57-476: Gravel ( / ˈ ɡ r æ v əl / ) is a loose aggregation of rock fragments . Gravel occurs naturally on Earth as a result of sedimentary and erosive geological processes; it is also produced in large quantities commercially as crushed stone . Gravel is classified by particle size range and includes size classes from granule - to boulder -sized fragments. In the Udden-Wentworth scale gravel
114-575: 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
171-558: A debris flow can come to a halt while still on moderately tilted ground. The flow then becomes consolidated under its own weight. Debris flow fans occur in all climates but are more common where the source rock is mudstone or matrix-rich saprolite rather than coarser, more permeable regolith . The abundance of fine-grained sediments encourages the initial hillslope failure and subsequent cohesive flow of debris. Saturation of clay-rich colluvium by locally intense thunderstorms initiates slope failure. The resulting debris flow travels down
228-419: A few tens of kilometers of their source outcrops. Gravel is deposited as gravel blankets or bars in stream channels; in alluvial fans ; in near-shore marine settings, where the gravel is supplied by streams or erosion along the coast; and in the deltas of swift-flowing streams. The upper Mississippi embayment contains extensive chert gravels thought to have their origin less than 100 miles (160 km) from
285-514: A hiatus of 70,000 to 80,000 years between the old and new fans, with evidence of tectonic tilting at 45,000 years ago and an end to fan deposition 20,000 years ago. Both the hiatus and the more recent end to fan deposition are thought to be connected to periods of enhanced southwest monsoon precipitation. Climate has also influenced fan formation in Death Valley , California , US, where dating of beds suggests that peaks of fan deposition during
342-643: A high hydraulic conductivity , sometimes reaching above 1 cm/s. Most gravel is derived from disintegration of bedrock as it weathers . Quartz is the most common mineral found in gravel, as it is hard, chemically inert, and lacks cleavage planes along which the rock easily splits. Most gravel particles consist of multiple mineral grains, since few rocks have mineral grains coarser than about 8 millimeters (0.31 in) in size. Exceptions include quartz veins , pegmatites , deep intrusions , and high-grade metamorphic rock . The rock fragments are rapidly rounded as they are transported by rivers , often within
399-664: A lag of gravel deposits that have the appearance of a network of braided streams. Where the flow is more continuous, as with spring snow melt, incised-channel flow in channels 1–4 meters (3–10 ft) high takes place in a network of braided streams. Such alluvial fans tend to have a shallower slope but can become enormous. The Kosi and other fans along the Himalaya mountain front in the Indo-Gangetic plain are examples of gigantic stream-flow-dominated alluvial fans, sometimes described as megafans . Here, continued movement on
456-443: A million people were rendered homeless, about a thousand lost their lives and thousands of hectares of crops were destroyed. Buried alluvial fans are sometimes found at the margins of petroleum basins. Debris flow fans make poor petroleum reservoirs, but fluvial fans are potentially significant reservoirs. Though fluvial fans are typically of poorer quality than reservoirs closer to the basin center, due to their complex structure,
513-502: A minimum, major structural flood control measures are required to mitigate risk, and in some cases, the only alternative is to restrict development on the fan surface. Such measures can be politically controversial, particularly since the hazard is not obvious to property owners. In the United States, areas at risk of alluvial fan flooding are marked as Zone AO on flood insurance rate maps . Alluvial fan flooding commonly takes
570-414: A minor component, making up less than 1% of all sedimentary rock. Alluvial fans likely contain the largest accumulations of gravel in the geologic record. These include conglomerates of the Triassic basins of eastern North America and the New Red Sandstone of south Devon . [REDACTED] Media related to Gravel at Wikimedia Commons Rock fragment A rock fragment , in sedimentary geology,
627-409: A part of the fan with a steeper gradient, where deposition resumes. As a result, normally only part of the fan is active at any particular time, and the bypassed areas may undergo soil formation or erosion. Alluvial fans can be dominated by debris flows ( debris flow fans ) or stream flow ( fluvial fans ). Which kind of fan is formed is controlled by climate, tectonics , and the type of bedrock in
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#1732772813098684-473: A streamflow-dominated alluvial fan shows nearly the same depositional facies as ordinary fluvial environments, so that identification of ancient alluvial fans must be based on radial paleomorphology in a piedmont setting. Alluvial fans 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 have also been found on other bodies of
741-419: A water content between 40 and 80 weight percent. Floods may transition to hyperconcentrated flows as they entrain sediments, while debris flows may become hyperconcentrated flows if they are diluted by water. Because flooding on alluvial fans carries large quantities of sediment, channels can rapidly become blocked, creating great uncertainty about flow paths that magnifies the dangers. Alluvial fan flooding in
798-483: Is a sand-sized particle or sand grain that is made up of multiple grains that are connected on the grain scale. These can include grains which are sand-sized themselves (a granitic rock fragment), or finer-grained materials ( shale fragments). This definition is used for QFR ternary diagrams , provenance analysis, and in the Folk classification scheme, mainly in sandstones. This article related to petrology
855-625: Is a stub . You can help Misplaced Pages by expanding it . Alluvial fans 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 1 square kilometer (0.4 sq mi) to almost 20,000 square kilometers (7,700 sq mi). Alluvial fans typically form where flow emerges from
912-525: Is an accumulation of sediments that fans out from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment . This accumulation is shaped like a section of a shallow cone , with its apex at the source of sediments. Alluvial fans vary greatly in size, from only a few meters across at the base to as much as 150 kilometers across, with a slope of 1.5 to 25 degrees. Some giant alluvial fans have areas of almost 20,000 square kilometres (7,700 sq mi). The slope measured from
969-414: Is an important commercial product, with a number of applications. Almost half of all gravel production is used as aggregate for concrete . Much of the rest is used for road construction, either in the road base or as the road surface (with or without asphalt or other binders.) Naturally occurring porous gravel deposits have a high hydraulic conductivity , making them important aquifers . Colloquially,
1026-438: Is approximately in equilibrium with erosion, so the river annually carries some 100 million cubic meters (3.5 × 10 ^ cu ft) of sediment as it exits the mountains. Deposition of this magnitude over millions of years is more than sufficient to account for the megafan. In North America , streams flowing into California's Central Valley have deposited smaller but still extensive alluvial fans, such as that of
1083-443: Is categorized into granular gravel (2–4 mm or 0.079–0.157 in) and pebble gravel (4–64 mm or 0.2–2.5 in). ISO 14688 grades gravels as fine, medium, and coarse, with ranges 2–6.3 mm (0.079–0.248 in) for fine and 20–63 mm (0.79–2.48 in) for coarse. One cubic metre of gravel typically weighs about 1,800 kg (4,000 lb), or one cubic yard weighs about 3,000 lb (1,400 kg). Gravel
1140-432: Is described as fanglomerate . Stream flow deposits tend to be sheetlike, better sorted than debris flow deposits, and sometimes show well-developed sedimentary structures such as cross-bedding. These are more prevalent in the medial and distal fan. In the distal fan, where channels are very shallow and braided, stream flow deposits consist of sandy interbeds with planar and trough slanted stratification. The medial fan of
1197-568: Is located adjacent to low-relief terrain. In Nepal , the Koshi River has built a megafan covering some 15,000 km (5,800 sq mi) below its exit from Himalayan foothills onto the nearly level plains where the river traverses into India before joining the Ganges . Along the upper Koshi tributaries, tectonic forces elevate the Himalayas several millimeters annually. Uplift
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#17327728130981254-438: Is most likely composed of round grains of water ice or solid organic compounds about two centimeters in diameter. Alluvial fans are the most important groundwater reservoirs in many regions. Many urban, industrial, and agricultural areas are located on alluvial fans, including the conurbations of Los Angeles, California ; Salt Lake City, Utah ; and Denver, Colorado , in the western United States, and in many other parts of
1311-535: Is sometimes distinguished from rubble, which is loose rock particles in the same size range but angular in shape. The Udden-Wentworth scale , widely used by geologists in the US, defines granular gravel as particles with a size from 2 to 4 mm (0.079 to 0.157 in) and pebble gravel as particles with a size from 4 to 64 mm (0.16 to 2.52 in). This corresponds to all particles with sizes between coarse sand and cobbles . The U.S. Department of Agriculture and
1368-404: Is widely and plentifully distributed, mostly as river deposits, river flood plains, and glacial deposits, so that environmental considerations and quality dictate whether alternatives, such as crushed stone , are more economical. Crushed stone is already displacing natural gravel in the eastern United States, and recycled gravel is also becoming increasingly important. The word gravel comes from
1425-625: The Apennine Mountains of Italy have resulted in repeated loss of life. A flood on 1 October 1581 at Piedimonte Matese resulted in the loss of 400 lives. Loss of life from alluvial fan floods continued into the 19th century, and the hazard of alluvial fan flooding remains a concern in Italy. On January 1, 1934, record rainfall in a recently burned area of the San Gabriel Mountains , California , caused severe flooding of
1482-737: The Kings River flowing out of the Sierra Nevada . Like the Himalayan megafans, these are streamflow-dominated fans. Alluvial fans are also found on Mars . Unlike alluvial fans on Earth, those on Mars are rarely associated with tectonic processes, but are much more common on crater rims. The crater rim alluvial fans appear to have been deposited by sheetflow rather than debris flows. Three alluvial fans have been found in Saheki Crater . These fans confirmed past fluvial flow on
1539-518: The Main Boundary Thrust over the last ten million years has focused the drainage of 750 kilometres (470 miles) of mountain frontage into just three enormous fans. Alluvial fans are common in the geologic record, but may have been particularly important before the evolution of land plants in the mid-Paleozoic. They are characteristic of fault-bounded basins and can be 5,000 meters (16,000 ft) or thicker due to tectonic subsidence of
1596-704: The Old French gravele or gravelle . Different varieties of gravel are distinguished by their composition, origin, and use cases. Types of gravel include: In locales where gravelly soil is predominant, plant life is generally more sparse. This is due to the inferior ability of gravels to retain moisture, as well as the corresponding paucity of mineral nutrients, since finer soils that contain such minerals are present in smaller amounts. Sediments containing over 30% gravel that become lithified into solid rock are termed conglomerate . Conglomerates are widely distributed in sedimentary rock of all ages, but usually as
1653-824: The Soil Science Society of America define gravel as particles from 2 to 80 mm (0.079 to 3.150 in) in size, while the German scale (Atterburg) defines gravel as particles from 2 to 200 mm (0.079 to 7.874 in) in size. The U.S. Army Corps of Engineers defines gravel as particles under 3 in (76 mm) in size that are retained by a number 4 mesh, which has a mesh spacing of 4.76 mm (0.187 in). ISO 14688 for soil engineering grades gravels as fine, medium, and coarse with ranges 2 mm to 6.3 mm to 20 mm to 63 mm. The bulk density of gravel varies from 1,460 to 1,920 kg/m (2,460 to 3,240 lb/cu yd). Natural gravel has
1710-678: The Solar System . Alluvial fans are built in response to erosion induced by tectonic uplift . The upwards coarsening of the beds making up the fan reflects cycles of erosion in the highlands that feed sediments to the fan. However, climate and changes in base level may be as important as tectonic uplift. For example, alluvial fans in the Himalayas show older fans entrenched and overlain by younger fans. The younger fans, in turn, are cut by deep incised valleys showing two terrace levels. Dating via optically stimulated luminescence suggests
1767-698: The Valles Marineris canyon system. These provide evidence of the existence and nature of faulting in this region of Mars. Alluvial fans have been observed by the Cassini-Huygens mission on Titan using the Cassini orbiter's synthetic aperture radar instrument. These fans are more common in the drier mid-latitudes at the end of methane/ethane rivers where it is thought that frequent wetting and drying occur due to precipitation, much like arid fans on Earth. Radar imaging suggests that fan material
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1824-488: The U.S., with a total value of about $ 12.6 billion. Some 960 million tons of construction sand and gravel were produced. This greatly exceeds production of industrial sand and gravel (68 million tons), which is mostly sand rather than gravel. It is estimated that almost half of construction sand and gravel is used as aggregate for concrete . Other important uses include in road construction, as road base or in blacktop ; as construction fill; and in myriad minor uses. Gravel
1881-604: The alluvial fan on which the towns of Montrose and Glendale were built. The floods caused significant loss of life and property. The Koshi River in India has built up a megafan where it exits the Himalayas onto the Ganges plain . The river has a history of frequently and capriciously changing its course, so that it has been called the Sorrow of Bihar for contributing disproportionately to India's death tolls in flooding. These exceed those of all countries except Bangladesh . Over
1938-409: The apex is generally concave, with the steepest slope near the apex (the proximal fan or fanhead ) and becoming less steep further out (the medial fan or midfan ) and shallowing at the edges of the fan (the distal fan or outer fan ). Sieve deposits , which are lobes of coarse gravel, may be present on the proximal fan. The sediments in an alluvial fan are usually coarse and poorly sorted, with
1995-431: The area feeding the flow onto the fan. Debris flow fans receive most of their sediments in the form of debris flows. Debris flows are slurry-like mixtures of water and particles of all sizes, from clay to boulders, that resemble wet concrete . They are characterized by having a yield strength, meaning that they are highly viscous at low flow velocities but become less viscous as the flow velocity increases. This means that
2052-768: The basin and uplift of the mountain front. Most are red from hematite produced by diagenetic alteration of iron-rich minerals in a shallow, oxidizing environment. Examples of paleofans include the Triassic basins of eastern North America and the New Red Sandstone of south Devon, the Devonian Hornelen Basin of Norway, and the Devonian- Carboniferous in the Gaspé Peninsula of Canada. Such fan deposit likely contain
2109-478: The bottom. Multiple braided streams are usually present and active during water flows. Phreatophytes (plants with long tap roots capable of reaching a deep water table ) are sometimes found in sinuous lines radiating from arid climate fan toes. These fan-toe phreatophyte strips trace buried channels of coarse sediments from the fan that have interfingered with impermeable playa sediments. Alluvial fans also develop in wetter climates when high-relief terrain
2166-423: The coarsest sediments found on the proximal fan. When there is enough space in the alluvial plain for all of the sediment deposits to fan out without contacting other valley walls or rivers, an unconfined alluvial fan develops. Unconfined alluvial fans allow sediments to naturally fan out, and the shape of the fan is not influenced by other topological features. When the alluvial plain is more restricted, so that
2223-420: The fan comes into contact with topographic barriers, a confined fan is formed. Wave or channel erosion of the edge of the fan ( lateral erosion ) sometimes produces a "toe-trimmed" fan, in which the edge of the fan is marked by a small escarpment. Toe-trimmed fans may record climate changes or tectonic processes, and the process of lateral erosion may enhance the aquifer or petroleum reservoir potential of
2280-400: The fan. Toe-trimmed fans on the planet Mars provide evidence of past river systems. When numerous rivers and streams exit a mountain front onto a plain, the fans can combine to form a continuous apron. This is referred to as a bajada or piedmont alluvial plain . Alluvial fans usually form where a confined feeder channel exits a mountain front or a glacier margin. As the flow exits
2337-745: The fan: Finer sediments are deposited at the edge of the fan, but as the fan continues to grow, increasingly coarse sediments are deposited on top of the earlier, less coarse sediments. However, a few fans show normal grading indicating inactivity or even fan retreat, so that increasingly fine sediments are deposited on earlier coarser sediments. Normal or reverse grading sequences can be hundreds to thousands of meters in thickness. Depositional facies that have been reported for alluvial fans include debris flows, sheet floods and upper regime stream floods, sieve deposits, and braided stream flows, each leaving their own characteristic sediment deposits that can be identified by geologists. Debris flow deposits are common in
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2394-516: The feeder channel and onto the surface of the fan. Debris flow fans have a network of mostly inactive distributary channels in the upper fan that gives way to mid- to lower-level lobes. The channels tend to be filled by subsequent cohesive debris flows. Usually only one lobe is active at a time, and inactive lobes may develop desert varnish or develop a soil profile from eolian dust deposition, on time scales of 1,000 to 10,000 years. Because of their high viscosity, debris flows tend to be confined to
2451-554: The feeder channel onto the fan surface, it is able to spread out into wide, shallow channels or to infiltrate the surface. This reduces the carrying power of the flow and results in deposition of sediments. Flow in the proximal fan, where the slope is steepest, is usually confined to a single channel (a fanhead trench ), which may be up to 30 meters (100 ft) deep. This channel is subject to blockage by accumulated sediments or debris flows , which causes flow to periodically break out of its old channel ( nodal avulsion ) and shift to
2508-400: The feeder channel. This results in sheetfloods on the alluvial fan, where sediment-laden water leaves its channel confines and spreads across the fan surface. These may include hyperconcentrated flows containing 20% to 45% sediments, which are intermediate between sheetfloods having 20% or less of sediments and debris flows with more than 45% sediments. As the flood recedes, it often leaves
2565-530: The form of short (several hours) but energetic flash floods that occur with little or no warning. They typically result from heavy and prolonged rainfall, and are characterized by high velocities and capacity for sediment transport. Flows cover the range from floods through hyperconcentrated flows to debris flows, depending on the volume of sediments in the flow. Debris flows resemble freshly poured concrete, consisting mostly of coarse debris. Hyperconcentrated flows are intermediate between floods and debris flows, with
2622-402: The form of stream flow rather than debris flows. They are less sharply distinguished from ordinary fluvial deposits than are debris flow fans. Fluvial fans occur where there is perennial, seasonal, or ephemeral stream flow that feeds a system of distributary channels on the fan. In arid or semiarid climates, deposition is dominated by infrequent but intense rainfall that produces flash floods in
2679-617: The largest accumulations of gravel in the geologic record. Alluvial fans have also been found on Mars and Titan , showing that fluvial processes have occurred on other worlds. Some of the largest alluvial fans are found along the Himalaya mountain front on the Indo-Gangetic plain . A shift of the feeder channel (a nodal avulsion ) can lead to catastrophic flooding, as occurred on the Kosi River fan in 2008. An alluvial fan
2736-450: The largest accumulations of gravel in the geologic record. Several kinds of sediment deposits ( facies ) are found in alluvial fans. Alluvial fans are characterized by coarse sedimentation, though the sediments making up the fan become less coarse further from the apex. Gravels show well-developed imbrication with the pebbles dipping towards the apex. Fan deposits typically show well-developed reverse grading caused by outbuilding of
2793-411: The last 25,000 years occurred during times of rapid climate change, both from wet to dry and from dry to wet. Alluvial fans are often found in desert areas, which are subjected to periodic flash floods from nearby thunderstorms in local hills. The typical watercourse in an arid climate has a large, funnel-shaped basin at the top, leading to a narrow defile , which opens out into an alluvial fan at
2850-471: The last few hundred years, the river had generally shifted westward across its fan, and by 2008, the main river channel was located on the extreme western part of the megafan. In August 2008 , high monsoon flows breached the embankment of the Koshi River . This diverted most of the river into an unprotected ancient channel and flooded the central part of the megafan. This was an area with a high population density that had been stable for over 200 years. Over
2907-424: The likely flood path, the likelihood of abrupt deposition and erosion of sediments carried by the flood from upstream sources, and a combination of the availability of sediments and of the slope and topography of the fan that creates extraordinary hazards. These hazards cannot reliably be mitigated by elevation on fill (raising existing buildings up to a meter (three feet) and building new foundations beneath them ). At
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#17327728130982964-485: The periphery of the embayment. It has been suggested that wind-formed ( aeolian ) gravel "megaripples" in Argentina have counterparts on the planet Mars . Gravel is a major basic raw material in construction . Sand is not usually distinguished from gravel in official statistics, but crushed stone is treated as a separate category. In 2020, sand and gravel together made up 23% of all industrial mineral production in
3021-722: The planet and further supported the theory that liquid water was once present in some form on the Martian surface. In addition, observations of fans in Gale crater made by satellites from orbit have now been confirmed by the discovery of fluvial sediments by the Curiosity rover . Alluvial fans in Holden crater have toe-trimmed profiles attributed to fluvial erosion. The few alluvial fans associated with tectonic processes include those at Coprates Chasma and Juventae Chasma, which are part of
3078-465: The proximal and medial fan even in a debris-flow-dominated alluvial fan, and streamfloods dominate the distal fan. However, some debris-flow-dominated fans in arid climates consist almost entirely of debris flows and lag gravels from eolian winnowing of debris flows, with no evidence of sheetflood or sieve deposits. Debris-flow-dominated fans tend to be steep and poorly vegetated. Fluvial fans (streamflow-dominated fans) receive most of their sediments in
3135-473: The proximal and medial fan. These deposits lack sedimentary structure, other than occasional reverse-graded bedding towards the base, and they are poorly sorted. The proximal fan may also include gravel lobes that have been interpreted as sieve deposits, where runoff rapidly infiltrates and leaves behind only the coarse material. However, the gravel lobes have also been interpreted as debris flow deposits. Conglomerate originating as debris flows on alluvial fans
3192-528: The term gravel is often used to describe a mixture of different size pieces of stone mixed with sand and possibly some clay. The American construction industry distinguishes between gravel (a natural material) and crushed stone (produced artificially by mechanical crushing of rock.) The technical definition of gravel varies by region and by area of application. Many geologists define gravel simply as loose rounded rock particles over 2 mm (0.079 in) in diameter, without specifying an upper size limit. Gravel
3249-574: The world. However, flooding on alluvial fans poses unique problems for disaster prevention and preparation. The beds of coarse sediments associated with alluvial fans form aquifers that are the most important groundwater reservoirs in many regions. These include both arid regions, such as Egypt or Iraq, and humid regions, such as central Europe or Taiwan. Alluvial fans are subject to infrequent but often very damaging flooding, whose unusual characteristics distinguish alluvial fan floods from ordinary riverbank flooding. These include great uncertainty in
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