AS: Slides

Slides

-       Occurs on a slide/ slip plane or what is known as a failure surface, which is lubricated by rain water which had infiltrated along this major line of weakness.

-       Slides may be rotational or translational (planar).

-       In the planar slide, the weathered rock moves downhill leaving behind it a flat rupture surface.

-       Where rotational movement occurs, a process sometimes referred to as slumping, a curved rupture surface is produced.  These are mostly slides, usually along more than one slip plane, which is curved. These have a variety of names but are essentially the same process. The more resistant/ permeable rock e.g. limestone remains, while the clay material slumps into blocks on slip planes.

-       Impacts on slopes: Stepped uneven profile, scars, slumped block and a toe of material at the base.

AS: Flows

Flow

-       Fast, need well lubricated material

-       The material behaves like a viscous fluid

-       Material size – large boulders – small grains

-       The debris avalanche (large boulders) is the fastest of the flows

-       Some other types of flows include earthflows and debris flows (small grain sized slows).

-       Occurs because there is a decrease in internal/ shear strength. Heavy rain infiltrates the regolith – lubricates the material by filling the pores thus increasing pore pressure. Shear strength < external stress i.e. gravity

-       Flows can be triggered by earthquakes would increase shear stress.

-       The most important point about flows is that there is a decrease in movement with depth. The top middle moves the fastest and the front extends the furthest (an area known as the ‘toe/lobe’). The internal deformation of material down the slope and as the material goes down the slope there is a decrease in velocity.  A scar is left at the top of the slope where the flow began. This is a steeper section of the slope.

-       The overall impact of the slope: Scar, gentler gradient at the base of the slope and material may spread widening slope foot.

-       Mudflows: Rapid movements, occurring on steeper slopes, exceeding 1km/hr. They are most likely to occur following periods of intensive rainfall, where both volume and weight are added to the soil giving it a higher water content than an earthflow

-       Earthflows: When the regolith slopes 5-15⁰ becomes saturated with water, it begins to flow downhill at a rate varying between 1 and 15 km per year. The movement of material may produce short flow tracks and small bulging lobes or tongues, yet may not be fast enough to break the vegetation.

Solifluction

-       This process meaning ‘soil flow’ is a slightly faster movement averaging between 5cm to 1m per year.

-       Often takes place under periglacial conditions where vegetation cover is limited.

-       During winter both the bedrock and regolith are frozen. In summer, the surface layer thaws but the underlying layer remains frozen and acts like impermeable rock. Because surface water cannot infiltrate downwards and temperatures are too low for effective evaporation, any topsoil will become saturated and will flow as an active layer over the frozen layers.

-       This process produces solifluction sheets and lobes (rounded tongue like features), and heads (a mixture of sand and clay formed in valleys and at the foot of sea cliffs).

AS: Heaves

Heaves

-       Slowest

-       Effects and movement – imperceptible and hardly visible

-       Most widespread – most occurring

-       Associated with relatively fine materials e.g. silt, unconsolidated material

Frost Heaves

-       Occurs at high lats. And altitudes in mid lats.

-       Water freezes during winter and produces a lens of ice beneath the particles because the particles conduct the cold. The cold, ice expands by 9% and pushes outwards.

Soil Creep

-       Mass movement process

-       Occurs on slopes of about 5⁰ and produces terracettes

-       The impact on the slope can be direct (microfeatures include terracettes (tiny ripples under the grass due to a accumulation of soil in very small ridges)).

-       The impact can be indirect where fences can break, bases of trees can turn downwards, telegraph pole tilt, cracks can form in the road and soil can accumulate behind walls.

-       Can cause a down slope breaking of bed rock or a rock outcrop.

AS: Rockfalls

•        Sub-aerial weathering e.g. physical weathering by changes in temperature wetting/ drying à block disintegration at the top of the slope
•        Rockfalls occur on slopes exceeding 40⁰.
•        Blocks at the top become loose and fall vertically down the free face (90 degrees) due to gradual weathering processes such as freeze thaw and/ or tectonic activity.
•        Weathered rock at the foot of the slope (scree or talus) at a 45 degree angle and is a boulder controlled concave slope.
•        Causes: Reduction in shear strength due to weathering à loose blocks. These fall under the influence of gravity (90⁰) à base of the slope to form a new gentler slope segments (45⁰).

AS: Types of mass movement

Types of mass movement

-       Falls

-       Heaves

-       Flows

-       Slides

Mass movement: 101 - AS

Classifications of mass movement

The types of processes can be classified in a number of different ways:

-       Speed of movement

-       Water content

-       Type of movement: flows, slides, slumps

-       Material

The mechanism of mass movement

Rock particles on slopes are held on the slope by friction in a state of dynamic equilibrium. Their steady state (not moving) represents a balance between the internal (within/ between the particles known as internal or shear strength) and external forces (known as external/shear stress). When shear strength = shear stress = no movement. If one is greater than the other = movement.

Water as a factor in changing the dynamic equilibrium of the slope:

Water is a very important factor in influencing slope stability. Particles in the soil stick together if it rains, the rain infiltrates via the pores and lubricates the weathered material therefore reduces friction and makes the weathered material easier to move down the slopes. Water may also increase external stress because it adds weight to the slope (because of an increase in pore pressure).

What keeps slopes in place?

Vegetation (Binds soil thus stabilising slopes), friction (will vary with the weight of the particle and slope angle, and can be overcome with the help of water), cohesive force (An act to bind the particles of the slope and prevalent is water-less clay) and pivoting (occurs in debris layers that contain material embedded in the slope).

What factors lead to increasing stress and decreasing shear resistance?

Water, weathering and the type of material can reduce resistance. Stress can be increased by steepening of a slope, undercutting of a slope, addition of a mass of regolith, dumping of mining waste, sliding from higher up the slope, vibrational shock and earthquakes.

Factors that contribute to increased shear stress:

·      Removal of lateral support through undercutting or slope steepening – Erosion by rivers, glaciers, wave action, faulting, previous rock falls or slides.  

·      Removal of underlying support –Undercutting by rivers, waves, sub-surface solution, loss of strength by extrusion of underlying sediments.

·      Loading of slope – Weight of water, vegetation, and accumulation of debris.

·      Lateral pressure – Water in cracks, freezing in cracks, swelling and pressure release.

·      Transient stresses – Earthquakes and movement of trees in the wind.

Factors that contribute to reduced shear strength:

·      Weathering effects – Disintegration of granular rocks, hydration of clay materials, dissolution of cementing minerals in rock or soil.

·      Changes in pore water pressure – Saturation or softening of material

·      Changes in structure – Creation of fissures in shales and clays, remoulding of sand and sensitive clay.

·      Organic effects – Burrowing of animals and decaying tree roots.

AS Mass movement: Carson and Kirkby’s classifications of mass movements

Although this is not directly needed, this diagram may appear in an insert and you may be required to interpret it in an exam.

AS: Droughts

•        The hydrological cycle accounts for 1% of the total water on the planet
•        The hydrological cycle is a closed system because water is neither added nor lost
•        Over the last 300 years the world’s population has increased x7 and demand for water has increased x40.
•        Severe water stress is experienced by 1.1 billion people in 80 countries

Reliability of rainfall

•        Few homes in LEDC’s have piped water
•        Few developing countries have the money or the technology to build dams to store water. If they have it was mostly like built with foreign aid
•        Torrential downpours give insufficient time to infiltrate the ground. Instead surface runoff create flash floods
•        The most vulnerable areas are desert margins and tropical interiors where average annual rainfall is low and rainy seasons are short
•        Many countries just experience wet/ dry seasons. If rain fails one year, the result of produce can be disastrous

Clean Water

•        1.1 billion people lack clean water
•        Rural areas use local rivers for drinking as well as washing and sewerage disposal
•        Shanty towns lack proper drainage for sewerage which may pollute water ways
•        Disease can break out
•        Villagers can help themselves out though by: building wells to reach a permanent supply, lining wells with concrete, using pumps and teaching about proper hygiene

AS: Atmosphere & Weather - Global Warming case study

Global warming

·      The concern about global warming is the build-up of gases within the atmosphere.

·      Deforestation has meant that the extra CO₂ that has been produced, can not be absorbed by trees and converted in oxygen.

·      An increase in greenhouse gases has meant that there is an increase in long-wave radiation being absorbed by these gases. This has meant the earth has gotten warmer.

Impacts of Global warming

·      Climatic

o   Increased storm activity

§  Tornadoes in the mid west

§  Hurricanes

§  Typhoons

§  Cyclones

o   Temperature increases

§  With no action, temperatures will increase by 2.5⁰C in the next 50 years.

o   Reduced rainfall

§  Leads to droughts

o   Rise in sea temperatures

·      Other

o   Forest fires

§  Increase of fires because of dry forests

o   Coral bleaching

§  From a rise in sea temperatures

§  For example reefs on the Great Barrier Reef

o   Water shortages

§  4 billion could face water shortages if temperatures increase 2⁰C.

o   Changes in agriculture

§  Samoln fishing could become obselte

§  A 35% decrease in yields if temperatures increase 3⁰C

§  However there could be an increase in timber yields

§  Increase in growing seasons in temperate and alpine areas

o   Flooding

§  By 2100 an overall 1m rise in sea levels if no action has occurred.

§  Flooding will occur in Delta areas more frequently e.g. New Orleans.

§  4 million km² is threatened

§  200 million could be at risk of loosing their homes from floods by 2050.

o   Changes to Tourism

§  Longer tourist seasons for summer tourist destinations

§  Winter tourism to ski fields and glaciers may decline.

o   Soil erosion

§  Especially in areas such as the amazon that is damaged by slash and burn practices

o   Spreading of disease

§  A 2⁰C rise could increase the number infected by Malaria by 60 million

o   Extinction of wildlife

§  If temperatures increase by 2⁰C, 40 % of species will become extinct

§  Habitats could decrease in range e.g. Polar Bears

§  Range of Species could increase e.g. Butterflies in the UK