Chapter 7: WEATHERING AND EROSION Objectives • Describe the two major kinds of rock weathering. • Identify three end products of weathering. • Explain the difference between weathering, erosion, and mass wasting. • Describe how ice, water, and air transport regolith across Earth’s surface. • Define and give examples of mass wasting by slope failure and/or sediment flow. Weathering-The First Step in the Rock Cycle Weathering-The First Step in the Rock Cycle • How rocks disintegrate – Weathering • The chemical and physical breakdown of rock exposed to air, moisture and living organisms – Regolith • A loose layer of fragments that covers much of Earth’s surface – Soil • The uppermost layer of regolith, The rock in the photo has weathered in which can support rooted plants place with little erosion, forming soil Weathering: Two Types • Mechanical engineering • The breakdown of rock into solid fragments by physical processes • Chemical composition of rock NOT altered • Chemical weathering • The decomposition of rocks and minerals by chemical and biochemical reactions • Joints – A fracture of rock , along which no appreciable movement has occurred – Sheet jointing or exfoliation – Frost wedging • Abrasion – The gradual wearing down of bedrock by the constant battering of loose particles transported by wind, water or The jointing in these rocks has ice exposed new surface area which has broken and smoothed due to wind, water and ice. Weathering exfoliation and frost wedging Frost wedging and biomechanical weathering Chemical weathering • Dissolution – The separation of materials into ions in a solution by a solvent, such as water or acid – Rainwater acts as weak solution of carbonic acid – Anthropogenic actions influence The marble grave marker has been attacked by acidity of rainwater acidic rain because of the calcite composition. The grave marker on the right, while old, has not been dissolved because of its granite composition Chemical weathering: ion exchange and the chemical breakdown of feldspar Factors affecting weathering • Tectonic setting – Young, rising mountains weather relatively rapidly – Mechanical weathering most common Factors affecting weathering • Rock composition – Minerals weather at different rates • Calcite weathers quickly through dissolution • Quartz is very resistant to chemical and mechanical weathering • Mafic rocks with ferromagnesian minerals weather more easily Factors affecting weathering • Rock structure – Distribution of joints influence rate of weathering • Relatively close joints weather faster Factors affecting weathering • Topography – Weathering occurs faster on steeper slopes • Rockslides Factors affecting weathering • Vegetation – Contribute to mechanical and chemical weathering – Promotes weathering due to increased water retention – Vegetation removal increases soil loss Vegetation can both hold water And increase weathering. If removed Rocks may also be vulnerable to abrasion Factors affecting weathering • Biologic activity – Presence of bacteria can increase breakdown of rock Factors affecting weathering • Climate – Chemical weathering is more prevalent in warm, wet tropical climates • Mechanical weathering less important here – Mechanical weathering is more prevalent in cold, relatively dry regions • Chemical weathering occurs slowly here Note: temperate regions such as at the center of the chart undergo both chemical and mechanical weathering, i.e. New York area Factors affecting weathering: color dots on map match colors on chart Products of Weathering • Clay – Tiny mineral particles of any kind that have physical properties like those of the clay minerals – Clays are hydrous alumino-silicate minerals Products of Weathering • Sand – A sediment made of relatively coarse mineral grains • Soil – Mixture of minerals with different grain sizes, along with some materials of biologic origin – Humus – Partially decayed organic matter in soil Erosion and Mass Wasting Erosion is the removal of weathering products from the source and most often occurs by water • Erosion – The wearing away of bedrock and transport of loosened particles by a fluid, such as water – Example: Sediment moved along the bottom of a stream Erosion and Mass Wasting • Erosion by wind Particles of sand are transported close to the surface. finer particles of silt and clay can be transported great distances • Erosion by ice – Glacier • A semi-permanent or perenially frozen body of ice, consisting of recrystallized snow, that moves under the pull of gravity Wind-blown fine sediments such as this dust cloud can Be transported across oceans Erosion and Mass Wasting Left: deposits of unsorted glacial till from glacier Right: rock polished and striated by glacier Erosion by ice: glacier removes, breaks and transports rock pieces glaciers scour valleys and deposit piles of debris as moraines Erosion and Mass Wasting • Mass wasting – The downslope movement of regolith and/or bedrock masses due to the pull of gravity • Slope failure – Falling, slumping or sliding of relatively coherent masses of rock Erosion and Mass Wasting: Rock slide, rock fall, and slumping result in downhill transport of broken rock Erosion and Mass Wasting • Flow: If water or air combines with the downward movement, the regolith can “flow” downhill • Creep – The imperceptibly slow downslope flow of regolith • Unstable slopes move very slowly over long periods of time Erosion and Mass Wasting Why do major landslides occur near plate boundaries? • Tectonics and mass wasting – World’s major historic landslides clustered near converging lithospheric plates • High mountains undergo rapid weathering • Earthquakes near plate boundaries can trigger landslides This massive slide was triggered by A magnitude 9 earthquake in Alaska near a subduction zone. Critical Thinking • On Earth, clay minerals are the most common products of weathering. Samples from the Moon do not contain any clay minerals. Why? • Why are some granite bodies extensively jointed, while others are essentially joint free? Critical Thinking • On Earth, clay minerals are the most common products of weathering. Samples from the Moon do not contain any clay minerals. Why? • Why are some granite bodies extensively jointed, while others are essentially joint free?
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