Lecture Outlines Physical Geology Earth Revealed, 6/e

1. Lecture OutlinesPhysical GeologyEarth Revealed, 6/e McGeary, Plummer & Carlson

2. Mass WastingPhysical Geology: Earth Revealed, 6/e Chapter 13 Steve Kadel, Glendale Community College

3. Mass Wasting • Mass wastingis 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. End of Chapter 13