Much of the world's limestone is made of dead sea creatures
For the past 540 million years, limestone has mostly been built by life: corals, shells and microscopic plankton piling up on shallow sea floors. That same rock now becomes cement, road base, toothpaste filler and paint pigment, and it holds about 30 percent of the world's oil reservoirs.
Limestone is a sedimentary rock made mainly of calcite and aragonite, two crystal forms of calcium carbonate. It forms when those minerals come out of calcium-rich water, sometimes chemically, but mostly through organisms that build skeletons and leave them behind. Carbonate rocks make up about a fifth to a quarter of all sedimentary rock, and limestone is most of that; the remainder is mostly dolomite, which often forms when limestone is chemically altered.
Look closely and the grains tell a story. Many are skeletal fragments whose chemistry reflects their makers: articulate brachiopods and floating foraminifera build from low-magnesium calcite, sea urchins and their relatives from high-magnesium calcite, and molluscs and corals from aragonite. Coral debris points to rough, current-swept water, while bryozoan fragments suggest calm seas. Ooids, sand grains under 2 millimetres coated in concentric carbonate layers, form in energetic settings like the Bahama platform. Peloids, tiny structureless grains, are often thought to be fecal pellets of marine animals.
Most of these grains sit in a matrix of carbonate mud called micrite, made of crystals smaller than 5 micrometres. In fresh sediment it is mostly needles of aragonite, which turn into calcite within a few million years. Pure limestone is white to grey, but abundant organic matter can make it nearly black, and traces of iron or manganese tint it yellow or red.
Physical properties vary widely. Porosity ranges from 0.1 percent in dense limestone to 40 percent in chalk. The rock is soft, only 2 to 4 on the Mohs scale, yet dense limestone can withstand crushing pressures up to 180 megapascals, far above the roughly 40 of ordinary concrete. Because rainwater slowly dissolves it, exposed limestone often erodes into karst landscapes riddled with caves.
Source: Limestone
