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SEDIMENTARY ROCKS

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Sedimentary Rocks are formed by the accumulation and subsequent consolidation of sediments into various types of rock.u00a0 The key is the sediments.u00a0 Sediments are unconsolidated material and have different origins.u00a0 Ultimately, the origin of these sediments is the weathering, erosion and/or the chemical breakdown of other rocks.u00a0 These "other" rocks could be igneous, metamorphic or even other sedimentary rocks.u00a0 The type and size of the sediments and how they are formed will lead to the classification of the different sedimentary rocks listed below.u00a0u00a0

There are three main subclasses of sedimentary rocks.u00a0 They include biochemical, clastic and evaporative sedimentary rocks.u00a0

Biochemical sedimentary rocks are formed from organic processes that involve living organisms producing the sediments.u00a0 These living organisms can be snails and clams whose discarded calcium carbonate shells can form limestone.u00a0 But it also includes swamp plants whose organic debris can produce coal if conditions are right. Although the origin of the sediments are organic, most of the chemicals that the living organisms used to produce their shells or their body parts have origins from previous rocks.u00a0 Thus these are sedimentary rocks, but with a biogenic intermediate so to speak.

Clastic sedimentary rocks are sometimes considered true sedimentary rocks as they are composed directly of the sediments or fragments from other rocks.u00a0 These fragments are called clasts, hence the term clastic.u00a0 Generally they are further defined by the size of their clasts which range from nearly boulder size in the coarsest conglomerates and breccias to very fine grain size (<0.004 mm) in shale.u00a0 Often clastic rocks can have their clasts analyzed and the original source rocks can be determined if the clasts have not been moved too far from the source or have not been worked and reworked into sedimentary rock after sedimentary rock.u00a0 Glacial till from a large continental glacier can be analyzed and the origin of the glacier's debris could be determined and therefore the possible path of the glacier could be determined.

Evaporative or chemical sedimentary rocks are formed from the generally inorganic deposition of chemicals, usually through evaporation of a chemical rich solution.u00a0 These chemicals generally have their origin from the chemical weathering of other rocks or other sediments.u00a0 The sodium and chlorine in halite (table salt) comes from the sodium and chlorine in other rocks that have dissolved over the years.u00a0 Unlike clastic sedimentary rocks, the direct origin of the chemicals is rarely easy to identify.u00a0 The chemicals could come from magma deep in the crust of the Earth, rocks that dissolved in the ocean billions of years ago or from an outcrop in the hillside next to a playa lake.u00a0 Sometimes the origin can be figured out and sometimes there is no way to know the originating source.

No matter what type of sedimentary rock, water is almost always a key component.u00a0 The only real exception to this is desert wind blown sediments.u00a0 All other rock types involve water is some way and generally in an important way.u00a0 The biochemical rocks come from water born organisms.u00a0 Clastic rocks are usually water transported, sorted and deposited.u00a0 Evaporative rocks are of course derived from chemicals dissolved in water.u00a0 Where there is water, there are sedimentary rocks being formed.u00a0 Sedimentary geologists must have an understanding of hydrology in order to understand their subjects.

Another common factor to sedimentary rocks is that they originate on the surface of the Earth, unlike most igneous and metamorphic rocks which originate in the interior of the Earth's crust.u00a0 Geologists can actually see many sedimentary rocks form or at least see the sediments that will become sedimentary rocks prior to their lithification (literally "turning into stone").u00a0u00a0

The lithification of the sediments is usually accomplished by a cementing agent.u00a0 Once the sediments are no longer loose sediments, but cemented together grains or crystals, they become a rock.u00a0 What happens to sediments from this stage on is called diagenesis.u00a0

Diagenesis is important to study in sedimentary rocks.u00a0 It includes the study of the compaction of the rock, physical conditions, chemical alterations and biological interactions.u00a0 Cementation is usually the first aspect of diagenesis, but cementation can be episodic, reversed and recementation can occur.u00a0 Compression of the rocks can change the banding and increase chemical alterations.u00a0 Chemicals can leave or enter a rock through pore space waters, and minerals can crystallize, or dissolve, or become hydrated or oxidized or chemically changed in other ways.u00a0 Temperature increases can alter the characteristics of the rock as well.u00a0 If too much heat and pressure occur during diagenesis then the rock may wander into the regime of metamorphism.u00a0 At times geologist will argue over the boundary between a highly diagenetically altered sedimentary rock and a low grade metamorphic rock.u00a0 Diagenesis is extremely important in understanding the history and the resulting effects on the characteristics of sedimentary rocks.

Geodes are another type of sedimentary rock. They form when minerals in aqueous solution crystallize on the interiors of cavities. This may occur in typical sedimentary formations, or as ground water penetrates igneous formations.

Common Sedimentary Rocks:


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