Difference between revisions of "Sedimentary rock" - New World Encyclopedia

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Two types of sedimentary rock, limey shale overlaid by limestone, as observed at Cumberland Plateau, Tennessee.

A sedimentary rock is one of the three main rock groups, the other two being igneous and metamorphic rocks. It is formed by the consolidation of sediments that have been deposited and accumulated in layers. The sediments may correspond to material derived from various sites, generated by such processes as (a) the weathering or erosion of earlier rock formations, (b) the breakdown of biological materials (such as bones or shells), or (c) the precipitation of minerals from solution. This material is transported by the action of various agents, particularly water, glaciers, and wind. The deposited material is then compacted (squeezed) or cemented (bound) together or recrystallized to form new rock.

Sedimentary rocks make up a relatively thin layer of rock that covers about 75 percent of the Earth's land area. They provide important information about the history of the Earth. Their composition offers clues about the original rock, and differences between successive layers suggest how the environment changed over time. They contain fossils, and they often form reservoirs of petroleum and other hydrocarbons. On a practical level, they are widely used as construction material. Common sedimentary rocks include chalk, limestone, sandstone, and shale.

Classification

Sedimentary rocks are classified into three groups, based on the material they were formed from. These groups are: clastic, biochemical (or biogenic), and chemical precipitate.

Clastic sedimentary rocks are composed of discrete fragments or clasts of materials derived from other rocks. They are composed largely of quartz, along with other common minerals such as feldspar, amphiboles, and clay minerals. They sometimes contain more exotic igneous and metamorphic minerals.

Biochemical sedimentary rocks contain materials generated by living organisms. They include fragments of bones and shells that are remnants of organisms.

Chemical precipitate sedimentary rocks consist of material that was precipitated out of solution, such as by the evaporation of water they were dissolved in.

General stages of formation

In general, a sedimentary rock is formed in four basic stages:

weathering or erosion of pre-existing rock;
transportation of the sediment by agents such as water, ice, or wind;
deposition of the sediment; and
conversion into new rock by processes such as compaction (squeezing together), cementation (binding together), and recrystallization.

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.

The weathering or erosion of pre-existing rock can be mechanical, chemical, or both. Mechanical weathering involves breakdown of the rock into particles without altering the chemical composition of the minerals it contains. Ice is the most important agent of mechanical weathering. Water percolates into cracks and fissures within the rock, freezes, and expands. The force exerted by the expansion is sufficient to widen cracks and break off pieces of rock. Heating and cooling of the rock, with its accompanying expansion and contraction, also aid the process. Mechanical weathering contributes to further breakdown of the rock by increasing the surface area exposed to chemical agents.

Chemical weathering corresponds to breakdown of the rock by chemical reaction. This process dissolves some minerals and converts others into particles that can be readily carried away. Air and water are both involved in many complex chemical reactions. Igneous rocks are commonly attacked by water, particularly acid or alkaline solutions, and all the common igneous rock-forming minerals (except quartz, which is very resistant) are changed in this way into clay minerals and chemicals in solution.

The weathered particles—in the form of clay, silt, sand, and gravel— and dissolved materials are transported by water, ice, and wind. These agents reduce the size of the particles, sort them by size, and deposit them at new locations, generally at lower elevation. The sediments dropped by streams and rivers form alluvial fans, flood plains, and deltas, or they reach the bottom of lakes and the seafloor. The wind may move large amounts of sand and other smaller particles. Glaciers transport and deposit great quantities of usually unsorted rock material as till.

As the sediment builds up, the overburden (or 'lithostatic') pressure squeezes it into layered solids in a process known as lithification ('rock formation'), and the original fluids are expelled. The term diagenesis is used to describe all the chemical, physical, and biological changes, including cementation, undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface weathering.

Sedimentary rocks are laid down in horizontal layers called beds or strata. Each new layer settles above older ones, in a process called superposition. There are usually some gaps in the sequence called unconformities. These represent periods in which no new sediments were laid down, or when earlier sedimentary layers were raised above sea level and eroded away.

Clastic sedimentary rocks

When the particles derived from earlier rocks become deposited, compacted, and cemented together, they form clastic sedimentary rocks. These rocks contain inert minerals that are resistant to mechanical and chemical breakdown, such as quartz, zircon, rutile, and magnetite. Quartz is one of the most resistant minerals, mechanically and chemically.

Sizes of clastic sedimentary rocks

Clastic sedimentary rocks may be regarded as falling along a scale of grain size. Shale is the finest, with particles less than 0.004 millimeters (mm); siltstone is a little bigger, with particles between 0.004 and 0.06 mm; sandstone is coarser still, with grains of 0.06 to 2 mm; and conglomerates and breccias are the coarsest, with grains 2 to 256 mm. Breccia has sharper particles, while conglomerate is categorized by its rounded particles. Arenite is a general term for sedimentary rock with sand-sized particles.

The classification of clastic sedimentary rocks is complex because many variables are involved. The size (or range of sizes) and composition of the particles, the cement, and the matrix (that is, the smaller particles between the larger grains) must all be taken into consideration. Shales, which consist mostly of clay minerals, are generally further classified on the basis of composition and bedding.

Coarser clastic sedimentary rocks are classified according to their particle size and composition. Orthoquartzite is a very pure quartz sandstone; arkose is a sandstone with quartz and abundant feldspar; greywacke is a sandstone with quartz, clay, feldspar, and metamorphic rock fragments present, formed from sediments carried by turbidity currents.

Biochemical sedimentary rocks

Biochemical (or biogenic) sedimentary rocks include carbonate minerals generated by organisms such as corals, molluscs, and foraminifera. Minerals such as calcite (calcium carbonate) cover the ocean floor and later form limestone. Other examples include stromatolites, the flint nodules in chalk (which is itself a biochemical sedimentary rock, a form of limestone), and coal (derived from the remains of tropical plants subjected to high pressure).

Chemical precipitate sedimentary rocks

Chemical precipitate sedimentary rocks are formed when mineral solutions, such as sea water, evaporate. Examples include the evaporite minerals halite and gypsum.

Significance of sedimentary rocks

Sedimentary rocks contain important information about the history of the Earth. They contain fossils, the preserved remains of ancient plants and animals. The composition of sediments provides us with clues about the original rock. Differences between successive layers indicate changes to the environment that have occurred over time. Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remnants.

The sedimentary rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only five percent of the total. As such, the sedimentary sequences we see represent only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks.

Sedimentary rocks are also significant in economic terms. Being relatively soft and easy to cut, they are often used as construction material. For example, the White House in Washington, DC, is made of sandstone. In addition, sedimentary rocks often form porous and permeable reservoirs in sedimentary basins in which petroleum and other hydrocarbons are found.

References
ISBN links support NWE through referral fees

Blatt, Harvey, and Robert J. Tracy. 1995. Petrology: Igneous, Sedimentary, and Metamorphic, 2nd ed. New York: W.H. Freeman. ISBN 0716724383.

Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412.

Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060.

Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321.

Skinner, Brian J., Stephen C. Porter, and Jeffrey Park. 2004. Dynamic Earth: An Introduction to Physical Geology. 5th ed. Hoboken, NJ: John Wiley. ISBN 0471152285.

Tucker, Maurice E. 2001. Sedimentary Petrology. 3rd ed. Oxford, UK: Blackwell Publishing. ISBN 0632057351.

External links

A Basic Sedimentary Rock Classification. Retrieved April 16, 2007.

EarthChem. Retrieved April 16, 2007.

PETDB: Petrological Database of the Ocean Floor. Center for International Earth Science Information Network (CIESIN), Columbia University. Retrieved April 16, 2007.

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@@ Line 2: / Line 2: @@
 [[Image:Limestoneshale7342.jpg|frame|300px|right|Two types of sedimentary rock, limey [[shale]] overlaid by [[limestone]], as observed at [[Cumberland Plateau]], [[Tennessee]].]]
-'''A Sedimentary rock''' is one of the three main [[Rock (geology)|rock]] groups (along with [[igneous rock|igneous]] and [[metamorphic rock|metamorphic]] rocks) and is formed in four main ways:
+A '''sedimentary rock''' is one of the three main [[Rock (geology)|rock]] groups, the other two being [[igneous rock|igneous]] and [[metamorphic rock|metamorphic]] rocks. It is formed by the consolidation of sediments that have been deposited and accumulated in layers. The sediments may correspond to material derived from various sites, generated by such processes as (a) the [[weathering]] or [[erosion]] of earlier rock formations, (b) the breakdown of biological materials (such as [[bone]]s or [[shell]]s), or (c) the [[precipitation]] of [[mineral]]s from solution. This material is transported by the action of various agents, particularly [[water]], [[glacier]]s, and [[wind]]. The deposited material is then compacted (squeezed) or cemented (bound) together or recrystallized to form new rock.
-* by the deposition of the [[weathering|weathered]] remains of other rocks (known as '[[clastic]]' sedimentary rocks);
+Sedimentary rocks make up a relatively thin layer of rock that covers about 75 percent of the [[Earth]]'s land area. They provide important information about the [[history of the Earth]]. Their composition offers clues about the original rock, and differences between successive layers suggest how the environment changed over time. They contain [[fossil]]s, and they often form reservoirs of [[petroleum]] and other [[hydrocarbon]]s. On a practical level, they are widely used as construction material. Common sedimentary rocks include [[chalk]], [[limestone]], [[sandstone]], and [[shale]].
-*It is created by water, ice and wind patterns. by the accumulation and the consolidation of sediments;
-* by the deposition of the results of biogenic activity; and
-* by precipitation from [[solution]].
-Sedimentary rocks include common types such as [[chalk]], [[limestone]], [[sandstone]] and [[shale]]. Sedimentary rocks cover 75% of the Earth's land area. Four basic processes are involved in the formation of a clastic sedimentary rock: [[weathering]] ([[erosion]])caused mainly by friction of waves, [[Transportation (sediment)|transportation]] where the sediment is carried along by a current, [[Deposition (sediment)|deposition]] and [[compaction]] where the sediment is squashed together to form a rock of this kind.
+==Classification==
-Sedimentary rocks are widely used as construction material. In addition, they often form reservoirs for [[petroleum]] and other [[hydrocarbons]].
-==Formation==
-Sedimentary [[rock (geology)|rock]]s are formed because of the [[overburden pressure]] as particles of [[sediment]] are [[settling|deposited]] out of air, ice, wind, or water flows carrying the particles in [[suspension (chemistry)|suspension]].  As sediment deposition builds up, the overburden (or 'lithostatic') pressure squeezes the sediment into layered solids in a process known as [[lithification]] ('rock formation') and the original [[connate fluids]] are expelled. The term [[diagenesis]] is used to describe all the chemical, physical, and biological changes, including [[cementation]], undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface weathering.
+Sedimentary rocks are classified into three groups, based on the material they were formed from. These groups are: clastic, biochemical (or biogenic), and chemical precipitate.
-Sedimentary rocks are laid down in layers called beds or strata. Each new layer is laid down horizontally over older ones in a process called superposition.There are usually some gaps in the sequence called unconformities. These represent periods in which no new sediments were being laid down, or when earlier sedimentary layers were raised above sea level and eroded away.
+* Clastic sedimentary rocks are composed of discrete fragments or clasts of materials derived from other rocks. They are composed largely of [[quartz]], along with other common minerals such as [[feldspar]], [[amphibole]]s, and [[clay]] minerals. They sometimes contain more exotic [[igneous]] and [[metamorphic rock|metamorphic]] minerals.
-Sedimentary rocks contain important information about the [[history of the Earth]]. They contain [[fossil]]s, the preserved remains of ancient [[plant]]s and [[animal]]s. The composition of sediments provides us with clues as to the original rock. Differences between successive layers indicate changes to the environment which have occurred over time. Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remnants.
+* Biochemical sedimentary rocks contain materials generated by living organisms. They include fragments of bones and shells that are remnants of organisms.
-The sedimentary rock cover of the continents of the Earth's [[Crust (geology)|crust]] is extensive, but the total contribution of sedimentary rocks is estimated to be only five percent of the total. As such, the sedimentary sequences we see represent only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks.
+* Chemical precipitate sedimentary rocks consist of material that was precipitated out of solution, such as by the evaporation of water they were dissolved in.
-==Classification==
+== General stages of formation ==
-Sedimentary rocks are classified into three groups. These groups are clastic, chemical precipitate and biochemical or biogenic.
+In general, a sedimentary rock is formed in four basic stages:
+* [[weathering]] or [[erosion]] of pre-existing rock;
+* [[Transportation (sediment)|transportation]] of the sediment by agents such as water, ice, or wind;
+* [[Deposition (sediment)|deposition]] of the sediment; and
+* conversion into new rock by processes such as [[compaction]] (squeezing together), cementation (binding together), and recrystallization.
-===Clastic===
+[[Image:Lower_antelope_3_md.jpg|thumb|350px|right|[[Antelope Canyon|Lower Antelope Canyon]] was carved out of the surrounding [[Sandstone|sandstone]] by both mechanical weathering and chemical weathering. Wind, sand, and water from [[Flash flood|flash flooding]] are the primary weathering agents.]]
-Clastic sedimentary rocks are composed of discrete fragments or clasts of materials derived from other rocks.  They are composed largely of [[quartz]] with other common minerals including [[feldspar]], [[amphibole]]s, [[clay minerals]], and sometimes more exotic [[igneous]] and [[metamorphic rock|metamorphic]] minerals.
-Clastic sedimentary rocks, such as breccia or sandstone, were formed from rocks that have been broken down into fragments by weathering, which then have been transported and deposited elsewhere.
+The weathering or erosion of pre-existing rock can be mechanical, chemical, or both. '''Mechanical weathering''' involves breakdown of the rock into particles without altering the chemical composition of the minerals it contains. Ice is the most important agent of mechanical weathering. Water percolates into cracks and fissures within the rock, freezes, and expands. The force exerted by the expansion is sufficient to widen cracks and break off pieces of rock. Heating and cooling of the rock, with its accompanying expansion and contraction, also aid the process. Mechanical weathering contributes to further breakdown of the rock by increasing the surface area exposed to chemical agents.
-[[Clastic rock|Clastic sedimentary rocks]] may be regarded as falling along a scale of [[grain size]], with [[shale]] being the finest with particles less than 0.004 mm, [[siltstone]] being a little bigger with particles between 0.004 to 0.06 mm, and [[sandstone]] being coarser still with grains 0.06 to 2 mm, and [[conglomerate (geology)|conglomerate]]s and [[breccia]]s being the coarsest with grains 2 to 256 mm.  Breccia has sharper particles, while conglomerate is catergorized by its rounded particles.  ''Arenite'' is a general term for sedimentary rock with sand-sized particles.
+'''Chemical weathering''' corresponds to breakdown of the rock by chemical reaction. This process dissolves some minerals and converts others into particles that can be readily carried away. Air and water are both involved in many complex chemical reactions. Igneous rocks are commonly attacked by water, particularly acid or alkaline solutions, and all the common igneous rock-forming minerals (except quartz, which is very resistant) are changed in this way into clay minerals and chemicals in solution.
-The classification of clastic sedimentary rocks is complex because there are many variables involved. Particle size (both the average size and range of sizes of the particles), composition of the particles, the cement, and the matrix (the name given to the smaller particles present in the spaces between larger grains) must all be taken into consideration.
+The weathered particles&mdash;in the form of clay, silt, sand, and gravel&mdash; and dissolved materials are transported by water, ice, and wind. These agents reduce the size of the particles, sort them by size, and deposit them at new locations, generally at lower elevation. The sediments dropped by streams and rivers form alluvial fans, flood plains, and deltas, or they reach the bottom of lakes and the seafloor. The wind may move large amounts of sand and other smaller particles. Glaciers transport and deposit great quantities of usually unsorted rock material as [[till]].
-Shales, which consist mostly of clay minerals, are generally further classified on the basis of composition and bedding.
+As the sediment builds up, the overburden (or 'lithostatic') pressure squeezes it into layered solids in a process known as [[lithification]] ('rock formation'), and the original fluids are expelled. The term [[diagenesis]] is used to describe all the chemical, physical, and biological changes, including [[cementation]], undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface weathering.
-Coarser clastic sedimentary rocks are classified according to their particle size and composition. Orthoquartzite is a very pure quartz sandstone; [[arkose]] is a sandstone with quartz and abundant feldspar; [[greywacke]] is a sandstone with quartz, clay, feldspar, and metamorphic rock fragments present, which was formed from the sediments carried by turbidity currents.
+Sedimentary rocks are laid down in horizontal layers called beds or strata. Each new layer settles above older ones, in a process called superposition. There are usually some gaps in the sequence called unconformities. These represent periods in which no new sediments were laid down, or when earlier sedimentary layers were raised above sea level and eroded away.
-All rocks disintegrate slowly as a result of mechanical and chemical [[weathering]].
+== Clastic sedimentary rocks ==
-[[Image:Lower_antelope_3_md.jpg|thumb|350px|right|[[Antelope Canyon|Lower Antelope Canyon]] was carved out of the surrounding [[Sandstone|sandstone]] by both mechanical weathering and chemical weathering.  Wind, sand, and water from [[Flash flood|flash flooding]] are the primary weathering agents.]]
-'''Mechanical weathering''' is the breakdown of rock into particles without producing changes in the chemical composition of the minerals in the rock. Ice is the most important agent of mechanical weathering. Water percolates into cracks and fissures within the rock, freezes, and expands. The force exerted by the expansion is sufficient to widen cracks and break off pieces of rock. Heating and cooling of the rock, and the resulting expansion and contraction, also aids the process. Mechanical weathering contributes further to the breakdown of rock by increasing the surface area exposed to chemical agents.
-'''Chemical weathering''' is the breakdown of rock by chemical reaction. In this process the minerals within the rock are changed into particles that can be easily carried away. Air and water are both involved in many complex chemical reactions. The minerals in igneous rocks may be unstable under normal atmospheric conditions, those formed at higher temperatures being more readily attacked than those which formed at lower temperatures. Igneous rocks are commonly attacked by water, particularly acid or alkaline solutions, and all of the common igneous rock forming minerals (with the exception of quartz which is very resistant) are changed in this way into clay minerals and chemicals in solution.
+When the particles derived from earlier rocks become deposited, compacted, and cemented together, they form clastic sedimentary rocks. These rocks contain inert minerals that are resistant to mechanical and chemical breakdown, such as quartz, [[zircon]], [[rutile]], and [[magnetite]]. Quartz is one of the most resistant minerals, mechanically and chemically.
-Rock particles in the form of clay, silt, sand, and gravel, are transported by the agents of erosion (usually water, and less frequently by ice and wind) to new locations and redeposited in layers, generally at a lower elevation.
+=== Sizes of clastic sedimentary rocks ===
-These agents reduce the size of the particles, sort them by size, and then deposit them in new locations. The sediments dropped by streams and rivers form alluvial fans, flood plains, deltas, and on the bottom of lakes and the sea floor. The wind may move large amounts of sand and other smaller particles. Glaciers transport and deposit great quantities of usually unsorted rock material as [[till]].
+[[Clastic rock|Clastic sedimentary rocks]] may be regarded as falling along a scale of [[grain size]]. [[Shale]] is the finest, with particles less than 0.004 millimeters (mm); [[siltstone]] is a little bigger, with particles between 0.004 and 0.06 mm; [[sandstone]] is coarser still, with grains of 0.06 to 2 mm; and [[conglomerate (geology)|conglomerate]]s and [[breccia]]s are the coarsest, with grains 2 to 256 mm. Breccia has sharper particles, while conglomerate is categorized by its rounded particles. ''Arenite'' is a general term for sedimentary rock with sand-sized particles.
-These deposited particles eventually become compacted and cemented together, forming clastic sedimentary rocks. Such rocks contain inert minerals which are resistant to mechanical and chemical breakdown such as quartz, [[zircon]], [[rutile]], and [[magnetite]]. Quartz is one of the most mechanically and chemically resistant minerals.
+The classification of clastic sedimentary rocks is complex because many variables are involved. The size (or range of sizes) and composition of the particles, the cement, and the matrix (that is, the smaller particles between the larger grains) must all be taken into consideration. Shales, which consist mostly of clay minerals, are generally further classified on the basis of composition and bedding.
-===Biochemical===
+Coarser clastic sedimentary rocks are classified according to their particle size and composition. Orthoquartzite is a very pure quartz sandstone; [[arkose]] is a sandstone with quartz and abundant feldspar; [[greywacke]] is a sandstone with quartz, clay, feldspar, and metamorphic rock fragments present, formed from sediments carried by turbidity currents.
-Biochemical sedimentary rocks contain materials generated by living organisms, and include carbonate minerals created by organisms, such as [[coral]]s, [[mollusc]]s, and [[foraminifera]], which cover the [[ocean floor]] with layers of [[calcite]] which can later form [[limestone]].  Other examples include [[stromatolite]]s, the [[flint]] nodules found in [[chalk]] (which is itself a biochemical sedimentary rock, a form of limestone), and coal (derived from the remains of tropical plants and subjected to pressure).
-===Chemical precipitate===
+==Biochemical sedimentary rocks==
-Precipitate sedimentary rocks form when mineral solutions, such as [[sea water]], evaporate.  Examples include the [[evaporite]] minerals [[halite]] and [[gypsum]].
+Biochemical (or biogenic) sedimentary rocks include carbonate minerals generated by organisms such as [[coral]]s, [[mollusc]]s, and [[foraminifera]]. Minerals such as [[calcite]] (calcium carbonate) cover the [[ocean floor]] and later form [[limestone]]. Other examples include [[stromatolite]]s, the [[flint]] nodules in [[chalk]] (which is itself a biochemical sedimentary rock, a form of limestone), and coal (derived from the remains of tropical plants subjected to high pressure).
-== Economic importance ==
+==Chemical precipitate sedimentary rocks==
-Sedimentary rocks are economically important in that they can easily be used as construction material because they are soft and easy to cut.  For example, the White House in Washington, DC, is made of sandstone. In addition, sedimentary rocks often form [[porosity|porous]] and [[Permeability (fluid)|permeable]] reservoirs in [[sedimentary basin]]s in which [[petroleum]] and other [[hydrocarbons]] can be found.
+Chemical precipitate sedimentary rocks are formed when mineral solutions, such as [[sea water]], evaporate. Examples include the [[evaporite]] minerals [[halite]] and [[gypsum]].
-==Additional information==
+== Significance of sedimentary rocks ==
-It is believed that the relatively low levels of [[carbon dioxide]] in the [[Earth]]'s [[Earth's atmosphere|atmosphere]], in comparison to that of [[Venus (planet)|Venus]], is because of large amounts of carbon being trapped in limestone and [[dolomite]] sedimentary layers. The flux of carbon from eroded sediments to marine deposits is part of the [[carbon cycle]].
+Sedimentary rocks contain important information about the [[history of the Earth]]. They contain [[fossil]]s, the preserved remains of ancient [[plant]]s and [[animal]]s. The composition of sediments provides us with clues about the original rock. Differences between successive layers indicate changes to the environment that have occurred over time. Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remnants.
-The shape of the particles in sedimentary rocks has an important effect on the ability of micro-organisms to colonize them. This interaction is studied in the science of [[geomicrobiology]]. One measure of the shape of these particles is the roundness factor, also known as the Krumbein number after the geologist [[William C. Krumbein|W. C. Krumbein]].
+The sedimentary rock cover of the continents of the Earth's [[Crust (geology)|crust]] is extensive, but the total contribution of sedimentary rocks is estimated to be only five percent of the total. As such, the sedimentary sequences we see represent only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks.
+Sedimentary rocks are also significant in economic terms. Being relatively soft and easy to cut, they are often used as construction material. For example, the White House in Washington, DC, is made of sandstone. In addition, sedimentary rocks often form [[porosity|porous]] and [[Permeability (fluid)|permeable]] reservoirs in [[sedimentary basin]]s in which [[petroleum]] and other [[hydrocarbon]]s are found.
 ==See also==