Chalk, also known as calcium carbonate, is composed of the elements calcium, carbon and oxygen and has the chemical sign CaCO3. Calcium carbonate is found in layers all over the world and is the main component of snail shells, pearls and the skeletons of marine organisms known as coccolithophores.
These coccolithophores thrive in warm seas, and they have laid down their fossilized remains on a vast scale. This is the main difference between chalk and the chemically identical alternative forms of calcium carbonate such as limestone, marble and calcite crystals. Chalk releases carbon dioxide in the presence of strong acids and readily forms the soluble compound calcium bicarbonate when exposed to carbon-rich water. The ease with which it then dissolves is primarily responsible for the erosion of carbonate rocks and the formation of large underground caverns. Dissolved chalk is also responsible for hard water when it is found in aquifers.
The coccolithophores that contain calcium carbonate in their cell walls take the calcium and carbon they need from the water they live in. Ultimately, this has the effect of scrubbing carbon from the atmosphere and transporting it to the deep sea sediments after the animal dies.