Stratification is a process that occurs due to layering in most sedimentary rocks and in those igneous rocks which are formed at the surface of the Earth, from lava flows and volcanic fragmental deposits. The layers range from several millimeters to many meters in thickness and greatly differ in shape. The strata may range from thin sheets that cover many square kilometers to thick bodies that extend only a few meters laterally. Stratification planes are the planes of parting or separation between individual rock layers. They are horizontal where deposition of sediments take place as flat-lying layers, and they exhibit inclination where the depositional site is a sloping surface. The bottom surface of this stratum roughly conforms to irregularities of the surface underneath.
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Formation of Stratified Rocks
Stratification in sedimentary rocks may result from several changes in their texture or composition during this process of deposition. It can also result from pauses in the deposition that allow the older deposits to undergo certain changes before they are covered by additional sediments. A series of color changes are also observed that are resulted from the differences in mineral composition, or merely as layers of almost similar aspects separated by distinct parting planes. There is no direct relationship between the thickness, extent of strata, the rate of deposition, and the time represented. For example, a stratum of limestone which is 2.5 cm thick may take longer to form than a stratum of sandstone which is 3 m in thickness. The most common cause of stratification is the variation in the transporting ability of the agent which causes this deposition to take place.
Stratification in volcanic rocks and sedimentary rocks differs in certain respects. Under the influence of gravity, particle size, and wind, fragmental volcanic material becomes sorted. Falling to the ground, well-sorted layers can be formed. If these sediments fall into any lake or sea, they become layered like any other detrital matter. The process of stratification may also result from continuous flows of liquid lava or alternations between flows and ashfalls.
All the sedimentary deposits are not stratified. The ones which are transported by ice only, landslide deposits, and residual soils, for instance, exhibit no stratification. Original stratification may be destroyed by animals and plants or by disturbances after deposition.
Cross-bedding and Graded Bedding
When layering within the stratum takes place at an angle to the main bedding plane, cross-bedding is said to take place. Cross-bedding is also known as cross-stratification. Here, the sedimentary structures formed are roughly horizontal units which compose of inclined layers. This process occurs when there is deposition on the inclined surfaces of bedforms like ripples and dunes, and it indicates that the depositional environment contains a flowing medium (mainly wind or water). Some examples of these bedforms are ripples, sand waves, dunes, anti-dunes, bars, and delta slopes.
A graded bed is a bed that is characterized by a systematic change in grain size from one of its sides to the other. Graded beds represent depositional environments that decrease in transport energy over time, but these beds can also form during rapid depositional movements. They are best represented where the sudden strong current deposits heavy, coarse sediments first with finer particles following them as the current weakens. They can also be formed in terrestrial stream deposits. In reverse or inverse grading, the bed becomes coarse upwards. This type of grading is pretty uncommon. This is also seen in Aeolian processes.
Some Facts on Stratified Rocks
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Limestone is a sedimentary rock which is mainly composed of mineral calcite, which is in the form of calcium carbonate. Limestone often contains variable amounts of silica along with amounts of clay, silt, and sand.
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Sandstone is a sedimentary rock that is formed under oceans, lakes, and rivers. They are composed of sand particles such as quartz or feldspar. This natural stone is pretty tough and resistant to weathering. It is also a popular material used in building and paving from ancient times.
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Shale rocks are rocks that are made up of clay-sized particles. They can form in rivers, basins, and oceans. They roughly cover 70% of the Earth’s surface and constitute about 55% of all sedimentary rocks.