Lithological Processes, Hazards and Management
1. Theory of Plate Tectonics
Sea-floor spreading:
 Hypothesis
o Mid-ocean ridges places of generation of new crust
o Continents split  magma welled up along line of rift  cool and form crust
 rift constantly replenished by upwellings from mantle
o Ocean floors grow outwards from rifts
 Supporting evidence
o Rock magnetism
 Earth’s polarity reversed at least 9 times in past 4.5b years
 Time of formation of new basaltic oceanic crust  magnetized in
direction of prevailing magnetic field  fossil magnetism
 Parallel and symmetrical about ridge
 Alternating bands of rocks and reverse polarities
 Eg. Reykjanes Ridge
 Date magnetism of bands  calculate rate of sea-floor spreading eg.
Iceland 1cm / year on each side of ridge
o Geothermal heat flow
 Measured using thermistor probe
 Much hotter than normal over ridge due to injection of mantle
material
o Centres of seismic activity
o Dating of volcanic activity
 Eg. Iceland: most recent activity down centre of ridge, progressively
older volcanoes to east and west of ridge
o Pattern of sedimentation
 Age and thickness of sedimentation increases with increasing distance
Subduction:
 Hypothesis: crust destruction
 Supporting evidence
o Most intense seismic activity
o Pattern of earthquake foci
 More recent activity at shallow depth
 Benioff zone about 45 degree angle: marks line of disturbance caused
by passage of ocean crust as it is subducted, zone of descending foci
o Geothermal heat flow
 Cooler than normal over trenches due to cold crust descending into
mantle
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Mantle convection currents:
 Crust attached to upper mantle and they move together
 Source of energy for tectonic movements: Earth’s internal heat transferred through
more liquid asthenosphere
 Hot material in mantle rises to base of lithosphere, move in lateral direction, cools
and descends to be reheated
2. Global Structural Landforms
Divergent plate boundaries:
 Zones of tension
o 2 convective flows in mantle are divergent  viscous drag on overlying plate
cause it to be torn apart
o Hot spots
 Heat flow well above surrounding average  dome overlying crust
 Rift valleys
o Formation
 Hot spots  crustal stretching and formation of tensional cracks
 As plates move from area of upwelling, broken slabs displaced
downwards  downfaulted valleys
 May be due to two parallel faults  valley floor sink between 2
inward-facing scarps
o Features
 Fault: parallel, step, grid
 Block mountains and intervening basins eg. Basin and Range country
of western North America
 Horsts (upstanding) and graben (downthrown) eg. Rhine Rift Valley
 Volcanoes due to crustal weaknesses set up by faulting eg. Central
Icelandic Depression coincides with belt of most recent activity
 Lakes at depressions
o Eg. East African Rift
 Mid-ocean ridges
o Formation
 Rift valleys lengthening and deepening, extending into ocean 
valley becomes narrow linear sea with outlet to ocean
 New sea floor continuously generated  mid-ocean ridges
o Features
 10 000s km long, 100s km wide
 0.6km – 3km or more above sea floor
 Central rift running down middle of mid-ocean ridge eg. Red Sea
 Transform faults  staggered paths
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Volcanic islands eg. Iceland: only major site where mid-ocean ridge
appears above sea level
o Eg. Mid-Atlantic Ridge
Constructive Plate Boundaries:
 Zones of compression
o Oceanic-oceanic collision: denser plate subducted
o Continental-oceanic collision: oceanic plate subducted eg. Nazca and South
American plate
o Continental-continental collision: fused into a single block with a mountain
range eg. India and Asian plate
 Ocean trenches
o Long narrow troughs in ocean bed that mark zones of subduction
o Fringes a continent due to continental-oceanic collision eg. Peru-Chile Trench
which borders west coast of South America
o Occur on deep ocean floor due to oceanic-oceanic collision, flanked by
volcanic island arc
 Continental volcanic arcs
o Continental-oceanic collision causing folding and faulting to form mountain
chains like the Andes
o Descending oceanic plate reaches depth of about 100km, partial melting of
water-rich oceanic crust and some overlying mantle occurs  newly formed
andesitic / granitic magma less dense than surrounding mantle rocks and will
slowly rise  emplaced in overlying continental crust  cools and
crystallises at depth  magma may migrate to surface
o Found 100-400 km from trench
 Island arc volcanoes
o Formation of chains of islands during oceanic-oceanic collision
o Eg. Japanese Islands
 Fold mountains
o Result of continental-continental collision
o Eg. the Himalayas due to collision between Australian-Indian plate and
Eurasian plate
o Sweeps up and deforms sediment accumulated along margins of both
continents
o Intense folding and faulting
Transform Plate Boundaries:
 Zones of shearing  limited construction and destruction
 Occurs along transform fault, marked by zones of intensely shattered rock
 Linear scars, offset stream channels, elongated ponds
 Eg. San Andreas fault
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Hot Spots:
 Flow of geothermal heat through crust from mantle considerably higher
 Sites of volcanism and lavas produced are rich in alkali metals
 Cause upwarping of curst forming domes
 Eg. Hawaiian Chain of Islands: only volcanoes above hot spot is active
3. Extrusive Volcanism
Components of Volcanic Eruptions:
 Lava: magma that reaches earth’s surface
o Basaltic (most common), andesitic, granitic (in increasing silica content)
o Pahoehoe flows
 Twisted and ropy structure
 Wrinkles due to advancement of still molten subsurface lava when
those at the top have started to congeal
 Nearer to volcanic eruptions
o Aa flows
 Surface of rough, jagged blocks with sharp edges and spiny
projections
 Surface of flow cools and forms a crust, interior molten so still
advances
 Hardened crust broken into angular blocks
 Further from volcanic eruptions
o Pillow lava
 Extruded underwater
 Lava chilled quickly  brittle chilled surface cracks, making an
opening for still molten magma inside to ooze out like toothpaste 
cycle repeats  pile of lava pillows
 Pyroclasts
o Gases in highly viscous magma difficult to escape  internal pressure
o Once released, superheated gases expand thousand fold as they blow
pulverized rock and lava from vent
o Ash fall, ash flow, bombs, blocks
o Lahar eg. Mt St Helens: Toutle River: destroyed nearly all homes and bridges
along river
 Gases
o Pyroclastic flows: hot gases combined with large rock fragments and ash
 Due to hot, buoyant gases, can travel downslope in nearly frictionless
environment
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Types of Volcanic Eruptions:
 Magma and viscosity
o Viscosity directly related to silica content due to the strong bonds of
networks that must be ruptured for flow to occur
 Granitic lava viscous, short thick flows vs. basaltic lava
o Higher temperature, lower viscosity
 When lava flow cools, it begins to congeal
o Gas content: dissolved gases increase fluidity of magma and escaping gases
provide enough force to propel molten rock from vent
 Magma and nature of eruption
o Low pressure, less amount of gas magma can dissolve
o Reduced confining pressure as magma moves from deep in the earth to nearsurface environment  gases expand to 100s of times their original volume
 come out of solution and form bubbles
o Fluid basaltic magmas allow expanding gases to migrate upward and escape
from vent easily vs. granitic magma of which gases collect as bubbles that
increase in size and pressure till they explosively eject the semi-molten rock
o Viscous magma more likely to solidify within and clog up vent  inhibits
further rise of magma below and builds up pressure
Features of Extrusive Volcanism:
 Shield volcanoes
o Basaltic lava
o Broad, domed structure with average surface slope of a few degrees
o Wide base over 100km in diameter
o Small percentage of pyroclastic material
o Built by successive flows of basaltic lava, capable of flowing great distances
down gentle slopes, forming thin sheets of nearly uniform thickness
o Slopes slight near the summit because magma will readily run down slope,
but slope steeper further from summit so that lava can flow more easily
o Common in oceanic area like the Hawaiian Islands
 Stratovolcanoes
o Most violent
o Large and nearly symmetrical eg. Mayon volcano in Philippines
o Inter-bedded lavas and pyroclastic deposits emitted from central vent
 Extrude viscous lava for long periods, then ejects pyroclastic material
o Significant proportion made up of lahars
o Steep summits and more gently sloping flanks: concave slopes due to finer
pyroclasts found further away
 Cinder cones
o Pyroclastic material ejected into atmosphere then fall back to surface to
accumulate around vent
o Small, steep-sided cones, may have large, bowl-shaped crater
o Parasitic cones and occur in groups
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o Form within calderas of large volcanic mountains to represent final stages of
activity eg. Wizard Island in Crater lake, Oregon
Basalt plateau
o Fissure eruption: extrusion of basaltic lava along extended fracture
o Very fluid lava can remain molten long enough to flow 150km from source 
creation of basalt plateau as basalt is resistant to erosion and the rock which
basalt lava-flows overlie may not be so resistant  basalt act as cap-rock
o Eg. Deccan Plateau of West India
Calderas
o Circular depressions that exceed 1km in diameter
o Summit of volcanic structure collapses into partially emptied magma
chamber below
Volcanic Hazard Management: look at supplementary notes for more:
 Difficulties
o Authorities dislike the panic and disorders a large-scale evacuation entails
and will be blamed for miscalculations
 Eg. Mt Pelee reached climax in electoral period in 1902. While
opposition urged evacuation, the governor demonstrated his
solidarity with the citizens and his confidence in their future.
o Psychological barriers surrounding danger perception will be raised
whenever predicted eruption fails to materialize
o More difficult to predict the end of eruption
o Authorities face problems with ensuring the comfort level and survival of
evacuees
o ¾ of world’s most dangerous volcanoes in poor countries which spend
limited resources on famine relief and disease prevention
 Eg. Annihilation of the town of Armero with the eruption of Nevado
del Ruiz in 1985 due to the lack of disaster planning compounded by
ineffective administration and conflicting advice from experts and
non-experts
 Vs. eg. Mt Pinatubo in Philippines in 1991: many lives saved due to
successful communication between experts and the civil and military
authorities
 Vs. eg. Mt St Helens in 1980: evacuation, small numbers, better
communications, danger more obviously understood
o Revival of extinct volcanoes eg. Mt Pinatubo in 1991
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4. Earthquake Hazards
Characteristics of Earthquakes:
 Faults
o Dip-slip faults (vertical displacement)
 Normal fault: hanging wall moves down relative to footwall
 Reverse fault: hanging wall moves up relative to footwall
 Results in fault scarp
o Strike-slip faults (horizontal displacement)
 Right-slip fault
 Left-slip fault
 Seismic waves
o Surface waves
 Low frequency  more likely to stimulate resonance in buildings
 Rayleigh waves: ripples
 Love waves: horizontal shearing of ground
o Body waves
 P-waves: parallel to travel direction
 S-waves: perpendicular to travel direction
Earthquake Measurement:
 Mercalli scale: measures earthquake intensity
o Variations in population density
o Building materials and methods
o Distance from epicenter
 Richter scale: measures earthquake magnitude
o Logarithmic
Effects of Earthquakes
 Intrinsic conditions: magnitude, type, location, depth
 Geologic conditions: distance from event, path of waves, types of soil
 Societal conditions: quality of construction, preparedness
 Tsunami
o Especially common for subduction earthquakes
o Long period between waves: crests very high but troughs very low
o Eg. Indian Ocean tsunami
 Landslides
o Mountains formed along convergent plate boundaries which have steep
slopes
o Soil layer on side of hill liquefies and flow as landslide
 Liquefaction
o Water-saturated sediment reorganized due to violent shaking
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o Sediment collapses, expelling water and causing the ground surface to
subside
o Suspend grains in waterlogged soil so they lose contact and friction with
other grains
o Liquefied sand layer can shoot to surface through cracks, forming sand boils
o Eg. Mexico City earthquake
Fire
o Water lines that feed the fire hydrant may be broken
o Fuelled by winds eg. typhoon: 30 000 people incinerated by fire storm
o Dependent on time when earthquake strikes eg. lunch time when people are
using fire to cook food
o Eg. Tokyo earthquake: fire the biggest cause of death
Property damage
o Worsened by population explosion, industrialization, building on soft or
filled-in soil
o Eg. 26th December earthquake in Bam, Iran: complete collapse of the Bam
Citadel; high death toll because people were trapped when their mud-brick
homes collapsed – completely disintegrated and buried people in piles of
earth, rather than trapping them in voids of air pockets between building
slabs, as would happen in the collapse of a concrete building
Seiche
o Resonance in water body
o Tide-like rises and drops in water levels in lakes
o Swimming pools especially prone eg. 1994 Northridge earthquake
Fluctuation in underground hydrology
o Water in wells rise and dip in wavelike pattern
o Water levels may remain permanently changed eg. 1999 Izmit earthquake
5. Classification of Rock Types
Igneous rocks:
 Solidification of molten magma from mantle
o As molten magma cools, crystallization of minerals cause rock to solidify
o Nature of rock determined by mineral content and rate of crystallization
 Most common rock type
 Intrusive igneous rocks
o Formed under surface as material from mantle forced its way through crust,
cooling and solidifying as it moved
o Large crystals due to slow rate of cooling  coarse-grained eg. granite
 Extrusive igneous rocks
o Formed above surface as lava erupted from volcanoes cooled on contact
with atmosphere or sea
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o Smaller crystals  fine-grained eg. basalt
Sedimentary Rocks:
 Clastic
o Material moves deeper as more sediment deposited above it  hotter and
greater pressure  diagenesis because sediment in an environment where it
becomes unstable  cause clasts to be bound together and turned into rock
o Compaction: sediment squeezed by pressure of overlying sediment, water
lost as porosity and permeability rate reduced
o Cementation: minerals crystallize in pore spaces from circulating fluids,
cementing grains together
o Eg. sand laid down, lithified to give sandstone
 Non-clastic
o Not the result of weathered rock laid down
o Chemical
 Evaporation of water leaving salts behind / precipitation of salts
around central point eg. flint
o Biological
 Remains of marine organisms subjected to diagenetic processes on
bed of warm, calm seas eg. limestone
Metamorphic Rocks:
 Forces produced by movement of plates
 Mineral stable at atmospheric temperature and pressure but becomes unstable as
the pressure and temperature increases  mineral composition changed
 Contact metamorphism
o Source of extreme heat eg. igneous intrusion changes balance of minerals
within rock
o Water lost with increased temperature, and minerals recrystallised or broken
down into constituent parts
o Eg. marble from limestone
o As distance from intrusion increases, degree of alteration decreases due to
the decrease in temperature and duration of processes
 Eg. mudstone can be slate, schist or gneiss
 Regional metamorphism
o Increase in temperature and pressure due to forces at plate boundaries
o Great pressure as plates converge / great heat and pressure as melted crustal
material forces its way upward through continental crust
Comparison of Rock Types:
 Heat and pressure: sedimentary, metamorphic, igneous
 Hardness: sedimentary (once cement dissolves, clast left), igneous (crystals can
interlock to form much stronger bonds than cementation of loose particles),
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metamorphic (undergone lots of compression which results in realignment of
minerals – most resistant)
6. Weathering
Geometry of Rock Breakup
 Block disintegration: breaking down of rocks into large blocks
o Well-developed bedding planes with joints intersecting at right angles eg.
sedimentary rocks like limestone
o Selective weathering along these lines of weaknesses  detachment of large,
frequently angular boulders
 Granular disintegration: breaking down of rocks into numerous small fragments
o Crystalline rocks made up of numerous small crystals  particular mineral
less resistant and selectively weathered  loosen entire rock structure 
coarse sand in which each grain consists of single mineral particle separated
from others
o Eg. granite
 Exfoliation
o Outer layer receives most of the sun, so it expands and contracts the most
and will peel off first
o Detachment of rocks in concentric slabs from underlying rock mass, leaving
behind successively smaller spheroidal bodies
 Frost shattering
o Disintegration of rock along new surfaces of breakage to produce highly
angular rocks with sharp corner edges
 Spheroidal weathering
o Rounded by weathering form an initial block shape because chemical
weathering acts more rapidly and intensely on corners
Weathering Processes:
 Physical weathering
o Application of physical force to break up rock  superficial weathering
o Pressure release: exertion of physical stresses
 Macro
 Rock that has been confined decompresses as load of rock and
regolith above it is removed by erosion
 Expansion generates stresses that will cause fractures if they
are greater than the strength of the rock
 Less serious: sheet joints
 More extreme: exfoliation then block disintegration
 Eg. granite
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Micro
 Release of strain energy sets up stress between minerals / in
the cement between grains  granular disintegration
o Freeze-thaw
 Macro
 Water penetrates joints and beddings  freezes and expands
about 9%, exerting pressure on rock walls
 Closed system produced if freezing proceeds rapidly from
surface downwards  seal in water contained within
openings of rock  maximum pressure easily exceeds tensile
strength of rocks
 Repeated stress  widening of fissures  block disintegration
 Micro
 Water penetrates pore spaces between minerals especially for
rocks of high permeability and porosity  minerals may break
along their boundaries  granular disintegration
o Insolation weathering
 Disintegration of rocks caused by expansion and contraction through
solar heating and cooling
 Macro
 Effects of diurnal heating confined to surface layers  sharp
thermal gradient  surface of rocks expands more than rock
at depth  formation of stresses  exfoliation in thin layers
 Micro
 Dark-coloured minerals expand more than light-coloured ones
 stress between minerals  granular disintegration
 Rocks are bad conductors of heat and effect of heating can only
penetrate a few mm into rock  exfoliation very small scale
 Blackwelder and Griggs experiment
 After 244 simulated years of insolation weathering, no
detectable disintegration
 Only with some water content then breakdown is observed
o Salt weathering
 Coastal areas
 Water within rock saturated with salt through evaporation or
temperature change, some salt forced to crystallize and exert
pressure  salt crystal grows with cycles  grains of sand
dislodged
 Arid environments
 High daytime temperatures, low and erratic rainfall, high
potential evaporation
 Water with contained salts drawn up by capillary action 
water evaporates leaving salts behind as surface salt coatings
or contained in rock pores and joints
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 Thermal expansion of salts within pores / hydration of salts
during high humidity / growth of salt crystals
Chemical weathering
o Alteration of chemical composition of rock minerals
o Can occur at great depths since water can infiltrate and percolate regolith
o Regolith produced predominantly fine-grained
o Solution
 Highly alkaline and acidic water from soil moisture and groundwater
more effective in dissolving rock
o Carbonation
 Weak carbonic acid from rainfall dissolves limestone as carbonate
minerals become bicarbonates
o Hydration
 Absorption of water into existing minerals
 Minerals may change colour / expand to weaken rock, show signs of
cracking, causing granular disintegration
o Hydrolysis
 Reaction of mineral with water eg. feldspar reacts with water to
produce potassium hydroxide (removed in solution) and aluminosilicic acid (which will break down into clay minerals)
o Oxidation
 Reaction of mineral with oxygen to form oxides / hydroxides eg. iron
Biological weathering
o Weathering by living organism or by-product of living organism
o Root prising  block disintegration eg. Angkorwat
 Commonly seen in urban areas as roots prise through concrete to
search for water, opening passageways that increases vulnerability to
other forms of weathering
o Decay of organic matter produces variety of chemicals  some organic acids
dissolved as water passes through soil  acidic  chelation: breakdown of
soil and eventual weathering of rock
Climate and Weathering:
 Humid tropics
o High precipitation and temperature
o Rapid chemical weathering
 Van’t Hoff: speed of chemical reaction increase 2.5 times with each
rise of 10 degree Celsius
 Abundant soil moisture for chemical reaction to occur
 Dense vegetation and dead matter increases biochemical weathering
 Chemical weathering 4x more rapid than in temperate regions
o Physical weathering limited
 Masking effect of thick regolith of 30 – 60m comprising sand, clay and
corestones: regolith removal less effective than regolith formation
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 Lack of exposures of solid rock
 Uniformly high temperature
Seasonally humid tropics
o High summer temperatures and heavy seasonal rainfall
o Chemical weathering more dominant than physical weathering
 Less vegetation in savanna
 Formation of low domes and inselbergs as rapid stripping of regolith
partially exposes fresh rock of basal surface of weathering
Hot arid
o High diurnal temperature and low precipitation
o Physical weathering dominant through insolation and salt weathering
 Shallow regolith: weathered material no time to accumulate before
being removed
Temperate
o Moderate temperature and rainfall
o More physical than chemical weathering especially during winter
o Shallow regolith: freezing rarely severe and does not penetrate to any great
depth
Glacial
o Abundant snowfall and low temperature
o Physical more than chemical weathering
 Freeze-thaw dominant process due to melt water
 Water from thawed snow contain much carbon dioxide as solubility
of carbon dioxide increased at low temperatures  carbonation
Peltier’s diagrams
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Strahkov’s diagram
Factors affecting Weathering:
 Rock strength and hardness
o Stronger and harder rocks more resistant to physical weathering
o Rock hardness
 Hardness of mineral constituents
 Strength of cementation between minerals
o Rock age
 Harder if more time for cementation to take place
 Compression from young rocks above and from crustal movements
help to bind rock constituents more tightly
 Chemical composition
o Determines if minerals are susceptible to chemical changes and breakdown
o Weathering of the rock itself or the cement
 Rock texture
o Coarse-grained
 Unstable constituents  selective chemical weathering  reduces
rock’s coherence
 Large voids between grains  high primary permeability increases
possibility of trapping water for chemical and frost weathering
o Fine-grained
 Numerous boundaries between tiny materials of fine-grained rocks
provide lines of weaknesses along which weathering agents can
penetrate
 Rock structure
o Joints: surface area of rocks that can be weathered increases due to high
secondary permeability
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Topography
o Gentle slope: weathered material may accumulate in situ and prevent access
to fresh rock  retards physical weathering
Altitude
o Places above tree-line experience more effective freeze-thaw due to reduced
insulation from vegetation and soil
o Too high an altitude decreases diurnal cycles
Aspect: impacts the sunlight received and temperature of the place
Landforms:
 Scree slopes and block fields
o Freeze-thaw weathering causes angular fragments of rocks to fall and
accumulate at bottom of steep slopes
o On gentler slopes, fragments remain
 Exfoliation domes
 Limestone pavements: clints and grykes due to chemical weathering along joints
7. Mass Movement
Factors affecting Mass Movement:
 Gravity, angle of slope and shear stress
o Shear stress: force trying to enact movement
o Gravitational force stronger on steeper slopes, pulls down regolith faster,
thinner regolith
 Nature of slope and shear strength
o Shear strength: resistance slope offers against movement
o Sands and gravel: friction between particles
o Silt and clay: cohesive forces between particles which is influenced by
moisture availability
o Rock slopes: solidification and crystallization create very strong chemical and
physical bonds
 Role of water
o Increases shear stress but decreases shear strength
 Reduce cohesion between particles due to pore pressure
 Lubricates contacts between soil particles  minimize friction and
allows them to slip over each other more easily, reducing resistance
to gravity
 Increases weight of sediment
 Triggering mechanisms
o Earthquakes: vibration reduces internal strength of sediments by shaking
material loose from supported position and reduces cohesion
o Undercutting of slopes by streams
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Classification and Types of Mass Movement:
 Carson and Kirby’s classification [draw diagram]
 Soil creep
o Gravity creep: soil particles set in motion when disturbed by plant growth or
activity of soil fauna
 As one particle moves, dislodges other particles and so on till initial
disturbance absorbed
o Soil heave: downslope movement of regolith
 Expansion and contraction due to heating and cooling / wetting and
drying / frost action
 Solifluction
o Summer: layer above permafrost melts and becomes unstable  permafrost
prevents downward drainage of melt water  water-logged soil  slide
 Fall
o Steep slope where angle of friction greatly exceeded
o Weathering done sufficient damage to allow block of rock to detach and fall
under gravity until it rests where the slope angle is low enough
 Slide
o Triggering mechanism usually needed
o Moving mass moves down a slide plane until it reaches bottom of the plane
where impact usually breaks it up
o Slide plane lubricated or selectively weathered
 Slump
o Rotational movement along curved slip plane
o Moisture concentrated at base
o Steplike terrace bounded by curved, wall-like scarp left behind
 Flow
o Soil moisture content high
o Velocity greatest at surface and decreases to nil at bottom of mass
o Slope materials with high proportion of fine particles (more than 35%) most
prone
o Internal deformation under its own weight, dependent on clay becoming
saturated with water to a percentage greater than the liquid limit
 Avalanches
Human Activities and Mass Movement
 Undercutting
o Construction of roadways especially in mountainous terrain
o Landslides common in the Himalayas, especially during the monsoon season,
but increasing number of roads and settlements built along foot of slopes
overlooking river valleys eg. Himachal Pradesh, India in 1995
 Construction
o Clifftop buildings: increase gravitational force applied to slope
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o Eg. Holbeck Hall Hotel in Scarborough in 1993 but it was following a number
of drier than average summers, and a particularly wet spring
 Top layers of cliff made of unconsolidated material deposited by
retreating glaciers
 Bounder clay had dried and cracked, so water can easily penetrate
 Foot of cliff attacked by sea, so not as stable
Deforestation
o Eg. Hong Kong expansion onto marginal land to accommodate demands of
population pressure
o Reduces stability of slope due to removal of binding properties of roots, and
exposes soil to direct impact of downpours
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