DO NOW Pick up notes and Review #32.

REVIEW How are wind and water erosion alike? How does gravity affect both? They both transport sediment The larger particles stay close to the ground.

MASS WASTING SES3c: Describe the processes and hazards associated with both sudden and gradual mass wasting.

What is Mass wasting? Mass wasting is the movement of large amounts of material downhill under gravity. Follows after weathering. May be fast or slow. Can occur on steep slopes or shallow dipping slopes. VIDEO LINK – no sound

Causes Gravity - The principle force tending to pull materials downslope.

CauseS 2. Friction - resists the downslope movement of material.

Causes 3. Slope: the steeper the slope, the more likely that the materials will move downslope – MOST important factor (no slope = no movement).

Factors: Composition of Materials Solid: Bedrock is generally stable = prohibits mass movements. Tectonic deformation & weathering produce fractures and joints = unstable.

Factors: Composition of Materials 2. Mechanical weathering – cracks?

Factors: Composition of Materials 3. Rock Planes: sedimentary layers and metamorphic foliation planes - those parallel to slope tend to slip easier (less stable) than those that are at right angles to the slope (more stable).

factors: Angle of Repose The Angle of Repose is the steepest angle at which a pile of unconsolidated grains remains stable. It is controlled by the frictional contact between the grains. The angle of repose is greater for angular pebbles because there is more friction!

factors: Angle of Repose Increases with increasing grain size: The larger the grain size, the greater the angle material will be stable. The smaller the grain size, the smaller the angle material will be stable.

FACTORS: Angle of repose

angle of repose for different materials

factors: Water Can cause previously stable slopes to slide. SMALL amounts can increase. cohesiveness = increases surface tension.

factors: Water Excessive water leads to slope failure. Friction and cohesion is reduced (lubricates the slope, promotes slippage). When pore spaces in sedimentary rock fill with water, the weight of the material increases and material becomes unstable.

Factors: Water

Factors: Rock Orientation Slope of rock in relation to: Slope of land Weak zones Downslope rock less stable than upslope rock. Weak zones perpendicular to rock slope more stable than parallel weak zones.

Factors: Vegetation Vegetation, especially those with deep root networks, helps hold material on a slope. Replanting along road cuts increases stability. Vegetation removed by forest fires, clear cuts, people, etc. decreases stability.

Types of Mass wasting Classified based on speed and way they travel downslope. Three Groups: Falls: Rockfall Flows: Creep and Solifluction Slides: Slump and Mudslides

TYPE: rock fall Rock fall is when a piece of rock on a steep slope becomes dislodged and falls down the slope. Fastest type. Rock or sediment breaks free and falls vertically or near vertically. Any size of material. Builds talus slopes.

Rockfall examples

TYPE: Creep Creep is the slowest type of flow (cm or mm per year) and is too slow to observe. Dry Occurs everywhere, even on gentle slopes. Affects only the top few meters; top moves faster than bottom (due to friction). Objects in the soil start to lean over – a way to positive identify creep.

Creep Examples

TYPE: Solifluction Solifluction is soil flow – cold wet, slow. 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 cannot percolate downward (because of permafrost), so soil gets water- saturated and flows downslope.

Solifluction Examples

Slide: mud or land slide A mud slide or land slide is a fast type of slide. Wet – often water saturated. Material may be soil, rock, or a combination of the two. May break apart during movement or remain intact. Includes “Glides” - rocks moves downslope along a more or less planar surface.

Examples of Mud/Land slides

slide: slump A slump is a type of slide. Separates along a curved slip surface. Material moves as a single unit— but sometimes has multiple blocks. Forms crescent-shaped scar at the head where the material detached – called a scarp. Can be triggered by heavy rains or earthquakes.

Slump examples

Other Types of Mass Wasting Falls: Topples – forward rotation around some pivotal point. Slides: Block slides – large mass moves together. Lateral spread –on gentle or flat slopes as a result of liquefaction of rock – possibly during earthquake.

Other Types of Mass Wasting Flows: Debris flow - a combination of loose soil, rock, organic matter, air, and water mobilize as a slurry that flows downslope

Dealing with risk Proper Land Use: What type of rock? Slope of rock layers? Angle of repose? Rock orientation?

Dealing with risk Structural Control Retaining Structures – support surface material. Drainage - water reduces friction. Terraces – reduce slope

Review Which road would have the least risk for mass wasting? Why? A The rock under road A has upslope which is more stable.

TO DO Work on Review #32 and anything else you are missing.

Lab: Angle of repose How does grain size affect the angle of repose? Measure the angle of repose of different materials: Sand – dry and wet Rice Aquarium gravel