Sedimentaryong bato

Ang sedimentary rock, rock na nabuo sa o malapit sa ibabaw ng Earth sa pamamagitan ng akumulasyon at lithification ng sediment (detrital rock) o sa pamamagitan ng pag-ulan mula sa solusyon sa normal na temperatura ng ibabaw (kemikal na bato). Ang mga sedimentary na bato ay ang pinaka-karaniwang bato na nakalantad sa ibabaw ng Earth ngunit lamang ng isang menor de edad na nasasakupan ng buong crust, na pinangungunahan ng mga malalaking bato at metamorphic na mga bato.

Ang mga sedimentary na mga bato ay ginawa ng pag-init ng mga bato ng preexisting at ang kasunod na transportasyon at pag-aalis ng mga produkto ng pag-uudyok. Ang pag-uulat ay tumutukoy sa iba't ibang mga proseso ng pisikal na pagkasira at pagkabulok ng kemikal na nagaganap kapag ang mga bato sa ibabaw ng Lupa ay nakalantad sa kalangitan (pangunahin sa anyo ng pag-ulan) at ang hydrosfos. Ang mga prosesong ito ay gumagawa ng lupa, hindi pinagsama-samang bato detritus, at mga sangkap na natunaw sa tubig sa lupa at runoff. Ang pagguho ay ang proseso kung saan ang mga produkto ng pag-uudyok ay inilipat ang layo mula sa lugar ng pag-uugat, alinman bilang solidong materyal o bilang mga natunaw na bahagi, sa kalaunan ay mai-deposito bilang sediment. Ang anumang hindi pinagsama-samang pagdeposito ng solid na naka-weather na materyal ay bumubuo ng sediment. Maaari itong mabuo bilang resulta ng pag-aalis ng mga butil mula sa paglipat ng mga katawan ng tubig o hangin,mula sa pagtunaw ng glacial ice, at mula sa pagbagsak ng pagbagsak (pagdulas) ng mga bato at lupa na pagtugon sa gravity, pati na rin sa pamamagitan ng pag-ulan ng mga nalusaw na produkto ng pag-uugnay sa ilalim ng mga kondisyon ng mababang temperatura at presyur na mananaig sa o malapit sa ibabaw ng Earth.

Ang sedimentary na mga bato ay ang lithified na katumbas ng mga sediment. Karaniwan ang mga ito ay ginawa sa pamamagitan ng semento, compacting, at kung hindi man solidifying preexisting unconsolidated sediment. Ang ilang mga uri ng sedimentary rock, gayunpaman, ay direktang tumubo sa kanilang solidong sedimentary form at nagpapakita ng walang intervening pagkakaroon bilang sediment. Ang mga organikong bahura at kama ng mga evaporite ay mga halimbawa ng nasabing mga bato. Sapagkat ang mga proseso ng pisikal (mekanikal) na pag-iilaw at kemikal na pag-iilaw ng kemikal ay magkakaiba-iba, bumubuo sila ng mga kapansin-pansing natatanging mga produkto at dalawang batayang magkakaibang mga uri ng sediment at sedimentaryong bato: (1) mga katutubo na maliliit na bato ng sedimentaryong bato at (2) allochemical at orthochemical sedimentary rock.

Clastic terrigenous sedimentary rocks consist of rock and mineral grains, or clasts, of varying size, ranging from clay-, silt-, and sand- up to pebble-, cobble-, and boulder-size materials. These clasts are transported by gravity, mudflows, running water, glaciers, and wind and eventually are deposited in various settings (e.g., in desert dunes, on alluvial fans, across continental shelves, and in river deltas). Because the agents of transportation commonly sort out discrete particles by clast size, terrigenous clastic sedimentary rocks are further subdivided on the basis of average clast diameter. Coarse pebbles, cobbles, and boulder-size gravels lithify to form conglomerate and breccia; sand becomes sandstone; and silt and clay form siltstone, claystone, mudrock, and shale.

Chemical sedimentary rocks form by chemical and organic reprecipitation of the dissolved products of chemical weathering that are removed from the weathering site. Allochemical sedimentary rocks, such as many limestones and cherts, consist of solid precipitated nondetrital fragments (allochems) that undergo a brief history of transport and abrasion prior to deposition as nonterrigenous clasts. Examples are calcareous or siliceous shell fragments and oöids, which are concentrically layered spherical grains of calcium carbonate. Orthochemical sedimentary rocks, on the other hand, consist of dissolved constituents that are directly precipitated as solid sedimentary rock and thus do not undergo transportation. Orthochemical sedimentary rocks include some limestones, bedded evaporite deposits of halite, gypsum, and anhydrite, and banded iron formations.

Sediments and sedimentary rocks are confined to the Earth’s crust, which is the thin, light outer solid skin of the Earth ranging in thickness from 40–100 kilometres (25 to 62 miles) in the continental blocks to 4–10 kilometres in the ocean basins. Igneous and metamorphic rocks constitute the bulk of the crust. The total volume of sediment and sedimentary rocks can be either directly measured using exposed rock sequences, drill-hole data, and seismic profiles or indirectly estimated by comparing the chemistry of major sedimentary rock types to the overall chemistry of the crust from which they are weathered. Both methods indicate that the Earth’s sediment-sedimentary rock shell forms only about 5 percent by volume of the terrestrial crust, which in turn accounts for less than 1 percent of the Earth’s total volume. On the other hand, the area of outcrop and exposure of sediment and sedimentary rock comprises 75 percent of the land surface and well over 90 percent of the ocean basins and continental margins. In other words, 80–90 percent of the surface area of the Earth is mantled with sediment or sedimentary rocks rather than with igneous or metamorphic varieties. The sediment-sedimentary rock shell forms only a thin superficial layer. The mean shell thickness in continental areas is 1.8 kilometres; the sediment shell in the ocean basins is roughly 0.3 kilometre. Rearranging this shell as a globally encircling layer (and depending on the raw estimates incorporated into the model), the shell thickness would be roughly 1–3 kilometres.

Despite the relatively insignificant volume of the sedimentary rock shell, not only are most rocks exposed at the terrestrial surface of the sedimentary variety, but many of the significant events in Earth history are most accurately dated and documented by analyzing and interpreting the sedimentary rock record instead of the more voluminous igneous and metamorphic rock record. When properly understood and interpreted, sedimentary rocks provide information on ancient geography, termed paleogeography. A map of the distribution of sediments that formed in shallow oceans along alluvial fans bordering rising mountains or in deep, subsiding ocean trenches will indicate past relationships between seas and landmasses. An accurate interpretion of paleogeography and depositional settings allows conclusions to be made about the evolution of mountain systems, continental blocks, and ocean basins, as well as about the origin and evolution of the atmosphere and hydrosphere. Sedimentary rocks contain the fossil record of ancient life-forms that enables the documentation of the evolutionary advancement from simple to complex organisms in the plant and animal kingdoms. Also, the study of the various folds or bends and breaks or faults in the strata of sedimentary rocks permits the structural geology or history of deformation to be ascertained.

Finally, it is appropriate to underscore the economic importance of sedimentary rocks. For example, they contain essentially the world’s entire store of oil and natural gas, coal, phosphates, salt deposits, groundwater, and other natural resources.

Several subdisciplines of geology deal specifically with the analysis, interpretation, and origin of sediments and sedimentary rocks. Sedimentary petrology is the study of their occurrence, composition, texture, and other overall characteristics, while sedimentology emphasizes the processes by which sediments are transported and deposited. Sedimentary petrography involves the classification and study of sedimentary rocks using the petrographic microscope. Stratigraphy covers all aspects of sedimentary rocks, particularly from the perspective of their age and regional relationships as well as the correlation of sedimentary rocks in one region with sedimentary rock sequences elsewhere. (For further information about these fields, see geologic sciences.)