Sedimentary rock facts for kids
Sedimentary rocks are rocks formed from sediment. They are deposited over time, and often show layers which can be seen in cliffs. Other types of rock are igneous rock and metamorphic rock.
Sediments are usually formed from matter which falls to the bottom of oceans and lakes. The matter includes tiny pieces of other rocks, and dead animals, plants and microorganisms. Also, inorganic chemicals may be precipitated from solution in the water. The three most common sedimentary rocks are limestone, sandstone and shale.
Sedimentary rocks cover 75–80% of the Earth's land area, but they make up only 5% of the Earth's crust. The relative abundances of the different types of sedimentary rocks are:
Consolidation
Squeezed and compressed over time, the sediments become 'consolidated' (made solid) into layers of rock. Stratigraphy is the study of those layers
Cementation occurs when dissolved mineral components deposit in the interstices of sediments. It is the sticking together of sediment that forms a rock.
Otherwise, rocks may have the water squeezed out, yet remain for long periods with their character unchanged. There are many quarries of unconsolidated deposits where sand and gravel are removed for the construction industry. Such quarries may be taking out sand which has lasted as sand since the Jurassic period. But consolidated sand, sandstone, can be a very hard rock.
Consolidated rocks may be changed some more, by water seeping through, or by heat and extreme pressure.
Types of sedimentary rock
Sediments are formed by three processes:
- Solids swept down from the land.
- Bits & pieces laid down in water, e.g. shells.
- Chemicals in solution, which may be precipitated.
These sedimentary processes – weathering, transportation, and deposition – reach three final end products: quartz sand, shale (from clay), and limestone (CaCO3). Most sedimentary rocks are variations on this general pattern.
Chalk, limestone, and dolomite are all basically made from calcium carbonate. This comes from a mixture of minerals and pieces of animals (especially animal shells). They are mostly formed in oceans. Shales, sandstones, and conglomerates are all clastic rocks. They are made from pieces of other rocks. The pieces may have come from erosion by water, ice or wind. Coal is made from ancient plants; oil and natural gas is also organic in origin.
Some sedimentary rocks are made of just one type of sediment, all about the same size, such as sand. Other sedimentary rocks will have large and small lumps, and pieces of different types of rock. Well-known sedimentary rocks are sandstone and limestone.
Sedimentary rocks may be found anywhere on Earth. When sedimentary rocks are heated and squeezed, they become metamorphic rocks. Igneous rocks have a volcanic origin. Over a very long time, rocks get recycled, in two ways. When marine sediments are raised above sea level, they get weathered, and the pieces carried down to the sea. On a much longer time-scale continental plates may collide. Then one plate goes under the other (is subducted), and all its material is recycled, emerging much later.
Fossils
Among the three major types of rock, fossils are most commonly found in sedimentary rock. Sedimentary rocks form at temperatures and pressures that do not destroy fossil remnants. Often these fossils may only be visible under magnification.
Fossils can be both the direct remains or imprints of organisms and their skeletons. Most commonly preserved are the harder parts of organisms such as bones, shells, and the woody tissue of plants. Soft tissue has a much smaller chance of being fossilized, and the preservation of soft tissue of animals older than 40 million years is very rare. Imprints of organisms made while they were still alive are called trace fossils, examples of which are burrows, footprints, etc.
As a part of a sedimentary rock, fossils undergo the same processes as does the host rock. For example, a shell consisting of calcite can dissolve while a cement of silica then fills the cavity. In the same way, precipitating minerals can fill cavities formerly occupied by blood vessels, vascular tissue or other soft tissues. This preserves the form of the organism but changes the chemical composition, a process called permineralization. The most common minerals involved in permineralization are various forms of amorphous silica (chalcedony, flint, chert), carbonates (especially calcite), and pyrite.
Related pages
Images for kids
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Uluru (Ayers Rock) is a large sandstone formation in Northern Territory, Australia.
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Claystone deposited in Glacial Lake Missoula, Montana, United States. Note the very fine and flat bedding, common for deposits coming from lake beds further away from the source of sediment.
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Sedimentary rock with sandstone in Malta
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Lower Antelope Canyon was carved out of the surrounding sandstone by both mechanical weathering and chemical weathering. Wind, sand, and water from flash flooding are the primary weathering agents.
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Outcrop of Ordovician oil shale (kukersite), northern Estonia
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Fossils of Nerinea marine gastropods of Late Cretaceous (Cenomanian) age, in limestone in Lebanon
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Cross-bedding and scour in a fine sandstone; the Logan Formation (Mississippian) of Jackson County, Ohio
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A piece of a banded iron formation, a type of rock that consists of alternating layers with iron(III) oxide (red) and iron(II) oxide (grey). BIFs were mostly formed during the Precambrian, when the atmosphere was not yet rich in oxygen. Moodies Group, Barberton Greenstone Belt, South Africa
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Global collage of sand samples. There is one square centimeter of sand on every sample photo. Sand samples row by row from left to right: 1. Glass sand from Kauai, Hawaii 2. Dune sand from the Gobi Desert 3. Quartz sand with green glauconite from Estonia 4. Volcanic sand with reddish weathered basalt from Maui, Hawaii 5. Biogenic coral sand from Molokai, Hawaii 6. Coral pink sand dunes from Utah 7. Volcanic glass sand from California 8. Garnet sand from Emerald Creek, Idaho 9. Olivine sand from Papakolea, Hawaii. [1]
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Burrows in a turbidite, made by crustaceans, San Vincente Formation (early Eocene) of the Ainsa Basin, southern foreland of the Pyrenees
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Cross-bedding in a fluviatile sandstone, Middle Old Red Sandstone (Devonian) on Bressay, Shetland Islands
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Flute casts, a type of sole marking on the base of a vertical layer of Triassic sandstone in Spain
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The swirls of tan, green, blue, and white are sediment in the shallow waters of the Gulf of Mexico off the Yucatan Peninsula. The blue-green cloud in this image roughly matches the extent of the shallow continental shelf west of the peninsula. This is a perfect example of a shallow marine depositional environment.
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Cyclic alternation of competent and less competent beds in the Blue Lias at Lyme Regis, southern England
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The Permian through Jurassic stratigraphy of the Colorado Plateau area of southeastern Utah that makes up much of the famous prominent rock formations in protected areas such as Capitol Reef National Park and Canyonlands National Park. From top to bottom: Rounded tan domes of the Navajo Sandstone, layered red Kayenta Formation, cliff-forming, vertically jointed, red Wingate Sandstone, slope-forming, purplish Chinle Formation, layered, lighter-red Moenkopi Formation, and white, layered Cutler Formation sandstone. Picture from Glen Canyon National Recreation Area, Utah.
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Distribution of detritus
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Sedimentary rocks on Mars, investigated by NASA's Curiosity Mars rover
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Steeply dipping sedimentary rock strata along the Chalous Road in northern Iran
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Stratified remains of Puʻu Mahana cinder cone.
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A regressive facies shown on a stratigraphic column
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See also
In Spanish: Roca sedimentaria para niños
