Lecture notes ‐ Bill Engstrom: Instructor Mass Wasting GLG 101 – Physical Geology

Although changes can occur slowly over geologic time, sometimes catastrophic changes occur as soil, rock debris, and/or bedrock move in bulk at the Earth's surface. This is called “mass wasting”, and it’s a form of erosion/transportation. The movement (downhill) is controlled by gravity as the main driving force.

Why it’s important

On average, forms of mass wasting such as landslides, mudslides and avalanches actually account for more loss of life and property damage than do earthquakes and volcanoes. You should be aware of the signs and hazards when buying a home, visiting an area/traveling, etc. We’ll cover some of the more important things you need to know in this section.

These are some of the gravitational forces/factors that control mass wasting

• Weight of material‐ mass acted upon by gravity is weight • Normal force‐ component of gravitational force perpendicular to slope • Shear force‐ component of gravitational force parallel to slope • Shear Strength‐ Cohesive force of material‐ resistance to deformation (breaking or flowing) • Friction‐ force resisting movement between two bodies

In short……gravity = the cause of Mass Wasting

Water – critical factor. The potential for mass wasting is directly related to the amount of water in the material (rock or debris etc.)

– A small amount of water ‐ helps hold material together

– Too much water – pushes grains apart/reduces shear strengh & material moves faster. This diminishes particle cohesion (friction)

– Water also adds weight

– Freeze/thaw ‐ breaks up material. Freezing = a lifting force – particles separated from underlying material & allows for “creep”.

– The expansion/contraction of clayey debris can also cause creep

Controls and Triggers of Mass Wasting. There are a number of factors we will cover later on. However, one of the most important is the slope angle.

• Oversteepening of slopes—slope angle

– Stable slope angle (angle of repose) is different for various materials. – Oversteepened slopes are unstable.

Classification of Mass Wasting – There are different ways to classify mass wasting

• Classification by Type of Material

• Debris (unconsolidated material)

• Bedrock

• Classification by Type of Movement

• Flow (viscous fluid – incoherent mass)

• Slip

• movement along defined planes

• coherent blocks

• Form slides and slumps

• Fall – free fall off cliffs

• Classification by Rate of Movement

• Creep (< 1 cm/year)

• Flows and Slides (1mm/day to 5km/hour)

• Falls and Avalanches (>5km/hour)

• Landslides = Flows, Slides, Falls, Avalanches

Bottom line ….. It is difficult to classify – Lets look at some examples of different types.

Types of Mass Wasting

Slumps

• Movement of a mass of rock or unconsolidated material as a unit along a curved surface

• Occurs along oversteepened slopes

Debris flow (mudflows/earthflows)

• Consists of soil and regolith with a large amount of water

• Often confined to channels • Material flows like a viscous liquid

Lahars = a special type

• Can happen when rainfall saturates ash and debris flows downhill,

Or

• When ice and snow are melted during and eruption, saturating ash and debris.

Slips and Slides

Rock slides

– Blocks of bedrock slide down a slope.

– Moving along planes of weakness (e.g. foliation)

– Generally very fast and destructive

– May break up into avalanches

Debris slides and slumps

– Coherent mass moves along plane of weakness

– Debris slumps ‐ rotational

Rock Falls

• Rocks freely fall from cliffs.

• May be caused by:

– wave action (undercutting)

– human activity (bulldozers); or

– from frost heaving along steep cliffs.

Debris Falls

– Starts as coherent mass

– May become a debris avalanche

Rock fall‐ Talus

Talus = apron of debris of broken rocks at base of large cliffs.

Factors that Influence Mass Wasting:

Slope Angle Steeper slope = higher shear force (less stable)

Relief

• Relief = difference in highest and lowest elevations.

• Higher relief – moving masses build up more momentum/speed.

Rock Type Loose debris‐less stable than solid rock

Vegetation Roots hold debris together More vegetation = more stable However, in solid rock roots can help break up material

Climate freeze/thaw‐expansion/contraction lifts materials followed by melting & movement (gravity) – “creep”

Climate – precipitation

• Frequent precipitation – inhibits mass wasting (less likely to move)

Materials held together/surface tension of water

• Heavy or intermittent ppt – may enhance mass wasting (more likely to move)

Less vegetation/quick saturation

Pre‐existing weaknesses (planes of weakness) Potential planes of weakness

• Foliations – metamorphic rocks

• Bedding – sedimentary rocks

• Sheet joints / cooling joints / fractures and faults

Planes of weakness parallel to a slope = unstable

Planes Perpendicular to a slope = stable

General rule‐ if planes are oriented perpendicular to slope, system is most stable.

Planes oriented parallel to shear force is most hazardous

Debris Thickness & Addition of Weight

Thick debris over bedrock – It weighs more and it is susceptible to holding large quantities of water

Addition of weight to upper slope‐ extra force applied on slope‐ a push

Earthquakes & Volcanism

Earthquakes can trigger mass wasting by the emination of shock waves moving the material; or by liquifaction, jostling water saturated sand to break grain contacts and allow flow of water around grains‐ quicksand.

Volcanism‐ Magmas rising beneath volcanoes can increase slope angles (Mt. St. Helens) and cause mass wasting.

Finding Mass Wasting Hazards

Recognition – What to look for or consider

• Look for signs of creep or any other mass wasting (scarps, hummocky toes)

• Look for planes of weakness (foliation in meta rx in Phx)

• Consider consequences of steepening slope as in making road or landscape cuts

• Beware of removal of vegetation

• Consider addition of weight • Systems such as sprinklers that add water

Preventing mass wasting

• Stabilizing slopes

Retaining walls with adequate drainage

Planting vegetation to hold material together

Rock bolting – to bedrock

• Removing unstable slopes (e.g. slopes with planes of weakness)

• Minimizing extra weight – construction/buildings

8/2011