Chalk cliffs are almost pure calcium carbonate.
Almost all carbonate rock is composed of calcium carbonate which has been extracted from sea water by living organisms as small as blue-green algae up to large clams and bivalves. The carbonate rocks tend to form in warm, shallow seas and can develop very thick beds, which trigger the development of limestone under the pressure of the overlying deposits.
Sedimentary: Limestones
Limestones come in a variety of forms depending on the size and depositional basin that they develop in. Most limestones are very fine grained and do contain some impurities. The Bahamas platform is a warm shallow sea east of Florida which is currently depositing and forming limestone today. Limestone is particularly sensitive to carbonic acid, which forms naturally as the carbon dioxide in the air is dissolved in water. Long-term chemical erosion has created most of the known cave formations. Within the cave environment, some of the calcium carbonate is redeposited as stalactites or stalagmites. Areas with limestone bedrock tend to have few streams or springs because the groundwater can escape downwards through fissures into the limestone.
Sedimentary: Chalk
Chalk is a very pure form of calcium carbonate that has been formed from the shells of small plankton whose skeletons collect on the bottom after they die. Chalk, depending on purity and weathering, will tend to be white to gray in color. Because it is very fine-grained, chalk is fairly resistant to weathering, so it is often found as bluffs or sea cliffs. The White Cliffs of Dover are one of the more famous examples.
Sedimentary: Dolomite
Dolomite is a form of carbonate rock that includes magnesium in its chemical makeup. The dolomite begins as a normal bed of calcium carbonate, or limestone rock, but if the groundwater includes dissolved magnesium and the right warm conditions are present, the limestone is chemically altered to create dolomite.
Metamorphic: Marble
Both limestone and dolomite can become various kinds of marble through metamorphic processes. Since they initially build in warm seas, one of the most common metamorphic environments is at subduction zones where the seabed is being pushed under continental rock. The increased heat and pressure as the formation is pushed closer to the mantle reheats and recrystallizes the limestone into marble, which though much harder, is still susceptible to acids.
Tags: calcium carbonate, warm shallow
