Coal Formation

Coal formation begins when thin layers of silt covers plant material that died and fell into a swamp or bog. Each year new layers of plant material falls into the swamp until the weight of the overlying sediments drives the water out forming peat. As the heat and pressure increased due to more overlying sediment peat turns into true coal when the carbon content reaches 25 percent.

Coal seams exposed in road cut near Sutton, Alaska, Photo by Myrna Martin

Coal layers showing on a roadside in Alaska, Myrna Martin

Peat is partially carbonized vegetable matter that formed in bogs and swamps. Peat is the first stage in the formation of coal. Peat is a brown to black accumulation of woody plant material. It has a high moisture content of about 75%. Peat burns freely when dried.

Lignite coal
Coal formation continues as increasing heat and pressure turns peat into lignite brown or black coal. It is the first true coal. Lignite coal is between 25-35% carbon. It often displays the original woody structure in the rocks. It burns readily with a smoky flame and has a strong odor. Generating plants use lignite coal to produce electricity.

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Bituminous coal structure
Bituminous coal is black and has a glassy luster with distinct bands and no evidence of a wood structure. Subbituminous coal has a carbon content of 35-45 percent.

Bituminous coal uses
Bituminous coal has a higher carbon content between 44 to 86 percent pure carbon. This coal does not crumble when exposed to air. It is used to make coke for the steel industry and supplies heat for industrial processes.

Anthracite coal
Anthracite coal is a metamorphic rock with a glassy luster. It has a shell-like fracture and is harder to ignite than sedimentary coals. Anthracite coal has the highest carbon content of all the coal. It is between 86 and 98 percent pure carbon. Anthracite coal is used for heating homes because it burns with little smoke or odor.

More Rock Cycle Links

Pahoehoe-Lava Find out how pahoehoe forms in the Hawaiian Islands.

Pyroclastic Rocks Find out how pyroclastic rocks form during volcanic eruptions.

Aa Lava Learn more about aa lava flows that occur on the Hawaiian Islands and the lava balls that often form on their surface.

Welded Tuff Find out how welded tuff is produced during violent eruptions and by supervolcanoes.

Intrusive Rocks Learn more about intrusive rocks that cool underground forming batholiths like the Sierra Nevada Mountains in California.

Coal Formation Find out how leaves falling into a swamp can turn into coal.

Rock Cycle Learn more about igneous, sedimentary and metamorphic rocks and how they form.

Kids Fun Science The links on our home page include information about volcanoes, science activities, plate tectonics, the rock cycle and much more

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