Connect the Spheres with the Coal Cycle

By Clary, Renee; Wandersee, James | Science Scope, October 2010 | Go to article overview

Connect the Spheres with the Coal Cycle


Clary, Renee, Wandersee, James, Science Scope


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Coal fueled the Industrial Revolution and, as a result, changed the course of human history. However, the geologic history of coal is much, much longer than that which is recorded by humans. In your classroom, the coal cycle can be used to trace the formation of this important economic resource from its plant origins, through its lithification, or rock-forming changes, to its final recovery as a fossil fuel. The coal cycle also incorporates Earth-system science: Its event sequence integrates the various spheres of the Earth system, including the atmosphere, biosphere, hydrosphere, geosphere, and even a human-created "technosphere." Likewise, the coal cycle, as a subset of the carbon cycle, also integrates with the hydrologic cycle, and the rock cycle.

Background

Coal is defined as a biochemical sedimentary rock. Unlike many rocks that are composed of one or more minerals, coal's components are not minerals because they originated as living, organic material. Coal forms when the remains of plants and animals accumulate in a low-oxygen, swampy environment. Over time, these organic layers are compacted and changed to produce a range of coal types, or ranks. After the original plant and animal material experiences some alteration and compression, peat can result. The peat can be further altered and compressed to form lignite, a low-rank coal composed of 25%-35% carbon. Further alteration can yield bituminous coal, then anthracite, a metamorphosed coal with the highest percentage of carbon (over 90%). Whereas lignite and bituminous coal are considered biochemical sedimentary rocks, anthracite can be classified as a metamorphic rock because of its higher degree of alteration (Figure 1). Anthracite represents only a small percentage of the coal found in the United States and is present in the Appalachian region.

Formation of coal from organic material requires time--millions of years--and specific conditions. Fortunately, the conditions for deposition of the original organic layers existed at various times throughout Earth's history. During the Pennsylvanian period, approximately 300 to 320 million years ago, continental ice accumulation and the melting of glaciers resulted in fluctuating sea levels. When massive glaciers melted, the seas rose. These rising waters drowned the lush vegetation in swampy areas, depositing the organic layers necessary for future coal formation. When glaciers again formed within polar regions, sea levels dropped, and vegetation could again grow in the once-flooded areas. The rising and falling sea level created cyclothems in the rock record, which are repeating marine and nonmarine rock units with coal interbedded within. In the United States, Pennsylvanian-age coals are found in the Appalachian region, as well as in the Midwest. Coals also formed more recently. Mesozoic-era coals (66-251 million years ago) are found in U.S. Rocky Mountain states and western North America. Lignite and sub-bituminous coals continued to be deposited in the early Paleogene period (23-66 million years ago) in the northern Great Plains states, and in the southern United States. Figure 2 maps the locations of the U.S. coal regions.

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Extending the coal cycle to the Earth's spheres

The coal cycle effectively focuses on one component of the carbon cycle (Figure 3). Furthermore, coal formation, extraction, and consumption illustrate how the coal cycle affects Earth's spheres, including the atmosphere, biosphere, hydrosphere, and geosphere. Photosynthesizing plants originally used carbon dioxide from the atmosphere to fix carbon for energy. These plants of the swamp forests are part of the biosphere, and they provided the original source material for the organic layers that eventually formed coal. Upon the plants' death, some of the carbon was returned to the atmosphere in the form of carbon dioxide as the plants decayed. …

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