DO NOW Pick up notes and Review 32

DO NOW • Pick up notes and Review #32.

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

H. T S E BO NG S I S H T E S C IT A O W W R D S P E S E T A H IA M T C L E O A B I SS DU R A A C S S R E D G D R D : ZA N A 3 C A S H N E E D S N D A UD S I T G N M A S S W S A

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 1. 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 1. 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 watersaturated 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? A Why? 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