Sedimentary rock
Sedimentary rocks record Earth's history and provide vital resources.
Sedimentary rocks form through the cementation of sediments, which are particles derived from either geological detritus—mineral fragments from the weathering and erosion of pre-existing rocks or from solidified lava blobs ejected by volcanoes—or biological detritus, such as the shells, bodies, and fecal matter of dead aquatic organisms that slowly accumulate on the floors of water bodies in a process known as marine snow. These particles are transported to deposition sites by agents of denudation, including water, wind, ice, or mass movement, and they settle through sedimentation; dissolved minerals can also precipitate directly from water solution to form sediment. Although sedimentary rocks cover about 73% of Earth’s land surface, they represent only an estimated 8% of the crust’s volume, forming a relatively thin layer atop a crust dominated by igneous and metamorphic rocks. They are typically deposited in horizontal layers called strata, creating a structure known as bedding, and often accumulate in large sedimentary basins; such rocks have also been identified on Mars. The study of sedimentary rock strata is fundamental to understanding Earth’s history, including past geography, climate, and the evolution of life, and it provides critical information for civil engineering projects like roads, tunnels, and canals. These rocks are also major sources of natural resources, including coal, fossil fuels, drinking water, and ores. The scientific discipline focused on sedimentary rocks is sedimentology, which overlaps with geology, physical geography, and other Earth sciences such as pedology and geochemistry. Sedimentary rocks are classified into four groups by origin: clastic, biochemical, chemical, and a minor category for rocks formed by impacts or volcanism. Clastic rocks, composed of cemented rock fragments called clasts, are subdivided by grain size into conglomerates and breccias (gravel), sandstones (sand), and mudrocks (mud), with further distinctions based on clast shape, composition, or matrix content.
- formation_process
- Cementation of sediments from geological or biological detritus
- land_surface_coverage
- 73% of Earth's land surface
- crust_volume
- 8% of Earth's crust volume
- primary_classifications
- Clastic, biochemical, chemical, and other sedimentary rocks
- key_discipline
- Sedimentology
- notable_occurrence
- Also found on Mars
Lore & Background
Sedimentary rocks are typically deposited in horizontal layers known as strata, a structure called bedding, and often accumulate in large geological features called sedimentary basins. They form a thin veneer over the Earth's crust, which is composed mainly of igneous and metamorphic rocks; while sedimentary rock covers about 73% of the current land surface, it makes up only an estimated 8% of the crust's volume. Their appearance varies widely based on composition and grain size. Clastic sedimentary rocks are composed of cemented rock fragments, or clasts, which can be individual grains of quartz, feldspar, clay minerals, or mica, as well as lithic fragments containing multiple minerals. These rocks are classified by dominant particle size using the Udden-Wentworth scale: gravel (over 2 mm diameter), sand (0.06 to 2 mm), and mud (further divided into silt and clay). Conglomerates contain rounded gravel, while breccias contain angular gravel. Sandstones are named based on the relative abundance of quartz, feldspar, and lithic grains, as well as the amount of muddy matrix present. Sedimentary rocks are also important sources of natural resources such as coal, fossil fuels, drinking water, and ores, and their study provides key information for understanding Earth's history, including paleogeography, paleoclimatology, and the history of life.
Reader's Guide
The study of sedimentary rocks and rock strata provides information useful for civil engineering, such as constructing roads, houses, tunnels, and canals. They are important sources of natural resources including coal, fossil fuels, drinking water, and ores. The sequence of sedimentary rock strata is the main source for understanding Earth's history, including palaeogeography, paleoclimatology, and the history of life. Sedimentology, the scientific discipline studying these rocks, is part of both geology and physical geography and overlaps with pedology, geomorphology, geochemistry, and structural geology. Sedimentary rocks are classified by origin into clastic, biochemical, chemical, and other types (e.g., volcanic tuff, impact breccias). Clastic rocks are subdivided by grain size into conglomerates/breccias, sandstones, and mudrocks. Biochemical rocks include limestone from calcareous skeletons, coal from compressed vegetation, and chert from siliceous organisms. Chemical rocks form by inorganic precipitation, such as evaporites like halite and gypsum. Compositionally, they are grouped into siliciclastic, carbonate, evaporite, organic-rich, and siliceous types.
Did You Know?
- Sedimentary rocks cover 73% of Earth's land surface but only 8% of the crust's volume.
- Sedimentary rocks have been found on Mars.
- The study of sedimentary rock strata is the main source for understanding Earth's history, including paleoclimatology and the history of life.
- Clastic sedimentary rocks are subdivided by grain size into gravel, sand, and mud fractions using the Udden-Wentworth scale.
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