Sedimentary rock facts for kids
Sedimentary rocks are special rocks formed from tiny bits of other rocks, plants, animals, and even chemicals. These bits, called 'sediment,' settle down over time, often at the bottom of oceans and lakes. You can often see these rocks in layers, like a giant cake, especially in cliffs! They are different from igneous and metamorphic rocks.
The most common types are limestone, sandstone, and shale. Sedimentary rocks cover most of Earth's land (about 75-80%), but they only make up a small part (5%) of the Earth's outer layer, called the crust.
Here's how much of each type we find:
Contents
How Sedimentary Rocks Form
Sediments are squeezed and pressed together over long periods. This process makes them solid, forming layers of rock. This is called 'consolidation.'
Sometimes, tiny dissolved minerals act like glue. They fill the spaces between sediment pieces, sticking them together. This is called cementation. It makes the sediment hard and turns it into rock.
Some sediments, like sand and gravel, can stay loose for a very long time. We dig these up from quarries to use in construction. But when sand becomes solid, it forms sandstone, which can be a very hard rock.
Types of Sedimentary Rocks
Sediments are formed in three main ways:
- Solid bits washed down from land.
- Pieces laid down in water, like shells.
- Chemicals dissolved in water that then turn solid.
These processes create three main types of sediment: quartz sand, clay (which forms shale), and calcium carbonate (which forms limestone). Most sedimentary rocks are variations of these.
Rocks from Shells and Minerals
Chalk, limestone, and dolomite are mostly made from calcium carbonate. This comes from a mix of minerals and tiny pieces of animals, especially their shells. These rocks usually form in oceans.
Rocks from Other Rock Pieces
Shales, sandstones, and conglomerates are called clastic rocks. They are made from pieces of other rocks. These pieces might have been broken off by erosion from water, ice, or wind.
Rocks from Living Things
Coal is made from ancient plants. Oil and natural gas also come from old organic material.
Some sedimentary rocks are made of just one type of sediment, all about the same size. An example is sand. Other sedimentary rocks can have large and small pieces, or even different types of rock mixed together.
Sedimentary rocks can be found all over Earth. When they get very hot and squeezed, they change into metamorphic rocks. Rocks are constantly being recycled over long periods. For example, when ocean sediments rise above sea level, they break down, and the pieces are carried back to the sea.
Fossils in Sedimentary Rocks
Fossils are most often found in sedimentary rocks. This is because sedimentary rocks form under conditions that don't destroy the remains of living things. Sometimes, these fossils are so tiny you need a microscope to see them!
Fossils can be the actual remains of organisms or their imprints. The hard parts, like bones, shells, and woody plant parts, are most commonly preserved. Soft parts of animals rarely become fossils, especially if they are older than 40 million years.
Imprints made by organisms while they were alive are called trace fossils. Examples include burrows or footprints.
As part of a sedimentary rock, fossils go through the same changes as the rock itself. For example, a shell made of calcite might dissolve, and then a new mineral like silica fills the empty space. This keeps the shape of the organism but changes what it's made of. This process is called permineralization. Common minerals involved are silica (like chalcedony or flint), carbonates (like calcite), and pyrite.
Related pages
Images for kids
-
Uluru (Ayers Rock) is a large sandstone formation in Northern Territory, Australia.
-
Claystone deposited in Glacial Lake Missoula, Montana, United States.
-
Sedimentary rock with sandstone in Malta.
-
Lower Antelope Canyon carved from sandstone by wind, sand, and water.
-
Outcrop of Ordovician oil shale (kukersite), northern Estonia.
-
Fossils of Nerinea marine gastropods of Late Cretaceous age, in limestone in Lebanon.
-
Cross-bedding and scour in fine sandstone; Logan Formation (Mississippian) of Jackson County, Ohio.
-
A piece of a banded iron formation, with alternating layers of iron oxides.
-
Global collage of sand samples from different locations.
-
Cross-bedding in fluviatile sandstone, Middle Old Red Sandstone (Devonian) on Bressay, Shetland Islands.
-
Flute casts, a type of sole marking on the base of a vertical layer of Triassic sandstone in Spain.
-
Sediment in the shallow waters of the Gulf of Mexico off the Yucatan Peninsula.
-
Layers of rock in the Blue Lias at Lyme Regis, southern England.
-
The Permian through Jurassic layers of the Colorado Plateau in southeastern Utah.
-
Distribution of detritus.
-
Sedimentary rocks on Mars, investigated by NASA's Curiosity Mars rover.
-
Steeply dipping sedimentary rock layers along the Chalous Road in northern Iran.
-
Layered remains of Puʻu Mahana cinder cone.
-
A regressive facies shown on a stratigraphic column.
,_Sunset.jpg)
See also
In Spanish: Roca sedimentaria para niños
