The oxygen cycle involves the movement of oxygen between the atmosphere, water bodies, land and the rocks beneath. Oxygen is a necessary element of life on earth. It occupies about 20 percent of the atmosphere by volume, and together with nitrogen which comprises almost 80 per cent of the atmosphere, the two make up virtually all of the composition of the atmosphere. As well as the amount in the atmosphere, oxygen also constitutes parts of the soil mantle, the rocks below this mantle, and is a constituent of water in the oceans, lakes and rivers.
A feature of oxygen is that it is very often in association with other elements, as in the case of water (H2O), carbon dioxide (CO2) and iron oxide (Fe2O3). It also combines with numerous many other elements that comprise the earth's crust. It is thus a part of many of the other major global cycles. Most of the earth's total oxygen is stored in rocks and minerals (about 99 per cent) which form part of the lithosphere, the outer solid layer of the earth's crust. The next largest amount is to be found in the atmosphere and a tiny fraction, though of great importance, is in the biosphere.
Some oxygen in the atmosphere is in the form of ozone (O3). This is found high in the atmosphere in the stratospheric zone. This ozone layer absorbs most of the ultraviolet rays from the sun. Such rays are harmful to living cells. There is now some concern that other changes in the atmosphere, induced by mankind, may reduce the concentration of ozone and thus the effectiveness of guarding humans against the ultraviolet rays.