All organic rocks are formed by living organisms. Animals produce limestone from calcium carbonate, which is the mineral calcite. They create shells for protection by extracting the calcium carbonate from seawater and turning it into limestone shells.
Coral reef teeming with life, NOAA
Chalk is a fine-grained limestone made from the shells of foraminifera which are microscopic one-celled animals. These tiny one-celled animals create tests, shells, of calcite that falls to the ocean floor when they die. Black board chalk is the compacted shells of these microscopic creatures mixed with clay.
Diatoms are minute sea creatures similar to foraminifera. They extract silica, quartz, from ocean water to build their tiny shells. When these microscopic creatures die their shells drift down forming layers of silica ooze.The silica ooze compacts to form diatomite, a soft light-colored rock. Diatomaceous Earth is made from diatomite and is used to filter water.
How coral reef begin to form
Coral reef begin to form when tiny polyps settle on a rocky floor of the ocean. They are smaller than the eraser on a pencil. They extract calcite from ocean water to form their homes of limestone. The limestone protects them from predators.
Enlarging the coral reef
Each generation of polyp builds on top and enlarges the coral reef as they build their homes. The Great Barrier Reef in Australia is so large that astronauts can see it from space.
Swampy areas in warm climates
Coal forms in swampy areas in warm climates that are slowly sinking over long periods of time. Leaves and other vegetable matter falls into the water where it is covered up. Each fall a layer of silt is brought into the swamp after a rainstorm covering the plant material.
The layers of plant material builds up and the water is driven out. Slowly the carbon content in the layers increases. When the carbon content reaches at least 25% it turns into lignite coal. Lignite is the first true coal that forms in a swamp.
Limestone Caverns Learn how limestone caverns form and where you can visit some of them.
Basalt Rocks Basalt rocks cover the floors of all the oceans, create cinder cones and form the largest volcanic mountains on Earth.
Salt Domes Learn more about how salt domes form, what they are used for and how they are sometimes a key to find oil.
Lava Caves Find out how pahoehoe lava flows form lava caves.
Ignimbrite Learn more about ignimbrite that forms during large volcanic eruptions that produce pyroclastic flows.
Organic Rocks Learn how animals extract calcite out of ocean water to form shells that are organic rocks.
Rock Cycle Learn more about igneous, sedimentary and metamorphic rocks and how they form.
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