Limestone facts for kids
| Sedimentary rock | |
Limestone outcrop in the Torcal de Antequera nature reserve of Málaga, Spain
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| Composition | |
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| Calcium carbonate: inorganic crystalline calcite or organic calcareous material |
Limestone is a common type of rock found all over the world. It's a carbonate sedimentary rock, which means it forms from layers of sediment that build up over time. Limestone is mostly made of calcium carbonate (CaCO
3), a mineral that comes from the shells and skeletons of tiny sea creatures. This rock is super important because it's the main source of lime, a material used to make cement and concrete. You can often find fossils in limestone, which are like clues that tell scientists about ancient environments and how life has changed on Earth.
About one-fifth to one-quarter of all sedimentary rocks are carbonate rocks, and most of these are limestone. Most limestone forms in shallow parts of the ocean, like continental shelves, but it can also form in other places. Because limestone can dissolve a little bit in rainwater, it often creates unique landscapes called karsts, with amazing features like caves. In fact, most cave systems are found in limestone!
Limestone has many uses. It's a key ingredient for making cement, which is used in roads and buildings. It's also used as a filler in things like toothpaste and paint, to improve soil, and even as a decorative rock in gardens. Plus, about 30% of the world's petroleum reservoirs (where oil is found) are in limestone formations.
Contents
What is Limestone?
Limestone is mainly made of the minerals calcite and aragonite. These are different crystal forms of calcium carbonate (CaCO
3). Sometimes, limestone can also contain a mineral called dolomite, which has magnesium in it. Most limestone is quite pure, with only small amounts of other materials like fine sand or clay.
Limestone usually looks white or gray. If it has a lot of organic matter, it can be almost black. Traces of iron or manganese can give it yellow or red colors. Limestone can be soft or quite dense. Even though it's relatively soft (you can scratch it with a knife), dense limestone can be very strong, much stronger than concrete!
Grains and Mud: The Building Blocks
Most limestone is made of tiny, sand-sized grains held together by a fine carbonate mud. These grains are often pieces of marine organisms. When creatures like corals or foraminifera (tiny ocean animals) die, their shells and skeletons, made of aragonite or calcite, are left behind. These pieces become the grains in limestone.
Other types of grains include ooids. These are tiny, round grains that look like little pearls. They have layers of calcium carbonate around a central piece of sand. Peloids are small, structureless grains, often thought to be tiny waste pellets from marine organisms.
The fine mud that holds these grains together is called micrite. It's made of very tiny crystals, often formed from seawater or by algae. Over time, this soft mud hardens into solid rock.
Testing Limestone in the Field
You can recognize limestone because it's softer than many other rocks. Also, if you put a drop of weak hydrochloric acid on it, limestone will bubble! This happens because the acid reacts with the calcium carbonate. Dolomite, a similar rock, reacts much less. Impurities like clay or iron can change the color of limestone, especially on surfaces exposed to weather.
How Does Limestone Form?
Limestone forms when calcite or aragonite minerals come out of water that has dissolved calcium. This can happen in two main ways: through living organisms or through non-biological processes. For the last 540 million years, living organisms have been the most important factor.
The amount of dissolved carbon dioxide (CO
2) in water greatly affects how much calcium carbonate can dissolve. If the water gets warmer or the pressure drops, less CO
2 can stay dissolved, causing calcium carbonate to form solid particles.
The oceans are full of calcium carbonate, but it doesn't always turn into rock right away. Tiny living things play a huge role. Much of the limestone we see today formed on carbonate platforms, which are shallow, flat areas in the ocean.
From Soft Sediment to Hard Rock
After sediments settle, they go through a process called diagenesis, where they are pressed together and turn into solid rock. During this process, soft aragonite often changes into a more stable form of calcite. This is also when fossils are preserved in amazing detail.
Water moving through the sediments helps to cement them together. As sediments get buried deeper, the weight of the layers above them squeezes them. Minerals can dissolve at points where grains touch and then re-deposit in empty spaces, making the rock even harder and less porous.
Types of Limestone You Might See
Limestone comes in many forms, depending on how and where it was made.
- Travertine is a type of limestone that forms in freshwater, often around waterfalls, hot springs, or in caves. It can be dense and banded, or very porous and spongy, which is called tufa. The beautiful stalagmites and stalactites in caves are also forms of travertine.
- Coquina is a limestone made mostly of broken pieces of shells, corals, or other fossil fragments that are loosely cemented together.
- Chalk is a very soft, fine-textured limestone. It's made from the tiny shells of planktonic microorganisms that lived in the ocean. The famous White Cliffs of Dover in England are made of chalk.
- Marl is a mix of carbonates and clay sediments, often found in earthy layers.
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Limestone and Living Organisms
Most limestone is formed thanks to living organisms, especially near coral reefs. These organisms build their skeletons or shells from calcium carbonate. When they die, their remains contribute to the limestone. Over millions of years, different types of organisms have been responsible for building reefs. For example, ancient structures called stromatolites were built by colonies of cyanobacteria.
Organisms help create limestone in two ways: by directly building their skeletons from it, and by removing carbon dioxide from the water through photosynthesis. This removal makes it easier for calcium carbonate to form solid rock.
Reefs are complex structures found in shallow, warm ocean waters. They are home to many different kinds of life. The organisms that build reefs have changed over Earth's history, from ancient sponges to corals and algae today. These reefs create massive limestone formations over time.
Limestone Landscapes: Caves and Karst
Limestone is partly soluble in water, especially in slightly acidic rainwater. This means it can dissolve over time, creating amazing landforms. These landscapes are called karsts.
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In karst regions, surface water often disappears quickly. It drains downwards through cracks in the limestone. Over thousands or millions of years, this water, mixed with natural acids from the soil, slowly widens these cracks. This process dissolves the calcium carbonate and carries it away. This is how most cave systems are formed in limestone bedrock. Cenotes, which are natural sinkholes or pits, are also common in limestone areas.
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Coastal limestones can also be eroded by organisms that bore into the rock. This process, called bioerosion, is common in tropical areas.
Limestone often forms spectacular rocky cliffs and islands. Famous examples include the Rock of Gibraltar, the White Cliffs of Dover, and the beautiful hills around the Lijiang River in China. The Florida Keys are also made mainly of limestone from ancient coral reefs.
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How We Use Limestone Every Day
Limestone is a very important raw material used all over the world in many ways.
Limestone in Construction and Art
Limestone has been used in architecture for thousands of years. Many famous landmarks, including the Great Pyramid of Giza in Egypt, were built with limestone. In Canada, Kingston, Ontario, is even nicknamed the 'Limestone City' because so many of its buildings are made from it. When limestone is heated and put under pressure, it changes into marble, which is used for statues, buildings, and tabletops.
Limestone is easy to cut into blocks and carve. Ancient American sculptors, like the Maya civilization, valued limestone for its carving properties. They used it to decorate their sacred buildings with detailed stories and messages from their kings. Limestone is durable and lasts a long time, which is why many ancient limestone ruins still stand today.
Many medieval churches and castles in Europe are also made of limestone. In the United States, Indiana limestone is famous for its high quality and has been used in many important buildings.
Other Important Uses of Limestone
Limestone is the raw material for making lime, which is used to treat soils, purify water, and in metal production. It's a key ingredient in many chemical industries.
Other uses include:
- Making quicklime (calcium oxide) and slaked lime (calcium hydroxide), which are used in cement and mortar.
- Crushed limestone is used as a base for roads and in asphalt concrete.
- It helps clean the air by reacting with sulfur dioxide from power plants.
- It's used in making soda–lime glass.
- As a white pigment or cheap filler in toothpaste, paper, plastics, and paint.
- Added to bread and cereals as a source of calcium.
- As a calcium supplement in animal feed.
- To make purified water less acidic and add back important nutrients.
- In blast furnaces, limestone helps remove impurities from iron.
Limestone formations are often porous, meaning they have tiny holes, and allow liquids to pass through. This makes them important places where petroleum reservoirs (oil and gas) are found. About a third of the world's oil reserves are in limestone. Limestone can also host metal ores, like lead and zinc.
Keeping Limestone Safe and Sound
Limestone is a very important industrial material. While companies have generally had enough limestone, it's important to manage this resource carefully as demand continues to grow.
When working with powdered limestone, it's a good idea to wear protective gear like masks. Breathing in too much dust can irritate your lungs, and getting dust in your eyes can cause discomfort. Limestone is generally considered safe, but it's always best to be careful with any dust.
Removing graffiti from limestone can be tricky because the rock is porous. Harsh cleaning chemicals can damage the stone, so special care is needed.
Images for kids
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Ooids from a beach on Joulter's Cay, The Bahamas
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Ooids in limestone of the Carmel Formation (Middle Jurassic) of southwestern Utah.
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Thin-section view of a Middle Jurassic limestone in southern Utah, U.S. The round grains are ooids; the largest is 1.2 mm (0.05 in) in diameter. This limestone is an oosparite.
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Chert nodule within soft limestone at Akçakoca, Turkey
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Stylolites in limestone
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A stratigraphic section of Ordovician limestone exposed in central Tennessee, U.S. The less-resistant and thinner beds are composed of shale. The vertical lines are drill holes for explosives used during road construction.
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Photo and etched section of a sample of fossiliferous limestone from the Kope Formation (Upper Ordovician) near Cincinnati, Ohio, U.S.
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A concretionary nodular (septarian) limestone at Jinshitan Coastal National Geopark, Dalian, China
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Limestone from Lake Tai, used in gongshi, a Chinese stone art
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Folded limestone layers on Cascade Mountain in Provo Canyon, Utah
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Fossils in limestone from the northern Black Sea region
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Limestone quarry at Cedar Creek, Virginia, US
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Limestone as building material
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Limestone is used worldwide as building material.
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See also
In Spanish: Caliza para niños
- Coral sand
- Charmant Som
- In Praise of Limestone
- Kurkar
- Limepit
- Sandstone
- Liming (soil)
