CH 09 MASS WASTING

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Lecture Outlines Physical Geology, 10/e Plummer, McGeary & Carlson Mass Wasting Physical Geology 10/e, Chapter Steve Kadel, Glendale Community College Mass Wasting • Mass wasting is downhill movement of masses of bedrock, rock debris or soil, under the pull of gravity • Landslides are much more costly over time 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 the major geologic hazards Classification of Mass Wasting • Types of mass wasting are classified based on: – Rate of movement • 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) – Type of movement • 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 along plane parallel to motion • Rotational slide (slump) - movement along a curved surface – Fall • Material free-falls or bounces down a cliff 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 Types of Mass Wasting • Creep – Very slow downslope movement of soil or unconsolidated debris – 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 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 of saturated debris 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 are 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 • 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 [...]...End of Chapter 9
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