2 + A. Mass Movement

Question 1

What is Denudation?

Answer 1

Denudation is the removal of rocks by the combined effects of weathering, erosion and mass movement

Question 2

What is a rock?
What structures can a rock have?

Answer 2

A rock is a solid aggregate of one or more minerals
Rocks can be:

• Unconsolidated
• Compacted
• Cemented
• Crystallised

Question 3

How does climate affect weathering?

Answer 3

Climate dictates which weathering processes will occur, and at what rate.

Question 4

How does the climate of polar and sub-polar areas affect weathering?

Answer 4

• Polar areas affected most by freeze-thaw weathering
• Chemical weathering is restricted by cold temperatures which slows chemical reactions, however ↓
• Carbon dioxide is more soluble at low temperatures, so carbonation can occur

Question 5

How does the climate of temperate areas affect weathering?

Answer 5

• Minimal physical weathering
• High rates of chemical weathering due to wet climate and blanket of vegetation
• Effects of pollution significant in urban areas

Question 6

How does the climate of arid / semi-arid areas influence climate?

Answer 6

• Rates of weathering slowest on Earth
• Chemical action may be slow due to lack of moisture
• Salt crystal growth and temperature-based weathering (block and granular disintergration) important due to large diurnal temperature range

Question 7

How does the climate of humid tropical areas affect climate?

Answer 7

• Fastest rates of weathering on Earth
• Rates of weathering accelerated by hot, wet conditions
• Increased ionisation of water makes hydrolysis very fast.

Question 8

Why do weaknesses in rocks make weathering more likely?

Answer 8

• Weaknesses in rocks make weathering more likely as it increases the surface area exposed to the elements
• They also create more cracks for water to seep in during freeze thaw

Question 9

How does vegetation cover affect weathering?

Answer 9

Generally, increased amounts of vegetation increase the rate of chemical action through the release of organic acids, important in processes such as chelation.

• Increased levels of CO2 from plant respiration forms carbonic acid when dissolved in water and increases rates of carbonation
• Rates of physical weathering decrease due to insulation of vegetation decreasing frost action and thermal effects.
• Rates of biological weathering incerase through growth of plant roots into joints and along bedding planes.

Question 10

How does relief affect weathering due to temperature?

Answer 10

Relief affects weathering through indirect affect on the climate:

• Freeze-thaw action is more important in mountainous areas as the diurnal temperature range include temperatures higher and lower than 0’C
• Rainfall totals tend to be higher in upland areas and temperatures tend to be colder - increases freeze-thaw and chemical weathering.
• In the Northern Hemisphere, North-facing slopes have greater rates of frost weathering as they recieve less direct sunlight

Question 11

Besides temperature, how else does relief affect weathering?

Answer 11

• Slope processes i.e. landslides can expose previously unexposed rock which then becomes susceptible to weathering
• In lowland areas, thick layers of soil and weathered material may protect unweathered rock, however, accumulation of water at bases of slopes may increase chemical weathering

Question 12

How does human activity increase rates of weathering?

Answer 12

• Human industry emissions increase amounts of chemical pollutants in the atmosphere - SO2, CO2, NOx, leading to increased chemical weathering from acid rain
• Deforestation reduces chemical and biological weathering through decreases in organic acids, but might increase physical weathering on now exposed rocks / soil

Question 13

What is slope failure?

Answer 13

Slope failure is when a stable slope gives way, or when is is no longer able to resist continuous stress.

Question 14

What are slides?

Answer 14

Slides are a form of mass movement where one rock layer slides over the layer below.

Example: Grindlewald, Switzerland

Question 15

When do slides occur?

Answer 15

Slides occur when an entire mass of material moves along a slide plane or shear plane i.e. a line of weakness / bedding plane within the rock

They are a dry mass movement and rely on weakness inherent within the rock itself.

Question 16

What is a rotational slide?

Answer 16

Rotational slides are a form of mass movement where one rock layer slides over the layer below on a curved / concave shear plane

Rotational slides can have multiple layers.

Example: Hookern landslide, Beer

Question 17

What is a rock fall?

Answer 17

A rock fall is a mass movement where rocks broken by weathering fall down a steep slope, often accumulating at the base of a slope / cliff to create talus or scree.

Example: Burton Bradstock, Dorset

Question 18

Where can falls occur?

Answer 18

Falls ocurr on steep slopes >70°, usually on bare rock faces where joints are exposed to weathering and erosion processes.

Question 19

What are flows?

Answer 19

Flows are the rapid movement of rock and weathered debris mixed with water down valleys as a turbulent and structureless mixture of sediment and water.

Question 20

What are creeps? (Slopes)

Answer 20

Creeps are the slow, downslope movement of unconsolidated material and soft rocks, rarely more than 1-2cm/year.

Question 21

What are the three types of creep (slopes) studied at A-level?

Answer 21

Clay-rich soil = plastic flow, more likely to happen on saturated, thick surface deposits on steeper slopes

Freezing and thawing of ice = heave, freeze-thaw leads to expansion and contraction of soil parallel to slope leading to net downslope movement

Wetting and drying = solifunction, accelerated creep - may also include viscous flow

Question 22

What was the 1966 Aberfan Disaster?

Answer 22

Morning of 21st October 1966 - a landslide of coal waste crashed into the village of Aberfan:

• 28 adults and 116 children killed.
• Half the town’s youth lost
• Only 5 survivors
• Pantglas Junior School was mostly destroyed
• Adjacent secondary school was also damaged

Question 23

What caused the 1966 Aberfan disaster?

Answer 23

• Aberfan’s coal mine (the primary source of employment for the village) had placed several spoil tips on top of the hill above the village due to limited space in the valley
• Tips 4,5 and 7 were also placed on top of springs and streams, decreasing the stability of the tips as tailings (fine coal and ash particles) are susceptible to water - develop properties similar to quicksand when wet
• Weeks of torrential rain before the disaster reduced the tips to water-saturated debris

Question 24

Which human factors increase shear stress?

Answer 24

• Dumping mining waste on hills
• Building on hillsides
• Building dams
• Explosions (quarrying / war)

Question 25

Which human factors reduce shear strength?

Answer 25

• Undercutting at base of a slope to build a road
• Ploughing
• Deforestation / clearing vegetation
• Building coastal defences starving beaches further down the coast

Question 26

Which physical factors increase shear stress?

Answer 26

• Slope angle
• Earthquakes / vibrations
• Heavy rain - saturated rock
• Poorly drained land e.g. porous rock which is not permeable
• Snowmelt on active volcano

Question 27

Which physical factors decrease shear strength?

These are mostly to do with the rock itself:

Answer 27

• Lack of vegetation
• Rock with lots of joints / faults
• Unconsolidated rock
• Lots of weathering e.g. granular disintergration
• Freezing of water in cracks
• Erosion by river or sea at base of cliff

Question 28

What is shear stress?

Answer 28

Shear stress is the size of the forces trying to pull a mass downslope.

Question 29

What is shear strength?

Answer 29

Shear strength is the size of resistant forces supporting a mass on a slope.

When shear stress > shear strength, mass movement occurs.

Question 30

What is aquaplaning?

Answer 30

If the spaces along a contact between the two rock masses of low permeability are filled with water, the water pressure bears part of the weight of the overlying rock mass, thereby reducing friction along the contact.

Question 31

What is ‘liquid limit’?

Answer 31

Liquid limit is the limit that a pore pressure can be before a fine-grained sediment (sand/silt/clay) loses strength and begins to flow.

Pore pressure increases as sediment becomes saturated with water

Question 32

How does a block wall at the bottom of a slope reduce creep?

Answer 32

A block wall at the bottom of a slope is a physical obstacle to the movement of material via creep.

• Gravel packs and weepholes ensure sufficient drainage through the wall reducing water-based mass movement such as flow or heave, and prevents water accumulation behind the wall.
• The building pad, foundation and enforcing rods provide further strength and security to the wall against creep

Question 33

How does drainage reduce the risk of mass movement?

Answer 33

Increased drainage directs water away from the slope, increasing shear strength and reducing shear stress.

Water is often a key factor in the formation of slides and flows - efficient removal reduces likelihood of mass movements.

Drainage may be necessary to protect slopes which are porous but impermeable e.g. clay, as well as to reduce mass movement caused by heavy rainfall which might saturate the soil.

Question 34

How does gradient reduction reduce the risk of mass movement?

Answer 34

Moving material from the top to the bottom of the slope reduces shear stress by decreasing the gradient of the slope.

Furthermore, weathered material is moved to the bottom of the slope from the cliff edge, which increases the shear strength of the slope as a whole as the recently uncovered material is less weathered.

Moving material like this also removes any undercutting or wave-cut notches which may cause mass movement in the undercut cliff to a significant degree

Question 35

How does afforestation reduce the risk of mass movement?

Answer 35

• Interception from trees and foliage reduces the overall saturation of the slope which increases shear strength, as well as protecting the soil below from certain types of weathering.
• Roots decrease saturation by absorbing water, which reduces the chance of mudflows (less surface runoff)
• Roots also increase shear strength by binding soil together and stabilising it, as well as slowing the velocity of runoff and collecting sediment which would otherwise reduce shear strength.

Question 36

Where is Lyme Regis?

Answer 36

Lyme Regis is a town in the middle of the Jurassic Coast World Heritage Site. It also happens to be built on one of the most unstable and rapidly eroding cliffs in Europe

Question 37

What evidence is there that Lyme Regis has suffered instability?

Answer 37

• The area has suffered landslides since the end of the Ice Age 12,000 years ago
• Some of the biggest landslides in Britain occur between Sidmouth and Charmouth
• Lyme Regis itself is constructed on relict landslides which extend up to 1km inland

Question 38

Which layers are contained in Lyme Regis cliffs?

Answer 38

Greensand (permeable)
Gault clay (impermeable) - water permeating through greensand collects here.
(Lias clay and limestone) - up to 185 million years old!

Question 39

How does Lyme Regis’ weather and location make it so unstable?
(Physical Factor)

Answer 39

• Large fetch of 8000km to mouth of Amazon allows larger storm waves into the area
• Heavy rainfall events recently and long history of big storms. Many mass movement events occur after high rainfall

Question 40

How does Lyme Regis’ geology make it so unstable?

Answer 40

• Rocks clip towards the sea, making the toes of cliffs susceptible to erosion
• Springs and streams emerge from cliffs due to greensand + gault clay
• Fault lines present in cliff e.g. two major faults below East cliff

Question 41

How do historic human activities make Lyme Regis so unstable?

Answer 41

• The Cobb (built in 1200s) has starved beaches to the east of sediment, and caused Monmouth beach to increase in size
• Quarrying / removal of sand and gravel has taken place around Lyme until 100 years ago.
• → Blue Lias rock quarries for lime and hydraulic cement, as well as the Lyme Regis cement works have weakened the area and rocks around Lyme Regis considerably
• Old landfill site above East Cliff used from Victorian Era to late 1970s. Washing machines and fridges came down onto the beach as part of the** 6 May 2008** landslide

Question 42

How do modern human activities make Lyme Regis so unstable?

Answer 42

• Building on slope (the entire town of Lyme Regis) has increased shear stress
• Digging fossils out of the rock, either for scientific research or as a hobby, has reduced the stability of the cliffs somewhat
• On the Jurassic Coast, places outside towns are not protected from erosion / mass movement

Question 43

Why is Lyme Regis worth protecting?

Answer 43

• Approximately 170 houses close to the water are at risk of falling into the sea
• Population of 5,000 rises to 15,000 in summer tourist season
• Cost : benefit ratio 1:6 - for every £1 spent on coastal defences etc, £6 pounds of assets are saved

Question 44

How much money has been spent protecting Lyme Regis?

Answer 44

More than £35 million has been spent on coastal protection and slope stabilisation in the area since 1994, spread out over several phases.

Question 45

What measures have been taken to protect Lyme Regis via the cliffs?

Answer 45

• Drainage pipes, weepholes and mesh netting placed all over Lyme Regis
• Re-vegetated slope above sea wall

Question 46

What measures have been taken to protect coastline on the east side of Lyme Regis?

Answer 46

• New sea wall and pedestrianised walkway to the east of Lyme Regis - £19 million scheme
• Riprap / rock armour built on east side of Lyme Regis with Lavikite from Norway, Limestone from the Mendips and Basalt from Ireland

Question 47

What measures have been taken to protect Lyme Regis beaches?

Answer 47

• 60,000 tonnes of shingle dredged from Isle of Wight to reinforce shingle beach (larger beach absorbs power of waves)
• Dividing sea wall between shingle and sand beaches built in 2007
• 40,000 tonnes of sand dredged from Normandy to reinforce sand beach (absorbs waves and important for tourism)

Question 48

How have Lister and Langmoor Gardens been used to protect Lyme Regis?

Answer 48

Lister and Langmoor gardens are stabilised with over 1000 deep bored piles as well as soil nailing, soil buttressing, drainage and plenty of vegetation.

Question 49

What monitoring devices are contained within Lyme Regis?

Answer 49

• 47 standpipe piezometers and 3 standpipes
• Inclinometers in 34 boreholes
• Slip indicators
• 68 permanent ground markers
• Field observatory

Question 50

How do 47 standpipe piezometers and 3 standpipes protect Lyme Regis?

Answer 50

47 standpipe piezometers dug 3-54m deep and linked to data loggers - measure water pressure in rocks

Question 51

How do inclinometers in 34 boreholes protect Lyme Regis?

Answer 51

Inclinometers in 34 boreholes dug 11-50m below ground to measure rate of lateral movement

Question 52

How do slip indicators protect Lyme Regis?

Answer 52

Slip indicators = lowered down piezometer tubes to measure distortion / shearing

Question 53

How are 68 permanent ground markers used to protect Lyme Regis?

Answer 53

Position of 68 permanent ground markers monitored every 3-4 months with GPS to measure vertical and horizontal movement

Question 54

How does a field observatory help protect Lyme Regis?

Answer 54

Serves as a base for structural inspections - roads, paths and seawall structures checked for damage.

Question 55

What is rock pinning?

Answer 55

Metal rods and plates are drilled into rock to secure the rock layer / resist movement

Question 56

What is netting and why is it used?

Answer 56

Metal netting is fastened to road cuttings to prevent loose blocks falling onto the road below

Question 57

What are gabions?

Answer 57

Gabions are boxes made of metal mesh that are filled with rocks to stabilise the toe of the landslip, as well as other uses e.g. sea defences

Question 58

How does drainage stabilise slopes?

Answer 58

Excess water on slopes provides lubrication and is a key factor in the formation of flows and slides - moving water away from vulnerable slopes protects them from collapse

Question 59

How does grading protect slopes?

(Grading reduces the relief of a slope)

Answer 59

Making the angle of a slope more gentle reduces shear stress of a slope

Question 60

How does afforestation protect slopes?

Answer 60

Afforestation increases interception and transpiration, and reduces surface runoff decreasing mass movement

Roots also bind soil together