Mass wasting

Mass wasting, also known as slope movement or mass movement, is the geomorphic process by which soil, sand, regolith, and rock move downslope typically as a solid, continuous or discontinuous mass, largely under the force of gravity, but frequently with characteristics of a flow as in debris flows and mudflows.[1] Types of mass wasting include creep, slides, flows, topples, and falls, each with its own characteristic features, and taking place over timescales from seconds to hundreds of years. Mass wasting occurs on both terrestrial and submarine slopes, and has been observed on Earth, Mars, Venus, and Jupiter's moon Io.

When the gravitational force acting on a slope exceeds its resisting force, slope failure (mass wasting) occurs. The slope material's strength and cohesion and the amount of internal friction between material help maintain the slope's stability and are known collectively as the slope's shear strength. The steepest angle that a cohesionless slope can maintain without losing its stability is known as its angle of repose. When a slope made of loose material possesses this angle, its shear strength perfectly counterbalances the force of gravity acting upon it.

Mass wasting may occur at a very slow rate, particularly in areas that are very dry or those areas that receive sufficient rainfall such that vegetation has stabilized the surface. It may also occur at very high speed, such as in rockslides or landslides, with disastrous consequences, both immediate and delayed, e.g., resulting from the formation of landslide dams.

Factors that change the potential of mass wasting include: change in slope angle, weakening of material by weathering, increased water content; changes in vegetation cover, and overloading.

Volcano flanks can become over-steep resulting in instability and mass wasting. This is now a recognised part of the growth of all active volcanoes. It is seen on submarine as well as surface volcanoes: Loihi in the Hawaiian volcanic chain and Kick 'em Jenny in the Caribbean volcanic arc are two submarine volcanoes that are known to undergo mass wasting. The failure of the northern flank of Mount St Helens in 1980 showed how rapidly volcanic flanks can deform and fail.

TalusConesIsfjorden
Talus cones produced by mass wasting, north shore of Isfjord, Svalbard, Norway.
Mass Waste Palo Duro 2002
Mass wasting at Palo Duro Canyon, West Texas (2002)

Role of water

Water can increase or decrease the stability of a slope depending on the amount present. Small amounts of water can strengthen soils because the surface tension of water increases soil cohesion. This allows the soil to resist erosion better than if it were dry. If too much water is present the water may act to increase the pore pressure, reducing friction, and accelerating the erosion process and resulting in different types of mass wasting (i.e. mudflows, landslides, etc.). A good example of this is to think of a sand castle. Water must be mixed with sand in order for the castle to keep its shape. If too much water is added the sand washes away, if not enough water is added the sand falls and cannot keep its shape. Water also increases the mass of the soil, this is important because an increase in mass means that there will be an increase in velocity if mass wasting is triggered. Saturated water, however, eases the process of mass wasting in that the rock and soil debris are easily washed down-slope.

Types

Based on how the soil, regolith or rock moves downslope as a whole, mass movements can be broadly classified as creeps and landslides.

Creep

Soil creep is a slow and long term mass movement. The combination of small movements of soil or rock in different directions over time are directed by gravity gradually downslope. The steeper the slope, the faster the creep. The creep makes trees and shrubs curve to maintain their perpendicularity, and they can trigger landslides if they lose their root footing. The surface soil can migrate under the influence of cycles of freezing and thawing, or hot and cold temperatures, inching its way towards the bottom of the slope forming terracettes. Landslides are often preceded by soil creep accompanied with soil sloughing — loose soil that falls and accumulates at the base of the steepest creep sections. [2]

Landslide

A landslide, also called a landslip, is a slow or rapid movement of a large mass of earth and rocks down a hill or a mountainside. Little or no flowage of the materials occurs on a given slope until heavy rain and resultant lubrication by the same rainwater facilitate the movement of the materials, causing a landslide to occur.

In particular, if the main feature of the movement is a slide along a planar or curved surface, the landslide is termed slump, earth slide, debris slide or rock slide, depending on the prevailing material.

Movement of soil and regolith that more resembles fluid behavior is called a flow. These include avalanches, mudflows, debris flows, earth flow, lahars and sturzstroms. Water, air and ice are often involved in enabling fluid-like motion of the material.

A fall, including rockfall and debris fall, occurs where regolith cascades down a slope, but is not of sufficient volume or viscosity to behave as a flow. Falls are promoted in rocks which are characterized by the presence of vertical cracks. Falls can also result from undercutting by running water as well as by waves. They usually occur at very steep slopes such as a cliff face. The rock material may be loosened by earthquakes, rain, plant-root wedging, and expanding ice, among other things. The accumulation of rock material that has fallen and resides at the base of the structure is known as talus.

Triggers

Soil and regolith remain on a hillslope only while the gravitational forces are unable to overcome the frictional forces keeping the material in place (see slope stability). Some factors that reduce the frictional resistance relative to the downslope forces, and thus can trigger slope movement, can include:

  • earthquakes
  • increased overburden from structures
  • increased soil moisture
  • reduction of roots holding the soil to bedrock
  • undercutting of the slope by excavation or erosion
  • weathering by frost heave or chemical dissolution
  • bioturbation

Mitigation

See also

References

  1. ^ Britannica
  2. ^ "Indicators of potentially unstable slopes" (PDF). Sound Native Plants. Retrieved 2019-01-22.

Further reading

  • Monroe, Wicander (2005). The Changing Earth: Exploring Geology and Evolution. Thomson Brooks/Cole. ISBN 0-495-01020-0.
  • Selby, M.J. (1993). Hillslope Materials and Processes, 2e. Oxford University Press. ISBN 0-19-874183-9.
  • Fundamentals Of Physical Geography (Class 11th NCERT). ISBN 81-7450-518-0

External links

Anseris Mons

Anseris Mons is an isolated massif (mountain) in the southern highlands of Mars, located at the northeastern edge of Hellas Planitia at longitude 86.65°E and latitude 29.81°S. The mountain is 58 km (36 mi) in diameter and rises to an elevation of approximately 4,200 m (13,780 ft) above datum (martian "sea" level) or about 6,200 m (20,300 ft) above the surrounding plains. The mountain lies in the southeastern quarter of the Iapygia quadrangle (MC-21), straddling the boundary with the adjoining Hellas quadrangle (MC-28) to the south.

Anseris Mons is named from Anseris Fons, a telescopic albedo feature mapped by Greek astronomer E. M. Antoniadi in 1930. The name was approved by the International Astronomical Union (IAU) in 1991.Anseris Mons is not a volcano. Geologically, the massif is thought to be the eroded remnant of an ancient crustal block uplifted from depths of several kilometers in the formation of the Hellas impact basin during the period of heavy bombardment. Anseris Mons is the type area for a large set of rugged mountain blocks (>25 km across) that occur in a relatively continuous band 200 to 500 km wide around the western, northeastern, eastern, and southeastern rim of the Hellas basin. Many of the blocks, particularly along the western rim, are concentric with the basin and bounded by faults.Rocks making up Anseris Mons and other massifs around Hellas are mapped as Noachian in age. However, work by Herbert Frey at NASA’s Goddard Spaceflight Center using Mars Orbital Laser Altimeter (MOLA) data indicates that the southern highlands of Mars contain numerous buried impact basins that are older than the visible Noachian-aged surfaces and which pre-date the Hellas impact. He suggests that the Hellas impact should mark the beginning of the Noachian period (base of the Noachian system). If Frey is correct, then Anseris Mons bedrock is actually pre-Noachian in age, perhaps dating back to over 4.1 billion years ago.The Anseris Mons massif has undergone a significant amount of erosion since it was uplifted. The flanks of the mountain have huge triangular re-entrants and associated spurs, which give the massif a broad, pyramidal shape. The re-entrants were likely produced through a variety of mass-wasting and periglacial/glacial processes. A large cirque-like re-entrant with channelized debris aprons or fans is present on the south side of the mountain.

Arête

An arête is a narrow ridge of rock which separates two valleys. It is typically formed when two glaciers erode parallel U-shaped valleys. Arêtes can also form when two glacial cirques erode headwards towards one another, although frequently this results in a saddle-shaped pass, called a col. The edge is then sharpened by freeze-thaw weathering, and the slope on either side of the arete steepened through mass wasting events and the erosion of exposed, unstable rock. The word ‘arête’ is actually French for edge or ridge; similar features in the Alps are described with the German equivalent term Grat.

Where three or more cirques meet, a pyramidal peak is created.

Cut bank

A cut bank, also known as a river cliff or river-cut cliff, is the outside bank of a water channel (stream), which is continually undergoing erosion. Cut banks are found in abundance along mature or meandering streams, they are located on the outside of a stream bend, known as a meander, opposite the slip-off slope on the inside of the bend. They are shaped much like a small cliff, and are formed by the erosion of soil as the stream collides with the river bank. As opposed to a point bar, which is an area of deposition, a cut bank is an area of erosion.

Typically, cut banks are nearly vertical and often expose the roots of nearby plant life. Often, particularly during periods of high rainfall and higher-than average water levels, trees and poorly placed buildings can fall into the stream due to mass wasting events. Given enough time, the combination of erosion along cut banks and deposition along point bars can lead to the formation of an oxbow lake.

Not only are cut banks steep and unstable, they are also the area of a stream where the water is flowing the fastest at a higher pressure and often deeper, making them rather dangerous. In geology, this is known as an area of high-energy.

Material eroded here is deposited downstream in point bars.

Danube Planum

Danube Planum is a rifted mesa on the surface of Jupiter's moon Io. It is located on Io's trailing hemisphere at 22.73°S 257.44°W / -22.73; -257.44. Danube Planum is 244.22 kilometers across and 5.5 km tall. The mountain is bisected by a 15-to-25-kilometer-wide, northeast-southwest-trending canyon, splitting the mountain into two main east and west mountains, with several additional blocks at the southern end of the fracture. The outer margin of the plateau is marked by 2.6-to-3.4-km-tall scarps. Mass wasting in the form of landslide deposits are visible along the base of the western half of Danube Planum. Two volcanic depressions, known as paterae, lie at northern and southern ends of mountain. The volcano at the northern end, Pele, is one of the most active volcanoes on Io. One of the faults that helped form Danube Planum may also act as a conduit for magma to rise to the surface at Pele.In 1985, the International Astronomical Union officially named the mountain after the Danube River, one of a number of places the mythological Io passed during her wanderings.

Denudation

In geology, denudation involves the processes that cause the wearing away of the Earth's surface by moving water, by ice, by wind and by waves, leading to a reduction in elevation and in relief of landforms and of landscapes. Endogenous processes such as volcanoes, earthquakes, and plate tectonics uplift and expose continental crust to the exogenous processes of weathering, of erosion, and of mass wasting.

Diamictite

Diamictite ( ; from Ancient Greek δια (dia-): through and µεικτός (meiktós): mixed) is a type of lithified sedimentary rock that consists of nonsorted to poorly sorted terrigenous sediment containing particles that range in size from clay to boulders, suspended in a matrix of mudstone or sandstone. The term was coined by Richard Foster Flint and others as a purely descriptive term, devoid of any reference to a particular origin. Some geologists restrict the usage to nonsorted or poorly sorted conglomerate or breccia that consists of sparse, terrigenous gravel suspended in either a mud or sand matrix.Unlithified diamictite is referred to as diamicton.

The term diamictite is often applied to nonsorted or poorly sorted, lithified glacial deposits such as glacial tillite, and diamictites are often mistakenly interpreted as having an essentially glacial origin (see Snowball Earth). The most common origin for diamictites, however, is deposition by submarine mass flows like turbidites and olistostromes in tectonically active areas, and they can be produced in a wide range of other geological conditions. Possible origins include:

glacial origin

meltwater flow deposition

unsorted moraine glacial till

basal melt-out

ice rafted sediments deposited by melting icebergs or disintegrating ice sheets (dropstones)

volcanic origin

lahars

lahar mass flows entering the ocean

marine origin

debris flow

turbiditic olistostromes

mixing of sediments by submarine landslides

tectonic origin

fault gouge

erosional origin

regolith, in the form of a debris flow

other mass wasting events

extraterrestrial origin

impact breccia

Earthflow

An earthflow (earth flow) is a downslope viscous flow of fine-grained materials that have been saturated with water and moves under the pull of gravity. It is an intermediate type of mass wasting that is between downhill creep and mudflow. The types of materials that are susceptible to earthflows are clay, fine sand and silt, and fine-grained pyroclastic material.When the ground materials become saturated with enough water, they will start flowing (soil liquefaction). Its speed can range from being barely noticeable to rapid movement. The velocity of the flow is dictated by water content: the higher the water content is, the higher the velocity will be. Because of the dependency on water content for the velocity of the flow, it can take minutes or years for the materials to move down the slope.

Erkenek Tunnel

Erkenek Tunnel (Turkish: Erkenek Tüneli), is a road tunnel in Malatya province, eastern Turkey opened in 2017 connecting Eastern Anatolia region with the Mediterranean Region.

Erkenek Tunnel is situated on the highway between Doğanşehir in Malatya Province and Gölbaşı in Adıyaman Province, west of Erkenek village. It connects Eastern Anatolia with the Mediterranean Region bypassing the Erkenek Pass, which made the heavy truck traffic difficult during the winter season. It is a twin-tube tunnel with a length of 1,816 m (5,958 ft).Construction of the tunnel began in 2011.As insufficient soil survey and boring works caused mass wasting and subsidence, it lasted six years due to additional ground reinforcement works. The cost of construction is 253 million (approx. US$ 72 million. The opening of the tunnel took place in presence of Minister of Transport, Maritime and Communication Ahmet Arslan, Minister of Customs and Trade Bülent Tüfenkci and some other high-ranked local officials on 28 May 2017.

Erosion and tectonics

The interaction between erosion and tectonics has been a topic of debate since the early 1990s. While the tectonic effects on surface processes such as erosion have long been recognized (for example, river formation as a result of tectonic uplift), the opposite (erosional effects on tectonic activity) has only recently been addressed. The primary questions surrounding this topic are what types of interactions exist between erosion and tectonics and what are the implications of these interactions. While this is still a matter of debate, one thing is clear, the Earth's landscape is a product of two factors: tectonics, which can create topography and maintain relief through surface and rock uplift, and climate, which mediates the erosional processes that wear away upland areas over time. The interaction of these processes can form, modify, or destroy geomorphic features on the Earth’s surface.

Mudflow

A mudflow or mud flow is a form of mass wasting involving "very rapid to extremely rapid surging flow" of debris that has become partially or fully liquified by the addition of significant amounts of water to the source material.Mudflows contain a significant proportion of clay, which makes them more fluid than debris flows; thus, they are able to travel farther and across lower slope angles. Both types are generally mixtures of various kinds of materials of different sizes, which are typically sorted by size upon deposition.Mudflows are often called mudslides, a term applied indiscriminately by the mass media to a variety of mass wasting events. Mudflows often start as slides, becoming flows as water is entrained along the flow path; such events are often called flow slides.Other types of mudflows include lahars (involving fine-grained pyroclastic deposits on the flanks of volcanoes) and jökulhlaups (outbursts from under glaciers or icecaps).A statutory definition of "flood-related mudslide" appears in the United States' National Flood Insurance Act of 1968, as amended, codified at 42 USC Sections 4001 and following.

Nivation

Nivation refers to the geomorphic processes associated with snow patches. The primary processes are mass wasting and the freeze and thaw cycle, in which fallen snow gets compacted into firn or névé. The importance of the processes covered by the term nivation with regard to the development of periglacial landscapes has been questioned by scholars, and the use of the term is discouraged.Nivation has come to include various subprocesses related to snow patches which may be immobile or semi-permanent. These sub-processes include erosion (if any) or initiation of erosion, weathering, and meltwater flow from beneath the snow patch.Weathered particles are moved downslope by creep, solifluction and rill wash. Over time, this leads to the formation of nivation hollows which, when enlarged, can be the beginnings of a cirque.

Rockslide

A rockslide is a type of landslide caused by rock failure in which part of the bedding plane of failure passes through compacted rock and material collapses en masse and not in individual blocks. While a landslide occurs when loose dirt or sediment falls down a slope, a rockslide occurs only when solid rocks are transported down slope. The rocks tumble downhill, loosening other rocks on their way and smashing everything in their path. Fast-flowing rock slides or debris slides behave similarly to snow avalanches, and are often referred to as rock avalanches or debris avalanches.

SVSlope

SVSLOPE is a slope stability analysis program developed by SoilVision Systems Ltd.. The software is designed to analyze slopes using both the classic "method of slices" as well as newer stress-based methods. The program is used in the field of civil engineering to analyze levees, earth dams, natural slopes, tailings dams, heap leach piles, waste rock piles, and anywhere there is concern for mass wasting. SVSLOPE finds the factor of safety or the probability of failure for the slope. The software makes use of advanced searching methods to determine the critical failure surface.

Scalloped margin dome

A scalloped margin dome is a type of volcanic dome, found on Venus, that has experienced collapse and mass wasting such as landslides on its perimeter. The margins of these domes have arcuate headscarps or 'scallops' separated by ridges that are a consequence of adjoining scallops. Sometimes debris or slumping can be found at the bottom of these scarps or scattered many tens of kilometers away. Many examples show no debris at all. The center of these domes is often, but not always, a depression. There is another theory that the radial ridges of scalloped margin domes are volcanic dikes.

During the first month of data from the Magellan spacecraft, the first of these features was found to the northeast of Alpha Regio, on Venus. It was one of the largest of these domes and therefore stood out. The strange feature was originally dubbed by the Magellan Project Science Team The Tick, because the many radiating ridges resembled the legs of a tick. Its concavity was likely confused as domelike as a tick's body, instead of the actuality which is that it is a bowl-shaped depression. Through the first year of Magellan image data The Tick was thought to be a unique feature until an aide to the science team catalogued inconspicuous similar features all over Venus. This resulted in referring to the features as 'ticks' which was later changed to 'scalloped margin domes'.

Slump (geology)

A slump is a form of mass wasting that occurs when a coherent mass of loosely consolidated materials or rock layers moves a short distance down a slope. Movement is characterized by sliding along a concave-upward or planar surface. Causes of slumping include earthquake shocks, thorough wetting, freezing and thawing, undercutting, and loading of a slope.

Translational slumps occur when a detached landmass moves along a planar surface. Common planar surfaces of failure include joints or bedding planes, especially where a permeable layer overrides an impermeable surface. Block slumps are a type of translational slump in which one or more related block units move downslope as a relatively coherent mass.

Rotational slumps occur when a slump block, composed of sediment or rock, slides along a concave-upward slip surface with rotation about an axis parallel to the slope. Rotational movement causes the original surface of the block to become less steep, and the top of the slump is rotated backward. This results in internal deformation of the moving mass consisting chiefly of overturned folds called sheath folds.

Slumps have several characteristic features. The cut which forms as the landmass breaks away from the slope is called the scarp and is often cliff-like and concave. In rotational slumps, the main slump block often breaks into a series of secondary slumps and associated scarps to form stairstep pattern of displaced blocks. The upper surface of the blocks are rotated backwards, forming depressions which may accumulate water to create ponds or swampy areas. The surface of the detached mass often remains relatively undisturbed, especially at the top. However, hummocky ridges may form near the toe of the slump. Addition of water and loss of sediment cohesion at the toe may transform slumping material into an earthflow. Transverse cracks at the head scarp drain water, possibly killing vegetation. Transverse ridges, transverse cracks and radial cracks form in displaced material on the foot of the slump.

Slumps frequently form due to removal of a slope base, either from natural or manmade processes. Stream or wave erosion, as well as road construction are common instigators for slumping. It is the removal of the slope's physical support which provokes this mass wasting event. Thorough wetting is a common cause, which explains why slumping is often associated with heavy rainfall, storm events and earthflows. Rain provides lubrication for the material to slide, and increases the self-mass of the material. Both factors increase the rate of slumping. Earthquakes also trigger massive slumps, such as the fatal slumps of Turnagain Heights Subdivision in Anchorage, Alaska. This particular slump was initiated by a magnitude 8.4 earthquake that resulted in liquefaction of the soil. Around 75 houses were destroyed by the Turnagain Slump. Power lines, fences, roads, houses, and other manmade structures may be damaged if in the path of a slump.

The speed of slump varies widely, ranging from meters per second, to meters per year. Sudden slumps usually occur after earthquakes or heavy continuing rains, and can stabilize within a few hours. Most slumps develop over comparatively longer periods, taking months or years to reach stability. An example of a slow-moving slump is the Swift Creek Landslide, a deep-seated rotational slump located on Sumas Mountain, Washington.

Slumps may also occur underwater along the margins of continents and islands, resulting from tidal action or a large seismic event. These submarine slumps can generate disastrous tsunamis. The underwater terrain which encompasses the Hawaiian Islands gains its unusual hummocky topography from the many slumps that have taken place for millions of years.

One of the largest known slumps occurred on the south-eastern edge of the Agulhas Bank south of Africa in the Pliocene or more recently. This so-called Agulhas Slump is 750 km (470 mi) long, 106 km (66 mi) wide, and has a volume of 20,000 km3 (4,800 cu mi). It is a composite slump with proximal and distal allochthonous sediment masses separated by a large glide plane scar.

Solifluction

Solifluction is a collective name for gradual mass wasting slope processes related to freeze-thaw activity. This is the standard modern meaning of solifluction, which differs from the original meaning given to it by Johan Gunnar Andersson in 1906.

Submarine canyon

A submarine canyon is a steep-sided valley cut into the seabed of the continental slope, sometimes extending well onto the continental shelf, having nearly vertical walls, and occasionally having canyon wall heights of up to 5 km, from canyon floor to canyon rim, as with the Great Bahama Canyon. Just as above-sea-level canyons serve as channels for the flow of water across land, submarine canyons serve as channels for the flow of turbidity currents across the seafloor. Turbidity currents are flows of dense, sediment laden waters that are supplied by rivers, or generated on the seabed by storms, submarine landslides, earthquakes, and other soil disturbances. Turbidity currents travel down slope at great speed (as much as 70 km/h), eroding the continental slope and finally depositing sediment onto the abyssal plain, where the particles settle out.About 3% of submarine canyons include shelf valleys that have cut transversely across continental shelves, and which begin with their upstream ends in alignment with and sometimes within the mouths of large rivers, such as the Congo River and the Hudson Canyon. About 28.5% of submarine canyons cut back into the edge of the continental shelf, whereas the majority (about 68.5%) of submarine canyons have not managed at all to cut significantly across their continental shelves, having their upstream beginnings or "heads" on the continental slope, below the edge of continental shelves.The formation of submarine canyons is believed to occur as the result of at least two main process: 1) erosion by turbidity current erosion; and 2) slumping and mass wasting of the continental slope. While at first glance, the erosion patterns of submarine canyons may appear to mimic those of river-canyons on land, due to the markedly different erosion processes that have been found to take place underwater at the soil/ water interface, several notably different erosion patterns have been observed in the formation of typical submarine canyons.Many canyons have been found at depths greater than 2 km below sea level. Some may extend seawards across continental shelves for hundreds of kilometres before reaching the abyssal plain. Ancient examples have been found in rocks dating back to the Neoproterozoic. Turbidites are deposited at the downstream mouths or ends of canyons, building an abyssal fan.

Tohil Mons

Tohil Mons is a mountain on Io, one of Jupiter's moons. It stands at 5400 m (18,000 feet) and is located in the Culann–Tohil region of Io's antijovian hemisphere. It was named after the Mayan weather god Tohil.

Tohil Mons is one of the most geologically complex mountains in the Solar System as the mountain shows evidence of forming from other mountains through tectonic and erosional processes, in combination with silicate volcanic rock or magma activity.The main massif of Tohil Mons has ridges and grooves, which indicate the presence of materials that have been tectonically displaced, including younger mottled crustal materials that were displaced during mass wasting processes.

The area between the mountain and the nearby volcanoes, Radegast Patera and Tohil Patera contain a number of dark and white silicate flows, which are thought to be lava ponds or small lava lakes.

Topographical studies of the terrain indicate that there is less than 1 km of relief in the Culann–Tohil region and that there is no discernible correlation between centers of active volcanism and topography.

UTEXAS

UTEXAS is a slope stability analysis program written by Stephen G. Wright of the University of Texas at Austin. The program is used in the field of civil engineering to analyze levees, earth dams, natural slopes, and anywhere there is concern for mass wasting. UTEXAS finds the factor of safety for the slope and the critical failure surface. Recently the software was used to help determine the reasons behind the failure of I-walls during Hurricane Katrina.

Soil
Foundations
Retaining walls
Stability
Earthquakes
Geosynthetics
Numerical analysis
Geologic principles and processes
Stratigraphic principles
Petrologic principles
Geomorphologic processes
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