Earth Science: Geology, the Environment, and the Universe

Chapter 6: Sedimentary and Metamorphic Rocks

Problem of the Week

Down 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.

  1. 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?

  2. 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?

  3. 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.

  1. If this estimate was made in 1995, what year will it be when the majority of our coal reserves are used up?

  2. How long will it have taken us to use up that which took millions of years to form?
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