How carbon in the atmosphere is added to rocks and is gone back through the atmosphere?
The movement of carbon between the atmosphere and the lithosphere (rocks) begins with rain. This is where atmospheric carbon combines with water to form a weak acid – carbonic acid – that falls to the surface in rain. The acid dissolves rocks – a process called chemical weathering – and releases calcium, magnesium, potassium, or sodium ions. The rivers then carry the ions on to the sea. In the ocean the sodium ions combine with bicarbonate ions to form calcium carbonate. In the ocean most of the calcium carbonate is made by shell-building (calcifying) organisms and plankton. After the organisms die they sink to the seafloor. Over time layers of shells and sediment are cemented together and turn rock, storing the carbon in rock – limestone and its derivatives. Another way carbon is made in this process is from living things. This is where heat and pressure compress the mud and carbon, over millions of years forming sedimentary rock such as shale. And in some cases, when dead plant matter builds up faster than decay, layers of organic carbon become oil, coal and gas (fossil fuels).
The cycle returns carbon to the atmosphere through volcanoes. Earth’s land and ocean surfaces sit on several moving crustal plates. When the plates collide, one sinks under the other, and the rocks it carries melts under the extreme heat and pressure. The heated rock recombines into silicate materials, releasing carbon dioxide. When volcanoes erupt, they vent the gas to the atmosphere and cover the land with fresh silicate rock to start the cycle again.