Slump (geology)

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The slump that destroyed Thistle, Utah, by creating an earthen dam that flooded the area Thistleslide.png
The slump that destroyed Thistle, Utah, by creating an earthen dam that flooded the area

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. [1] 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.

Mass wasting geomorphic process by which soil, sand, regolith, and rock move downslope

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. 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.

Translational slumps occur when a detached landmass moves along a planar surface. [2] 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. [3] 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. [4] 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.

Escarpment Steep slope or cliff separating two relatively level regions

An escarpment, or scarp, is a steep slope or long cliff that forms as an effect of faulting or erosion and separates two relatively level areas having differing elevations. Usually scarp and scarp face are used interchangeably with escarpment.

Earthflow

An earthflow 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.

Slumped chalk slopes at Mupe Bay in Dorset, England Mupe Bay cliffs.jpg
Slumped chalk slopes at Mupe Bay in Dorset, England

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.

The Swift Creek Landslide is an active, slow moving landslide located in western Washington, USA, due east of Everson on Sumas Mountain. Asbestos laden sediment originating from the landslide has recently generated much interest in this area. Asbestos is a known carcinogen. EPA sampling has documented asbestos in sediments in Swift Creek and in downstream Sumas River. Average asbestos levels in Swift Creek dredged material exceed the level that, in construction materials, triggers worker safety requirements and material handling and disposal regulations.

Sumas Mountain mountain in United States of America

Sumas Mountain is a mountain located in Whatcom County, Washington, 15 miles northeast of Bellingham and southwest of Vedder Mountain. Located in the Skagit Range the mountain is notable for its high biodiversity and year-round hiking trails. It is sometimes referred to as American Sumas to distinguish it from an identically named mountain just to the north in British Columbia. It is largely owned by the Washington Department of Natural Resources, but some parcels are privately held. While the public land is open for recreation, it is managed primarily for timber harvest. Clearcuts are present on many slopes and most all the remaining forest is in varying stages of recovery and regrowth.

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.

<i>Tsunami</i> Series of water waves caused by the displacement of a large volume of a body of water

A tsunami or tidal wave,, also known as a seismic sea wave, is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explosions above or below water all have the potential to generate a tsunami. Unlike normal ocean waves, which are generated by wind, or tides, which are generated by the gravitational pull of the Moon and the Sun, a tsunami is generated by the displacement of water.

Hawaiian Islands An archipelago in the North Pacific Ocean, currently administered by the US state of Hawaii

The Hawaiian Islands are an archipelago of eight major islands, several atolls, numerous smaller islets, and seamounts in the North Pacific Ocean, extending some 1,500 miles from the island of Hawaiʻi in the south to northernmost Kure Atoll. Formerly the group was known to Europeans and Americans as the Sandwich Islands, a name chosen by James Cook in honor of the then First Lord of the Admiralty John Montagu, 4th Earl of Sandwich. The contemporary name is derived from the name of the largest island, Hawaii Island.

Topography The study of the shape and features of the surface of the Earth and other observable astronomical objects

Topography is the study of the shape and features of land surfaces. The topography of an area could refer to the surface shapes and features themselves, or a description.

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. [5]

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Erosion Processes which remove soil and rock from one place on the Earths crust, then transport it to another location where it is deposited

In earth science, erosion is the action of surface processes that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location. This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, air (wind), plants, animals, and humans. In accordance with these agents, erosion is sometimes divided into water erosion, glacial erosion, snow erosion, wind (aeolic) erosion, zoogenic erosion, and anthropogenic erosion. The particulate breakdown of rock or soil into clastic sediment is referred to as physical or mechanical erosion; this contrasts with chemical erosion, where soil or rock material is removed from an area by its dissolving into a solvent, followed by the flow away of that solution. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres.

Landslide type of natural disaster, geological phenomenon

The term landslide or, less frequently, landslip, refers to several forms of mass wasting that include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows and debris flows. Landslides occur in a variety of environments, characterized by either steep or gentle slope gradients: from mountain ranges to coastal cliffs or even underwater, in which case they are called submarine landslides. Gravity is the primary driving force for a landslide to occur, but there are other factors affecting slope stability which produce specific conditions that make a slope prone to failure. In many cases, the landslide is triggered by a specific event, although this is not always identifiable.

Alluvial fan A fan- or cone-shaped deposit of sediment crossed and built up by streams

Alluvial fans are triangular-shaped deposits of water-transported material, often referred to as alluvium. They are an example of an unconsolidated sedimentary deposit and tend to be larger and more prominent in arid to semi-arid regions. These alluvial fans typically form in elevated or even mountainous regions where there is a rapid change in slope from a high to low gradient. The river or stream carrying the sediment flows at a relatively high velocity due to the high slope angle which is why coarse material is able to remain in the flow. When the slope decreases rapidly into a relatively plain or plateau, the stream loses the energy it needs to move its sediment. Deposition subsequently occurs and the sediment ultimately spreads out, creating an alluvial fan. Three primary zones occur within an alluvial fan which includes the proximal fan, medial fan, and the distal fan.

Soil liquefaction

Soil liquefaction occurs when a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress such as shaking during an earthquake or other sudden change in stress condition, in which material that is ordinarily a solid behaves like a liquid.

Submarine canyon A steep-sided valley cut into the seabed of the continental slope

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, eroding the continental slope and finally depositing sediment onto the abyssal plain, where the particles settle out.

Fault scarp A small step or offset on the ground surface where one side of a fault has moved vertically with respect to the other

A fault scarp is a small step or offset on the ground surface where one side of a fault has moved vertically with respect to the other. It is the topographic expression of faulting attributed to the displacement of the land surface by movement along faults. They are exhibited either by differential movement and subsequent erosion along an old inactive geologic fault, or by a movement on a recent active fault.

Rockslide type of landslide caused by rock failure

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.

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.

There have been known various classifications of landslides and other types of mass wasting.

Landslide mitigation refers to several man-made activities on slopes with the goal of lessening the effect of landslides. Landslides can be triggered by many, sometimes concomitant causes. In addition to shallow erosion or reduction of shear strength caused by seasonal rainfall, landslides may be triggered by anthropic activities, such as adding excessive weight above the slope, digging at mid-slope or at the foot of the slope. Often, individual phenomenon join together to generate instability over time, which often does not allow a reconstruction of the evolution of a particular landslide. Therefore, landslide hazard mitigation measures are not generally classified according to the phenomenon that might cause a landslide. Instead, they are classified by the sort of slope stabilization method used:

Causes of landslides

The causes of landslides are usually related to instabilities in slopes. It is usually possible to identify one or more landslide causes and one landslide trigger. The difference between these two concepts is subtle but important. The landslide causes are the reasons that a landslide occurred in that location and at that time. Landslide causes are listed in the following table, and include geological factors, morphological factors, physical factors and factors associated with human activity.

Sedimentary structures include all kinds of features formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different particle sizes are deposited on top of each other. These beds range from millimeters to centimeters thick and can even go to meters or multiple meters thick.

Submarine landslide Landslides that transport sediment across the continental shelf and into the deep ocean

Submarine landslides are marine landslides that transport sediment across the continental shelf and into the deep ocean. A submarine landslide is initiated when the downwards driving stress exceeds the resisting stress of the seafloor slope material causing movements along one or more concave to planar rupture surfaces. Submarine landslides take place in a variety of different settings including planes as low as 1° and can cause significant damage to both life and property. Recent advances have been made in understanding the nature and processes of submarine landslides through the use of sidescan sonar and other seafloor mapping technology.

A bedrock river is a river that has little to no alluvium mantling the bedrock over which it flows. However, most bedrock rivers are not pure forms; they are a combination of a bedrock channel and an alluvial channel. The way one can distinguish between bedrock rivers and alluvial rivers is through the extent of sediment cover.

River bank failure

River bank failure can be caused when the gravitational forces acting on a bank exceed the forces which hold the sediment together. Failure depends on sediment type, layering, and moisture content.

The Truttman Sink is an earthflow within the Humboldt Lagoons State Park, along the coast of Humboldt County, California. It is located between Trinidad to the south and Orick to the north. It deposits materials into the northern end of Big Lagoon and the Pacific Ocean, especially during periods of heavy rain. The soil characteristics, geology, and vegetation along the slope of this mass-wasting feature suggest a combination of an earthflow movement and a rotational slump.

Hillslope evolution is the changes in the erosion rates, erosion styles and form of slopes of hills and mountains over time.

References

  1. Tarbuck, E.J.; Lutgens, F.K. (1998), Earth, an introduction to Physical Geology (6th ed.), Prentice Hall, pp. 219–220, ISBN   978-0139741227
  2. Girty, G. H. (2009), "Landslides" (PDF), Perilous Earth: Understanding Processes Behind Natural Disasters, Montezuma Publishing, pp. 1–17
  3. Easterbrook, D. J. (1999), Surfaces Processes and Landforms (2nd ed.), Prentice Hall, ISBN   978-0138609580
  4. Hansen, M. C. (2000), Earthquakes and Seismic Risk in Ohio, Ohio Department of Natural Resources, Division of Geological Survey
  5. Uenzelmann-Neben, G.; Huhn, K. (2009). "Sedimentary deposits on the southern South African continental margin: Slumping versus non-deposition or erosion by oceanic currents?" (PDF). Marine Geology. 266 (1–4): 65–79. doi:10.1016/j.margeo.2009.07.011.