About 75% of the surface of the globe is coveredsedimentary rocks. This category includes more than half of the minerals extracted from the bowels of the planet. They are mainly concentrated on the continents, the shelf and the continental slope, and only about 15% are at the bottom of the oceans.
Sedimentary rocks are formed whenredeposition of weathering products and deposition of the substance mainly in the aquatic environment (less often on land), as well as in the process of the activity of glaciers. Precipitation can occur in different ways. Depending on its nature, different groups of sedimentary rocks are distinguished:
- clastic - are the result of predominantly physical weathering, subsequent transport and redeposition of the material;
- colloidal-sedimentary - are formed as a result of chemical decomposition, accompanied by the formation of colloidal solutions;
- chemogenic - formed when the precipitate is precipitated from aqueous solutions as a result of chemical reactions or caused by various causes of supersaturation of solutions;
- biochemical - the result of chemical reactions, taking place with the participation of living organisms;
- biogenic, or organogenic - the result of the vital activity of organisms.
Often there are overlapping signs of differentgroups of sedimentary rocks. The classification given above relates them to any one group. For example, limestone, which may have clastic, organogenic, chemogenic or biochemical origin, belongs to biochemical rocks.
By chemical composition, sedimentary rocksdiffer from magmatic by a large number of rock-forming components and a much higher differentiation. This is due to the fact that the material for their formation is the weathering products of metamorphic, magmatic and older sedimentary rocks, as well as gases and mineral components dissolved in natural waters, the products of the life activity of organisms, products of volcanic eruptions and even fragments of meteorites (for example, nickel-iron balls ). In addition, they often contain traces of long-extinct life - fossil flora and fauna. As a rule, the age of such fossils is equal to the age of the rocks themselves, but there are also older, redeposited organic remains.
Some rocks (diatomites, nummulitic andmosshank limestones and others) are entirely composed of organisms (biomorphic) or their fragments (detritus). In biomorphic (all-porous) structures, fossils are isolated from each other, bound with cement (brachiopod shells) or grow on each other, forming growth textures (coral or bryozoan limestones). Detritus structures are formed in shallow water under the influence of destructive shells of excitement or as a result of the vital activity of predators crushing shells in order to extract food.
Sedimentary rocks are formed as followsway: when the parent rocks are destroyed, the initial substance is formed, which is transported by water, wind or glaciers, and then deposited on the land surface and at the bottom of the water basins. As a result, a precipitate consisting of dissimilar components is formed, completely or partially saturated with water and having a loose and porous structure. This sediment, modified over time, is the sedimentary rock.
The sedimentation process is influenced byclimate and tectonic regime. In conditions of humid (warm and humid) climate, aleurites, clays, diatomites, caustobioliths (combustible minerals) are formed. For arid (arid) zones, limestones, gypsum, anhydrite, and rock salt are characteristic. In the folded areas, powerful layers of sedimentary rocks accumulate, which are characterized by such features as variability in space and a motley composition of detrital material. On the platforms, on the contrary, there are layers of low-power rocks with a more uniform composition of detrital material.
Due to the fact that the conditions of sedimentation inpast geological epochs were close to modern or similar to them, on the basis of data on the nature of the distribution of sedimentary rocks on the earth's surface, one can recreate the paleotectonic and paleogeographic situation on the planet.
