geo rocks and weathering - slopes

what is mass movement?

all downhill movements of weathered material in response to gravity

what is creep?

the slow downward progression of rock and soil down a low grade slope

what is a terracette

a step-like feature formed by soil creep

what is solifluction?

the gradual movement of wet soil or other material down the slope, especially where frozen subsoil acts as a barrier to the percolation of water

what is heave?

the movement of particles at right angles to the slope

what is regolith

all weathered material which sits above the rock layer

what is scree/talus

the loose rock created by weathering usually found at the base of a slope

when describing a slope what do you talk about

1) slope shape/incline

2) type of slpoe

3) any features

4)rock type

what are rills and gullies

rills: small, shallow channels formed by runoff rainfall

gullies: larger deeper channels that develop when rills combine due to continued water erosion

when is a slope unstable (shear strength and stress)

when shear stress exceeds shear strength which leads to mass movement

when is a slope stable?

when shear strength is greater than shear stress

what is shear strength?

the resisting forces: vegetation binding soil together, structure of the rock and water content

what is shear stress?

the driving forces: slope angles, humans adding/taking away, weight of material on the slope (e,g rainfall, vegetation)

how does humid climates affect slope processes

humid: more chemical weathering = round slope shape

type of mass movement: soil creep

how do arid climates affect slope processes

weathering is physical = jagged or straight slopes

type of mass movement: sheetwash

what are the 5 factors affecting slope processes

1) climate

2) faults, angle of dips

3) soil

4) aspect

5) vegetation

how does the faults and angles of dip affect the slope processes

lots of faults: more weathering and weaker

high angle of dips = prone to more slips

how does soil affect slope processes

structure and texture determines how much water it can hold

clay soils = hold more water - more stress (movement)

sandy soils = hold less water

clay soils with lots of vegetation = lots of mass movement

how does aspect affect slope processes

North hemisphere:

north facing slope = shaded and rarely rose above freezing

south facing slopes = more cycles of freeze thaw

south hemispher:

north facing = sunnier and drier - rock falls dry landslides

south facing = cooler and wetter = solifluction

how does vegetation affect slope processes

decreases surface runoff as the roots uptake water and intercept rainfall

deforested slope = more erosion and gullying

what are the 5 types of mass movement

heaves

flows

falls

slides

flows

what is are heaves

a slow, small scale process which can wet or dry

where and when do heaves normally occur?

they occur in regions with a wet climate

more effective when freeze thaw takes place so in NZ it occurs mostly in winter

what are the causes of heaves?

small scale processes of freeze-thaw , heating and cooling or wetting and drying that rearrange the soil particles. These things reduce the shear strength of the slope material

what is the process of heaves?

water infiltrates into the soil

water freezes and expands

soil particles are pushed to the surface at a right angle

soil particles fall vertically under gravity

the repetition of this creates a downslope movement

what is the outcome (landforms) caused by heaves?

Landform: terracettes

other outcomes: bent tree trunks, tilted power poles

what is a flow?

fast and wet mass movement that consist of saturated fine particles that move downslope like a viscous liquid

what speeds can flows reach?

over 50km/h

what are the two types of flows?

mudflow - faster more fluid

earthflow - slower, thicker but deeper

what are the conditions needed for flows

slopes over 10 degrees

wet climate as they tend to occur after abundant rainfall

presence of fine-grained material (over 33% clay)

what is the cause of flows

the sediment absorbs water, becomes saturated and starts to behave like a liquid

what is the process of flows?

abundant rainfall

material (regolith) absorbs water

the regolith becomes saturated and begins to behave like a liquid rather than solid

material moves downslope at a high speed until it reaches a flat area

what are the outcomes of flows?

scar - hollow located at the top section of the flow; it is created by the removal of material

track. - channel created by the movement of material

toe - accumulation of material that spreads out at the base of flow

what are falls?

fast and dry mass movement where the moving fragment are completely detached at some points

what are the conditions of falls

steep slopes over 40 degrees

mostly on bare rock faces, exposed to weathering (no soil or vegetation

cold climate which development of vegetation and accelerates mechanical weathering

what are the causes of falls?

mechanical weathering or erosion detaches fragments of rocks

what is the process of falls?

rock face exposed to mechanical weathering

fragments of rock get detached from the cliff

fragments fall due to gravity:

• vertical - free fall

• steep - bounce

• moderate - roll

what are outcomes of falls

landform: scree slope

straight scree - when the fall is short

concave scree - when the fall is long

other outcomes:

retreat of cliffs

source of debris

what are slides?

landslides: relatively fast and dry mass movements that occur when an entire mass of material moves along a slip plane

what are the conditions needed for a slide

weak rocks

presence of a slip plane

steep slopes (20-40 degrees)

active undercutting

what is the causes of slides

downward force exceeds the resistance produced by friction between particles due to factors that decrease shear strength or increase shear stress

• the presence of water supports this process

what are outcomes of slides?

scar, slide plane (exposed by the downslope movement), toe

what are slumps?

relatively fast and wet mass movement that consists in the movement of saturated material along a curved slip plane and over a short distance

what are conditions needed for slumps to occur

weaker rocks, especially clay

wet climate

steep slope (20-40 degrees)

what is the processes of slump formation

clay absorbs water

clay becomes saturated

slope material slips along a curved slip plane

what is a rainsplash erosion

rainsplash is a process in which soil particles are knocked into the air by raindrop impact

how does rainsplash erosion occur in flat surfaces

rainsplash dislodges particles equally in all directions , the soil becomes loose

how does rainsplash erosion occur on steep slopes

there is a transportation of the detached particles and at the botton a deposition of the detached particles

what are factors that influence the rate of rainsplash erosion

rainfall intensity

slope angle

moment in a rainfall episode - at the start vert effective as soil is loose

what is surface runoff

occurs when the soils infiltration capacity is exceeded

what is sheetwash

unchanelled flow of water over a soil surface

what are 6 human activities that impact the shear strength and stress of slopes

building

deforestation

piling up waste soil and rocks

construction

footpath trampling

quarrying

what are the different types of strategies to reduce mass movement?

subsurface drainage

retaining walls

netting

pinning

grading

what mm does subsurface drainage target

slides, slumps and flows

how does subsurface drainage work

the water that infiltrates is collected underground, then drained through a pipe

the sediment is then maintained relatively dry avoiding saturation and lubrication of the surface that separated two rock beds

what mm does retaining walls target?

soil creep, landslide

what is netting

used to help collect fragments of scree, made of heavy duty steel mesh

what mm does netting target

rockfalls

what mm does pinning target

slides, rockfalls, slumps

what is pinning

used to attack a wire net or concrete blocks to a rock face or slope so that the risk of rockfalls is reduced or the risk of erosion is reduced

what mm does grading target

landslides, flows