. ^[19]

Uses

Chalk in different colors

Child drawing with sidewalk chalk

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". ^{[20] [5]}

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. ^[21]

Glazing putty mainly contains chalk as a filler in linseed oil. ^[22]

Chalk and other forms of limestone may be used for their properties as a base. ^[23] Chalk is a source of quicklime by thermal decomposition, or slaked lime following quenching of quicklime with water. ^[24] In agriculture, chalk is used for raising pH in soils with high acidity. ^[25] Small doses of chalk can also be used as an antacid. ^[26] 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. ^[27] Polishing chalk is chalk prepared with a carefully controlled grain size, for very fine polishing of metals. ^[28]

French chalk (also known as 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). ^[29]

Chalk beds form important petroleum reservoirs in the North Sea ^[30] and along the Gulf Coast of North America. ^[13]

Previous uses

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. ^[31]

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. ^[32] 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. ^[33]

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. ^{[34] [35]} 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. ^[36]

A mixture of chalk and mercury can be used as fingerprint powder. However, because of the toxicity of the mercury, the use of such mixtures for fingerprinting was abandoned in 1967. ^[37]

See also

• Blackboard  – Reusable writing surface
• Chalk carving  – Method of shaping chalk
• Chalk line  – Tool for marking straight lines
• Chalking the door  – Christian tradition of blessing one's home
• Chalk (military)  – group of paratroopers that deploy from a single aircraftPages displaying wikidata descriptions as a fallback
• Climbing Chalk  – List of manmade gear
• Clunch  – Traditional building material of chalky limestone rock
• Hill figure  – Type of landscape design
• Pastel  – Powdered-pigment-based art medium
• Sanguine  – Colored chalk variant
• List of types of limestone  – Limestone deposits listed by location

References

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2. Boggs, Sam (2006). Principles of sedimentology and stratigraphy (4th ed.). Upper Saddle River, N.J.: Pearson Prentice Hall. ISBN   0131547283.
3. Hancock, Jake M. (January 1975). "The petrology of the Chalk". Proceedings of the Geologists' Association. 86 (4): 499–535. Bibcode:1975PrGA...86..499H. doi:10.1016/S0016-7878(75)80061-7.
4. "Chalk". Craven & Pendle Geological Society. Archived from the original on 20 June 2009.
5. King, Hobart M. "Chalk: A biological limestone formed from shell debris". Geology.com. Retrieved 1 March 2021.
6. "Introducing the Chalk". Chalk East. Archived from the original on 3 June 2012.
7. Van Der Voert, E.; Heijnen, L.; Reijmer, J.J.G. (2019). "Geological evolution of the Chalk Group in the northern Dutch North Sea: inversion, sedimentation and redeposition". Geological Magazine . 156 (7): 1265–1284. Bibcode:2019GeoM..156.1265V. doi:10.1017/S0016756818000572. hdl: 1871.1/76ca3535-823f-483f-9a38-cb070ab65e32 . S2CID   134798076.
8. Blatt, Harvey; Middleton, Gerard; Murray, Raymond (1980). Origin of sedimentary rocks (2d ed.). Englewood Cliffs, N.J.: Prentice-Hall. p. 508. ISBN   0136427103.
9. Blatt, Middleton & Murray 1980, p. 576.
10. Glossary of Geology (3rd ed.). Washington, D.C.: American Geological Institute. 1972. p. 165.
11. Palamakumbura, R.N.; Robertson, A.H.F. (2018). "Pliocene–Pleistocene sedimentary–tectonic development of the Mesaoria (Mesarya) Basin in an incipient, diachronous collisional setting: facies evidence from the north of Cyprus". Geological Magazine. 155 (5): 997–1022. Bibcode:2018GeoM..155..997P. doi:10.1017/S0016756816001072. S2CID   16436977.
12. Prothero, Donald R. (9 July 2018). "The Story of the Earth in 25 Rocks: Tales of Important Geological Puzzles and the People Who Solved Them". doi:10.7312/prot18260-021.{{cite journal}}: Cite journal requires |journal= (help)
13. Pearson, Krystal (2012). "Geologic models and evaluation of undiscovered conventional and continuous oil and gas resources: Upper Cretaceous Austin Chalk". U.S. Geological Survey Scientific Investigations Report. Scientific Investigations Report. 2012–5159. doi: 10.3133/sir20125159 .
14. Stephenson, Lloyd W.; Monroe, Watson H. (1938). "Stratigraphy of Upper Cretaceous Series in Mississippi and Alabama". AAPG Bulletin. 22. doi:10.1306/3D933022-16B1-11D7-8645000102C1865D.
15. Longman, M.W.; Luneau, B.A.; Landon, S.M. (1998). "Nature and distribution of Niobrara lithologies in the Cretaceous Western Interior Seaway of the Rocky Mountain region". The Mountain Geologist. Retrieved 28 April 2021.
16. Tewksbury, B. J.; Hogan, J. P.; Kattenhorn, S. A.; Mehrtens, C. J.; Tarabees, E. A. (1 June 2014). "Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt". Geology. 42 (6): 479–482. Bibcode:2014Geo....42..479T. doi:10.1130/G35362.1.
17. Henderson, Robert A.; McNAMARA, Kenneth J. (October 1985). "Taphonomy and ichnology of cephalopod shells in a Maastrichtian chalk from Western Australia". Lethaia. 18 (4): 305–322. Bibcode:1985Letha..18..305H. doi:10.1111/j.1502-3931.1985.tb00710.x.
18. Shipboard Scientific Party (2001). "Ocean Drilling Program Leg 192 Preliminary Report: Basement drilling of the Ontong Java Plateau" (PDF). Ocean Drilling Program Preliminary Report 92. Ocean Drilling Program and Texas A&M University. Retrieved 15 February 2023.
19. "Chalk Mines | KURG". www.kurg.org.uk. Archived from the original on 2 February 2017. Retrieved 27 January 2017.
20. 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
21. "How chalk is made - material, making, used, processing, procedure, product, industry". www.madehow.com. Archived from the original on 3 November 2017.
22. Rohleder, Johannes (2001). "The beginnings: Calcium carbonate in glazing putty and rubber". Calcium Carbonate. pp. 138–159. doi:10.1007/978-3-0348-8245-3_6. ISBN   978-3-0348-9490-6.
23. Blatt, Middleton & Murray 1980, p. 445.
24. Blount, Bertram (1990). Chemistry for Engineers and Manufacturers: Chemistry of manufacturing processes. University of Wisconsin – Madison.
25. Oates, J. A. H. (11 July 2008). Lime and Limestone: Chemistry and Technology, Production and Uses. John Wiley & Sons. pp. 111–3. ISBN   978-3-527-61201-7.
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27. Baxter, P. M.; Davis, W. B.; Jackson, J. (January 1981). "Toothpaste abrasive requirements to control naturally stained pellicle.: The relation of cleaning power to toothpaste abrasivity". Journal of Oral Rehabilitation. 8 (1): 19–26. doi:10.1111/j.1365-2842.1981.tb00471.x. PMID   6935391.
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30. Hardman, R.F.P. (1982). "Chalk reservoirs of the North Sea" (PDF). Bulletin of the Geological Society of Denmark. 30 (3–4): 119–137. doi:10.37570/bgsd-1981-30-12. S2CID   149452185 . Retrieved 27 April 2021.
31. Sieveking, G. De G.; Bush, P.; Ferguson, J.; Craddock, P. T.; Hughes, M. J.; Cowell, M. R. (August 1972). "Prehistoric flint mines and their identification as sources of raw material". Archaeometry. 14 (2): 151–176. doi:10.1111/j.1475-4754.1972.tb00061.x.
32. "Archived copy" (PDF). Archived (PDF) from the original on 29 October 2013. Retrieved 24 October 2013.
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34. Walker, Peter; et al. (2005). Rammed earth: design and construction guidelines. Bracknell, England: Building Research Establishment. p. 5. ISBN   9781860817342.
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Further reading

• 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. Archived from the original on 7 June 2009. Retrieved 1 December 2005. The "White Rocks" is the name given to cliffs to the east of Portrush in County Antrim, Northern Ireland.