Earth Science: Geology, the Environment, and the UniverseChapter 6:
Sedimentary and Metamorphic RocksProblem of the WeekDown in the Coal Mines In North America coal has been, and still is,
an important source of energy, but to geologists coal is an organic rock.
This makes coal different from most other rocks. Most of the material
in rocks and minerals are by definition inorganic. The formation of coal
is associated with sedimentary rock. The coal mined today was formed
from thick deposits of plant material, which accumulated in warm swampy
areas somewhat like the conditions we now see in the Everglades, in Florida,
or the Dismal Swamp in North Carolina and Virginia. Plant matter in these
areas accumulated quickly and over many thousands of years the layers piled
up. Sediments eventually covered them. As the sediments above continued
to build up and grow thicker, the plant material was compacted and subjected
to increased pressure and heat. Depending on conditions such as depth, amount
of compaction, and exposure to heat, four different stages or grades of
coal have been identified; stage one – peat; stage two - lignite (lowest
quality coal); stage three – bituminous coal; and anthracite (highest grade
coal). Problem: At each stage of coal formation, compaction is
a major factor in removing water and volatiles, improving the grade of
coal. A. Given the following information solve these
problems. - A lignite seam is estimated to be 20% as thick as the original thickness
of the peat bed it developed from.
- If the peat bed were 750 m thick, how thick would the lignite
layer be?
- A bituminous coal seam is 3 meters thick (top to bottom). A bituminous
coal seam is estimated to be 5 – 10% of the original thickness of the
peat bed.
- About how many meters thick was the original peat layer?
- Every 30.5 centimeters (12 inches) of coal thickness represents approximately
10,000 years of continuous peat accumulation.
- If we find a peat formation that is 5.4 meters thick, how long
would it have taken for this accumulation to build up?
Just a thought: Coal formation takes millions of years. The use
and mining of coal in North America began in some places, such as Nova
Scotia, as early as the mid 1700s. (For the purposes of this problem,
let’s say 1750.) Based on current consumption rates, it is estimated that
there are approximately 265 years of coal reserves left in North America.
- If this estimate was made in 1995, what year will it be when the majority
of our coal reserves are used up?
- How long will it have taken us to use up that which took millions
of years to form?
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