 Organic sedimentary rocks form from the build up and decay of plant and animal material. This usually forms in swamp regions in which there is an abundant supply of growing vegetation and low amounts of oxygen. The vegetation builds so quickly that new layers of vegetation bury the dead and decaying material very quickly. The bacteria that decay the vegetation need oxygen to survive. Because these decaying layers are buried so fast the bacteria use up what oxygen there is available and can not finish the decomposition of the vegetation. The overlaying layers become so heavy that they squeeze out the water and other compounds that aid in decay.
This compressed vegetation forms coal. The longer and deeper that coal is buried makes it of higher quality. Peat is the first stage of coal formation. Lignite is the next grade of coal followed by bituminous and the highest grade, anthracite. Anthracite is actually a metamorphic rock. It forms during mountain building when compaction and friction are extremely high. This form of coal burns very hot and almost smokeless. It is used in the production of high grade steel.
Click on the "Next" button to learn about the most common sedimentary rock.
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