Coal

Coal is a combustible black or brownish-black

sedimentary rock

, formed as rock strata called

coal seams

. Coal is mostly

carbon

with variable amounts of other elements, chiefly

hydrogen

,

sulfur

,

oxygen

, and

nitrogen

.
Coal is formed when dead plant matter decays into peat and is converted into coal by the heat and pressure of deep burial over millions of years. Vast deposits of coal originate in former

wetland

s—called coal forests—that covered much of the Earth's tropical land areas during the late Carboniferous ( Pennsylvanian) and Permian times. However, many significant coal deposits are younger than this and originate from the Mesozoic and

Cenozoic

eras.

Coal is primarily used as a fuel. While coal has been known and used for thousands of years, its usage was limited until the Industrial Revolution. With the invention of the

steam engine

, coal consumption increased. In 2020 coal supplied about a quarter of the world's primary energy and over a third of its electricity. Some

iron

and

steel

making and other industrial processes burn coal.

The extraction and use of coal causes premature deaths and illness. The use of coal damages the environment, and it is the largest anthropogenic source of

carbon dioxide

contributing to climate change. 14 billion tonnes of carbon dioxide was emitted by burning coal in 2020, which is 40% of the total fossil fuel emissions and over 25% of total global greenhouse gas emissions. As part of the worldwide energy transition many countries have reduced or eliminated their use of coal power. The UN Secretary General asked governments to stop building new coal plants by 2020. Global coal use peaked in 2013. But in China it reached higher levels than ever in 2021. To meet the Paris Agreement target of keeping

global warming

to below coal use needs to halve from 2020 to 2030, and phasing down coal was agreed in the Glasgow Climate Pact.

The largest consumer and importer of coal is China. China mines almost half the world's coal, followed by India with about a tenth. Australia accounts for about a third of world coal exports, followed by

Indonesia

and Russia.

Etymology

The word originally took the form ''col'' in Old English, from Proto-Germanic *''kula''(''n''), which in turn is hypothesized to come from the Proto-Indo-European root *''g''(''e'')''u-lo-'' "live coal".

Germanic

cognates include the Old Frisian ''kole'', Middle Dutch ''cole'', Dutch ''kool'', Old High German ''chol'',

German

''Kohle'' and Old Norse ''kol'', and the Irish word ''gual'' is also a cognate via the Indo-European root.

Geology

Coal is composed of macerals,

minerals

and water.

Fossils

and

amber

may be found in coal.

Formation

The conversion of dead vegetation into coal is called coalification. At various times in the geologic past, the Earth had dense forests in low-lying wetland areas. In these wetlands, the process of coalification began when dead plant matter was protected from biodegradation and

oxidation

, usually by mud or acidic water, and was converted into peat. This trapped the carbon in immense

peat bog

s that were eventually deeply buried by sediments. Then, over millions of years, the heat and pressure of deep burial caused the loss of water, methane and carbon dioxide and increased in the proportion of carbon. The grade of coal produced depended on the maximum pressure and temperature reached, with

lignite

(also called "brown coal") produced under relatively mild conditions, and sub-bituminous coal,

bituminous coal

, or

anthracite

(also called "hard coal" or "black coal") produced in turn with increasing temperature and pressure.

Of the factors involved in coalification, temperature is much more important than either pressure or time of burial. Subbituminous coal can form at temperatures as low as while anthracite requires a temperature of at least .

Although coal is known from most geologic periods, 90% of all coal beds were deposited in the Carboniferous and Permian periods, which represent just 2% of the Earth's geologic history. Paradoxically, this was during the Late Paleozoic icehouse, a time of global

glaciation

. However, the drop in global sea level accompanying the glaciation exposed

continental shelf

s that had previously been submerged, and to these were added wide

river delta

s produced by increased

erosion

due to the drop in base level. These widespread areas of wetlands provided ideal conditions for coal formation. The rapid formation of coal ended with the coal gap in the Permian–Triassic extinction event, where coal is rare.

Favorable geography alone does not explain the extensive Carboniferous coal beds. Other factors contributing to rapid coal deposition were high

oxygen

levels, above 30%, that promoted intense

wildfire

s and formation of

charcoal

that was all but indigestible by decomposing organisms; high

carbon dioxide

levels that promoted plant growth; and the nature of Carboniferous forests, which included lycophyte trees whose determinate growth meant that carbon was not tied up in

heartwood

of living trees for long periods.

One theory suggested that about 360 million years ago, some plants evolved the ability to produce

lignin

, a complex polymer that made their

cellulose

stems much harder and more woody. The ability to produce lignin led to the evolution of the first

trees

. But bacteria and fungi did not immediately evolve the ability to decompose lignin, so the wood did not fully decay but became buried under sediment, eventually turning into coal. About 300 million years ago, mushrooms and other fungi developed this ability, ending the main coal-formation period of earth's history. However, a 2016 study largely refuted this idea, finding extensive evidence of lignin degradation during the Carboniferous, and that shifts in lignin abundance had no impact on coal formation. They suggested that climatic and tectonic factors were a more plausible explanation.

One likely tectonic factor was the Central Pangean Mountains, an enormous range running along the equator that reached its greatest elevation near this time. Climate modeling suggests that the Central Pangean Mountains contributed to the deposition of vast quantities of coal in the late Carboniferous. The mountains created an area of year-round heavy precipitation, with no dry season typical of a

monsoon

climate. This is necessary for the preservation of peat in coal swamps.

Coal is known from

Precambrian

strata, which predate land plants. This coal is presumed to have originated from residues of algae.

Sometimes coal seams (also known as coal beds) are interbedded with other sediments in a cyclothem. Cyclothems are thought have their origin in glacial cycle

{{Redirect category shell, 1=
{{R from move
{{R from ambiguous term
{{R from other ca ...