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Megafan
An alluvial megafan is a large cone or fan-shaped deposit built up by complex deposition patterns of stream flows originating from a single source point known as an apex. Megafans differ from Alluvial fans in their sheer size. Due to their larger size, they may be formed by different geomorphic processes. The criterion of what differentiates megafans from typical alluvial fans is an artificial one of scale. The scale divide varies in the literature, with the most common being a 100-km apex-to-toe length. Alternative values as little of 30-km apex-to-toe length have been proposed, as well as alternative metrics like coverage areas of greater than 10,000 square-km. Formation The flow source from the apex occupies a portion of the fans apron, building up that portion with depositions. Through complex processes like avulsion, the stream changes course over time occupying different areas of the fan. Over long periods of time, sediment builds up creating a fan formation. Generally the thr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Avulsion (river)
In sedimentary geology and fluvial geomorphology, avulsion is the rapid abandonment of a river channel and the formation of a new river channel. Avulsions occur as a result of channel slopes that are much less steep than the slope that the river could travel if it took a new course. Deltaic and net-depositional settings Avulsions are common in river deltas, where sediment deposits as the river enters the ocean and channel gradients are typically very small. This process is also known as delta switching. Deposition from the river results in the formation of an individual deltaic lobe that pushes out into the sea. An example of a deltaic lobe is the bird's-foot delta of the Mississippi River, pictured at right with its sediment plumes. As the deltaic lobe advances, the slope of the river channel becomes lower, as the river channel is longer but has the same change in elevation. As the slope of the river channel decreases, it becomes unstable for two reasons. First, water under the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Alluvial Fan
An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but are also found in more humid environments subject to intense rainfall and in areas of modern glaciation. They range in area from less than to almost . Alluvial fans typically form where flow emerges from a confined channel and is free to spread out and infiltrate the surface. This reduces the carrying capacity of the flow and results in deposition of sediments. The flow can take the form of infrequent debris flows or one or more ephemeral or perennial streams. Alluvial fans are common in the geologic record, such as in the Triassic basins of eastern North America and the New Red Sandstone of south Devon. Such fan deposits likely contain the largest accumulations of gravel in the geologic record. Alluvial fans have also been found on Mars ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Alluvial Plain
An alluvial plain is a largely flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which alluvial soil forms. A floodplain is part of the process, being the smaller area over which the rivers flood at a particular period of time, whereas the alluvial plain is the larger area representing the region over which the floodplains have shifted over geological time. As the highlands erode due to weathering and water flow, the sediment from the hills is transported to the lower plain. Various creeks will carry the water further to a river, lake, bay, or ocean. As the sediments are deposited during flood conditions in the floodplain of a creek, the elevation of the floodplain will be raised. As this reduces the channel floodwater capacity, the creek will, over time, seek new, lower paths, forming a meander (a curving sinuous path). The leftover higher locations, typically natural levees at the margins ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sedimentology
Sedimentology encompasses the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation (erosion and weathering), transport, deposition and diagenesis. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary structures. Sedimentary rocks cover up to 75% of the Earth's surface, record much of the Earth's history, and harbor the fossil record. Sedimentology is closely linked to stratigraphy, the study of the physical and temporal relationships between rock layers or strata. The premise that the processes affecting the earth today are the same as in the past is the basis for determining how sedimentary features in the rock record were formed. By comparing similar features today to features in the rock record—for example, by comparing modern sand dunes to dunes preserved in ancient aeolian sandstones—geologists reconstruct past environmen ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Subaqueous Fan
A subaqueous fan is a fan-shaped deposit formed beneath water (similar to deltas or terrestrial alluvial fans), and are commonly related to glaciers and crater lakes. Subaqueous fan deposits are generally described as coarse to fine gravel and/or sand, with variable texture and sorting. Underflows (meltwater denser than lake water) tends to produce subaqueous fans with channels and levees. Subaqueous fans can be formed by the influence of glacier movement and by underwater currents typically found at a river delta. The sediment size and composition that makes up the subaqueous fan is dependent on the type of rock that the water flow or glacial ice sheet moves over. Sedimentary structures found in subaqueous fans are heavily dependent on the strength of the water flow. Glacial Formation of the Subaqueous Fan Background on Glacial Deposition Ice is a more efficient agent of erosion compared to wind and water. Glaciers can carry a heavy load of sediment to the ice front of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
River Delta
A river delta is a landform shaped like a triangle, created by deposition (geology), deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot carry away the supplied sediment. It is so named because its triangle shape resembles the Greek letter Delta. The size and shape of a delta is controlled by the balance between watershed processes that supply sediment, and receiving basin processes that redistribute, sequester, and export that sediment. The size, geometry, and location of the receiving basin also plays an important role in delta evolution. River deltas are important in human civilization, as they are major agricultural production centers and population centers. They can provide Coast, coastline defense and can impact drinking water supply. They are also Ecology, ecologically important, with different species' assemblages ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Placer Deposit
In geology, a placer deposit or placer is an accumulation of valuable minerals formed by gravity separation from a specific source rock during sedimentary processes. The name is from the Spanish word ''placer'', meaning "alluvial sand". Placer mining is an important source of gold, and was the main technique used in the early years of many gold rushes, including the California Gold Rush. Types of placer deposits include alluvium, eluvium, beach placers, aeolian placers and paleo-placers. Placer materials must be both dense and resistant to weathering processes. To accumulate in placers, mineral particles must have a specific gravity above 2.58. Placer environments typically contain black sand, a conspicuous shiny black mixture of iron oxides, mostly magnetite with variable amounts of ilmenite and hematite. Valuable mineral components often occurring with black sands are monazite, rutile, zircon, chromite, wolframite, and cassiterite. Early mining operations were likely a result ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Floodplain
A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudie, A. S., 2004, ''Encyclopedia of Geomorphology'', vol. 1. Routledge, New York. The soils usually consist of clays, silts, sands, and gravels deposited during floods. Because the regular flooding of floodplains can deposit nutrients and water, floodplains frequently have high soil fertility; some important agricultural regions, such as the Mississippi river basin and the Nile, rely heavily on the flood plains. Agricultural regions as well as urban areas have developed near or on floodplains to take advantage of the rich soil and fresh water. However, the risk of flooding has led to increasing efforts to control flooding. Formation Most floodplains are formed by deposition on the inside of river meanders and by overbank flow. Whereve ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Alluvium
Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluvium is also sometimes called alluvial deposit. Alluvium is typically geologically young and is not consolidated into solid rock. Sediments deposited underwater, in seas, estuaries, lakes, or ponds, are not described as alluvium. Floodplain alluvium can be highly fertile, and supported some of the earliest human civilizations. Definitions The present consensus is that "alluvium" refers to loose sediments of all types deposited by running water in floodplains or in alluvial fans or related landforms. However, the meaning of the term has varied considerably since it was first defined in the French dictionary of Antoine Furetière, posthumously published in 1690. Drawing upon concepts from Roman law, Furetière defined ''alluvion'' (the F ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Okavango Delta
The Okavango Delta (or Okavango Grassland; formerly spelled "Okovango" or "Okovanggo") in Botswana is a swampy inland delta formed where the Okavango River reaches a tectonic trough at an altitude of 930–1,000 m in the central part of the endorheic basin of the Kalahari. All the water reaching the delta is ultimately evaporated and transpired and does not flow into any sea or ocean. Each year, about of water spreads over the area. Some flood waters drain into Lake Ngami. The area was once part of Lake Makgadikgadi, an ancient lake that had mostly dried up by the early Holocene. The Moremi Game Reserve, a national park, is on the eastern side of the delta. The delta was named as one of the Seven Natural Wonders of Africa, which were officially declared on 11 February 2013 in Arusha, Tanzania. On 22 June 2014, the Okavango Delta became the 1000th site to be officially inscribed on the UNESCO World Heritage List. Geography Floods The Okavango is produced by seasonal flood ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Xinjiang
Xinjiang, SASM/GNC: ''Xinjang''; zh, c=, p=Xīnjiāng; formerly romanized as Sinkiang (, ), officially the Xinjiang Uygur Autonomous Region (XUAR), is an autonomous region of the People's Republic of China (PRC), located in the northwest of the country at the crossroads of Central Asia and East Asia. Being the largest province-level division of China by area and the 8th-largest country subdivision in the world, Xinjiang spans over and has about 25 million inhabitants. Xinjiang borders the countries of Mongolia, Russia, Kazakhstan, Kyrgyzstan, Tajikistan, Afghanistan, Pakistan and India. The rugged Karakoram, Kunlun and Tian Shan mountain ranges occupy much of Xinjiang's borders, as well as its western and southern regions. The Aksai Chin and Trans-Karakoram Tract regions, both administered by China, are claimed by India. Xinjiang also borders the Tibet Autonomous Region and the provinces of Gansu and Qinghai. The most well-known route of the historic Silk Ro ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Taklamakan Desert
The Taklimakan or Taklamakan Desert (; zh, s=塔克拉玛干沙漠, p=Tǎkèlāmǎgān Shāmò, Xiao'erjing: , dng, Такәламаган Шамә; ug, تەكلىماكان قۇملۇقى, Täklimakan qumluqi; also spelled Taklimakan and Teklimakan) is a desert in Southwestern Xinjiang in Northwest China. It is bounded by the Kunlun Mountains to the south, the Pamir Mountains to the west, the Tian Shan range to the north, and the Gobi Desert to the east. Etymology While most researchers agree on being the Persian word for "place", etymology of ''Takla'' is less clear. The word may be a Uyghur borrowing of the Persian , "to leave alone/out/behind, relinquish, abandon" + ''makan''. Another plausible explanation suggests it is derived from Turki ''taqlar makan'', describing "the place of ruins". Chinese scholars Wang Guowei and Huang Wenbi linked the name to the Tocharians, a historical people of the Tarim Basin, making the meaning of "Taklamakan" similar to "Tocharistan". A ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
