DENUDATION AND DEPOSION

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• 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.
• Denudation incorporates the mechanical, biological and chemical processes of erosion, weathering and mass wasting. Denudation can involve the removal of both solid particles and dissolved material

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• The river is one of the active agents of denudation that erodes, transports and deposits sediments on the earth's surface to produce erosion and depositional landforms.
• The other agents of denudation are ground water, wind, glaciers and sea waves. Rivers have the ability to erode the land over which they flow.

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• DENUDATION refers to the wearing down and stripping and leveling of the earths surface. They are also gradational because they grade/level the earth's surface. Weathering is the breakdown of rock and minerals.
• Erosion is the movement of these weathered materials.

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WEATHERING

• Weathering refers to the breaks down and loosens the surface minerals of rock so they can be transported away by agents of erosion such as water, wind and ice.
• Weathering is also a term which describes the general process by which rocks are broken down at the Earth’s surface into such things as sediments, clays, soils and substances that are dissolved in water.

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• Factors Which Control the Rates of Weathering
• Properties of the Parent Rock
• The mineralogy and structure of a rock affects it’s susceptibility to weathering.
• Different minerals weather at different rates. Mafic silicates like olivine and pyroxene tend to weather much faster than felsic minerals like quartz and feldspar.

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• Climate
• Rainfall and temperature can affect the rate in which rocks weather. High temperatures and greater rainfall increase the rate of chemical weathering.
• Rocks in tropical regions exposed to abundant rainfall and hot temperatures weather much faster than similar rocks residing in cold, dry regions.

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• Soil
• Soils affect the rate in which a rock weathers. Soils retain rainwater so that rocks covered by soil are subjected to chemical reactions with water much longer than rocks not covered by soil. Soils are also host to a variety of vegetation, bacteria and organisms that produce an acidic environment which also promotes chemical weathering.
• Minerals in a rock buried in soil will therefore break down more rapidly than minerals in a rock that is exposed to air.

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• Length of Exposure
• The longer a rock is exposed to the agents of weathering, the greater the degree of alteration, dissolution and physical breakup.
• Lava flows that are quickly buried by subsequent lava flows are less likely to be weathered than a flow which remains exposed to the elements for long periods of time.

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Types of weathering

• Chemical Weathering
• Chemical weathering is a process where minerals in a rock may be converted into clays, oxidized or simply dissolved.
• CW is any of the various weathering processes that cause exposed rock to undergo chemical decomposition, changing the chemical and mineralogical composition of the rock:

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• CW refers to the erosion or disintegration of rocks, building materials, etc., caused by chemical reactions (chiefly with water and substances dissolved in it) rather than by mechanical processes.

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• Chemical weathering is the weakening and subsequent disintegration of rock by chemical reactions.
•  These reactions include oxidation,  hydrolysis, and carbonation. These processes either form or destroy minerals, thus altering the nature of the rock’s mineral composition.

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• Oxidation is the reaction of rock minerals with oxygen, thus changing the mineral composition of the rock.
• When minerals in rock oxidize, they become less resistant to weathering. Iron, a commonly known mineral, becomes red or rust colored when oxidized.

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• From the Figure . The iron in olivine (Fe₂SiO₄) is reduced and the iron in limonite (Fe₂O₃.H₂O)  is oxidized. In addition, the release of silicon and hydration makes the mineral more susceptible to physical weathering. 

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• Carbonation is the process of rock minerals reacting with carbonic acid. Carbonic acid is formed when water combines with carbon dioxide. Carbonic acid dissolves or breaks down minerals in the rock.

CO₂ + H₂O  →  H₂CO₃�  �(carbon dioxide + water  →  carbonic acid)�  

CaCO₃ + H₂CO₃  →  Ca²⁺ + 2HCO_3-��  �(calcite + carbonic acid  →  calcium + bicarbonate)

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• Hydrolysis is a chemical reaction caused by water. Water changes the chemical composition and size of minerals in rock, making them less resistant to weathering.

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• A hydrolysis reaction of orthoclase (alkali feldspar), a common mineral found in igneous rock, yields kaolinite, silicic acid, and potassium.

2KAISi₃O₈ + 2H⁺ + 9H₂0  →  H₄Al₂Si₂O₉ + 4H₄SiO₄ + 2K⁺

(orthoclase + water  → kaolinite + silicic acid + potassium)

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• Hydration is the absorption of water into the mineral structure.  A good example of hydration is the absorption of water by anhydrite, resulting in the formation of gypsum.

Hydration expands volume and also results in rock deformation.

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• solution; as rain falls, it dissolves small amounts of carbon dioxide from the air, forming a weak acid that is able to dissolve limestone. It dissolves even more carbon dioxide as it seeps through the soil.

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• The acidic water, seeping into joints (cracks) in the rock gradually widens them and may produce “limestone pavements” like this one.
• As it sinks further, the water may begin to flow through the rock along larger joints and bedding planes, eventually widening them out to form cave systems.

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Physical weathering

• refers to the geological process of rocks breaking apart without changing their chemical composition. Over time, movements of the Earth and environment can break apart rock formations, causing physical weathering.
• Can also refer to other things in the environment breaking down, like soil and minerals. Pressure, warm temperatures, water and ice can cause physical weathering. �

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• Physical weathering is caused by the effects of changing temperature on rocks, causing the rock to break apart. The process is sometimes assisted by water.
• There are two main types of physical weathering:�Freeze-thaw occurs when water continually seeps into cracks, freezes and expands, eventually breaking the rock apart.

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• Exfoliation occurs as cracks develop parallel to the land surface a consequence of the reduction in pressure during uplift and erosion.
• Once the cracks develop, water enters and causes chemical weathering leading to the formation of new low-density minerals.
• This enhances the cracks and encourages slabs of rock to detach from the surface.

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Biological weathering

• This is a type of weathering that occurs when a force or pressure is applied in order to break rocks apart or degrade the minerals in them.
• By increasing the exposed surface area of rocks, they make it possible for other physical factors to speed up their degradation.

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BY PLANTS

• Plants can grow anywhere as long as there is water.
• Roots of trees or plants in general can biologically weather rocks by growing into the cracks and fractures of rocks and soil.
• As a result, they become more prone to breakage and eventually fall part.

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BY ANIMAL

• Burrowing animals like shrews, moles, earthworms, and even ants contribute to biological weathering.
• In particular, these animals create holes on the ground by excavation and move the rock fragments to the surface.
• As a result, these fragments become more exposed to other environmental factors that can further enhance their weathering.

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MASS WASTING

• Mass wasting/mass movement/slope movement, is defined as the large movement of rock, soil and debris downward due to the force of gravity.
• In other words, the earth's outer crust is being 'wasted' away on a 'massive' scale and falling to lower elevations.

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TYPES OF MASS WASTING

• A landslide, also known as a landslip, is a geological phenomenon that includes a wide range of ground movements, such as rock falls, deep failure of slopes and shallow debris flows.
• Landslides can occur in offshore, coastal and onshore environments.
• Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the original slope stability.

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• 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.

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• Soil Creeps is a long term process (slow movements). The combination of small movements of soil or rock in different directions over time are directed by gravity gradually downs lope.
• 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. This happens at a rate that is not noticeable to the naked eye.

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• The rate of soil creep down a slope depends on the steepness (gradient) of the slope, water absorption and content, type of sediment and material, and lastly vegetation.
• The rate of creep will take into account all of these factors to decide whether or not the hillside will progress downward.
• Creep is what is responsible for the rounded shape of hillsides.

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• Flows; 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.

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• Slump; a slipping of coherent rock material along the curved surface of a decline. Slumps involve a mass of soil or other material sliding along a curved surface (shaped like a spoon).
• It forms a small, crescent-shaped cliff, or abrupt scarp at the top end of the slope. There can be more than one scarp down the slope.

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• Falls; A fall, including rockfall, 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 characterised by presence of vertical cracks. Falls can result from undercutting by running water as well as from undercutting 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.

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Factors affecting mass movement

1.  Slope �     - gravity provides the energy to move the materials. �     - rapid movements  (e.g. landslide, slumping) are commonly found on steep slopes while slow movements (e.g. soil �        creep) are found on gentle slopes. �2.  Water �      - rainwater acts as a lubricant to weathered materials. �      - rainwater adds weight to weathered materials. �

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3.   Rock structure �      - joints dipping downslope usually assists landslides, rockfalls, rockslide, rock creep..... etc. 4.  Vegetation �      - lack of vegetation cover to hold the loose particles. �5.  Vibrations �      - earthquakes. �      - construction processes and mining operations.

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GLACIATION

• Glacial landforms are landforms created by the action of glaciers.
• Most of today's glacial landforms were created by the movement of large ice sheets during the Quaternary glaciations.

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• As the glaciers expanded, due to their accumulating weight of snow and ice, they crush and abrade scoured surface rocks and bedrock.
• The resulting erosional landforms include striations, cirques, glacial horns, arêtes, trim lines, U-shaped valleys, roches moutonnées, overdeepenings and hanging valleys.

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• Truncated spur – The triangular facet produced where the lower end of a ridge has been eroded by glacial ice.
• Rock-basin lake – A lake occupying a depression caused by glacial erosion of bedrock.
• Cirque – A steep-sided, amphitheater-like hollow carved into a mountain at the head of a glacial valley.
• Horn – A sharp peak formed where cirques cut back into a mountain on several sides.
• Arètes – A sharp ridge that separates adjacent glacial valleys.

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• The weight of the moving ice sheet produces rounded knobs and striated bedrock. 
• The orientation of the striations can be used to interpret the direction of movement of the ice sheet
• Thick ice sheets may bury mountain ranges, rounding and streamlining them in the direction of ice movement.

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Deposition by Glaciers

• Glaciers pick up rock fragments from the underlying bedrock and from the valley walls.
• Glaciers carry the debris without tumbling or sorting.  The deposited debris is called glacial till (angular, poorly sorted, and unlayered).  When till becomes lithified, it is called tillite.
• Large boulders transported large distances by glaciers are called erratics.

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• Moraine- An elongate deposit of glacial till.  There are several types:

1. Lateral moraine- Forms along the sides of a valley glacier as rocks fall from the steep cliffs of glacial valleys.

2. Medial moraine- Form where two tributary glaciers come together and adjacent lateral moraines join.

3. Recessional moraine- Deposits produced as glaciers recede during melting.

4. End moraine- A type of recessional moraine that forms at the terminus of a glacier during glacial recession as debris piles up long the front of the ice.  They tend to be crescent shaped.

5. Terminal moraine- Special type of end moraine that marks the farthest extent of the glacier.

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