Peat
Lignite
Bituminous Coal (a sedimentary rock) Anthracite
Coal (a metamorphic rock)Coal Formation
Bituminous Coal is classified as a Sedimentary Rock.
What makes coal unique among other Sedimentary Rocks is that coal does not
have a mineral composition. The composition of coal is the element carbon.
It is not formed from rock fragments; rather it is formed from biologic (organic)
remains, specifically plant matter that grew in swampy areas as long as 300-400
million years ago.
As trees and other vegetation die they may fall into a swamp, sinking to the
bottom, where plant matter accumulates, one layer upon another. A thick, soggy
mass of decaying vegetation eventually forms Peat, the first stage in coal formation.
Rivers in the area of the swamp deposit sediments on top of the peat and any
other decayed organic matter that may have accumulated there. The sediments
can lithify into sedimentary rock, further burying and compressing the material
under great weight and squeezing out all water from the Peat.
This continued burial and pressure changes Peat to Coal. The youngest layers
of coal found in swampy areas in the southeast United States are approximately
1 million years old.
What conditions are necessary for coal to form?
There are 4 distinct stages of coal formation:
Peat
Lignite
Bituminous Coal (a sedimentary rock) Anthracite
Coal (a metamorphic rock)
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The first stage in coal formation is Peat. Millions of years ago, as vegetation in swampy areas fell into swamps and swampy lakes, it piled up and decayed forming soft peat which is partially altered vegetation that, when burned, is very smoky with low energy output. |
| The second stage in coal formation is Lignite. Additional layers of decaying vegetation along with sediments deposited into the swamp or swampy lake further compact the peat forming Lignite, a soft brown coal which, when burned, produces moderate energy. |
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Further heat and pressure caused by continued burial and
greater compaction transforms the Lignite into Bituminous Coal, classified
as a Sedimentary Rock. It is a soft, black coal with a high energy output
and it is a major coal source for generating power, especially in industry. At this point in time, the climate has changed and the swamps are long gone. What remains is a layer of buried coal with sedimentary rock strata above it. |
| In the Rock Cycle, the processes of metamorphism can change
the physical properties of coal as well. Extreme heat and compressional
forces can change soft coal into hard coal. Anthracite Coal, a hard, dense
and shiny coal is classified as a Metamorphic Rock. Anthracite burns more
cleanly than Bituminous Coal with lower sulfur emissions given off into
the atmosphere as a by-product. It has a high energy output and is also
used in industry. In this last illustration in the series, tectonic forces have folded the region. The Bituminous Coal has been metamorphosed into Anthracite. The surrounding sedimentary rocks have also been changed to metamorphic varieties and the top layer of sandstone has been eroded away. |
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Observe an animation showing Coal Formation.
