Resources landslides.usgs.gov Murck, Skinner, and Porter, 1997. Dangerous Earth: An Introduction to Geologic Hazards, John Wiley & Sons, 300 p. Bloom, 1998. Geomorphology: A Systematic Analysis of Late Cenozoic Landforms, Prentice Hall,482 p.
Mass Wasting Collective term for all gravitational or downslope movements of weathered rock debris The term “landslide” is not a real geologic term Flowing water not involved, but water is very important in mass wasting
Role of Water in Mass Wasting Adds weight to rock mass Generates seepage pressure by groundwater flow
The empty can on the left slides on a dry surface when the angle reaches about 17 degrees. With a little water dripping thgough a small hole in the bottom of the can on the right, sliding will occur at a much lower angle
A slope is needed to give a horizontal component of force
Angle of repose is the natural steepnees of a pile of unconsolidated material. It varies depending on grain size, grain shape, and water content.
Types of Mass Movements Slides—blocks Flows—internal shear Heaves—vertical Subsidence—vertical but no free surface
Modifiers Composition –rock, soil, mud, mixture Speed of movement –Fast, moderate, slow
Basic Types of Mass Movements Slides Flows Creep Subsidence There is no single perfect way to classify all of the types of mass movements
Slides Cohesive blocks of material Moven on well-defined surface No internal shearing Velocity profile of pure slide: surface slide plane
Flows Differential shearing within mass No clear plane of movement at base Velocity of flow decreases with depth in flow Velocity profile of pure flow: surface base of flow
Heave Forces act perpendicular to ground surface by expansion and contractin of material Slow, downslope movement by gravity Velocity profile of pure heave: surface base of heave
Subsidence No free surface Downward settling of material Little or no horizontal motion Main cause is slow removal of material below subsiding mass
Speed of Movement Creep—very slow Slides—fast Avalanche—really fast
One way to classify mass movements is by speed of movement and by composition—slurry flows versus granular flows.
Geologic Controls on Mass Movements Passive Controls –Nature of material Degree of lithification Degree of weathering Interstitial water –Stratigraphy –Structure –Topogoraphy –Climate –Organic (vegetation/critters) Active Controls –Support removal –Slope oversteepening –Saturation with water –Earthquakes
Examples of Passive Controls Nature of material –Degree of lithification (sed rocks) –Degree of weathering –Insterstitial water –All pretty self explanatory
Passive Control—Stratigraphy –Alternating resistant and nonresistant rocks Perched water table SS SH
Passive Control—Stratigraphy –Alternating resistant and nonresistant rocks SS SH collapse Erosion removes support
Passive Control—Structure Dipping beds, jointed rock
Mudflows from 1980 eruption of Mount St. Helens reached speeds as high as 40 m/s and traveled as far as the Columbia river, ovee 90 km away.
A 1963 landslide into the Vaiont Reservoir, Italy, caused a giant wave to overflow the dam
The 1920 Grand Banks earthquake caused a submarine landslide that broke underwater telegraph cables and disrupted communications between North America and Eurpoe. It also proved the existence of turbidity currents.
There are significant landslide hazards in the eastern U.S.
La Conchita, CA, landslide, Spring 1995. USGS photo.
Mameyes, Puerto Rico, landslide, October 1985. USGS photo.
Landslide susceptibility map and recommended land use for Congress Springs area near San Francisco