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Lecture Outlines Physical Geology, 12/e Plummer & Carlson Copyright © The McGraw-Hill Companies, Inc Permission required for reproduction or display Mass Wasting Physical Geology 12/e, Chapter Mass Wasting • Mass wasting is downhill movement of masses of bedrock, rock debris or soil, driven by the pull of gravity • Landslides have been far more costly in the U.S., in terms of both lives and dollars, than all other geologic and weather hazards combined • Mass wasting is, with proper planning, perhaps the most easily avoidable of all major geologic hazards Classification of Mass Wasting • Types of mass wasting are classified based on: – Rate of movement (speed) • Wide range from < 1cm/year to >100 km/hour – Type of material • Did moving mass start out as solid bedrock or as debris (unconsolidated material at Earth’s surface) – Movement Type • Flow, slide, or fall Classification of Mass Wasting • Types of movement – Flow • Descending mass moves downhill as a viscous fluid – Slide • Descending mass remains relatively intact, and descends along well-defined surfaces • Translational slide - movement to motion along plane parallel • Rotational slide (slump) - movement surface along a curved – Fall • Material free-falls or bounces down a cliff Mass Wasting Chart • List the three classifications for which mass wasting is determined (Not the three types of movements.) Some types of Mass Wasting Page 222 What are the variables? How is the data chart set up? How does the chart specify material? Tell the specific type of mass wasting: flow, slow velocity, soil = creep bedrock, fall, fastest = rockfall debris, flow, varying speeds = debris flow bedrock, slide, varying speeds = rockslide Factors Controlling Mass Wasting page 226-228 explain each one Factors Controlling Mass Wasting • Factors making mass wasting likely: – Steep slopes • Shear forces maximized by gravity – Large relief • (large elevation change from top of mountains/hills to valley floor) – Thick layer(s) of loose rock, debris, soil – Presence of water • Lubricates moving rocks/debris/soil – Lack of vegetation • No roots to hold rock/soil in place – Seismic (earthquake) activity Factors Controlling Mass Wasting Common Types of Mass Wasting • Creep (or soil creep) – Very slow downslope movement of soil – Major contributing factors include water in soil and daily freeze-thaw cycles – Can be costly to maintain homes, etc., on creeping ground as foundations, walls, pipes and driveways crack and shift downslope over time Common Types of Mass Wasting • Debris flow - mass wasting in which motion takes place throughout the moving mass (flow) – Earthflow - debris moves downslope, slowly or rapidly, as a viscous fluid • Commonly occurs on steep hills, with thick debris cover, after heavy rains • Solifluction is an example – Mudflow - flowing mixture of debris and water, usually down a channel • Most likely to occur on steep unvegetated slopes with thick debris cover • Heavy rains on the slopes of stratocone volcanoes with fresh ash layers often triggers – Debris avalanches are very rapid and turbulent • Can reach speeds of several hundred km/hr Types of Mass Wasting • Rockfall - when a block of bedrock breaks free and falls or bounces down a cliff – Commonly an apron of fallen rock fragments (talus) accumulates at cliff base • Rockslide - the rapid sliding of a mass of bedrock along an inclined surface of weakness • Rock avalanche - a very rapidly moving, turbulent mass of broken-up bedrock • Debris slide - a coherent mass of debris moving along a well-defined surface • Debris fall - a free-falling mass of debris Preventing Landslides page 238 – 243 explain how to prevent mass wasting Preventing Landslides • Preventing mass wasting of debris – Construct retaining wall with drains – Don’t oversteepen slopes during construction • Preventing rockfalls and rockslides on highways – Remove all rock that is prone to sliding – “Stitch” together outcrop • Important to know the susceptibility of land to mass wasting before building any road or structure! End of Chapter
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