Stephen Fein, age 10, of Youngstown, Ohio, for his question:
What is limestone formed from?
The basic ingredient is calcium carbonate, alias calcite. It is a chemical compound of calcium, carbon and oxygen. These elements are quite abundant and there is plenty of the calcite both in the ground and dissolved in the seas. The earth his many recipes for using it to form layers and massive beds of limestone. Each patient process is a fascinating story. However, almost always a variety of other minerals manage to get mixed with the main calcite ingredient.
The restless winds and rains shift countless tons of rocky dirt. The restless earth moves slowly, but it never stops shifting around its rocky layers with their dozens of different chemical minerals. These restless activities patiently mix these minerals and shape them into hundreds of different rocks. The making of limestone requires lots of the mineral calcite, plus a helping hand from some water. The recipe is slightly different for each deposit, which is why no two limestone samples are exactly alike.
Calcite, the main ingredient is a chemical compound of 56 per cent calcium oxide and 44 per cent carbon dioxide. It forms a softish white mineral. But pure calcite limestone usually forms only in the deep ocean and samples are hard to find. Most limestones are mixed with assorted clays and sands, iron oxide and other minerals Sometimes the calcium in the basic calcite is partly replaced by magnesium. The various impurities tint the limestone with yellows or browns, greys or greens, rusty reds or even black.
Water dissolves calcite into invisible fragments. When the water evaporates, the calcite is left behind as a solid mineral. This is how layers of limestone are deposited around hot mineral springs. But most of the earth's limestones were formed with the help of bygone plants and animals that lived in the seas, many millions of years ago. Countless marine animals extracted calcite from the water and used it to build their bones and protective shells. A few seaweeds stiffened their foliage with calcite. As all these animals and plants died, their sturdy materials sank and made thick layers on the seabeds.
As the restless earth shifted its crust, large slabs of the seabed often lifted above the waves. Then the submerged deposits drained dry and became solid layers of limestone. Many of them had been deposited under shallow seas. There the turbulence had mixed lots of silty sand and muddy clay with the basic ingredeint. The clays formed cements that stuck bits of calcite together in solid masses.
Some limestones are formed mainly from the stoney houses built by soft little corals. Some are cemented masses of shells made by sea dwellers that became extinct millions of years ago. Some are made mostly from the tiny shells of one celled animals. And some limestones contain wads of stiff seaweeds and algae that got set into the mixture.