Mass Movement - Unit 2

Mass movement

Downslope movement of geologic materials due to gravity ; Also known as mass wasting

as _____ change, new material is exposed

slopes

alteration of the slope changes a _____ terrain into a ______ terrain

rugged; subdued

Several factors determine if mass movement will occur

controls, enablers, triggers

controls

Establish limits for onset of mass movement; - Angle of Repose

- Water content

enablers

Make mass movement more likely;

- Oversteepened slopes

- Removal of vegetation

- Weakened material

- Water content

triggers

Set mass movement in motion;

- Earthquakes

- Water content

Angle of Repose

Control; Maximum slope angle that can be

maintained by unconsolidated, non-cohesive sediment before mass movement occurs

Water content (small amounts)

Small amounts of water can make

sediment stick together;

- Moist unconsolidated sediments can maintain a steeper slope than dry sediments

water content (large amounts)

Large amounts of water (enough to reach

water saturation) have opposite effect

- Can eliminate ability of sediments to

maintain any slope

o Lubrication effect

o Added weight

Oversteepened slopes

Enabler; Slopes that exceed the angle of repose

- Can develop naturally or through human activities

Removal of vegetation

Enabler; Plants can stabilize slopes

- Roots

- Above ground growth

Weakened material

Enabler; Weathering weakens materials

- Could result in mass movement

on a slope without a major trigger

Earthquakes

Trigger; Can dislodge rocks and unconsolidated materials

Mass movement processes are primarily classified by...

- Type of material involved, and

- Type of motion or Rate of movement

Type of material involved

- Soil and regolith

- Rock

Type of motion

- Fall

- slide

- flow

- creep

Material: soil and regolith

- Debris, mud and earth qualifiers are used; e.g., Debris flow, Mudflow, Earthflow

Material: Rock

- Rock and block qualifiers are used;

e.g., Rock fall, Block slide

Motion: Fall

- Free fall of detached pieces

- Material distinctions (qualifiers)

o Rock fall

o Debris fall

Movement: Slide

- Movement along a zone of weakness

- Material distinctions (qualifiers)

o Debris slide

o Rock slide

o Block slide

Transitional slide

Material moves along a relatively flat surface

Rotational slide

Also known as slumps; Material moves along a surface that is concave up

Motion: flow

Material moves as a fluid with a relatively high viscosity

- Material distinctions (qualifiers)

o Debris flow

o Mud flow

o Earthflow

Motion: creep

Gradual downhill movement of soil and regolith; Often aided by heave movement

heave movement

Alternate expansion and contraction of surface material

Fast movement flow

Start-to-finish takes only seconds to minutes

o Falls

o Slides/slumps

o Most flows

Slow movement flow

mm to m per day

o Earthflows

Very slow movement flow

mm to cm per year

o Creeps

Avalanches

Most rapid form of mass movement

- Type of movement may change during event

- Material distinctions (qualifiers)

§ Rock avalanche

§ Debris avalanche