Natural calcium carbonate deposits may appear massive and immobile, but they do in fact undergo gradual change.
Within this system, atmospheric influences alone drive the cyclic processes. Water evaporates and is transformed into gas or water vapor that is carried away by the wind and returns to the earth's surface as rain.
Fine droplets of water (e.g. rain) make effective absorbers of atmospheric impurities like CO2, SO2 and NOx, which dissolve to form acids. This "acid rain" attacks limestone and decomposes it.
Calcium bicarbonate is water-soluble, meaning it is washed away into the oceans. This chemical process supplements the mechanical erosion of hills and mountains by wind and water, and is a principal cause of the increasing salinity in the world's oceans. In the oceans, changes in temperature, pressure, pH, CO2 content etc. may reverse the reaction, causing the carbonate to precipitate and gradually form new sedimentary rocks on the ocean floor.
