›Landslides are relatively rapid and perceptible movements of rock debris down slopes (p. 42).
›The materials involved in landslides are typically relatively dry (p. 42).
›The size and shape of the detached mass in a landslide depend on rock discontinuities, degree of weathering, and slope steepness (p. 42).
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Landslides are a category of mass movements characterized by relatively rapid and perceptible transfer of rock debris down slopes, primarily under the direct influence of gravity. The materials involved in landslides are typically relatively dry. The specific size and shape of the detached mass depend on the nature of discontinuities within the rock, the degree of weathering the rock has undergone, and the steepness of the slope.
Several types of landslides are identified based on the movement of materials:
* **Slump**: Involves the slipping of one or more units of rock debris with a backward rotation relative to the slope. This is particularly common where permeable debris or rock layers overlie impermeable strata, such as clay, allowing water to create a slippery surface.
* **Debris Slide**: Characterized by the rapid rolling or sliding of earth debris without any backward rotation of the mass.
* **Debris Fall**: Refers to a nearly free fall of earth debris from a vertical or overhanging face.
* **Rockslide**: Involves the sliding of individual rock masses down bedding, joint, or fault surfaces, often very fast and destructive over steep slopes.
* **Rock Fall**: Represents the free falling of rock blocks over any steep slope, keeping itself away from the slope. Rock falls primarily affect superficial layers of the rock face, distinguishing them from rockslides which involve materials up to a substantial depth.
Landslides and debris avalanches are common in tectonically active regions like the Himalayas, which are composed of sedimentary rocks and unconsolidated or semi-consolidated deposits, coupled with very steep slopes. They also occur in regions like the Nilgiris and Western Ghats, driven by steep slopes, pronounced mechanical weathering, and heavy short-period rainfall, despite these regions being relatively tectonically stable with hard rocks.
Landslides are often triggered by factors such as undercutting of steep slopes by rivers or the sea, seismic activity like earthquakes, volcanic disturbances that loosen rocks, or the lubricating action of rain-water which collects in joints or bedding planes, allowing layers to slide. Human activities, including artificially steepening slopes (e.g., road or rail cuttings) and clearing natural vegetation for agriculture or housing, significantly enhance the possibility of landslides by altering slope stability and increasing water penetration into the soil and rocks.
All key facts
›Landslides are relatively rapid and perceptible movements of rock debris down slopes (p. 42).
›The materials involved in landslides are typically relatively dry (p. 42).
›The size and shape of the detached mass in a landslide depend on rock discontinuities, degree of weathering, and slope steepness (p. 42).
›Specific types of landslides include Slump, Debris slide, Debris fall, Rockslide, and Rock fall (p. 42).
›**Slump** involves slipping of rock debris with a backward rotation (p. 42).
›**Debris slide** is rapid rolling or sliding of earth debris without backward rotation (p. 42).
›**Debris fall** is a nearly free fall of earth debris from a vertical or overhanging face (p. 42).
›**Rockslide** involves individual rock masses sliding down bedding, joint, or fault surfaces, which can be very fast and destructive on steep slopes (p. 42).
›**Rock fall** is the free falling of rock blocks from superficial layers of a steep slope, distinguishable from rockslide by affecting less depth (p. 43).
›Landslides and debris avalanches are frequent in the Himalayas due to tectonic activity, sedimentary and unconsolidated rocks, and steep slopes (p. 43).
›They also occur in the Nilgiris and Western Ghats, though less frequently, due to steep slopes, mechanical weathering, and heavy rainfall (p. 43).
›Landslides are very rapid movements occurring when a large mass of soil or rock falls suddenly (p. 40).
›They commonly occur on steep slopes in mountainous areas, on cliffs, or where human activities, such as road or rail cuttings, have artificially steepened slopes (p. 40).
›Causes of landslides include steep slopes being undercut by a river or the sea, gravitational force, earthquakes, or volcanic disturbances that loosen rocks (p. 40).
›Man-made steepening of slopes contributes to landslides by undercutting the slope and setting up vibrations that loosen rocks or soil (p. 40).
›A frequent cause of landslides is the lubricating action of rain-water, which collects in joints or bedding planes in rocks, causing one layer to slide over another, especially in areas of tilted strata (p. 40).
›Slumping is particularly common where permeable debris or rock layers overlie impermeable strata (e.g., clay), with water saturating the permeable material and providing a smooth, slippery surface on the impermeable layer for the upper layers to slide (p. 40).
›Water can collect at the base of regolith (weathered material) where it sinks readily, but more slowly into the solid rock beneath, allowing the regolith to slide away from the underlying rock (p. 40).
›Human activities, such as clearing natural vegetation for agriculture or housing, enhance the possibility of landslides by allowing more water to penetrate the soil and rocks (p. 40).
›Extensive landslides, whether natural or human-induced, can have disastrous consequences, including burying villages, railway lines, or people (p. 40).
›Examples of areas experiencing landslides include the Cameron Highlands (Malaysia), South Wales, British Columbia, and Hong Kong (p. 40).
mass movementweatheringearthquakesvolcanoesgroundwater
Exfoliation
›Exfoliation is an effect of weathering, specifically falling under physical weathering processes. — NCERT Class 11 — India: Physical Environment, ch05-landforms.md
›It is considered a result, not a geomorphic process. — NCERT Class 11 — India: Physical Environment, ch05-landforms.md
›Exfoliation involves the flaking off of curved sheets or shells from rocks or bedrock. — NCERT Class 11 — India: Physical Environment, ch05-landforms.md
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Exfoliation is a specific effect of physical or mechanical weathering, characterized by the flaking off of curved sheets or shells from the surface of rocks or bedrock. It is important to note that exfoliation is considered a *result* or an outcome, rather than a geomorphic *process* itself. This phenomenon leads to the formation of smooth and rounded rock surfaces. The primary causes of exfoliation include expansion and contraction induced by temperature changes, as well as pressure release, also known as unloading. Exfoliation can lead to the formation of distinct landforms such as exfoliation domes, which are typically a result of unloading, and tors, which are formed due to thermal expansion. This process is also known as 'onion peeling' due to the successive splitting off of outer layers. While temperature changes in deserts are a known cause, exfoliation is not confined to desert areas and can also be significantly induced by repeated wetting and drying, particularly in tropical regions.
All key facts
›Exfoliation is an effect of weathering, specifically falling under physical weathering processes. — NCERT Class 11 — India: Physical Environment, ch05-landforms.md
›It is considered a result, not a geomorphic process. — NCERT Class 11 — India: Physical Environment, ch05-landforms.md
›Exfoliation involves the flaking off of curved sheets or shells from rocks or bedrock. — NCERT Class 11 — India: Physical Environment, ch05-landforms.md
›The outcome of exfoliation is the creation of smooth and rounded rock surfaces. — NCERT Class 11 — India: Physical Environment, ch05-landforms.md
›It can occur due to expansion and contraction caused by temperature changes. — NCERT Class 11 — India: Physical Environment, ch05-landforms.md
›Exfoliation also relates to unloading (pressure release) and salt weathering. — NCERT Class 11 — India: Physical Environment, ch05-landforms.md
›Specific landforms like exfoliation domes result from unloading. — NCERT Class 11 — India: Physical Environment, ch05-landforms.md
›Tors are a result of thermal expansion-induced exfoliation. — NCERT Class 11 — India: Physical Environment, ch05-landforms.md
›Exfoliation is also known as 'onion peeling' because the various layers split off one after another, resembling the layers of an onion. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch04-weathering-mass-movement.md
›Exfoliation is not confined to desert areas. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch04-weathering-mass-movement.md
›Similar stresses causing exfoliation can be set up by repeated wetting and drying of the surface layers of rocks, especially in tropical regions like Malaysia where short downpours are followed by quick drying from the hot sun. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch04-weathering-mass-movement.md
›In deserts, repeated wetting and drying of rocks is likely just as important as temperature changes in mechanical weathering, as rocks dry very quickly after brief rainstorms. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch04-weathering-mass-movement.md
›Exfoliation is recognized as a result of temperature changes in deserts. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch04-weathering-mass-movement.md
Soil Creep
›Soil creep is a slow, gradual, and more or less continuous movement of soil down hill slopes.
›The movement is not very noticeable, especially on gentle slopes or when the soil is well-covered with vegetation.
›It is most common in damp soils where water acts as a lubricant, allowing individual soil particles to move over each other and the underlying rock.
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Soil creep represents a form of mass movement characterized by the slow, gradual, and generally continuous downhill movement of soil on slopes. This movement is often subtle and not readily noticeable, particularly when occurring on gentle gradients or in areas well-covered by grass or other vegetation.
The phenomenon is most prevalent in damp soils, where the presence of water acts as a lubricant, facilitating the movement of individual soil particles over each other and across the underlying rock. Another contributing factor to soil creep is the continuous trampling by grazing animals on slopes, which generates vibrations that loosen the soil and encourage its downward shift. Despite its slow pace, which makes direct observation challenging, the effects of soil creep become evident over time through various indicators.
All key facts
›Soil creep is a slow, gradual, and more or less continuous movement of soil down hill slopes.
›The movement is not very noticeable, especially on gentle slopes or when the soil is well-covered with vegetation.
›It is most common in damp soils where water acts as a lubricant, allowing individual soil particles to move over each other and the underlying rock.
›Continuous trampling by grazing animals, which sets up vibrations, can also loosen the soil and cause it to move.
›The movement is slow and cannot readily be seen in action.
›Evidence of soil creep includes the gradual tilting of trees, fences, and posts rooted in the soil.
›Soil accumulation can be observed at the foot of slopes or behind obstacles like walls, with the weight potentially bursting these obstacles.
Physical or Mechanical Weathering
›Mechanical weathering is the physical disintegration of a rock by the actual prising apart of separate particles (ch04-weathering-mass-movement.md).
›Processes of physical weathering work more easily when the rock surface has been weakened by chemical weathering (ch04-weathering-mass-movement.md).
›Dry climates provide good conditions for physical or mechanical weathering (ch04-weathering-mass-movement.md, page 37).
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Physical or mechanical weathering refers to the physical disintegration of a rock, where separate particles are prised apart. This process can occur even in completely fresh rock, but it is significantly facilitated when the rock's surface has already been weakened by chemical weathering. Mechanical weathering acts through several distinct mechanisms, including repeated temperature changes, cycles of wetting and drying, frost action, and biotic factors. Each of these processes leads to the breakdown of rocks into smaller fragments without altering their chemical composition. Dry climates, for instance, offer favorable conditions for physical or mechanical weathering, contrasting with warm, wet climates which promote rapid chemical weathering.
All key facts
›Mechanical weathering is the physical disintegration of a rock by the actual prising apart of separate particles (ch04-weathering-mass-movement.md).
›Processes of physical weathering work more easily when the rock surface has been weakened by chemical weathering (ch04-weathering-mass-movement.md).
›Dry climates provide good conditions for physical or mechanical weathering (ch04-weathering-mass-movement.md, page 37).
›**Repeated Temperature Changes:**
›In deserts, intense heating during the day causes outer rock layers to expand faster than the cooler interior, pulling away from the rest (ch04-weathering-mass-movement.md, page 37).
›Rapid temperature drops at night cause outer layers to contract faster than the interior, setting up internal stresses (ch04-weathering-mass-movement.md, page 37).
›These daily stresses cause rocks to crack and split, often along existing joints or cracks into rectangular blocks (ch04-weathering-mass-movement.md, page 37).
›Crystalline rocks like granite experience disintegration due to different expansion/contraction rates of constituent minerals (ch04-weathering-mass-movement.md, page 37).
›The gradual splitting away of sharp corners from rectangular blocks rounds them (ch04-weathering-mass-movement.md, page 37).
›When surface layers of rounded boulders split off, it's called onion peeling or exfoliation (ch04-weathering-mass-movement.md, page 38).
›**Repeated Wetting and Drying:**
›Similar stresses can be set up by repeated wetting and drying of surface layers, especially in tropical regions and at coasts (ch04-weathering-mass-movement.md, page 38).
›Outer layers absorb moisture and expand when wet, then quickly shrink as moisture evaporates when dry (ch04-weathering-mass-movement.md, page 38).
›Repeated cycles cause outer layers to split off (ch04-weathering-mass-movement.md, page 38).
›In deserts, wetting and drying by brief rainstorms is as important as temperature changes for mechanical weathering (ch04-weathering-mass-movement.md, page 38).
›**Frost Action:**
›Potent in temperate latitudes, where water or snow collects in rock cracks, joints, or pore spaces (ch04-weathering-mass-movement.md, page 38-39).
›When water freezes, it expands by one-tenth its volume, exerting a bursting pressure of almost 140 kg per square cm (ch04-weathering-mass-movement.md, page 39).
›Repeated freezing deepens and widens cracks, breaking rock into angular fragments (ch04-weathering-mass-movement.md, page 39).
›This process creates sharp pinnacles and angular outlines in mountain peaks, described as frost-shattered peaks (ch04-weathering-mass-movement.md, page 39).
›Angular rock fragments form screes at the foot of slopes (ch04-weathering-mass-movement.md, page 39).
›**Biotic Factors:**
›Plants sprouting in rock crevices extend roots that penetrate rocks, typically along joints and lines of weakness, prising them apart (ch04-weathering-mass-movement.md, page 39).
›Micro-organisms and plants like mosses or lichens can act as agents of both chemical and mechanical weathering by absorbing chemical elements and producing organic acids (ch04-weathering-mass-movement.md, page 37).
›Men also contribute to mechanical weathering through mining, road construction, and farming, which excavate rocks and make them more vulnerable (ch04-weathering-mass-movement.md, page 39).
Festoons (Island Arcs)
›Festoons are classified as a type of continental island, specifically under the sub-category of "archipelagos or island groups." (p. 97)
›They are defined as an archipelago forming a loop shape around the edge of a mainland. (p. 97)
›These island arcs mark the continuation of continental mountain ranges. (p. 97)
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Festoons, also known as island arcs, are a type of continental island formation. They consist of an archipelago, or group of islands, arranged in the shape of a loop or arc around the edge of a mainland. These islands are not isolated oceanic formations but are geologically linked to the continent, representing a continuation of mountain ranges that can be traced back to the mainland.
As a category of continental islands, festoons were formerly part of the mainland and became detached. This separation could have been caused by the subsidence of a part of the land or a rise in sea level that submerged the lowland links. The historical connection to the continent is often evident in the similar physical structure, flora, and fauna found on both the islands and the adjacent mainland.
All key facts
›Festoons are classified as a type of continental island, specifically under the sub-category of "archipelagos or island groups." (p. 97)
›They are defined as an archipelago forming a loop shape around the edge of a mainland. (p. 97)
›These island arcs mark the continuation of continental mountain ranges. (p. 97)
›Examples of festoons include the East Indies, the Aleutian Islands, the Ryukyu Islands, and the Kurile Islands. (p. 97)
›Like other continental islands, their separation from the mainland can be attributed to land subsidence or a rise in sea level. (p. 97)
›A former connection to the mainland can be identified through similarities in physical structure, as well as shared flora and fauna. (p. 97)
continental islandsoceanic islands
Mid-Oceanic Ridges
›Mid-Oceanic Ridges form an interconnected chain of mountain systems submerged within the ocean.
›They constitute the longest mountain-chain on the surface of the Earth.
›They are characterized by a central rift system at the crest, a fractionated plateau, and flank zones.
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Mid-Oceanic Ridges are extensive, interconnected mountain systems that are submerged within the ocean basins, forming the longest mountain chain on the surface of the Earth. These ridges represent one of the three major divisions of the ocean floor, typically situated between continental margins and abyssal plains.
A mid-oceanic ridge is characterized by a central rift system at its crest, accompanied by a fractionated plateau and flank zones extending along its length. The rift system at the crest is a region of intense volcanic activity, where frequent eruptions bring significant amounts of lava to the surface. This volcanic activity, combined with shallow-depth earthquake occurrences, indicates that mid-oceanic ridges are geologically very active areas.
These geological features are fundamental to the concept of "sea floor spreading." Scientists like Hess (1961) proposed that constant eruptions at the crest of oceanic ridges cause the oceanic crust to rupture. New lava then wedges into this rupture, pushing the oceanic crust outwards on either side, thereby causing the ocean floor to spread. This process means that mid-oceanic ridges are "spreading sites," also known as divergent boundaries, where new crust is continuously generated as tectonic plates move apart. The Mid-Atlantic Ridge is a prime example of such a divergent boundary.
Studies of rocks from mid-oceanic ridges reveal that rocks located equidistant on either side of the crest share remarkable similarities in their constituents and age. Rocks found closer to the ridge crest are the youngest and exhibit normal magnetic polarity, with their age progressively increasing as the distance from the crest grows. The oceanic crust itself is significantly younger than continental areas, with rocks in the oceanic crust rarely exceeding 200 million years in age. The magnetic patterns observed parallel to these ridges are crucial for determining the rates of plate movement.
All key facts
›Mid-Oceanic Ridges form an interconnected chain of mountain systems submerged within the ocean.
›They constitute the longest mountain-chain on the surface of the Earth.
›They are characterized by a central rift system at the crest, a fractionated plateau, and flank zones.
›The rift system at the crest is a zone of intense volcanic activity.
›Mid-oceanic ridges are one of the three major divisions of the ocean floor, located between continental margins and abyssal plains.
›Rocks found on either side of the crest of oceanic ridges, at equi-distant locations, show similarities in their constituents and age.
›Rocks closer to the mid-oceanic ridges are the youngest and exhibit normal magnetic polarity.
›The age of oceanic rocks increases as one moves away from the crest of the ridge.
›Oceanic crust rocks are much younger than continental rocks, with an age of nowhere more than 200 million years.
›Mid-oceanic ridge areas are associated with shallow-depth earthquake occurrences.
›They represent divergent boundaries, also known as spreading sites, where new crust is generated as plates pull apart.
›The Mid-Atlantic Ridge is a well-known example of a divergent boundary.
Denudation
›Denudation is the process of wearing away the Earth's surface, causing its general lowering and levelling out.
›It is carried out in four distinct phases that occur simultaneously in different parts of the world.
›The four phases of denudation are:
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Denudation is a fundamental external geological process responsible for wearing away the Earth's surface, leading to its general lowering and levelling. It results from the vigorous action of external forces that counterbalance the internal forces which create new relief features. This comprehensive process involves four distinct but simultaneously occurring phases.
The first phase is **weathering**, which refers to the gradual disintegration of rocks by atmospheric or weather forces. Following weathering, **erosion** takes over, actively wearing away the Earth's surface through the action of moving agents such as running water, wind, ice, and waves. The third phase, **transportation**, involves the removal of the eroded debris from its original location to new positions. Finally, **deposition** is the process where this transported debris is dumped in specific parts of the Earth, where it can accumulate and potentially form new rocks.
These four phases of denudation occur concurrently across different regions globally, though their rates can vary significantly. The rate and effectiveness of denudation are influenced by several factors, including the nature of the local relief, the structural characteristics of the rocks, the prevailing climate, and human interference.
All key facts
›Denudation is the process of wearing away the Earth's surface, causing its general lowering and levelling out.
›It is carried out in four distinct phases that occur simultaneously in different parts of the world.
›The four phases of denudation are:
›**Weathering**: The gradual disintegration of rocks by atmospheric or weather forces.
›**Erosion**: The active wearing away of the Earth's surface by moving agents like running water, wind, ice, and waves.
›**Transportation**: The removal of the eroded debris to new positions.
›**Deposition**: The dumping of the debris, where it may accumulate to form new rocks.
›The rate of denudation is influenced by the nature of the relief, the structure of the rocks, the local climate, and human interference.
Chemical Weathering
›Chemical weathering is the basic process by which denudation proceeds.
›It is the extremely slow and gradual decomposition of rocks due to exposure to air and water.
›Air and water contain chemical elements that set up chemical reactions in exposed rock surface layers.
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Chemical weathering is a fundamental process of denudation, involving the extremely slow and gradual decomposition of rocks. This occurs when rocks are exposed to air and water, which contain chemical elements that initiate reactions within the surface layers of the exposed rocks. These reactions can either weaken or entirely dissolve certain components of the rock, thereby loosening other crystals and diminishing the overall strength of the surface. For instance, in granite, felspar minerals are more rapidly weathered than quartz, leading to the loosening of quartz crystals and the formation of a coarse sandy residue.
When a rock surface undergoes chemical weathering, some loosened material is removed by agents like wind or water, exposing a fresh surface. However, much of the weathered material, known as regolith, can remain in place and form the base for soil. The presence of a soil cover generally enhances chemical weathering of the underlying rocks, as the soil absorbs rainwater, keeping the rocks in continuous contact with moisture. This rainwater, having absorbed organic acids from the soil, becomes a more potent weathering agent than pure rainwater acting on bare rock.
There are three primary chemical weathering processes: solution, oxidation, and decomposition by organic acids. Solution involves water dissolving minerals, a process significantly intensified when rainwater contains carbon dioxide, forming a weak acid. This is particularly effective in limestone regions, dissolving calcium carbonate and widening cracks to form caves. Oxidation is the reaction of oxygen in air or water with rock minerals, such as iron, forming easily erodible iron oxide (rust). Decomposition by organic acids is driven by bacteria in soil thriving on decaying organic matter, producing acids that accelerate weathering. Micro-organisms and plants like mosses or lichens on damp bare rock also contribute by absorbing chemical elements and producing organic acids. The rate of chemical weathering is significantly influenced by climate, with warm, wet conditions promoting rapid reactions, while dry climates inhibit it.
All key facts
›Chemical weathering is the basic process by which denudation proceeds.
›It is the extremely slow and gradual decomposition of rocks due to exposure to air and water.
›Air and water contain chemical elements that set up chemical reactions in exposed rock surface layers.
›These reactions can weaken or entirely dissolve certain rock constituents, loosening crystals and weakening the surface.
›In granite, felspar weathers more quickly than quartz, leading to the loosening of quartz crystals and forming a coarse sandy residue.
›Weathered material (regolith) may stay in position, forming the basis of soil.
›A soil cover usually enhances chemical weathering of underlying rocks because soil absorbs rainwater, keeping rocks in contact with moisture.
›Rainwater absorbs organic acids from the soil, becoming a stronger weathering agent than pure rainwater on bare rock.
›There are three major chemical weathering processes: Solution, Oxidation, and Decomposition by organic acids.
›**Solution:** Many minerals are dissolved by water, especially when rainwater contains carbon dioxide, making it a weak acid.
›Solution is the most potent weathering process in limestone regions, attacking and dissolving calcium carbonate.
›All rocks are subject to solution to some extent, though slower than with limestone.
Soil Impoverishment and Erosion in Agriculture
›Soil fertility is influenced by its mineral composition (e.g., iron, calcium, potash) and the presence of organic matter, or humus, which is derived from the decay of plants and organisms. (p. 240)
›Soils rich in humus give heavy yields, but they need to be replenished with manure or fertilizers when exhausted. (p. 240)
›**Overcropping**, where too many crops are grown on the same land without replacing nutrients, leads to soil impoverishment. (p. 241)
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Soil impoverishment and soil erosion are described as "twin evils" that result from unsound farming practices. Soil impoverishment refers to the deterioration of the mineral and organic content of the soil, making it highly deficient in plant nutrients and leading to low yields. Soil erosion is the process where the fertile top layer of soil is completely removed, sometimes beyond replacement. (p. 241)
Human interference and mismanagement in agriculture can greatly accelerate natural erosion processes. While natural conditions replenish soil fertility through processes like leaf-fall and organic decomposition, farming activities such as ploughing and harvesting crops upset this balance. Practices like overcropping, improper cultivation on slopes, and deforestation leave the soil exposed and vulnerable to erosion by agents like running water and wind. (p. 241-242)
All key facts
›Soil fertility is influenced by its mineral composition (e.g., iron, calcium, potash) and the presence of organic matter, or humus, which is derived from the decay of plants and organisms. (p. 240)
›Soils rich in humus give heavy yields, but they need to be replenished with manure or fertilizers when exhausted. (p. 240)
›**Overcropping**, where too many crops are grown on the same land without replacing nutrients, leads to soil impoverishment. (p. 241)
›**Monoculture**, the practice of growing the same crop year after year, exhausts minerals vital to those specific plants. (p. 241)
›**Slopewise cultivation** can create furrows that act as channels for rainwater, accelerating erosion and leading to the formation of gullies. (p. 241)
›Clearing forests for farming without replanting exposes bare soil to rain, which can cause **sheet-erosion** and wash away the fertile top-soil. (p. 242)
›Soil deposited in rivers from erosion can make them shallower and more prone to flooding. (p. 242)
›The 'slash-and-burn' method of **shifting cultivation** exposes barren surfaces to heavy tropical downpours, causing widespread sheet erosion. (p. 242)
›If the fallow period after shifting cultivation is only a few years, the land cannot regain its minerals and humus, becoming permanently impoverished. (p. 242)
›Degraded soil lacking in humus, which helps bind soil particles together, is more readily eroded. (p. 242)
›Soil Flow (Solifluction) is a type of mass movement where soil moves downslope due to gravitational forces.
›It occurs when the soil is completely saturated with water. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch04-weathering-mass-movement.md
›When saturated, individual soil particles become almost suspended in water, facilitating their easy movement over each other and the underlying rock. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch04-weathering-mass-movement.md
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Soil Flow, also known as Solifluction, is a distinct type of mass movement characterized by the downslope movement of soil that has become completely saturated with water. In this state, the individual soil particles are nearly suspended in water, allowing them to move easily over one another and over the underlying rock, causing the soil mass to behave much like a liquid.
This phenomenon manifests under specific climatic and environmental conditions. In arid regions, a soil flow or mud-flow can be triggered when a layer of weathered debris becomes fully saturated with rainwater following a storm, resulting in its downslope movement as a semi-liquid mass. In temperate and tundra regions, solifluction commonly occurs during spring when the surface layers of previously frozen ground begin to thaw. The melt-water acts as a lubricant, enabling the soil and rock debris to flow readily over the still-frozen subsoil. Furthermore, in areas with extensive peat soils, which possess a high moisture absorption capacity, a soil flow can occur if the peat reaches its saturation point, leading to its downslope movement. In Ireland, such flows involving peaty soil are specifically referred to as 'bog-bursts'.
All key facts
›Soil Flow (Solifluction) is a type of mass movement where soil moves downslope due to gravitational forces.
›It occurs when the soil is completely saturated with water. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch04-weathering-mass-movement.md
›When saturated, individual soil particles become almost suspended in water, facilitating their easy movement over each other and the underlying rock. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch04-weathering-mass-movement.md
›The saturated soil acts like a liquid, resulting in a soil-flow or mud-flow. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch04-weathering-mass-movement.md
›In arid regions, weathered debris can become saturated by storm rainwater and flow downslope as a semi-liquid mass. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch04-weathering-mass-movement.md
›In temperate and tundra regions, soil flows are common when the surface layers of frozen ground thaw in spring. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch04-weathering-mass-movement.md
›In these cold regions, melt-water lubricates the soil and rock debris, enabling them to flow over the underlying frozen subsoil. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch04-weathering-mass-movement.md
›In areas with peat soils, if the saturation point is reached, the peaty soil may flow downslope. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch04-weathering-mass-movement.md
›In Ireland, flows of peaty soil are known as 'bog-bursts'. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch04-weathering-mass-movement.md
›
Strips of normal and reverse magnetic field parallel to mid-oceanic ridges help determine rates of plate movement.
›The Arctic Ridge has the slowest plate movement rate, less than 2.5 cm/yr.
›Transform faults are generally perpendicular to mid-oceanic ridges.
›The boundary between the Indian and Antarctic plate is an oceanic ridge, representing a divergent boundary.
›Volcanic islands can emerge from the submarine ridges of the oceans.
›The rate of solution is affected by mineral composition, rock structure, and the density of joints or cracks.
›In tropical countries with heavy rainfall and warm climates, chemical weathering (including solution) often proceeds very rapidly, producing deep regoliths and core-stones.
›Core-stones are pieces of solid rock that resist weathering due to fewer joints or cracks.
›**Oxidation:** This is the reaction of oxygen in air or water with minerals in the rock.
›For example, iron in rocks reacts with air to form iron oxide (rust), which crumbles easily and is more easily eroded.
›Iron oxide removal loosens the overall rock structure and weakens it.
›**Decomposition by organic acids:** Bacteria in soil, thriving on decaying organic material, produce acids that speed up weathering when dissolved in water.
›Micro-organisms and plants (e.g., mosses, lichens) on damp bare rock absorb chemical elements and produce organic acids, acting as chemical weathering agents.
›Warm, wet climates promote rapid chemical weathering.