Mass movement

The process that transports Earth material (bedrock, sediment, soil) down slopes by the pull of gravity

Follows after weathering

May be fast or slow

Can occur on steep slopes or shallow dipping slopes

Occur when factors that drive materials downslope overcome the factors that resist downslope movement

Mass movements

In 1970, an earthquake-induced rock and snow avalanche on Mt. Huascaran, Peru, buried the towns of Yungay and Ranrahirca.

The death toll from the earthquake and landslide was 66,700.

The avalanche started as a sliding mass of glacial ice and rock.

The avalanche swept about 16.5 km to Yungay at a speed of 200 km/hr.

The fast-moving mass picked up glacial deposits and by the time it reached Yungay, it is estimated to have consisted of about 50100 million m3 of water, mud, and rocks.

Mass movements

Mass movementsYungay, Peru, May 31, 1970

Mass movements

Mass movement

I. Causes

A. Gravity, Friction, and Slope

1. Gravity: The principle force tending to pull materials downslope

2. Friction: resists the downslope movement of material

3. Slope: (all else being equal) the steeper the slope, the more likely that the materials will move downslope (mass movement)

Mass movements

Mass movement

I. Causes

B. Composition of Materials

1. Solid bedrock: generally stable = prohibits mass movements.

Tectonic deformation produce fractures and joints = unstable

a. Sedimentary bedding planes may slide along

b. Mechanical weathering has created cracks (exfoliation, freeze-thaw)

c. Metamorphic rock with foliation, if foliation lies parallel to the slope then the rock can slide along it

Mass movements

Highway 20 east of Newhalem.

Mass movements

The United States Geological Survey will install an instrument on the hillside to collect data on vibration and movement. Unstable rocks fill Afternoon

Creek. Compare the rocks with the full grown trees on the left and right.

Mass movement

I. Causes

B. Composition of Materials

1. Solid bedrock

Mass movements

Mass movement

I. Causes

B. Composition of Materials

2. Unconsolidated (loose) sediment

a. Slopes of loose material can only get so steep before the particles on their slopes start sliding down

b. Angle of repose—maximum angle at which unconsolidated material is stable

Sand = 30 to 35Cinder = 30 to 40

Mass movements

Mass movement

I. Causes

B. Composition of Materials

2. Unconsolidated (loose) sediment

a. Slopes of loose material can only get so steep before the particles on their slopes start sliding down

b. Angle of repose—maximum angle at which unconsolidated material is stable

Sand = 30 to 35Cinder = 30 to 40

Mass movements

Mass movement

I. Causes

C. Vegetation

Vegetation, especially those with deep root networks, helps hold material on a slope

Replanting efforts along roadcuts susceptible to mass movements)

Vegetation removed by forest fires, clear cuts, people (farmers)

Mass movements

Mass movement

I. Causes

D. Water

Most important factor causing previously stable slopes to slide

A small amount can increase cohesiveness = binds by surface tension

Excessive water promotes slope failure

Reduces friction between surface material and underlying rocks

Reduces cohesiveness/friction between individual grains = easier to flow

Mass movements

Mass movement

I. Causes

D. Water

Most important factor causing previously stable slopes to slide

A small amount can increase cohesiveness = binds by surface tension

Excessive water promotes slope failure

Reduces friction between surface material and underlying rocks

Reduces cohesiveness/friction between individual grains = easier to flow

Mass movements

Mass movements

II. Types of Mass Movements

Classified based on the speed and manner in which they travel downslope

A. Slow

1. Creep Slowest form (cm or mm per

year)

Moves unconsolidated material down-slope

Occurs everywhere, even on gentle slopes

Affects only the top few meters; top moves faster than bottom (due to friction) so objects in the soil start to lean over

Mass movements

Caused by: freeze-thaw, or wetting-drying events

or: rain drop splashes, gravity, animals burrowing, wind

Mass movements- Creep

Mass movements

II. Types of Mass Movements

2. Solifluction (soil flow)

Unconsolidated material

Occurs in very cold environments where surface is frozen to hundreds of meters

In summer, warm sun melts top meter or so of frozen ground

Water can’t percolate downward (because of permafrost), so soil gets water-saturated and flows downslope

Same process can occur where there’s impermeable bedrock or a clay layer

2-6” per year

Mass movements

II. Types of Mass Movements

2. Solifluction (soil flow)

Unconsolidated material

Occurs in very cold environments where surface is frozen to hundreds of meters

In summer, warm sun melts top meter or so of frozen ground

Water that is created can’t go down (because of permafrost), so soil gets water-saturated and flows downslope

Same process can occur where there’s impermeable bedrock or a clay layer

2-6” per year

Mass movements

II. Types of Mass Movements

B. Rapidkm/hour or even meters/second

1. Falls

Fastest type

Rock or sediment breaks free and falls vertically or near vertically

Any size of material

Main way that talus slopes are built

Mass movements

II. Types of Mass Movements

B. Rapid

2. Slide (rock slide, debris slide)

Rock or sediment breaks loose and moves in contact with an underlying slope along a preexisting plane of weakness

Bedding, fault, fracture, foliation

Moves as a single, intact mass

Small displacement of soil over bedrock to whole mountainside slabs of rock

Slip plane is generally flat

Mass movements

Mass movements

II. Types of Mass Movements

B. Rapid

3. Slump

A slide that separates along a concave up slip surface

Generally create a slip plane within unconsolidated material

Material moves as a coherent unit—sometimes multiple blocks

Forms crescent-shaped scar at the head where the material detached = scarp

Does not travel far

Does not travel very fast

Mass movements

II. Types of Mass Movements

B. Rapid

4. Flow

Mixture of sediment and soil moves downslope as a highly viscous fluid

Most are water saturated

Velocity depends on: water content, transport materials, underlying material, slope

a. Earthflows:Relatively dry soil, slow, small particles, high viscosity, slow (meters/hour – meters/min)

Mass movements

II. Types of Mass Movements

B. Rapid

4. Flow

b. Mudflows:saturated with water, fast, generally particles smaller than sand, consistency (wet concrete to muddy water), flow down canyons

c. Debris flows:Similar to mudflows water-saturated, fast (1-25 mph), composed of debris larger than sand size-boulders, need steep slopes

d. Debris or Rock avalanches: Special kind that moves very fast, very steep slopes, they “float” on a cushion of air trapped beneath

Mass movements

Mayflower Mountain debris flow in the Ten Mile Range near Climax, Colorado, in 1961. Note the large boulders transported by the flow. Finer material has been eroded from the top of the flow.

Due Monday, March 6st at beginning of class

Go to www.geology.cwu.edu and enter 101 in left navbar search field.

On G101 web page, download Problem Set #3, PDF file.

Discharge, stream velocity, wetting perimeterSimple calculationsNeed to understand units of calculations (e.g., m/sec or m3/sec)No late assignments accepted

Problem Set 3

Sheep Mountain Slide

Sheep Mountain Slide

Turtle Mountain Landslide

Sheep Mountain Slide

This house, which has been the subject of several news stories, is located on East Boston Terrace in the Capitol Hill area of Seattle. It hangs precariously over the headscarp of a reactivated old landslide.

This house is in the same drainage as, and a few hundred feet upslope of, a house destroyed by a landslide in 1942. That landslide killed one resident and seriously injured another.

One of the larger Seattle landslides along Perkins Lane on the southwest side of the Magnolia neighborhood. This is an area of continuing large-scale instability. Immediately following the February 1996 storms, a 1,500 -yd³ landslide slid from the upper portion of the bluff into the back yard of the home on the right.

Geo-engineers attempted to mitigate the problem by regrading and revegetating the upper slope. The slide was reactivated in December 1996, damaging at least five houses.

This view to the west over the Magnolia Bridge, a major artery into downtown Seattle, shows the landslide that forced the closure of the bridge and the "red-tagging" (condemning or declaring uninhabitable) of at least five homes along the headscarp of the slide.

This slide occurred after the rains had ceased.

Notice the displaced bridge trusses, the debris on the house at the base of the slope, and the broken water main just below the fallen truss and above the house.

Useless Bay area of Whidbey Island can cause periodic retreat of the bluff edge by as much as 20 feet or so in seconds. During this recent slide, a portion of the fence in front of the large house was lost.

Such episodes commonly are preceded and followed by decades of little erosion, making estimates of average bluff retreat rates potentially meaningless.

Continuing slide activity has made this home near Cape George in Jefferson County uninhabitable. At this site, the perching layer is a small area of ancient lakebed silt that lies beneath the house at about mid-bluff level, now covered with grass dropped from the back lawn.

Note the old slide mass at beach level (lower left), now covered by alder trees that are all of approximately the same age (here, perhaps 25 years old).