Page 282 - Ultimate Visual Dictionary (DK)

GEOLOGY, GEOGRAPHY, AND METEOROLOGY

Mineral resources STAGES IN THE FORMATION OF COAL

Stalk Leaf
MINERAL RESOURCES CAN BE DEFINED AS naturally occurring
substances that can be extracted from the Earth and are useful
as fuels and raw materials. Coal, oil, and gas – collectively
called fossil fuels – are commonly included in this group, but
are not strictly minerals, because they are of organic origin. Coal PLANT MATTER
formation begins when vegetation is buried and partly decomposed Decayed
to form peat. Overlying sediments compress the peat and transform plant matter
it into lignite (soft brown coal). As the overlying sediments
accumulate, increasing pressure and
temperature eventually transform
the lignite into bituminous and
hard anthracite coals. Oil and gas
are usually formed from organic About 60% PEAT
matter that was deposited in marine carbon About 70%
sediments. Under the effects of heat carbon
and pressure, the compressed
organic matter undergoes complex
OIL RIG, NORTH SEA chemical changes to form oil and
gas. The oil and gas percolate
upwards through water-saturated, permeable rocks and they may
rise to the Earth’s surface or accumulate below an impermeable
layer of rock that has been folded or faulted to form a trap – Crumbly
an anticline (upfold) trap, for example. Minerals are inorganic texture LIGNITE (BROWN COAL) Powdery
texture
substances that may consist of a single chemical element,
such as gold, silver, or copper, or combinations of elements
(see pp. 268-269). Some minerals are concentrated in
mineralization zones in rock associated with crustal movements
or volcanic activity. Others may be found in sediments as placer
deposits – accumulations of high-density minerals that have
been weathered out of rocks, transported, and About 80%
deposited (on riverbeds, for example). carbon
Increasing layers of
HOW COAL IS FORMED overlying sediment

Increasing layers of
overlying sediment Increasing
Vegetation
pressure and
Increasing temperature
pressure and
temperature
Shiny BITUMINOUS COAL
surface About 95%
carbon

Peat (about Lignite (about Bituminous coal
60% carbon) 70% carbon) (about 80% carbon)
PEAT LIGNITE (BROWN COAL) BITUMINOUS COAL ANTHRACITE COAL
280