Carbon ranks nineteenth in the order of abundance of the elements comprising about 0.2% of the earth's outer crust. In the earth's atmosphere carbon is present as carbon dioxide in amounts up to 0.03% by volume. Though it is widely distributed in Nature, mainly in the combined form, only minor amounts are found in the free or elemental state. It is present as main constituent of all animal and vegetable matter. Coal, petroleum and natural gas are also composed essentially of carbon. Various minerals, such as limestone, dolomite, and marble as well as certain marine deposits such as oyster shells all contain carbon in the form of carbonates.
Carbon plays a vital role in the so-called carbon life cycle. Carbon dioxide from the air, together with water, is absorved by plants and converted into carbohydrates in the process of photosynthesis. Animals consume the carbohydrates, returning the carbon dioxide to the atmosphere by the processes of respiration, excretion, fermentation and decay under bacterial action.