Types of weathering • Biological Weathering• Physical [mechanical] Weathering • Chemical Weathering
Biological weathering• Simple breaking of particles, by the consumption of soils particles by animals.• Particles can also fracture because of animal burrowing or by the pressure put forth by growing roots
Roots and Burrowing• The growth of roots or the burrowing of animals.• Tree roots are probably the most significant agents of biological weathering as they are capable of prising apart rocks by growing into cracks and joints
Physical [mechanical] weathering• This is the breakdown of mineral or rock material by entirely mechanical methods brought about by a variety of causes• Some of the forces originate within the rock or mineral, while others are applied externally.• Both of these stresses lead to strain and the rupture of the rock.• The processes that may cause mechanical rupture are frost shattering and pressure release.
Frost shattering• The transformation from liquid to solid crystalline form produces a volumetric change which in turn causes the necessary mechanical action for rupture.• They occur with ice and salt. Upon freezing the volumetric change of water from liquid to solid is 9%.• This relatively large volumetric change upon freezing has potentially a great rupturing effect.• The threshold temperature for frost action is at least - 5° Celsius
Pressure release• As erosion brings rock formations to the surface, they become subjected to less pressure.• This unloading of pressure causes the rocks to fracture horizontally with an increasing number of fractures as the rock approaches the Earths surface.
Chemical Weathering• Involves the alteration of the chemical and mineralogical composition of the weathered material.• The most common chemical weathering processes are oxidation, reduction, carbonation, and solution.
Oxidation• The reaction that occurs between compounds and oxygen.• The net result of this reaction is the removal of one or more electrons from a compound, which causes the structure to be less rigid and increasingly unstable.• Reduction is simply the reverse of oxidation, and is thus caused by the addition of one or more electrons producing a more stable compound.
Carbonation• The reaction of carbonate and bicarbonate ions with minerals. The formation of carbonic acid, a product of carbon dioxide and water, is important in the solution of carbonates and the decomposition of mineral surfaces because of its acidic nature
Hydrolysis• The weathering reaction that occurs when the two surfaces of water and compound meet. It involves the reaction between mineral ions and the ions of water (OH- and H+), and results in the decomposition of the rock surface by forming new compounds, and by increasing the pH of the solution involved through the release of the hydroxide ions
solution• Water and the ions it carries as it moves through and around rocks and minerals can further the weathering process. The effects of dissolved carbon dioxide and hydrogen ions in water. Molecules mix in solution to form a variety of acidic decompositional compounds. The extent, however, of rock being subjected to solution is determined primarily by climatic conditions.
Mass Movement• Mass movement is the down slope movement of earth materials under the influence of gravity. The detachment and movement of earth materials occurs if the stress imposed is greater than the strength of the material to hold it in place
Types of mass movement• Soil creep• Slide• Slumps• Solifluction• Flow• Rock fall
Soil creep• Is the slowest of all types of mass movement. Soil creep generally occurs in the top few meters of the surface and is accomplished by expansion and contraction of the soil• Trees growing on surfaces undergoing creep will have curved trunks or roots that are curved. Broken retaining walls and curved railroad tracks also indicate creep in action
Slide• A slide is a sheet of material that slips over a failure plane. Slides produce concave scars while slumps tend to produce a scarp or cliff exposure• Largely retain their internal structure and move large mass• Concave scar typically produced by a slide
Slumps• characterized by a backward rotation of the earth material as it moves along a curved failure plane resulting in a reverse slope.• Slumps take place as an intermittent movement of earth or rock material, often as several independent units, creating a number of step-like "terracettes.”• Undercutting of slopes by stream erosion, waves, and road building are common causes of slumping
Soilifluction• Down slope movement of soil over a permanently frozen subsurface.• Solifluction is common on slopes underlain by permafrost. During the summer when the upper permafrost is activated.• waterlogged soil mass slowly moves down slope to form solifluction lobes or terraces
Flow• The down slope movement of water-saturated soil, regolith, weak shale, or weak clay layers.• Earth flows are fairly slow, occurring over a few hours or so slow that they are almost imperceptible.• Earth flows are accompanied with slumping, but unlike slumping, there is no backward rotation. Earth flows differ from mudflows in that they• (1) tend to be slower• (2) are not confined to channels• (3) are more common in humid areas than dry, and• (4) have a lower water content
Rock fall• The most sudden forms of mass movement.• Rock fall occurs when blocks of rock shed from a cliff face and collect at the base.• Talus is a term that is applied to an accumulation of rock by rock fall.
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