Mass Wasting Physical Geology 1 Chapter 9

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: 1.Rate of movement Wide range from < 1cm/year to >100 km/hour 2.Type of material Did moving mass start out as solid bedrock or as debris (unconsolidated material at Earth’s surface) 3.Type of movement Flow, slide, or fall ?

Types of movement Flow Slide Fall 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: 1.Steep slopes Shear forces maximized by gravity 2.Large relief (large elevation change from top of mountains/hills to valley floor) 3.Thick layer(s) of loose rock, debris, soil 4. Presence of water Lubricates moving rocks/debris/soil 5. Lack of vegetation No roots to hold rock/soil in place 6. Seismic (earthquake) activity

Types of Mass Wasting 1. 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 con’t. 2. Debris flow - mass wasting in which motion takes place throughout the moving mass (flow) 3. 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 4. 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 5. Debris avalanches are very rapid and turbulent Can reach speeds of several hundred km/hr

Types of Mass Wasting con’t. 6. 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 7. Rockslide - the rapid sliding of a mass of bedrock along an inclined surface of weakness 8. Rock avalanche - a very rapidly moving, turbulent mass of broken-up bedrock 9. Debris slide - a coherent mass of debris moving along a well-defined surface 10.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 9