About 75% of the earth's surface covered with sedimentary rocks.This category includes more than half of the minerals extracted from the bowels of the planet.They mainly focused on the continents, the shelf and continental slope, and only about 15% is at the bottom of the oceans.
Sedimentary rocks are formed by redeposition of weathering products and deposition of material mainly in the aquatic environment (at least on the land), as well as in the process of activity of glaciers.Precipitation can occur in different ways.Depending on its nature, emit different groups of sedimentary rocks:
- clastic - are the result of mainly physical weathering, subsequent transfer and redeposition of the material;
- colloid-sedimentary - formed by chemical decomposition, accompanied by the formation of colloidal solutions;
- chemogenic - are formed by precipitation from aqueous solution as a result of chemical reactions or caused by different reasons supersaturated solution;
- biochemical - the result of chemical reactions taking place with the participation of living organisms;
- biogenic or Organogenic - the result of the activity of organisms.
Often there are signs of combining different groups of sedimentary rocks.The classification given above relates them to any one group.For example, limestone, which can be clastic, organogenic, chemogenic or biochemical origin, belongs to the biochemical species.
The chemical composition of sedimentary rocks differ from igneous rock forming a large number of components and a much higher differentiation.This is due to the fact that the material for their formation are products of weathering metamorphic, igneous and older sedimentary rocks, as well as gases and dissolved in natural waters, mineral components, the waste products of organisms, products of volcanic eruptions, and even fragments of meteorites (such as balls of nickel iron).In addition, in their composition often they found traces of long-vanished life - fossil flora and fauna.As a rule, the age of these fossils is the age of the rock, but there are also older, redeposited organic remains.
Some breeds (diatomite, nummulitovye and mshankovye limestone and others) is entirely composed of organisms (biomorphic) or their fragments (detritusovye).The biomorphic (tselnorakovinnyh) structures of the fossils are isolated from each other by communicating cement (brachiopod coquina) or picking at each other, forming a texture growth (mshankovye coral or limestone).Detritusovye structures formed in shallow water under the influence of destructive shells unrest or as a result of vital activity of predators, fragmenting the shell for the purpose of obtaining food.
Sedimentary rocks are formed as follows: the destruction of source rocks formed the starting material, which is transferred to water, wind and glaciers and then deposited on the land surface and the bottom of the basins.The result is made up of heterogeneous components precipitate, entirely or partially saturated with water and having a friable and porous structure.This precipitate, modified over time, - and has a sediment.
sedimentation process takes place under the influence of climate and tectonic regime.In humid conditions (hot and humid) climate formed silts, clays, diatomite, caustobioliths (fossil fuels).For the arid (dry) areas are characterized by limestone, gypsum, anhydrite, rock salt.The folded regions accumulate thick layers of sedimentary rocks, which are characterized by such features as the variability in space and variegated composition of clastic material.On the platforms, on the other hand, lie subterranean low power with more uniform composition of clastic material.
Due to the fact that the conditions of sedimentation in the past geological epochs were close to the modern or the like, on the basis of data on the distribution of sedimentary rocks on the Earth's surface can recreate paleotectonic and paleogeographic situation on the planet.