Chapter 15 Mass Wasting

Mass wasting The downslope movement of rock, regolith, soil, etc. under the direct influence of gravity Does not require a transporting medium (water, air, ice)

Plays a role in developing landforms (surface features) Weathering, by itself, doesn’t produce significant landforms Landforms are developed as weathering products are removed from their original place

Figure 15.2

Weathering weakens & breaks rock apart Mass wasting transports the debris downslope Generally to a stream, which carries material away This material may then be deposited anywhere downstream Ultimate destination – the ocean

For mass wasting to occur, need slopes that is, differences in elevation Most rapid mass wasting occurs in rugged, geologically-young mountains as mtn building subsides, mass wasting & erosion lowers the land leads to a gentler terrain

Causes of mass wasting Water Slope Vegetation Other “triggers”

Water’s role a major “trigger” heavy rains or snowmelt saturate surface materials pores in sediment fill w/water, reducing cohesion between particles particles can then slide apart easily (ex.: use slightly wet sand to make a sand castle. Add more water to the sand, what happens?)

Figure 15.4

Clay A “dry” clay is fairly rigid Wet clay is very “slick” Water also adds mass (“weight”), and helps to start movement of material downslope

Effects of slope If slope is too steep, material will move (oversteepening) Examples: Stream undercutting a valley wall Waves pounding against a cliff Human activity (that is, stupidity)

Angle of repose A characteristic of unconsolidated, granular materials (sand size or larger) The steepest angle where a pile of the material is stable Generally 25 – 40º from horizontal Larger, more angular fragments form the steepest slopes

Vegetation Root systems bind soil materials together Plants also shield underlying materials from erosional effects of raindrops More important when the vegetation removed Build a house on a scenic hillside, remove the natural vegetation so you have a view & a “normal” house – wait for a few heavy rains Wildfires (or clear-cutting) Removes vegetation Land exposed for erosion

Other triggers Earthquakes – can dislodge rock & unconsolidated materials that were semi-stable Liquefaction – shaking during EQ causes water-saturated sediments to lose strength & behave as a fluid Some movements occur without any obvious trigger

Mass Wasting Processes 4 main processes Slump Rockslide Debris flow Earthflow

These classes based on: Type of material Kind of motion Rate of motion

Type of material Type of motion Unconsolidated or bedrock Fall, slide, or flow

Fall: The freefall of detached, individual pieces (of any size) “Watch for falling rock”

Figure 15.8B

Figure 15.8A

Slide The material remains fairly coherent Moves along a well-defined surface This surface may be parallel to the slope, or curved “Landslide” – geologically, no definition. Yet, we all use the term to describe much mass wasting.

Flow Material moves downslope as a thick fluid Material almost always saturated w/water

Rate of movement Most movement is quite slow (more in a bit) Very rapid movement – generally asso. with rock avalanches Speeds >125 mph As best we can tell, air becomes entrapped beneath a mass of debris, creating a “rock hovercraft”

Slumps Downward sliding of a mass of rock/ unconsolidated material along a curved surface Material generally doesn’t move very fast or travel very far Often happens in multiple “blocks”

Figure 15.7A

Slumps often the result of oversteepening of a slope Examples (again) Valley wall cut by river Waves Overloading a slope (material deposited on top, humans build on edge of a slope)

Figure 15.12

La Conchita, California, 1995 U.S. Geological Survey photo by R. L. Schuster

Rockslide Blocks of bedrock break loose, slide downslope If material relatively unconsolidated, called a debris slide These tend to be the fastest & most destructive movements In areas where rock strata are inclined, or has joints/fractures parallel to the slope

Figure 15.7B Editor’s goof

Figure 15.14

Debris flow Involves the flow of soil/regolith containing large amounts of water Also called mudflows Generally seen in semiarid mountainous regions, and slopes of some volcanoes Flows often follow existing canyons & stream channels

Figure 15.7C

Flows in semi-arid areas Heavy rain or rapid snowmelt results in sudden floods Large amounts of soil, etc., washed into nearby streams Rate of flow depends on area slopes & water content of material If flow dense enough, can carry or push large boulders, trees, houses

Lahars A type of debris flow – defined as having 40% or more by weight of volcanic debris Mostly volcanic materials on the flanks of volcanoes May occur during eruption or when volcano quiet Unconsolidated layers of ash & debris become water saturated & flow downslope Caused by heavy rains, or melting of snow/ice as a result of pre-eruption heatflow

Earthflow Generally occurs on hillsides in humid areas as a result of heavy precipitation or snowmelt Material involved often rich in clay/silt-sized particles Movement generally slow (up to several meters per day) And, this isn’t the slowest movement

Figure 15.7D

Creep The slowest of the movements The gradual downhill movement of soil/regolith Often results from the alternate expansion/contraction of surficial materials by freeze/thaw cycles or wet/dry periods

Figure 15.18

Other causes of creep: Impact of raindrops Disturbance of material by plant roots and/or burrowing critters Saturation of ground with water (that pesky fluid again!)

Creep is a very slow process We can’t sit there and observe it happening (unless you have little else to do) We can see the effects of it, after a bit of time

Figure 15.19

Soil Creep, Yosemite National Park, California (A.A. Webb, Oct 2000) 

Fenceposts and telephone poles, on the other hand, don't grow vertically and merely tilt on creeping slopes.

Another fence

Some other ideas Solifluction The down-slope flow of water-saturated soil Occurs where water can’t escape from a saturated surface layer Usually due to an underlying dense clay layer or an impermeable rock layer Lewis Hills, Gros Morne National Park, Newfoundland

Geologic dictionary defines this as occurring in high elevations in regions underlain by frozen ground, which acts as a downward barrier to water movement Rate of movement 0.5 to 15 cm/yr

Permafrost A layer of unconsolidated material containing water which is frozen May be 30 cm to over 1000 m thick (1 to 3000 ft) When surface thaws (for whatever reason), water cannot seep down due to frozen material deeper down Surface becomes a mushy muck

Figure 15.G

Figure 15.I Log cabin shown here; any structure which generates heat

Upper, thawed layer can slowly flow Alaska pipeline A few years back Pipeline from the North Slope to the coast Oil needs to be warm to flow The pipeline would wreck havoc to Alaska’s permafrost area, not to mention the wildlife (oh, that’s biology or environmental science)

Geological activity has damaged the pipeline on several occasions Geological activity has damaged the pipeline on several occasions. Since its completion in 1977, the pipeline has transported over 15 billion barrels of oil.

View of the pipeline's underside, heat pipes, and radiators View of the pipeline's underside, heat pipes, and radiators. Outside Fairbanks, Alaska August 1980.

Mass wasting also occurs below the ocean surface These just aren’t seen as much Slides Along flanks of volcanic islands & seamounts Along continental margins, especially near deltas Contribute to tsunamis

A final thought on mass wasting…….

The depth of weathering is visible in the distant cut The depth of weathering is visible in the distant cut. These homes (costing $200,000 and up, when the photo was taken) were built on landslide deposits. The landslide deposits contain the ruins of the previous housing development built here. (http://www.uwgb.edu/dutchs/EarthSC202Slides/SOILSLID.HTM)