Concretion

Concretion

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Concretions in Western Kazakhstan

A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles. Concretions are often spherical or oval in shape, although irregular shapes can also occur. They are typically found in sedimentary rock or soil and can vary greatly in size, from small pebbles to large boulders.

Formation

Concretions form when mineral cement precipitates around a nucleus within the sediment. This process can occur in a variety of environments, including marine, freshwater, and terrestrial settings. The minerals that form concretions are often carbonates, such as calcite or dolomite, but can also include silica, iron oxide, or other minerals.

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Concretion in a geological garden, Prague

The formation of concretions is influenced by factors such as the composition of the sediment, the availability of mineral-rich fluids, and the conditions of pressure and temperature. Concretions can form rapidly or over millions of years, depending on these factors.

Types of Concretions

Concretions can be classified based on their composition, shape, and the environment in which they form. Some common types include:

• Carbonate concretions: These are composed primarily of calcium carbonate and are often found in marine sediments.
• Silica concretions: Formed from silica, these are typically found in volcanic or siliceous sedimentary rocks.
• Iron oxide concretions: These contain iron minerals and are often found in red beds or other iron-rich sediments.

Occurrence

Concretions are found worldwide and can occur in a variety of geological settings. They are often discovered in sedimentary rock formations, such as sandstone, shale, and limestone.

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Marlstone aggregate concretion from Michigan, USA

In some cases, concretions can be found in soil, where they may form as a result of pedogenic processes. They are also common in certain desert environments, where they can be exposed by erosion.

Notable Examples

Some well-known examples of concretions include the Moeraki Boulders in New Zealand, the Cannonball Concretions in North Dakota, USA, and the Kettle Point Concretions in Ontario, Canada.

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Concretion rock from the Middle Jurassic of Iran

Significance

Concretions are of interest to geologists because they can provide information about the conditions of the environment in which they formed. They can also serve as indicators of past fluid flow and mineralization processes.

Related Pages

• Sedimentary rock
• Mineral
• Geology
• Erosion

Rock concretions from Hells Hollow State Park, Pennsylvania

Gallery

• Concretions embedded in sandstone, Anza Borrego Desert State Park, California

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  Vaqueros Sandstone, Sanborn County Park

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  Soil mosaic with concretions