l>Sedimentary RocksSedimentary Rocks

Igneous rocks room sometimes thought about primary rocks since they crystallize indigenous a liquid. In that case, sedimentary rocks are acquired rocks since they are formed from fragments of pre-existing rocks.

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Formation that Sedimentary Rocks

Sedimentary rocks room the product of 1) weathering the preexisting rocks, 2) transport that the weathering products, 3) deposition that the material, followed by 4) compaction, and also 5) cementation the the sediment to type a rock. The last two procedures are dubbed lithification.



When rocks (igneous, sedimentary, or metamorphic) space at or close to the surface ar of the earth they space exposed come the processes of weathering.

In mechanical weathering rocks are damaged up right into smaller piece by frost-wedging (the freezing and thawing the water within cracks in the rock), root-wedging (tree and other tree roots growing into cracks), and abrasion brought about by, because that example, sand-blasting the a cliff face by blowing sands in the dessert, or the scouring of water transported sand, gravel, and also boulders ~ above the bedrock of a mountain stream. Mechanically weathering breaks rocks into smaller and smaller pieces but without otherwise transforming the minerals.

In chemical weathering mineral are changed into brand-new minerals and mineral byproducts. Part minerals like halite and also calcite may dissolve completely. Others, specifically silicate minerals, are altered by a chemical procedure called hydrolysis. Hydrolysis is the reaction of minerals in weakly acidic waters. Most organic surface waters are slightly acidic since carbon dioxide indigenous the air dissolves in the water. Some of the dissolved CO2 reacts v the water forming the chemical compound carbonic acid.

Complete weathering that silicate rocks will yield:

solid materials

1) clays

2) quartz sand (if the rock originally included quartz)

dissolved materials

3) soluble silica

4) metal cations

Rock fragments will likewise remain wherein the rocks space not completely weathered.

Not only is quartz the most stable the the usual rock creating minerals in chemical weathering, the high hardness and lack that cleavage do it quite resistant to mechanically weathering. Quartz is chin an certified dealer of mechanical weathering in the form of blowing dessert sand.


As the process of weathering proceeds the commodities are lugged off. The most vital transporting certified dealer is water. Water carries or rolls corpuscle in rivers, from the the smallest suspended clay particles to the biggest boulders. Boulders and also smaller absent fragments continue to be damaged up and also chemically transformed as lock tumble downstream. Water also carries liquified minerals, such as silica and cations downstream and also in the groundwater. Various other transporting agents incorporate wind which blows dust and sand, glaciers, i beg your pardon carry big amounts the gravel and large boulders in enhancement to smaller sized particles, and mass wasting on hillslopes. In enhancement to diminish the particle size, together sedimentary product is transported the is also sorted into similar sized particles as a result of an altering energy (velocity) in the transporting medium (water or wind), and also rounded by ongoing abrasion.


Sediments are transported only once there is enough power in the delivering medium, for example, as soon as a currently is flowing rapidly enough to bring a offered size the sedimentary particle. Steep hill streams have the right to move large boulders throughout spring flood yet these boulders will never be transported out into a placid lowland river. So the largest sediments (boulders, cobbles, and also pebbles) which survive the weathering process, often tend to be deposited close to to your source, for instance at the suggest where a mountain stream flows the end onto a sink floor. Sediments of a provided size space deposited whenever they move right into an atmosphere with insufficient power to transport them. For example, silt lugged by a flooding flow will work out out in the quiet backwaters outside the river banks (perhaps enriching someone"s farmland - while wrecking their home).

Sediments room deposited layer ~ above layer. The layers are deposited horizontally.

Sorting. As soon as a river encounters the s it begins to deposit its suspended sediments. Increasingly finer sediments are deposited moving away indigenous the shoreline. All fine products are winnowed the end leaving sands in the wave-dominated beach and nearshore environment. The sands continue to be in this high energy environment. In deeper/calmer water silt settles out. In water deep sufficient not come be impacted by surface ar wave action the clay portion begins to work out out.

The dissolved load in water will precipitate the end (crystallize) if it encounters a supersaturated environment. Gypsum, halite, and other salts, precipitate out of seawater in arid areas, prefer the east Mediterranean, wherein evaporation is high (thus raising the salinity) and influx of fresh seawater is low.

Compaction and also Cementation

As precipitate continues, the earlier deposited sediments room laden with raising overburden. They are compacted, to reduce the available pore room and expelling lot of the pore-water.

Dissolved minerals in the floor water precipitate (crystallize) native water in the pore spaces creating mineral crusts on the sedimentary grains, progressively cementing the sediments, thus creating a rock. Calcite (calcium carbonate), silica, and hematite (red iron oxide) room the most usual cementing agents. You may be familiar with calcite (or lime) encrustation top top old plumbing fixtures, showerheads, and also inside hot water heaters.

Types of Sedimentary Rocks

Sedimentary rocks may be split into three basic categories:

1) Clastic (detrital) sedimentary rocks space composed that the solid products of weathering (gravel, sand, silt, and also clay) cemented with each other by the liquified weathering products.

2) Biogenic (biochemical) sedimentary rocks are those composed of materials formed by the activity of life organisms such together coal (compacted undecayed plant matter) and also many limestones which are comprised of the shells or other skeletal fragments from maritime organisms.

3) Chemically precipitated (chemical) sedimentary rocks space those such together halite and also gypsum, and also some limestones, which kind direct precipitation (crystallization) that the liquified ions in the water.

Clastic Sedimentary Rocks

Clastic sedimentary rocks may first be classified according to their grain size. Clay-sized particles are too tiny to be seen through a microscope. Rock formed from clay-size particles are called shale. Silt-sized particles space visible v a microscope. Rock developed from these are dubbed siltstone. Sand-sized grains room visible to the naked eye and selection from 1/16 mm to 2 mm. Sand is further subdivided into really fine, fine, medium, coarse, and an extremely coarse. Rock created from these are referred to as sandstone. "Gravel"-sized grains selection from > 2 mm granules come very huge boulders. Rock containing these large size corpuscle are called conglomerate and also are typically really poorly sorted (e.g., they might contain, sand, gravel, and boulders every in one rock). If the gravel particles are little weathered and also are still angular (un-rounded) the rock is called breccia.

Biogenic Sedimentary Rocks

Carbonate Rocks (based on CO3). While part carbonate rocks type as straightforward chemical precipitates most carbonate rocks space the product of maritime organisms such as molluscs and corals. They precipitate calcite (calcium carbonate, CaCO3) or other comparable carbonate minerals straight from the dissolved chemicals in the water to develop their shells. Limestone is the product. In ~ some later on time (e.g.,. After ~ burial) calcite may be transformed into dolomite, CaMg(CO3)2. Calcite will react vigorously v dilute hydrochloric mountain (HCl). Powdered dolomite will react sluggishly with HCl.

Coal is also formed by biological activity but in this situation the product is organic matter from decaying plants that may accumulate if plant growth is much faster than the rate of decay. The organic matter will it is in buried and also compacted through layer ~ above layer of partially decayed plants, eventually ending up being coal.

Chemically precipitation Sedimentary Rocks

Where the liquified ions encounter supersaturated conditions they come the end of solution and combine together developing an orderly setup of atoms (that"s best - minerals). They are said to precipitate - go from the liquid, dissolved state come the solid decision state. Rocks formed in this way include halite, gypsum, anhydrite, and also some limestones. Class of precipitated rocks are referred to as evaporite deposits due to the fact that they typically form where evaporation is high in arid regions choose the desert southwest and in the eastern Mediterranean. Salt flats in the desert southwest and also elsewhere contain large deposits the chemically precipitated layers that formed as feather runoff from the bordering mountains brought dissolved ions out top top the flats whereby the waters then evaporated in the summer sun, leave behind the salts.

Sedimentary Structures

Most sedimentary rocks contain inner layering referred to as bedding or stratification. Stratification may range from a bed thickness of many meters under to fine millimeter-size laminations. Bedding is generally horizontal or virtually so.

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Internal stratification in ~ a bigger bed may be parallel or there might be cross-stratification caused by ripples, sand bars, and also dune structures. Ripple marks, a couple of millimeters come centimeters in size, are usual features in water laid sediments. Large scale cross-bedding in sandstone, in ~ horizontal layers a couple of to numerous feet thick, suggests deposition in sand dunes.

Ripple marks show deposition in a current. Assymetric ripples (one side steeper than the other) indicates a constant current direction together in streams. Symetric ripples indicate oscillating (waves) or weak currents.

Mudcracks are created by dry of wet muds. Raindrop impression may additionally be maintained in sediments. They show deposition in a terrestrial setting.

Fossils are very important indicators of depositional environment. Fossils include preserved skeleton fragments, tree roots, etc., and likewise trace fossils such together burrows, footprints, leaf impressions, etc. Coral and also many covering fossils indicate marine deposition. Leaves suggest terrestrial deposition.