భూమిలోపల బొగ్గు ఎలా ఏర్పడుతుంది?

భూమిలోపల బొగ్గు ఎలా ఏర్పడుతుంది? How is coal formed?

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.
… Some iron and steel making and other industrial processes burn coal.
The extraction and use of coal causes many premature deaths and much
illness. 

Coal formed millions of years ago when the earth was covered with huge
swampy forests where plants – giant ferns, reeds and mosses – grew. 
As the plants grew, some died and fell into the swamp waters.  New
plants grew up to take their places and when these died still more
grew.  In time, there was thick layer of dead plants rotting in the
swamp.  The surface of the earth changed and water and dirt washed
in, stopping to decaying process.  More plants grew up, but they too
died and fell, forming separate layers.  After millions of years many
layers had formed, one on top of the other.  The weight of the top
layers and the water and dirt packed down the lower layers of plant
matter.  Heat and pressure produced chemical and physical changes in
the plant layers which forced out oxygen and left rich carbon deposits. In
time, material that had been plants became coal.

Coals are classified into three main ranks, or types: lignite, bituminous
coal, and anthracite.  These classifications are based on the amount
of carbon, oxygen, and hydrogen present in the coal.  Coal is defined
as a readily combustible rock containing more than 50% by weight of
carbon. Coals other constituents include hydrogen, oxygen, nitrogen, ash,
and sulfur.  Some of the undesirable chemical constituents include
chorine and sodium.  In the process of transformation
(coalification), peat is altered to lignite, lignite is altered to
sub-bituminous, sub-bituminous coal is altered to bituminous coal, and
bituminous coal is altered to anthracite.