Carbon can stay trapped inside rocks for hundreds of millions of years. But wind, water, ice and biological activity can release the carbon - returning it to the atmosphere as carbon dioxide gas, or into rivers and sea as dissolved carbon.
Plants accelerate weathering. They push open cracks in the rocks with their roots, and, in the process, the rock is gradually worn away. Carbon in exposed rocks is more easily dissolved, beginning its journey into the biological carbon cycle.
Carbon in exposed organic rocks, like black shales or coal, can also be released directly to the atmosphere as carbon dioxide. Some rock breaks off in particles, along with any carbon it contains, and becomes soil and sediment.
The carbon in the sediments and the carbon in solution are transported together in rivers - until the rivers meet the sea.