Chalk ( /ˈtʃɔːk/) is a soft, white, porous, sedimentary carbonate
rock, a form of limestone composed of the mineral calcite.
an ionic salt called calcium carbonate or CaCO3. It forms under
reasonably deep marine conditions from the gradual accumulation of
minute calcite shells (coccoliths) shed from micro-organisms called
Flint (a type of chert) is very common as bands
parallel to the bedding or as nodules embedded in chalk. It is
probably derived from sponge spicules 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).
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 presumably indicates very stable
conditions over tens of millions of years.
Chalk curves" situated at Western Negev,
Israel are chalk
deposits formed in the
Mesozoic era's Tethys Ocean
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.
5.1 Previous uses
6 See also
8 External links
Former underground chalk mine in Meudon, France
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.
Chalk Group is a European stratigraphic unit deposited during the
Cretaceous Period. It forms the famous
White Cliffs of Dover
White Cliffs of Dover in
Kent, England, as well as their counterparts of the
Cap Blanc Nez
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
Jasmund National Park
Jasmund National Park in
Germany and at
Møns Klint in
Denmark – both once formed a single island.
Ninety million years ago what is now the chalk downland of Northern
Europe was ooze accumulating at the bottom of a great sea.
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 sea-water. 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 Alps raised these former
sea-floor deposits above sea level.
The chemical composition of chalk is calcium carbonate, with minor
amounts of silt and clay. It is formed in the sea by
sub-microscopic plankton, which fall to the sea floor and are then
consolidated and compressed during diagenesis into chalk rock.
Most people first encounter the word "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 manufacture now may utilize 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, calcium-based
chalk can be made where the crumbling particles are larger and thus
produce less dust, and is marketed as "dustless chalk". Colored
chalks, pastel chalks, and sidewalk chalk (shaped into larger sticks
and often colored), used to draw on sidewalks, streets, and driveways,
are primarily made out of gypsum.
Open chalk pit, Seale, Surrey, UK
Child drawing with sidewalk chalk
Chalk is a source of quicklime by thermal decomposition, or slaked
lime following quenching of quicklime with water. 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
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.
Chalk can also be used as fingerprint powder.
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. However, it is
now usually made from talc (magnesium silicate).
Chalk was also 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. In
gymnastics, rock-climbing, weight-lifting 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. 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.
List of types of limestone
^ Huxley, T. H. 1868. On a piece of chalk. Macmillan's Magazine
^ 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.
^ Blount, Bertram (1990). Chemistry for Engineers and Manufacturers:
Chemistry of manufacturing processes. University of Wisconsin –
^ Information on polishing powders, from the 1879 book "The Workshop
^ Walker, Peter; et al. (2005). Rammed earth: design and construction
guidelines. Bracknell, England: Building Research Establishment.
p. 5. ISBN 9781860817342.
^ Whitaker, William (1872). Memoirs of the Geological Survey of Great
Britain. 4. London: Longmans, Green. p. 389.
^ Easton, David (1996). The Rammed Earth House. White River Junction,
VT: Chelsea Green Publishing. p. 15.
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