Figure 2.8 The formation of limestone sediments. The continuous addition of calcium minerals into oceans from streams and rivers causes calcium to combine with CO2 to become calcium carbonate. Because calcium carbonate is not very soluble in water, it precipitates out of the water to form massive sediments over millions of years. This site of limestone sediments located in Victoria, Australia, was once under water but is now above the water due to changes in ocean depth.
Photo by Phillip Hayson/Photo Researchers, Inc.