Gradation What is gradation? Gradation refers to the processes which bring a land surface to a state of uniform grade by eroding the land. Another word, denudation also refers to the levelling processes which erode land features. Degradation Degradation refers to the processes of weathering, erosion and transportation. Weathering Weathering is the disintegration or decomposition of rocks on or near the earths surface. There are two types of weathering Physical weathering in which rock materials are eroded by pressure applied to them by heat, wind, water, ice etc and Chemical weathering in which the compounds of rock materials are chemically changed by contact with water or air. Physical weathering leaves angular broken fragments while chemical weathering leaves rounded surfaces. Kinds of Physical Weathering Frost shattering is when water seeps into rock fractures, freezes, expands and splits a rock apart. If this happens regularly due to fluctuating temperatures above and below zero, this is referred to as ice wedging. Thermal Expansion When rocks composed of different minerals (like granite) are heated causing thermal expansion and then cooled (as in a desert), the minerals expand and contract at different rates which eventually cause the rock to fragment apart. Pressure Release When rocks that have been under pressure have overlying layers eroded away, the lower pressure can cause cracks and fractures to develop. Exfoliation (the flaking off of the outer parts of a rock) and sheeting (the splitting off of sheets of rock from its outside) are two terms used to describe rocks that are breaking up due to pressure release. The Physical Activity of Animals and Plants Burrowing animals expose rock materials to water and air while plant roots probing rock cracks and growing in width may split rock materials. Growing plant roots can exert phenomenal pressure to crack rock materials. Crystal Growth When water seeps into rock pores, dissolving minerals in the rocks and then evaporates, it often leaves behind salt crystals which expand in the pores causing pressure which splits the rock apart. This is a common process in desert areas leading to caves at the base of sandstone formations like Mesa Verde. Kinds of Chemical Weathering - Solution Solution weathering is a kind of chemical weathering in which some chemical is dissolved in water and this solution acts on rocks, dissolving them, causing pores and cracks. A very common kind of solution weathering is when rain passing through the air or water passing through soil reacts with carbon dioxide gas to form hydrogen ions (H + - acid) and bicarbonate ions (HCO 3 - ). Magnesium carbonate (MgCO 3 ) and calcium carbonate (limestone - CaCO 3 ) rocks dissolve with constant exposure to hydrogen ions. Chemical Weathering - Oxidation Oxygen in air reacts with many compounds in rocks, making oxides. Iron oxide forms reddish brown stains where oxygen from the air has combined with iron in rocks. Iron oxide has a tendency to expand and crack rocks apart. Chemical Weathering - Hydrolysis Water ionizes as follows: H 2 O H + + OH -. When these ions of water replace ions of a mineral, clay materials are formed, a substance called kaolinite. This kind of chemical weathering occurs commonly in warm moist climates. Erosion Erosion is the removal and movement of rock debris and associated organic matter. Transportation Transportation is the movement of eroded debris by running water, ice wind or wave action. Aggradation Aggradation is the building up of a land surface by the deposition (depositing) of rock materials. Rate and Depth of Weathering: Temperature and Moisture At colder temperatures, physical weathering is most common. The greatest and deepest weathering takes place in forest biomes. The greater the moisture and temperature, the greater and deeper the chemical weathering of rock substrates. Rate of Weathering: Joints and Planes Joints are cracks in rocks caused by tectonic stresses. Bedding planes are parallel surfaces to rocks layers. The greater the number of joints and bedding planes, the faster rock materials will weather because the cracks in rocks create more surface area for water, solutions, and gases to pass in and through the rocks. Weathering Affected by Temperature and Moisture High temperatures and moisture conditions favour strong chemical weathering while colder temperatures and less moisture favours strong physical weathering. Mass Wasting Mass wasting is the downhill movement of weathered materials resulting from the pull of gravity. The more water content a weathered slope contains, the more likely mass wasting will be. Soil Creep Soil Creep is the slow movement of loose rock and soil downward. It causes a downslope tilt on fences, stone walls, tree trunks, gravestones. Soil Flow or Solifluction Solifluction is a downhill flow of water-saturated rock and soil materials. Solifluction is common in colder climates during periods when the soil melts and flows. Rock Glaciers and Avalanches Rock glaciers are large volumes of rock fragments that accumulate and move downhill, flowing slowly like ice glaciers. Rock glaciers are common in high mountains of humid environments. Rapid flows of snow, ice, earth and/or rock masses are called avalanches. Rock Slides A rock slide is a rapid movement of weathered rocks splitting off along bedding planes in a mountainous region. A Slump A slump is when a block of clay-rich soil and rock slips along a concave surface. Debris Slide A debris slide is a rapid movement of loose soil and rock in a steeply sloping terrain that is shaken loose by an earthquake or is undercut by a river or glacial erosion. Talus and Scree Talus and scree refer to the rubble that accumulates in fan-shaped piles from physical weathering of uplifted rocks or mountains. Talus technically refers more to the sloping debris shape while scree refers to the rock debris itself. Factors Affecting Mass Wasting Both friction and gravity affect how much mass wasting a terrain will experience. Friction is the internal resistance to movement and is reduced by the weigh and lubricating effect of water. Gravity is the earths force that pulls on slopes causing them to move. The steeper a land surface the greater the moving force of gravity becomes. The Effect of Climate on Mass Wasting Warm, moist climates foster more mass wasting while cooler and drier climates produce less mass wasting. End of Presentation