Mass Movement

Question 1

What is mass movement?

Answer 1

• large scale downslope movement of material under influence of gravity
• caused when stress exerted exceeds internal strength of hillslope, causing instability

Question 2

Draw a rough sketch of a diagram classifying mass movements.

Answer 2

• triangle w bottom labeled slow to fast and right slope labeled dry to wet
• bottom left of triangle = creep
• 2/3 right of triangle = landslide
• top of triangle = lahar

Question 3

How does material the slope is made from affect mass movement processes?

Answer 3

• rock (e.g. sandstone, slate) has highest internal strength
• cohesive materials (e.g. silts + clays) has middle internal strength
• non-cohesive materials (e.g. coarse grains like sand) has lowest internal strength

Question 4

What are the several types of mass movement?

Answer 4

• heaves
• flows
• slides
• falls

Question 5

What is heave?

Answer 5

• particles pushed towards surface at right angles by wetting, freezing or heating
• e.g. freeze-thaw causes water to expand causing soil to bulge
• then sinks down by gravity, slowly moving downslope
• creates terracettes on slopes made of cohesive materials
• occurs on slopes made from cohesive + non-cohesive materials
• 2 types: frost heave + soil creep
• slowest + most prevalent form of mass movement

Question 6

What are flows?

Answer 6

• controlled by moisture in slope
• when soil is oversaturated, cohesive bonds break causing soil to move downhill
• creates a scar + toe of flow
• occurs on slopes of cohesive materials
• higher water content leads to mudflows, lower = earthflows
• faster than heaves, slower than slides + falls

Question 7

What are falls?

Answer 7

• rocks fall under gravity on steep slopes >45°
• not dependent on rainfall as internal strength not weakened when wet
• occurs on slopes made from rocks
• happen suddenly + have dramatic effects
• slopes steep due to strong internal strength of rocks

Question 8

What are slides?

Answer 8

• rockslides + landslides are when entire masses of material moves along a slip plane (usually a bedding plane)
• can break when reaches bottom of slope
• occurs on weak rocks w steep slopes that have active undercutting