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Chapter 15 Weathering, Mass Wasting & Erosion Preliminaries to Erosion: Weathering and Mass Wasting Weathering Breaking down of rock into smaller components by atmospheric and biotic agents Mass Wasting Involves the downslope movement of broken rock due to gravity Erosion More extensive and distant removal of fragmented rock material Denudation Impacts of Weathering & Mass Wasting Weathering = fragmentation of rock in place – no obvious movement Mass Wasting = involves down slope movement – empty space above and debris strewn below Erosion = wearing away of the land by the action of water, ice or wind Denudation Weathering followed by mass wasting followed by erosion Results in a lowering of the landscape over millions of years Colorado River is major cause of erosion. Weather and mass wasting have also played a part in the carving of Grand Canyon Weathering Jointing Weathering Agents – Mechanical Weathering – Chemical Weathering – Biological Weathering Climate and Weathering Weathering 1st step in external process of rock disintegration Think of weathering as “aging of rock surfaces” Where lithosphere and atmosphere meet (land/air interface) Mechanical or chemical Exposed bedrock is prone to weathering – it’s more “fragile” Underlying rock is “stronger” Faults and Joints faults = displacement of land joints = no displacement of land no displacement displacement Faults The cracking and displacement of adjacent blocks of rock as a result of the severe stresses set up by tectonic activity Jointing Vertical Horizontal Cracks resulting from stress – very common Show no displacement Weathering Agents Mechanical Chemical Biological Most are atmospheric – easy for gas to penetrate bedrock Water can also penetrate Temperature change Biotic influence Mechanical Weathering Disintegration of rock material No change in chemical composition Mostly occurs at or near the surface Frost Wedging = freeze/thaw action of water Salt Wedging = salt crystallizes Temperature change = heat expansion & cold contraction Exfoliation = layers peel off Frost Wedging Water permeates cracks/fissures in rocks, then freezes, then melts Exfoliation Curved layers peel off bedrock Occurs at different scales Exfoliation - hill Exfoliation - single boulder Impact of Mechanical Weathering As rock is fragmented, the amount of rock surface increases More surface area exposed = more fragmentation Chemical Weathering Decomposition of rocks by alteration of minerals Chemical weathering requires moisture More in humid regions than arid Requires oxygen, water & carbon dioxide 3 processes are: Oxidation Hydrolosis Carbonation Oxidation Oxidation is the combination of a substance with oxygen Combines with metallic elements to form new compounds Usually softer and more easily eroded Red Rocks - Oxidation Oxygen combines with iron-bearing silicate minerals causing "rusting" Iron oxides are produced Iron oxides are red, orange, or brown in color "Georgia Red Clay" derives its color from the oxidation of iron bearing minerals Hydrolysis Decomposition of a chemical compound by reaction with water Igneous rock is very susceptible Increases volume of rock and brings on disintegration Carbonation Reaction between carbon dioxide in water & carbonate rocks Produces calcium bicarbonate (very soluble) Easily removed by runoff or percolation Biological Weathering Plants & Animals Roots, lichens Burrowing animals Climate and Weathering Weathering is enhanced by climate More chemical weathering with high temperatures and lots of rain Mass Wasting Fall Slide – Landslide – Slump Flow – Earthflow – Mudflow – Debris Flow Creep Mass Wasting 2nd step in external process of rock disintegration Movement of rock and soil downslope under the influence of gravity Denudation: weathering followed by mass wasting followed by erosion Fall: Falling of pieces of rock downslope Slide: Landslide – instantaneous collapse of a slope Flow: Gravity impelling force with water to bring section of a slope down i.e. mudflow Creep: Slowest and least perceptible gradual downhill movement Talus Slope Pieces of rock become wedged in slope, then are loosened and fall to bottom Visual Evidence of Creep Freeze –Thaw Processes Freezing lifts particle perpendicular, then melting deposits it further downslope
