The carbon cycle is the biogeochemical cycle in which carbon is continuously exchanged among the biosphere, atmosphere, pedosphere, hydrosphere and geosphere.
Carbon, which is the main component of biological compounds as well as a major component of many minerals, is present in the atmosphere in the form of carbon dioxide, which is absorbed by plants during photosynthesis. It accumulates in animals locked in various compounds when they consume these plants or consume other animals which have eaten these plants, and it is released into the atmosphere when these organisms die, as well as when fires burn and volcanoes erupt. Some of the carbon that is not released back into the atmosphere eventually becomes petroleum, coal and other fossil fuels.
Additionally, some of the carbon that is not released back into the atmosphere instead eventually becomes limestone, in the oceanic carbon cycle. This occurs because some species of marine animals absorb carbon dioxide from the ocean to make their shells. When these animals die, their bodies decompose and leave behind their hard shells, which accumulate on the seafloor and are eventually broken down and compacted under enormous pressure to form limestone. When this limestone is eventually exposed to air, it gets weathered and the carbon in it is released back into the atmosphere as carbon dioxide.