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Coal
the organic rock
Everything You Need to
Know About Coal
by
Andrew Alden
Coal is an enormously
valuable fossil fuel that
has been used for hundreds of years in industry.
It is made up of organic
components; specifically, plant matter that has been buried in an anoxic, or
non-oxygenated, environment and compressed over millions of years.
Fossil, Mineral or Rock?
Because it is organic,
coal defies the normal standards of classification for rocks, minerals, and
fossils:
· A
fossil is any evidence of life that has been preserved in rock. The plant
remains that make up coal have been "pressure cooked" for millions of
years. Therefore, it is not accurate to say that they have been
preserved.
· Minerals
are inorganic, naturally-occurring solids. While coal is a naturally-occurring
solid, it is composed of organic plant material.
· Rocks
are, of course, made up of minerals.
Talk to a geologist,
though, and they'll tell you that coal is an organic sedimentary rock.
Even though it doesn't
technically meet the criteria, it looks like a rock, feels like a rock and is
found between sheets of (sedimentary) rock. So, in this case, it is a
rock.
Geology isn't like
chemistry or physics with their steadfast and consistent rules. It is an Earth
science; and like the Earth, geology is full of "exceptions to the rule."
State legislators
struggle with this topic as well: Utah and West Virginia list coal as
their official state
rock while Kentucky named coal its state mineral in
1998.
Coal: the Organic Rock
Coal differs
from every other kind of rock in that it is made of organic carbon: the actual
remains, not just mineralized fossils, of dead plants.
Today, the vast majority
of dead plant matter is consumed by fire and decay, returning its carbon to the
atmosphere as the gas carbon dioxide.
In other words, it
is oxidized.
The carbon in coal, however, was preserved from oxidation and remains in a
chemically reduced form, available for oxidation.
Coal geologists study
their subject the same way that other geologists study other rocks.
But instead of talking
about the minerals that make up the rock (because there are none, just bits of
organic matter), coal geologists refer to the components of coal as macerals.
There are three groups of
macerals: inertinite, liptinite, and vitrinite.
To oversimplify a complex
subject, inertinite is generally derived from plant tissues, liptinite from
pollen and resins, and vitrinite from humus or broken-down plant matter.
Where Coal Formed
The old saying in geology
is that the present is the key to the past.
Today, we can find plant
matter being preserved in anoxic places: peat bogs like those of Ireland or
wetlands like the Everglades of Florida.
And sure enough, fossil
leaves and wood are found in some coal beds.
Therefore, geologists
have long assumed that coal is a form of peat created by the heat and
pressure of deep burial.
The geologic process of
turning peat into coal is called "coalification."
Coal beds are much, much
larger than peat bogs, some of them tens of meters in thickness, and they occur
all over the world.
This says that the
ancient world must have had enormous and long-lived anoxic wetlands when the
coal was being made.
Geologic History of Coal
While coal has been
reported in rocks as old as Proterozoic (possibly
2 billion years) and as young as Pliocene (2 million years old), the great
majority of the world's coal was laid down during the Carboniferous Period, a
60-million-year stretch (359-299 m.y.a.)
when sea level was high and forests of tall ferns and cycads grew in gigantic
tropical swamps.
The key to preserving the
forests' dead matter was burying it.
We can tell what happened
from the rocks that enclose the coal beds: there are limestones and shales on
top, laid down in shallow seas, and sandstones beneath laid down by river
deltas.
Obviously, the coal
swamps were flooded by advances of the sea. This allowed shale and limestone
to be deposited on top of them.
The fossils in the shale
and limestone change from shallow-water organisms to deep-water species, then
back to shallow forms.
Then sandstones appear as
river deltas advance into the shallow seas and another coal bed is laid down on
top. This cycle of rock types is called a cyclothem.
Hundreds of cyclothems
occur in the rock sequence of the Carboniferous. Only one cause can do that - a
long series of ice ages raising and lowering the sea level.
And sure enough, in the
region that was at the south pole during that time, the rock record shows
abundant evidence of glaciers.
That set of circumstances
has never recurred, and the coals of the Carboniferous (and the following
Permian Period) are the undisputed champions of their type.
It has been argued that
about 300 million years ago, some fungus species evolved the ability to digest
wood, and that was the end of the great age of coal, although younger coal beds
do exist.
A genome study in Science gave that theory more support in 2012. If
the wood was immune to rot before 300 million years ago, then perhaps anoxic
conditions were not always necessary.
Grades of Coal
Coal comes in three main
types or grades. First, the swampy peat is squeezed and heated to form a brown,
soft coal called lignite.
In the process, the
material releases hydrocarbons, which migrate away and eventually become
petroleum.
With more heat and
pressure lignite releases more hydrocarbons and becomes the higher-grade bituminous coal.
Bituminous coal is black,
hard and usually dull to glossy in appearance.
Still greater heat and
pressure yields anthracite, the highest grade of
coal. In the process, the coal releases methane or natural gas.
Anthracite, a shiny, hard
black stone, is nearly pure carbon and burns with great heat and little
smoke.
If coal is subjected to
still more heat and pressure, it becomes a metamorphic rock as the macerals
finally crystallize into a true mineral, graphite.
This slippery mineral
still burns, but it is much more useful as a lubricant, an ingredient in
pencils and other roles.
Still more valuable is
the fate of deeply buried carbon, which at conditions found in the mantle is
transformed into a new crystalline form: diamond.
However, coal probably
oxidizes long before it can get into the mantle, so only Superman could perform
that trick.
Andrew Alden
Professional geologist,
writer, photographer, and geological tour guide
Thirty-seven years of
experience writing about geological subjects
Experience
Andrew Alden is a former
writer for ThoughtCo who contributed hundreds of articles for more than 17
years. Andrew works as a geologist, writer, editor, and photographer. He has
written on geological subjects since 1981 and participates actively in his field.
For example, Andrew spent six years as a research guide with the U.S. Geological Survey,
leading excursions on both land land and at sea. And since 1992, he has hosted
the earthquakes conference for the online discussion platform, The Well,
which began as a dialogue between the writers and readers of the Whole Earth
Review.
In addition, Andrew is a
longtime member of the member of the Geological Society of
America — an international society that serves members in
academia, government, and industry; and the American Geophysical Union — a community of
earth and space scientists that advances the power of science to ensure a
sustainable future.
Andrew lives in Oakland,
California; and though he writes about the whole planet and beyond, Andrew
finds his own city full of interest too and blogs about its
geology.
Education
Andrew Alden holds a
bachelor's (B.A.) degree in Earth Science from the University of New
Hampshire, College of Engineering and Physical Sciences, in Durham, N.H.
Awards and Publications
ThoughtCo and Dotdash
ThoughtCo is a premier
reference site focusing on expert-created education content. We are one of the
top-10 information sites in the world as rated by comScore, a leading Internet
measurement company. Every month, more than 13 million readers seek answers to
their questions on ThoughtCo.
For more than 20 years, Dotdash brands
have been helping people find answers, solve problems, and get inspired. We are
one of the top-20 largest content publishers on the Internet according to
comScore, and reach more than 30% of the U.S. population monthly. Our brands
collectively have won more than 20 industry awards in the last year alone, and
recently Dotdash was named Publisher of the Year by Digiday, a leading industry
publication.
Why Is
Graphite Soft, But Diamond Is So Hard?
