Rocks And Weathering (1) Flashcards
Describe falls?
Movements of dry material that occur very quickly
They can create cones of material at the base of slopes known as Talia fones
The space that they vacate is known as a scar
Describe slides?
Movements of wet material, which tend to happen relatively quickly
Water in the earth or rock tends to create a slip planes on which the material slides
Examples of flows?
Mud flow
Debris flow
Examples of falls?
Rock falls
Land falls
Cliff toppling
Examples of creeps?
Soil creep
Earth spreads
Describe creeps?
Very slow movements of relatively moist earth and soil
That can continue for hundreds of years
Tetracettes can form on slope surfaces and features such as fences and trees can start to lean and move down slopes too
Example of creeps?
Soil creep
Earth spreads
What is a slope?
An inclined surface on earth
Mostly the term refers to hillsides which can be found within any environment
Describe slope processes?
Downslope movement of material
Moving under the influence of gravity
What influences slope development?
Climate Rock type Vegetation Soil Human activity Hydrology within a drainage basin
Define aspect?
Direction a slope faces
Northern hemisphere slopes that face north
Get less sunlight are in shadow more
Define gullies?
Channels on a steep slope created by fast movement of water
Smaller gullies known as rills
Define regolith?
Late angular rock fragments that are partially decomposed parent rock
Loose regolith is more prone to moving down slope
Define parent rock?
Rock that underlies a slope
Weathered parent rock is major constituent of the soil that sits on it
Define sheetwash?
The process by which soil is washed down slope following rainfall
After soil particles are loosened by the impact of rain splash and material is then moved away by surface wash/surface flow/ overland flow
Define subaerial?
Processes that take place on slopes where they are exposed to the open air
Eg erosion
How does tectonics (divergence) influence the development of slopes?
The African Rift Valley is formed by divergence of plates (constructive margin) and the sinking of land relative to the surrounding area, creating steep cliffs bounding flat land
How does soils influence the development of slopes?
Clay soils hold more water than sandy sooils and are so much prone to water based movement as they are heavy and slippery
How does tectonics (convergence) influence the development of slopes?
Creates fold mountain which are then eroded by ice and water
These processes together create the slope forms within an area
Smaller scale convergence forces the land to buckle and fold, creating and folding will influence a rock angle of dip
How does vegetation influence the development of slopes?
Slopes with a lot of vegetation have a high density of roots so more stable slopes
Plants take up water which results in an increase in sheaf strength of the slope (it is less slippery so material can stay on the slope more easily)
Roots also anchor the soil
Vegetated slopes can therefore be stepped
How does weathering influence the development of slopes?
High amounts of chemical weathering in warm, wet areas will produce clays which result in low angles slopes
Weathering in arid and cold areas which tends to be physical weathering
Will tend to result in loose angular material, jagged cliffs and steep slopes
How does climate influence the development of slopes?
Climate affects weathering and vegetation both of which affect slope form
Heavy rain means more saturation of soil and more slumping which reduces angles
How does slope aspect influence the development of slopes?
Frozen slopes facing sun more prone to seasonal melting - soil and debris flow downslope
Slopes tend to be flatter and longer in northern hemisphere
Slopes in shadow not prone to melting
North hemisphere north facing slopes are shorter and steeper
Define slides?
Movement of a cohesive unit with minimal internal deformation
Same volume or width
Change in position not form or shape
Define flows?
Material is distorted by the land over which it travels
It may flow within a channel for instance
Define heave?
Material expands perpendicular to the surface on which it sits and moves as a result
What is shear stresses?
Forces attempting to pull a mass downslope
What opposes downslope movement?
Friction will keep material on a slope this can be overcome by adding water
Cohesion (such as cohesiveness of clay particles)
Vegetation (roots bind soil together, this may allow build up of soil moisture)
Describe how organic effects contribute to reduced shear strength?
Burrowing of animals
Decay of tree roots
Factors that contribute to increased shear stress?
Lateral pressure Loading of slope Removing of underlying support Removal of lateral support through undercutting of slope steepening Transient stresses
What type of mass movement is very slow?
Soil creep
What type of mass movement is fast?
Avalanches
What type of mass movement is dry?
Rockfalls
What type of mass movement is very fluid?
Mudflows
What type of rocks do slumps occur on?
Weaker rocks
Especially clay
Differences between types of flows?
Mudflows are faster and more fluid than earth flows
Earth flows are thicker and deeper
Contrast flows and slumps?
Flows are more continuous less jerky and more likely to contort the mass into a new form
What is talus creep?
The slow movement of fragments on a scree slope
How does vegetation oppose downslope movement?
Bonds the soil and thereby stabilised slopes
However vegetation may allow soil moisture to build up making landslides more likely
What does water do to the cohesion of particles on a slope?
Reduced them by saturation
Water pressure in saturated soils decreases the frictional strength of the solid material
This weakens the slope
When do slides commonly occur?
Combination of
Weak rocks
Steep slopes
Active undercutting
When does rock move downslope?
They will do so whenever the downward force exceeds the resistance produced by friction and cohesion
What are landslides most sensitive to?
Water content
When do falls occur?
On steep slopes
Greater than 40 degrees
Especially on bare rock faces where joints are exposed
What initially causes falls?
Weathering eg freeze thaw weathering or disintegration or erosion prising open lines of weakness
Once the rocks are detached they fall under the k fluency of gravity
When does surface wash occur?
When the soils infiltration capacity is exceeded
How does sheetwash erosion of soil occur?
Through raindrop impact and subsequent transport by water flowing overland rather than in channels
What is a rill?
A relatively shallow channel, generally less than tens of centimetres deep and carrying water and sediment only a short period
How do raindrops influence flat surfaces?
Raindrops compact the soil and dislodge particles equally in all directions
Describe a slide?
Movement of a cohesive unit with minimal internal dislocation and deformation
Describe heave?
The slow expansion and movement of debris to the surface perpendicular to slope
Heave raised the slope profile often in a series
Describe flow?
Travels like a viscous liquid
Distorting and shaping the land
The rate of flow is influenced by gravitational potential
Outline two causes of soil creep?
Alternate wet and dry periods
Freeze thaw
Describe how wet and dry periods alternating can cause soil creep?
During wet periods the soil absorbs more water, becomes heave and moves downhill under the influence of gravity
In period with little or no rainfall the soil dries out and contract
What slopes do soil creep occur at?
Slopes greater than 5 degrees
Series of horizontal tidges running across slopes with a similar shape to steps
How can freeze thaw cause soil creep?
During freezing ice crystals expand forcing regolith closer the surface of the slope
When the ground layer thaws the displaced material falls back but under the influence of gravity will end up further down the slope
Difference in saturation of earth flow and mudflow?
Mudflows require higher saturation level
Results from extreme rainfall or rapid glacial melting
Fundamental difference between slides and flows?
Slides suffer interactions deformation whilst slides move together and are not affected by internal derangement
Causes of rotational slides?
Caused by saturated weaker rocks such as clays slide over stronger impermeable rock
When do rockfalls occur?
On exposed slopes exceeding 40 degrees with rocks that become fragmented due to structure
What is a debris avalanche?
A sudden catastrophic collapse from an unstable steep sided mountain, often on volcanos
Define deforestation?
Clearing of the earths surface ofyem resulting in land degradation and disturbed ecosystems
What is quarrying?
Form or extraction where rock material is removed through cutting and blasting of exposed rock faces
Background information on the causes of Aberfan?
South Wales coalfield was the largest coal deposits across britan
Waste was removed but with little space to put in narrow welsh valleys
Deposited on high ground away from the surrpunding settlements
Facts about Aberfan disaster?
21st October 1966
100,000 m cubed of debris engulfed part of the town in minutes
Results of Aberfan disaster?
147 deaths
116 children
Only a few lives saved - extreme negligence
Since then legislation passed regarding the removal and storage of waste material
Describe how water leakage can leak to landslides?
Water gets absorbed in the land and makes it heavier
The soil gets heavier gravity has more influence and the excess moisture can act as lubricant, reducing friction and increasing slide potential
Describe structural engineering?
The design and implementation of stabilising strategies and equipment
Describe soft engineering approaches?
Utilise more natural approaches in keeping with the surrounding local envirobment
Example of soft engineering?
Using plant vegetation o bind soil and intercept rainfall
Improved drainage channels prevent slopes becoming saturated and divert moisture away from lines of weakness (joints and bedding planes)
Landscaping to reduce gradient of slope or rock face
Descrive hard engineering?
Constructing physical structure that may support or hold earth and rock in place
Descrive slope stability analysis?
Designed to asses human made or natural slopes using mathematical models to measure the angles of slopes and the weight of soil, rock and debris and whether the forces of gravity and friction are in balance
What is a lahar?
Large mud and debris flow due to volcanic activity
Advantages of slope safety systems?
Risk of landslides cut by half since 1977
Advantages of installing drainage systems?
Protects from infiltration and water erosion
Copes with severe rains fall
Disadvantages of installing drainage systems?
Vulnerable to block
Ownership confusion leads to logistical problems
How does installing drainage systems work?
Reduced effect of heavy rainfall and thus surface runoff
As it intercepts and takes water away from slopes
How do does slope safety systems work?
Investigating and reaserchIng the causes of significant landslides
Context of Hong Kong slope instability?
Fast development - housing and infrastructure on slopes
Steep slopes - 30 degrees
Extreme rainfall
Tropical climate causes granite weathering which leads to thick regolith
Describe how a mudflow occurs?
Caused by high water content of regolith
Reduced internal cohesion
Movement under gravity occurs downslope
Define heave?
Movement of soil particles by water or freezing so they fall down
Define slide?
The downslope movement of material along a slide plane
How does rock type affect slope shape?
Resistant blocky rocks may produce free faces where rock falls can occur
Less resistant rocks will not maintain steep slopes
How does vegetation affect slopes?
Has the effect of binding together soil and hence protecting the slope from weather and erosion
So the slope is more stable
How do human activities affect slopes?
Undercutting, mining, waste dumping
Removal of vegetation destabilises slopes
Extent to which these factors affect slopes is influenced by climate
Can be positive or negative
Explain the factors and processes that may lead to rock falls?
Weathering - freeze thaw, seismic shakes from earthquakes
Undercutting by rivers
Human activity
Rock type is a factor (sucepiribilitu to weathering)
Climate (if diurnal more freeze thaw)
Define subduction?
The process where the oceanic crust is forced downwards under a continental plate
Or another oceanic plate is melted on the subduction (Benioff zone)
Define convection currents?
Created by heat within the earth generated by radioactive decay
Occurs in the semi molten mantle
Responsible for the movement of crustal plates
Effectiveness of physical weathering processes in different climates?
Insulation - effective in hot and dry climates
Freeze thaw - ineffective (needs diurnal temperature range)
Dry - all freeze thaw requires moisture
Describe the properties of granite?
Intrusive
Igneous rock
Medium to coarse texture
Composed of (usually) feldspar quartz and mica
Hard rock but has sets of joints which increase permeability and allow weathering
Describe the way granite is weathered?
Physical - freeze thaw
Water seeps into cracks
Freezes expands by 9%
Melts causing granite to break off
Pressure release - unloading of pressure causes joints to be created
Chemical - hydrolysis
Affects feldspar mostly but also mica
Quartz remains inert as residue
Results in the formation of kaolin
Explain how mudflows occur?
Input of water is essential (rainfall or snowmelt)
Water needs to thoroughly mix with clay in an upper accumulation zone
This caused internal cohesion to be reduced as a result of increased pore water pressure
Reduces strength of the material making it capable of flowing down even gentle slopes
What mass movements are slow?
Solifluctuatioj
Creep
Slump
What mass movements are fast?
Landslide
Mudflow
Rock fall
Earthflow
Describe mudflow?
High moisture content as material with water
Leads to lack of friction, more lubrication
Resulting mass moves down slope when volume and weight of water added to soil
Rapid movement s occur after a period of intense rainfall
Describe soil creep?
Low moisture content Slow Heave through wetting or drying Freezing and thawing Low angle slopes
Descrive Aberfan disaster?
1966
Heavy rain added water to waste materials in spoil tips constucted over a spring on slopes 25 degrees above the town
Internal cohesion reduced and the spoil heap moved downslope rapidly as mudflow
Describe how temperature affects chemical weathering?
As temperature increases
Rate of chemical weathering doubles with every 10 degree increase
Acts best with increase in rainfall
Define carbonation?
Rainwater contains carbon dioxide in solution
Forming carbonic acid
Which reacts with calcium carbonate
Define hydration?
Certain rocks able to absorb water into their structure
Causing them to swell
And become more suceptible to further breakdown
How does heave lead to soil creep?
Lifting of soil particles at right angles to the slope through water or freezing
Soil particles then drop downslope when dry or nelted
Giving rise to slow movement caused soil creep
How do slides occur?
Caused by slope failure of a block of material that moves downslope on a slide plane due to the overocoming of shear strength by shear stress
Define spheroidal weathering?
The rounding of detached blocks of rocks through chemical weathering at depth
Define salt crystal growth?
Crystal growth is the development of salt crystals in pores
Cracks in rocks which then put pressure on the rock structure
Why does heave occur?
Wetting/drying
Freezing/melting
Facts about soil creep?
Occurs on slopes over 5 degrees
Radley exceeds speeds of 1cm per year
How does soil creep occur?
Heave causes the lift of soil particles perpendicular to the surface
When dry or unfrozen, the particles fall back vertically under the influence of gravity
This leads to a slow downslope movement called soil creep
What is the maximum volume of rockslides?
100,000 cubic metres
Difference between rockslides and rockfalls?
Small fragments of rock don’t split down their path in ROCKFALLS
Blocks split as they travel downslope by bounding or rolling in ROCKSLIDES
What do both rockslides and rockfalls have in common?
Both in steep terrain (40 degree or more slopes)
Explain how rotational slide occurs?
Occurs along a curved slip plane
Due to shear strength being overcome by shear stress
Occur in clay and in I consolidated slope material
Why do rotational slides happen?
Increased SHEAR STRESS
undercutting result in failure
Shocks and vibrations mobilise water content increasing pore pressure
Changes in water (rainfall, increased weight or pore pressure)
The material that has not been moved in a slide is called what?
The crown
Define an ocean ridge?
Large scale mid ocean mountain range
With a rift along its spine where magma escapes
Define sea floor spreading?
The lateral extension of the ocean floor as oceanic plates move apart
Magma is added to the ocean floor
Describe an island arc?
Chain of islands
In a curved line
Usually volcanic close to or parallel to a destructive subduction oceanic plate margin
What is chelation?
The weathering of rocks through the action of organic substances
Including bacteria and humid acids
What is humic acid?
Acids released by the decay of organic materials
Explain how hydration weathers rock?
A mechanical weathering process
Whereby water is added to a mineral and it expands
Leading to grandiose disintegration
Define flow?
Movement of water laden material
With internal deformation
Define slide?
Movement of material along a slide plane
With minimal internal deformation
Descrive how heave occurs?
Slow movement associated with soil creep
Particles of earth are lifted perpendicular to the ground surface
By wetting or freezing
They then fall vertically down under gravity due to drying or melting
Descrive how flows can affect the shape of slopes?
Leave debris at the foot of the slope
Hollow left where flow commences
What is fall?
The movement away from the slope often involving large blocks of rock
Little contact with the slope surface until it hits the slow base
Describe how a mudflow can affect the shape of slopes?
Leave bowl shaped scar Where movement starts Flow tracks produced Decrease in angle at the base Steepened upper slope
What type of mass movement is undercutting likely to lead to?
Sliding
Rock fall
how can human activity impact slipe stability?
overloading the slipe
undercutting the slip by roads quarrying or mining
vegetatjon removal in deforestation
average thickness of continental and oceanic crust
35-70 continental
6-10 oceanic
average age of continental and oceanic crust
continental - very old over 1,500 million
oceanic - very young uber 200 million
average minerals of continental and oceanic crust
continental - silica aluminium and oxygen
average nature of rocks of continental and oceanic crust
continental - numerous types granitic is most common
oceanic - few types mainly basaltic
facts about ocean ridges
1000-4000km wide
crests that rise 2-3km
5km deep
facts about depths of ocean trenches
over 6000m to 11,000m
define heave
the lifting of usually soil particles out of the slope followed by a drop to the surface
define flow
the downslope movement of material
minimal internal deformation
high water content
how can rock falls affect the shape of slopes
steeper
screw accumulates at slope base
gentler angle
explain how fold mountains form
movement of plates leads to the folding of sediments trapped between the plates and subsequent uplift
resulting in mountain chains
explain why earthquakes may occur at a subduction plate boundary
oceanic plate forced under continental plate
resulting in a pressure increase
triggering earthquakes as this pressure is suddenly released
what are island arcs
chain of volcanic islands formed when two oceanic plates converge
oxidation
form of chemical weathering
oxygen in water reacts with rock or soil
causing it to become reddened
hydrolysis
form or chemical weathering
minerals are broken down the the reaction with hydrogen ions
define fall (in terms of mass movement)
the movement away from the slope often involving large blocks of rock
little contact with the slope surface until it hits the base
scree or talus is left behind
what is entrainment
the process whereby sediment is picked up by the channel flow is dependent upon the balance between the velocity and size of particle
briefly describe how abrasion occurs in a river channel
channel load is being transported and comes into contact with the channel bed and sides
this can produce erosion thag widens and deepens the channel
why do bluffs occur
erosion induced by the meandering channel
how can fold mountains occur at a subduction boundary
sediments carried on continental plate may be crushed and folded upwards
describe how a mud flow occurs?
high water content of revolting
internal cohesion reduced
movement under gravity occurs doenslope
how thick is oceanic crust
6-10 km
how thick is continental crust
35-70km
responses to boscastle flood
120 winched to safety (7 helicopters ove r3 hours)
largest RAF rescue since 1979
1000 people affected only 58 houses damaged
£250,000 costs in damage
why did the boscastle floods happen
50mm of rain in 2 hours
previous saturation of ground
moreland - little infiltration
impermeable standalone - little infiltration
overland flow
small rounded drainage basin - short lag time
steep valley sides
3 rivers in village all busy banks simultaneously
effects of Pakistan flood
1.2 million homes damaged
11,000 schools closed
1,600 deaths (secondary)
15% cotton crop fail
What processes lead to rock falls
Weathering
Seismic shakes from earthquakes
Undercutting by (rivers glaciers humans)
how can mineral makeup affect heating and cooling?
when a rock is composed of more than one thing instead of all one mineral
how does colour affect heating and cooling?
darker absorbs more heat
eg dark colour of mica in granite
define island arc
series of volcanos where two oceanic plates meet
explain how the slump mass movement has occurred
slump is a downward movement of rock moving at the same speed along a slip plane
slumps occur on weaker rocks such as clay when having absorbed water and become unstable
slide along slip plane when shear stress exceeds shear strength
why might a mass movement of slumping or rotational sliding happen
increase in moisture producing greater weight increased lubrication decreased strength along a slip plane undercutting
describe how abrasion occurs in a river channel?
erosion happens by particles being thrown against the channel sides or rubbed along the channel bed
role of turbulence and high velocity
what is the difference between alluvial fans and deltas?
deposition in water
importance of clay flocculation as fresh water meets salt water
formation of an alluvial fan
deposition as stream with lots of sediment
emerges out of a restricted valley
drop in gradient
and the river now spreads laterally
associated with flash floods
briefly when two plates converge how are fold mountains formed
sediment is crushed as the plates approach eachother
describe mudflows
high moisture content
material is saturated with water
lack of friction
increased lubrication
resulting mass moves down slope when volume and weight water added to soil
rapid movements occur on steep slopes after period of intense rainfall
describe soil creep
low moisture content heave through wetting or drying freezing or thawing low angles slopes
how did Aberfan 1966 influence slope stability
heavy rain added to waste material in spoil tips constructed over a spring on slopes over 25 degrees above town
internal cohesion reduced
spoil heap moved downslope rapidly we mudflow
