Limestone

Limestone is a type of sedimentary rock. It is made mostly of the minerals calcite and aragonite. These minerals come from calcium carbonate CaCO₃.

Limestone forms when these minerals come out of water that has dissolved calcium. This can happen in nature, such as when corals and shells build up in the sea.

Limestone often has fossils. These fossils help scientists learn about old environments and the evolution of life. Most limestone forms in shallow water, like near continental shelves or platforms.

Limestone is very useful. It helps make lime for cement, which is needed for concrete. It is also used for roads, in products like toothpaste or paint, as a soil conditioner, and in rock gardens. Limestone holds about 30% of the world's petroleum reservoirs.

Description

Limestone is made mostly of minerals called calcite and aragonite. These minerals are different crystal shapes of calcium carbonate (CaCO₃). Limestone forms when these minerals come out of water that has calcium dissolved in it.

Limestone is usually white or gray, but it can have other colors too. It can be soft and sometimes has small holes inside. Tiny bits of animals and plants called fossils are often found in limestone.

Formation

Limestone forms when minerals called calcite or aragonite come out of water that has calcium dissolved in it. This can happen in different ways, both with and without living things. The amount of a gas called carbon dioxide in the water helps decide if calcium carbonate will come out of the water.

Most of the calcium carbonate in the ocean comes from tiny living things. These living things help make up the limestone we see today. Scientists are still learning exactly how limestone forms from very small particles in the water.

During a process called diagenesis, loose sediments turn into solid rock. This process changes the minerals in the rock and can preserve fossils very well. As limestone gets buried deeper, it gets squeezed and changed even more. Sometimes, limestone changes into a different rock called dolomite when it meets certain kinds of water.

Occurrence

Limestone is a common rock on Earth. It is made mostly from tiny minerals called calcite and aragonite, which are forms of calcium carbonate. Limestone forms when these minerals come out of water that has calcium in it.

Limestone is found in places where water used to be, like old sea beds. It can be very old, forming more than 2.7 billion years ago. Most limestone forms in warm, shallow ocean waters, where tiny sea creatures and plants help make it. These places are a small part of the ocean, but they are where most limestone is found.

Limestone often has fossils in it, showing what kinds of creatures lived there long ago. It can also form in rivers and lakes, but these limestones usually have fewer kinds of creatures in them.

Limestone can change over time. In deep ocean waters, limestone can dissolve because of pressure and gases in the water. This is why we don’t find limestone very deep in the ocean today.

Limestone also forms because of living things, like tiny sea plants and animals that build their shells from it. Over time, these shells pile up and turn into limestone. Some very old limestone was made by simple creatures called stromatolites, but today, corals and other reef animals are more common.

Limestone can look different depending on where it formed. Some kinds have layers, while others are more mixed. Tiny creatures and plants often change the limestone as they move through it. In places without many moving creatures, like some lakes, limestone can have very thin layers.

Sometimes, limestone forms in places without much ocean, like where water dries up. In these places, minerals like calcite can form first as the water disappears.

Limestone landscape

Main article: Karst topography

Limestone can dissolve, especially when it touches acid. This creates many interesting shapes on the land. These shapes include flat areas, deep holes, hidden water entrances called cenotes, and long caves and narrow valleys. Places shaped by limestone are called karsts.

Limestone is not as strong as some rocks, but it is stronger than many other rocks made from tiny bits of old sea creatures. Because of this, limestone often forms hills and open lands. It is usually found together with other soft rocks like clay.

In areas where limestone is deep under the ground, there are often very few ponds or rivers on the surface. This is because water easily flows down through cracks in the limestone. Over many thousands or even millions of years, this water, mixed with natural acids from the soil, makes the cracks bigger and dissolves the limestone. Most big cave systems are formed this way in limestone areas.

Near the coast, special animals and plants can wear away limestone by drilling into it. This is most common in warm places.

Limestone often appears above ground in rocky cliffs and islands. Some famous places made of limestone include the Rock of Gibraltar, the Burren in Ireland, Malham Cove in England, the Great Orme in Wales, Fårö near Gotland in Sweden, the Niagara Escarpment in Canada and the United States, Notch Peak in Utah, Ha Long Bay in Vietnam, and the hills around the Lijiang River and Guilin in China.

The Florida Keys, islands off the southern coast of Florida, are made mostly of a special kind of limestone called oolitic limestone and the remains of old coral reefs that grew when the sea level was higher.

Uses

See also: ashlar

Limestone is used in many ways around the world, like in building, farming, and industry. It is very common in places like Europe and North America. Famous buildings like the Great Pyramid in Giza, Egypt and many structures in Kingston, Ontario, Canada, were made from limestone. When limestone is changed by heat and pressure, it becomes marble, which is used for statues, buildings, and tabletops. On the island of Malta, a special kind of limestone called Globigerina limestone has been used for years to build homes and sculptures.

Limestone can be made into many different things, like bricks, cement, or powder. It is easy to cut and shape, which made it popular for detailed carvings in the past. For example, the Maya civilization in Ancient Mexico used limestone to create beautiful sculptures and decorations in their buildings. Even though limestone lasts a long time outside, it is very heavy and expensive for tall buildings.

Limestone was very popular in the late 1800s and early 1900s for making train stations, banks, and other important buildings. Some modern skyscrapers use thin pieces of limestone just for the outside look. Places like Bloomington, Indiana, and Portland, England, are known for their high-quality limestone.

Limestone is also used to make lime, which helps treat soil and clean water. However, limestone can be damaged by acid rain, which harms statues and buildings made from it. To protect against this, sometimes crushed limestone is spread on lakes to help balance the water’s chemistry.

Other uses for limestone include:

• Making quicklime, slaked lime, cement, and mortar.
• Using crushed limestone to improve soil and make it less acidic (agricultural lime).
• Crushing it for use in roads and in making asphalt.
• Helping clean the air in factories by reacting with harmful gases.
• Making glass, toothpaste, paper, plastics, and paint.
• Adding it to bread and cereals to provide calcium.
• Using it in animal feed to give animals extra calcium.
• Helping keep water safe and clean in pipes.
• Mixing it with other materials in factories to clean up impurities.

Limestone formations can also hold important oil reserves and metal ores, like lead and zinc.

Scarcity

Limestone has always been needed for making iron and steel, and it is still in high demand today. In the past, the main problems were making sure there was enough limestone close to where it was needed and moving it to factories. Today, new materials have been developed, so limestone is not as hard to replace as it once was.

Occupational safety and health

While powdered limestone used in food is safe, dust from limestone can irritate the lungs, skin, and eyes. Special rules have been made to protect workers who handle limestone dust.

United States

Rules set by workplace safety groups limit how much limestone dust workers can be around in a day to keep them healthy.

Graffiti

Because limestone is a soft and porous stone, cleaning off graffiti can be very tricky without damaging the surface. Special care must be taken to clean it properly.