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The Coal Section
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What is Coal?
Coal is a combustible, dark-brown to black, carbonaceous sedimentary rock derived from degraded terrestrial
plant matter that accumulates in wetland environments known as mires: bogs, marshes, and swamps. The dominant
type of vegetation that accumulates in a mire determines the type of coal to be formed. There are two types
of coal, sapropelic coal and humic coal: sapropelic coal forms from mesoscopic (small) vegetation such as
sphagnum moss, while humic coal forms from macroscopic (large) vegetation such as sedges, grasses, shrubs,
and trees. Just as a rock is a collection of various minerals, coal is a collection of various macerals
(the altered remnants of plant matter). There are three primary maceral groups: vitrinite (derived from
woody tissue, primarily lignin and cellulose), liptinite (derived from waxes and resins), and inertinite
(derived from intensely altered and oxidized plant material such as charcoal).
What is Coalification?
Peat is the layers of plant matter that accumulate on the bottom of a mire. Coalification is the slow
alteration of that plant matter into coal, and it proceeds in two phases. The first phase is biogenesis: plant
matter is biogenically changed into serial forms of peat, ending as a form of peat called gytta. The second phase
is thermogenesis: the gytta is changed by heat into serial ranks of coal.
Biogenesis
In a mire, the subaqueous (underwater) accumulation of dead plant matter forms a deposit called peat. Within the
vertical succession of accumulated peat (called the peat column), a sequence of zones develops, from top to bottom
the fibric, humic, sapropelic, and hydro-gel or gytta zones. The zones in a peat column represent successive levels
of bio-degradation. In each zone, bacteria physically and chemically reduce and break down the peat. The uppermost
and shallowest part of the peat column contains free oxygen that supports aerobic bacteria; below this zone,
anaerobic bacteria take over. The end product of the continuous bacterial reduction of accumulated plant
material is the bottom zone, the layer of gytta. Gytta is a semi-amorphous, organic-rich gel. It marks the end
of the biogenic process, and is the precursor of coal. As biogenisis progresses over time, the gytta zone
increases in thickness, sometimes to hundreds of feet thick, while the thickness of the upper three zones in
the peat column remains relatively constant. The thickness of a coal bed directly corresponds to the final
thickness of the gytta zone underlying the peat column.
Thermogenesis
The second phase of coalification is a heat-driven process called thermogenesis. This phase of coalification
thermo-chemically removes water and concentrates carbon, converting gytta to coal. The thermogenic phase of
coalification is a function of time and temperature. In order for gytta to be thermogenically converted to coal,
the gytta must be buried and heated to temperatures between 150 and 215 degrees Fahrenheit over a long period of
time. These temperatures are generated by several different geologic processes: the natural geothermal gradient
of the earth (temperature increase with depth), emplacement of igneous rocks (intrusive and extrusive), and
naturally occurring fires in stratigraphically adjacent coal beds or overlying peat.
The rank of a coal depends on the duration (amount of time) and degree (maximum temperature) of thermogenesis.
The result of little or no thermal maturation is the transition from gytta to a thermoplastic resin called
lignite, the lowest rank of coal. As thermal maturation proceeds, the coal becomes enriched in hydrocarbons
and increases in rank through subbituminous, then to bituminous, and finally to anthracite, the highest rank of
coal.
When did peat accumulate in Wyoming?
During Wyoming’s geologic past there were several periods when widespread peat-forming wetlands existed.
Wyoming’s major deposits of coal are from the Cretaceous (138-65 mya) and the early Tertiary (65-50 mya).
There is little or no evidence of coal in Wyoming from the Carboniferous Period (360-290 million years ago),
a major coal producing time elsewhere.
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