Concretion
From The Art and Popular Culture Encyclopedia
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A concretion is a hard, compact mass of sedimentary rock formed by the precipitation of mineral cement within the spaces between the sediment grains. Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'. Concretions form within layers of sedimentary strata that have already been deposited. They usually form early in the burial history of the sediment, before the rest of the sediment is hardened into rock. This concretionary cement often makes the concretion harder and more resistant to weathering than the host stratum.
There is an important distinction to draw between concretions and nodules. Concretions are formed from mineral precipitation around some kind of nucleus while a nodule is a replacement body.
Descriptions dating from the 18th century attest to the fact that concretions have long been regarded as geological curiosities. Because of the variety of unusual shapes, sizes and compositions, concretions have been interpreted to be dinosaur eggs, animal and plant fossils (called pseudofossils), extraterrestrial debris or human artifacts.
See also
- Calcrete, CaCO3 concretions in arid and semi-arid soils
- Caliche (mineral), synonym of calcrete
- Dinocochlea in the Natural History Museum, London
- Gypcrust, CaSO4 concretions in arid and semi-arid soils
- Klerksdorp sphere
- Martian spherules
- Moeraki Boulders (New Zealand)
- Mushroom Rock State Park, Kansas
- Moqui Marbles
- Nodule (geology), a replacement body, not to be confused with a concretion
- Rock City, Kansas
- Speleothems, CaCO3 formations in caves
