How Did the First Limestone Form in the Precambrian?
The Precambrian period dates from the Earth's origins to approximately 600 million years ago. During this era, the planet's crust formed, oxygen accumulated in the atmosphere and Earth cooled to a temperature that could sustain life. Single-celled and multicellular organisms evolved during the Precambrian period. Some of these animals helped form the first limestone beds.
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What is Limestone?
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Limestone is a sedimentary rock composed of at least 50 percent calcium carbonate crystals known as calcite. Limestone often has some dolomite, silica, clay, sand or other substances mixed in with it. Due to the many materials that combine with calcite when it forms into limestone, limestone has a wide array of colors and textures.
How Does Limestone Form
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Animals such as shellfish and corals pull calcium carbonate from the water to form their shells. When the animals die, their shells fall the to seabed. Bacteria also pull calcium carbonate from the water and their excretions and dead cellular matter also contribute to the limestone bed. Finally, calcium carbonate naturally precipitates out of the water and the precipitate falls to the sea floor. Over time all this material accumulates, compresses and eventually forms into limestone. The formation of limestone began with animals and bacteria in the Precambrian and continues today.
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Where is Limestone Found?
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The ideal environment for limestone to form is in calm and warm marine waters that aren't too deep. Large limestone beds are found wherever there was once an ancient sea. This includes large areas of North America, Europe and Asia. Exposed limestone can be seen in the west of Ireland, England's Cliffs of Dover and the tufa outcropping of Orvieto, Italy.
Limestone Trivia
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Limestone makes up about 10 percent of the sedimentary rocks on Earth. Because the main component of limestone, calcite, is easily dissolved by slightly acidic cool water, caves and unique rock formations remain in places where limestone has come in contact with water for long periods of time. Sometimes when mountains form, limestone reaggregates into marble. Since limestone is composed of ancient animals, it is one of the most likely places to find fossils.
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References
- Photo Credit limestone rocks image by Maria Brzostowska from Fotolia.com
