Unit 5: Rocks and Weathering Flashcards
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Lithosphere
The rigid outer layer of the earth that includes the crust and the upper mantle
Asthenosphere
The upper layer of the earths mantle below the lithosphere in which there is relatively low resistance to plastic flow and convection is thought to occur
Igneous rocks
Forms from magma or lava solidification
Hard, no layers
Intrusive (granite): slow magma cooling
Extrusive (obsidian): rapid lava cooling
Sedimentary rocks
Forms fro sediment compaction
Crumbly, layers
Clastic (sandstone): compacted broken rocks
Chemical (limestone): compacted dissolved minerals
Organic (coal): compacted biogenic matter
Metamorphic rocks
Forms by transformation of other rocks
Relatively hard, may or may not have layers
Foliated (slate): has layers
Non-foliated (marble): no layers
Continental plates
Sial - silica and aluminium
30-40 km thick, 60-70 km under mountain chains
Comprises of numerous rocks, granite most common
Lighter, density of 2.7g/cm^3
Very old mainly over 1500 million years
Oceanic plates
Sima - silica and magnesium
6-10 km thick on average
Few types of rock, mainly basalt
Heavier, density of 3g/cm^3
Very young mainly under 200 million years
Convection current theory
States hughe convection currents occur in the earths interior. Hot magma rises through the core to the surface and spreads out at mid-ocean ridges. Cold solidified crust shrinks into earths interior because it is heavier and denser than surrounding materia. Cause as radioactive decay in the core
Dragging theory
Plates are dragged or subducted by their oldest edges which are cold and heavy. Plates are hot at the mid-ocean ridge but cold as they move away. Complete cooling takes about 1m years. As cold plates descend at the trenches pressure causes the rock to change and become heavier
Volcanic hotspots
A hotspot is a plume of lava that rises vertically through the mantle. Most are near plate margins and may have caused original rifting of the crust. They can cause movement outward flow of viscous rock from the centre may cause a drag force on the plates causing movement. A volcanic hotspot is an area of the mantle that experiences a plume of ultra hot magma causing volcanic activity on the surface. Narrow plume rises vertically from seismically slow lower pantle. Hotspot is stationary. Lithosphere moves
DIvergent (constructive) plate boudary
Plates moving apart from each other resulting in minor earthquakes and eruptions
Convergent (destructive) plate boundary
Subduction occurs resulting in large earthquakes and eruptions. Ocean trenches are also formed
Convergent (collision) plate boundary
2 or more continental plates collide with minimal subduction and creation of fold mountains. No eruptions but large earthquakes
Conservative/transform plate boundary
Plates slide past each other without subduction. Major earthquakes but no eruptions
Constructive boundary
Plates pulled apart by convection currents (sea floor spreading)
Both plates are oceanic
Mid-ocean volcano formed
Destructive boundary
Heavier oceanic plate
Oceanic plate sinks under continental plate (subduction)
Oceanic plate melts as it enters the hot mantle. Causes an increase in pressure
Volcanic eruption when too much pressure in mantle
Volcano formed above where plate melts
Lighter continental plate
Conservative boundary
Plates can be oceanic or continental
Pressure builds causing large earthquakes
Plates moving in opposite directions at slightly different speeds or angles
Alfred Wegener and Francis Bacon evidence
In 1912, Alfred Wegener had the idea of continental drift. Francis Bacon in 1620 stated how the shape of Africa was similar to that of South America. Wegener proposed continents were slowly drifting about the earth. Starting with the Carboniferous period 250 million years ago a large continent, Pangea, broke and began to drift forming continents
Harry Hess evidence
In the 20th century Harry Hess suggested convection currents would force molten magma up in the interior and crack the crust above. In the 1960’s research on rock magnetism supported this. The rocks of the mid-Atlantic ridges were magnetised in alternate directions in identical bands on both sides. This suggested that fresh magma came up through the centre and forced rocks apart. Increasing the distance from the ridge means rocks were older. Supported the idea that new rocks were being created at the centre of the ridge and older rocks are pushed apart
J Wilson evidence
In 1965 J Wilson linked continental drift and seafloor spreading into a concept of mobile belts and rigid plates, forming the basis of plate tectonics
Evidence of plate tectonics
Past and present distribution of earthquakes
Changes in earths magnetic field
Fit of continents
Glacial deposits in Brazil match those in west Africa
Fossil remains in India match those of Australia
Geological sequence of sedimentary and igneous rocks in parts of Scotland match those in Newfoundland
Ancient mountains can be traced from east Brazil to west Africa and from Scandinavia through Scotland to Newfoundland and the Appalachians
Fossil remains of na aquatic reptile, Mosasaurus (270 million years ago) are only in part of Brazil and south west Africa
Sea-floor spreading
The process by which new oceanic crust is formed at mid-ocean ridges and spreads outwards pushing older crust away from the ridge
Sea-floor spreading evidence
Wegener’s hypothesis of continental drift wasn’t widely accepted because he had no mechanism to explain how continents move. The idea was not received until new technology made ocean floor exploration possible. Harry Hess proposed seafloor spreading in which basaltic magma from the mantle rises to create new ocean floor at mid ocean ridges
What happened in 1948?
A survey of the floor of the Atlantic ocean revealed a continuous ridge running from north to south
1000 km wide
Heights of 2.5 km
Composed of volcanic rock
Similar found in Pacific ocean
Palaeomagnetism
Iron particles in lava erupted on the ocean floor are aligned with the earth’s magnetic field
As the lavas solidify these particles provide a permanent record of the earths polarity at the time of eruption
Geomagnetic polarity reversals
The earths polarity reverses at regular intervals (400000 years)
The result as a series of magnetic stripes with rocks aligned alternately towards the north and south poles
The striped pattern which is mirrored exactly on either side of a mid-ocean ridge suggest the ocean crust is slowly spreading away from the boundary and new rocks are being added equally on both sides
Age of the ocean floor
Very young places on or near ridges
Much older ages were recorded for floor rocks near continental masses (200m years)
Older crust is continuously being pushed aside by new crust
Ocean ridges
Giant submarine mountain ridges that have heights up to 3000m
Found on constructive plate boundaries where new lithosphere is created
The first ocean ridge discovered was the mid-Atlantic ridge. It was found when engineers were attempting to lay a submarine cable from north America to Europe
Subduction zone
The area where an oceanic plate meets another plate and sinks below
The oceanic plate has a similar density to the asthenosphere below and can easily be pushed down below the continental
Subduction zones dip between 30 and 60 degrees but each will get steeper with depth. The older a crust is the steeper the dip due to the increased density relative to the asthenosphere
The size of the earth is constant so the amount of land destroyed at the subduction zone is equal to the amount of land created at constructive plate boundaries
Ocean trenches
Long, narrow depressions in the ocean floor with depths of 6000-11000m
They occur as a result of subduction zones and are often found near land or island arcs
They are asymmetrical with the steeper side closest to the land mass
Benioff and Wadati zone
AN area that extends below ocean trenches to a depth of 68-km where earthquakes often occur
Was named after seismologists Hugo Benioff and Kiyoo Wadati
Known for deep-focus earthquakes
Zone of Seismicity corresponds to the down-going slab in a subduction zone
Dip can be 30-60 degrees
The Benioff zone may extend from near surface to depths of 650-700 km
Most earthquakes occur within 1000 C isotherm due to internal deformation and dehydration embrittlement of the subducting slabs
What is a hotspot?
Volcanic hotspots are created as plumes of magma rise through the earths mantle. Mantle plumes are long-lived areas of high levels of heat flow in the mantle. Plumes consist of an upwelling long thin conduit and a bulbous head with spreads at the base of the lithosphere and produces a lot of magma, due to partial melting with arises from a drop in pressure. These have their origin at the Gutenberg Discontinuity which is the boundary that divides the outer core and mantle (2900km). It is believe that heat from the core is passed to the mantle by conduction and heat portions of the lower mantle become less dense and buoyant and rise as diapirs which become mantle plumes. The magma build-up will create volcanic activity on the ocean floor as it breaks through weak areas. The volcanoes may break through the surface of the ocean waters to form islands. The hot spot stays in the same place but because the plate above moves over time it creates a chain of islands with the oldest being furthest from the hotspot. Hotspots have seismic and volcanic activity. Volcanic earthquakes are generated by magma and tectonic earthquakes occur because of structural weakness at the base of volcanoes or in the crust below
Fold mountain building
Created when 2 or more tectonic plates are pushed together. As they collide, compressing boundaries, rocks and debris are warped/folded into rocky outcrops ,hills, mountains and ranges. Created through orogeny. This event takes millions of years. Nappes are common, dramatic folded rocks or rock formations. Earths crust is warped into folded form. Often associated with continental crust. Created at convergent boundaries (collision or compression). At a compression zone tectonic activity forces crustal compression at the leading edge of the crust so most fold mountains are found on the edge or former edge of a continental boundary. Rocks on the edge are weaker and less stable so more susceptible to folding and warping. Most are made of sedimentary rock and metamorphic rock formed under high pressure and low temperatures
Types of folds
Fold mountains are defined by folds and their shape. Usually more than 1 type in a mountain. Anticlines and synclines are the most common up and down that result from compression. Anticline is an upside down U shape with the oldest rocks in the centre. A syncline has a U shape with the youngest rocks in the centre. A dome is a series of symmetrical anticlines. Oldest in the centre. A basin is a depression in the earths surface. Youngest in the centre
Monocline
A type of fold in which all layers incline or dip in the same direction
Chevron fold
A sharp, straight fold where rock layers are like zig-zags
Slump fold
A result of slope failure. This happens when sediments were soft before they became a single mass. Lithified to become a slump
Ptygmatic folds
A type of slump fold created when the folding material is more viscous than the surrounding material. Many are created as metamorphic rock melts and intrudes into another layer forming a dike
Disharmic folds
Where different rock layers have different fold shapes
Volcanic island arcs
Long, curved chain of oceanic islands associated with intense volcanic and seismic activity and orogenic processes. Most consist of 2 parallel, arcuate rows of islands. The inner row is composed of a string of explosive volcanoes and the outer row is made up of non-volcanic islands. With single arcs, many constituent islands are volcanically active. An island arc typically has a land mass or partially enclosed, shallow area on its concave side. Along the convex side there is a long, narrow deep-sea trench. Destructive earthquakes occur often. These are deep focus seismic events from 370 miles below an arc. They tend to have a foci of progressively greater depth towards the concave side
How are volcanic island arcs formed?
Formed when 2 lithospheric plates converge and one oceanic is forced into the partly molten lower mantle below the continental. An island arc is built from the surface of the overriding plate by extrusion of basalts and andesites. The basalts are from semi-molten mantle and andesites from the partial melting of the descending plate
Features of volcanic island arcs
Ahead of the subduction zone there is a low bulge on the sea floor (trench outer rise) causing by the bending of the plate as it subducts. The outer slope of the trench is generally gentle but broken by faults as the plate bends. The floor of the trench is flat and covered by sediment/ash. Te trench inner slope is steeper and contains fragments of the subducting plate. The subduction complex (or accretionary prisms) is the slice of descending slab and may form significant landforms. Most subduction zones contain an island arc parallel to a trench on the overriding plate. 150-200 km from trench
Weathering
The decomposition and disintegration of rocks in situ
Decomposition
Chemical weathering
Disintegration
Mechanical weathering that breaks the rock into smaller fragments
In situ
In the natural or original position or place
Weathering vs erosion vs deposition
Weathering is the breaking down or dissolving of rocks and minerals on earths surface
Erosion is the transportation or movement of the weathered materials
Deposition is the dropping of the weathered material
Features of weathering
Many minerals are formed under high pressure in the earths core. As they cool nearer the surface they stabilise
Can cause irreversible changes to rocks. Some can change from a solid to clastic (fragmented) state. Some can change from solid to plastic (pliable) state
Can change the volume, density, grain size, surface area, permeability, consolidation and strength of rocks
It can create new minerals and solutions
Some minerals can be very resistant to it
Minerals and salts may be removed, transported, concentrated or consolidated
It creates new landforms and features
Freeze-thaw weathering
Also called frost action is weathering due to freezing temperatures. Water gets into the cracks of rocks, freezes and expands by about 10%. This puts pressure on a rock causing it to shatter and break off
Heating/cooling processes in weathering
Repeated heating and cooling of rocks can cause them to be broken down and weathered away. Rocks expand rapidly when hot and contract rapidly when cold causing the breaking apart of layers of rock. The changes in temperature cause stress on the outer layers of the rock so they peel off (exfoliation)
Salt weatheirng
Occurs due to salt crystal growth in the cracks and pores in rocks. When saline solutions get into cracks and evaporate, it leaves the salt crystals that were in the solution. As these crystals accumulate over time, the build up of pressure expands the gap in the rocks. This causes rocks to break off or disintegrate
Pressure release weathering
Caused when rocks that are under a lot of pressure no longer have to bear a heavy load causing expansion and fracturing. When there is a removal of weight, the underlying rocks will expand when the pressure is released causing fractures to form on the rock surface
Biological weathering
The action of burrowing animals or growing roots to destabilise rocks. As the plant grows, the roots enter cracks in the rock under the soil. As the plant and roots grow, the roots cause the crack to get larger and rock breaks away
Chemical weathering
The decomposition of rock from a chemical change. This is often the result of the interaction between rocks and moisture which leads to dissolved particles and the formation of clays. More likely to take place in warm, moist vegetated areas
Oxidation
Occurs when rocks become exposed to air. Often can be seen as earth is moved, holes are dug or mass movement exposes underlying rock and soil. Iron-rich soils and rocks may appear grey or blue until exposed to air. Previously ferrous soil and rock will oxidise and change to a ferric state, turning rusty. This also occurs in iron-rich metals that have become exposed to air
Hydrolysis
Particularly significant in the decomposition of rocks to form clays. It is the process by which chemical bonds are broken and the components partner to form different elements. It is a reaction involving the breaking down of a bond in a molecule with water. Mainly occurs between a hydrogen ion in water and changes a solutions pH. On granite landscapes, hydrogen in water reacts with minerals in rock and are washed through attaching to silicic acid and potassium hydroxyl in chelation. Positive ions attach to negative ions. The product is a fine grey clay (kaolin)
Acidification
The process by which liquids become acidic and is a common way in which rocks and minerals are dissolved. The greater the concentration of acids, the greater the effect of weathering
Carbonation
The process where carbon dioxide, often from rainwater produced carbonic acid. This is a weak acid solution that reacts with calcium carbonate rocks
Acid rain
Caused by greater concentrations of carbon dioxide, sulphur dioxide and nitrogen oxide due to human activities. These gases form acids as they combine with water vapour as rain
Hydration
Related to the absorption of water by rocks. Certain rocks are particularly capable of absorbing water into pores and cracks. the rocks swell sometimes repeatedly in dry and wet conditions and can change state
Peltiers diagram
Shows the relationship between temperature and moisture availability in shaping the nature of weathering in an environment
Describe the relationship between annual rainfall and chemical weathering
There is stronger chemical weathering when mean annual precipitation is higher and decreases with a fall in precipitation
Why is strong chemical weathering on the graph where it is?
When there is carbon dioxide, sulphur dioxide and nitrogen in the atmosphere they combine with water vapour to form acid rain which is a process of chemical weathering. More of this acid rain will increase chemical weathering in an area
Describe the temperature and rain characteristics if very slight weathering and why
Slight weathering where mean annual rainfall is 0-1000mm, and where mean annual temperature is-14-30C. This is because a lot of weathering occurs with high rainfall and this area of the graph has low rainfall. Therefore there is only slight weathering
Where would freeze-thaw be on the graph and why?
Around 0-10C and medium annual precipitation so that enough rainfall can enter cracks and there is enough temperature variation for the process to occur
Glacial/periglacial climatic region
Frost very important. Susceptible to frost increases with increasing grain size
Taiga: High soil leaching, low rates of organic matter decomposition
Tundra: Low precipitation, low temperatures, permafrost (moist conditions, slow organic production and breakdown)
May have slow chemical weathering
Algal, fungal and bacterial weathering may occur
Granular disintegration occurs
Hydrolytic action reduced on sandstone, quartzite, clay, calcareous shales, phyllites and dolerites
Hydration weathering common due to high moisture
CO2 is more soluble at low temperatures
Temperate climatic regions
Precipitation and evaporation fluctuate
Mechanical and chemical weathering
Iron oxides leached and redeposited
Carbonates deposited in dry areas, leached in wet areas
Increased precipitation, lower temperatures, reduced evaporation
Moderate to high organic content, moderate breakdown
Silicate clays formed and altered
Deciduous forest: Abundant bases, high nutrient status, moderate to high biological activity
Coniferous areas: Acidic, low biological activity, leaching common
Arid/semi-arid climatic region
Evaporation exceeds precipitation
Low rainfall
High seasonal temperatures
Low organic content
Mechanical weathering, salt weathering, granular disintegration, dominant in dry areas
Possible thermal effects
Low organic input relative to decomposition
Light leaching produces CaCO3 in soil
Sulphates and chlorides accumulate in dry areas
Increased precipitation and decreased evaporation in semi-arid areas and steppes yield thick organic layers, moderate leaching and CaCO3 accumulation
Humid tropical climatic regions
High seasonal rainfall
Long periods of high temperatures
High moisture availability
Weathering products removed or accumulate to yield red and black clay soils, ferruginous and aluminous soils and calcium-rich soils
Calcareous rock generally heavily leached where silica content is high, soluble weathering products removed and parent silica in stable products are sandy
Where products remain, iron and aluminum are common
Usually intense deep weathering, iron and alumina oxides and hydroxides predominate
Organic content high but decomposition high
Rock type
Chemical composition
The nature of cements in sedimentary rocks
Joints and bedding planes
Chemical composition
Limestone consists of calcium carbonate and so is prone to carbonation-solution weathering. Granite is susceptible to hydrolysis because it contains feldspar
Sedimentary rocks
Fluids flowing through the rock and organisms may precipitate new minerals in the pore spaces between grains to form a cement that holds the sediment together. Common cements are quartz, calcite, hematite (and iron oxide) and silica. Iron oxide based cements are prone to oxidation. Quartz cements are very resistant
Joints and bedding planes
The more joints and bedding planes there are in rocks the greater likelihood there is for weathering to take place. Joint patterns in rocks heavily influence water movement. They act as lines of weakness creating differential resistance within the same rock type. Weathering is accelerated. Grain size can influence the speed of weathering. Coarse-grained rocks weather quickly due to a large void space and high permeability. Fine-grained rocks offer a greater surface area for weathering so are highly susceptible to weathering also
Porosity and permeabiltiy
Porosity is a measure of void space in a material
Permeability is a measure of the ability of a material to transmit fluids
A permeable rock contains many air space so has more surface area than less permeable rocks. Surface area affects the rate of weathering. Since more surface is exposed to the substances or forces in the environment, weathering is faster
Porosity and permeability examples
Does not always match but is quite closely related. Granite and basalt match at the lowest for each but clay with the highest porosity has the third lowest permeability and gravel which has a quite low porosity has the highest permeability
Goldich’s weathering system
Generally as mineral stability increases, susceptibility to weathering decreases. Olivine, augite and line plagioclase are most likely to weather. Lime soda plagioclase, biotie, orthoclase and muscovite quartz are least likely to weather
Vegetation
Factors that influence weathering:
-moisture contents soils
-root depth of plants
-acidity of humus (decaying material at the top of soil profiles)
Deep soils can protect rocks beneath from weathering but may increase weathering by supporting the growth of more plants
how vegetation weathers rocks:
-secretion of organic acids aids chemical weathering
-growth of roots causing physical weathering
Relief
Is the shape of the land
Weathered material needs to be removed for further weathering to occur. This won’t happen on gentle slopes or flat land
On steep slopes water may flow off too quickly and not allow for as much chemical or freeze-thaw weathering
Intermediate slopes therefore tend to have the most weathering
Slope aspect (direction) might also impact weathering especially for freeze-thaw weathering
Slopes
Usually considered to be inclined surfaces sometimes called hillslopes
Watersheds
The steepest slopes are usually found closer to the source of streams and river with the flatter land nearer the mouth
Types of slope
Sub-ariel: Exposed slopes on the surface of the earth
Sub-marine: Underwater slopes
Degredational: Erosional or eroded slopes
Aggradational: Depositional or created by deposits
Transportational: Slopes created when material is moved
Climate slope process
In humid areas, slopes are rounder due to chemical weathering, soil creep and fluvial transport. In arid regions, slopes are jagged or straight due to mechanical weathering and sheetwash. Climatic geomorphology studies how different process operate in different climatic zones and produce different slope forms
Soil slope process
A part of regolith. Structure and texture determines how much moisture it can hold. Clay soils hold more than sandy soils. Deep clay where vegetation has been removed will not be resistant to mass movement
Vegetation slope process
Can decrease overland runoff through interception and moisture storage. Deforested slopes are exposed to intense erosion and gullying. Vegetation can increase the change of major landslides. Dense forests reduce surface wash causing a build-up of soil between trees, deepening the regolith and increasing the potential for failure
Geology slope process
Includes faults, angle of dip and vulcanicity. These influence the strength of a rock and creates likes of weakness. Rock type and character affect vulnerability to weathering and degree of resistance to downslope movement. Slopes composed of many different types of rocks are more vulnerable to landslides due to differential erosion. Less resistant rock is worn away and leads to the undermining of more resistant rock.
Aspect slope process
Is the direction in which a slope faces. In some areas, past climatic conditions varied due to this. During the cold periglacial period in the NH, in an east-west valley, the southern slope (faced north) stayed in the shade. Temperature were rarely above freezing. The northern slope (faced south) had more freeze-thaw cycles. Solifluctuation and overland runoff lowered the level of the slope and streams removed the debris from the valley. Result was an asymmetric valley
Mass movement definition
Large scale movements of the earths surface that are not accompanied by a moving agent such as rivers, glaciers or the sea
Mass movements
Very slow
Fast
Dry
Fluid
Classifying mass movements
Mass movements occur at different magnitudes, frequencies and scale
-speed of movement
-water content
-type of movement
-material
Type of movement
Falls: movement of rock/debris under the effect of gravity
Flows: movements of a mass of soil or rock that contain a significant amount of water
Heaves/creeps: slow movements of a material up to the surface of a slope and down slope
Slides: an entire ass of material moving along a slip plane
Movement classified by material
Rock
Debris
Mud
Soil
Causes of mass movements
The likelihood of a slope failing can be expressed by its safety factor. This is the relative strength or resistance of the slope compared with the force trying to move it. The most important factors that determine movement are gravity, slope angle and pore pressure. Gravity acts to move the material downslope and to stick the particle to the slope. The downslope movement is proportional to the weight of the particle and slope angle. Water lubricates particles and can fill spaces between then. This forces them apart under pressure. Pore pressure will increase the ability of the material to move. This is especially important in movement of wet material on low-angle slopes
Shear strength
The internal resistance of the slope
Shear stress
The forces attempting to pull a mass downslope
Slope safety
In MEDCs, slopes in densely populated areas are often assessed for how likely they are to move suddenly. The safety factor is determined by the relative shear strength of a slope compared to shear stress. Slope failure occurs when shear strength is reduce or shear stress is increased
Factors that contribute to increased shear stress
Removal of lateral support through undercutting or slope steepening (erosion by rivers and glaciers, wave action, faulting, previous rockfall’s or slides)
Removal of underlying support (undercutting by rivers and waves, subsurface solution, loss of strength by extrusion of underlying sediments)
Loading of slope (weight of water, vegetation ,accumulation of debris)
Lateral pressure (water in cracks, freezing in cracks, swelling, pressure release)
Transient stresses (earthquakes, movement of trees in wind)
Factors the contribute to reduced shear strength
Weathering effects (disintegration of granular rocks, hydration of clay minerals, dissolution of cementing minerals in rock or soil)
Changes in pore-water pressure (saturation, softening of material)
Changes of structure (creation of fissures in shales and clays, remoulding of sand and sensitive clays)
Organic effects (burrowing of animals, decay of tree roots)
Opposing force
Mass movements are not inevitable on slopes. There are many forces working to oppose downslope material
Friction opposing force
WIll vary with the weight of particle and slope angle. Can be overcome on gentle slope angles of water is present (solifluctuation can occur on 3 degree slopes)
Cohesive forces opposing forces
Act to bind the particles on the slope. Clay may have high cohesion but this may be reduced if the water content becomes so high that the clay liquefies when it loses its cohesive strength
Pivoting opposing force
Occurs in the debris layers which contain material embedded in the slope
Vegetation opposing force
Binds the soil and so stabilises slopes but vegetation may allow soil moisture to build up and make landslides more likely
Soil creep description
A slow, small-scale process that occurs mostly in winter. It is one of the most important slope processes in environments where flows and slides are not common. Talus creep is the slow movement of fragments on a scree slope
Soil creep speed
Slow at 1-3 mm per year in temperate regions and up to 10 mm per year in tropical rainforest. In a well vegetated humid temperate area, soil creep can be 10x more important than slope wash. In periglacial areas it cam be 300 mm per year. Small-scale variations in slope, compaction, cohesion and vegetation will have a significant effect on the rate of creep
Common causes of soil creep
Individual soil particles are pushed or heaved to the surface by wetting, heating or freezing of water. About 75% of the soil creep movement is induced by moisture changes and associated volume change. Freeze-thaw and normal temperature controlled expansion and contraction are important in periglacial and tropical climates
Evidence of soil creep
Observation is difficult. Qualitative evidence such as bent trees is misleading and now largely discredited. The slow rate of movement may mean that measurement errors are serious
Slump description
Occurs on weaker rocks, especially clay and have a rotational movement along a curved slip plane
Slump speed
The speed of slump varies from metres per second to metres per year. Sudden slumps usually occur after earthquakes or heavy continuing rains and can stabilise in a few hours. Most slumps develop over longer periods, taking months or years to reach stability
Common causes of slumps
Clay absorbs water, becomes saturated and exceeds its liquid limit. It then flows along a slip plane. Often the base of a cliff has been undercut and weakened by erosion, reducing its strength
Evidence of slumps
Leads to cliff-like slopes after a period of time. Creates dramatic elevation changes
Flows description
More continuous, less jerky and more likely to contour the mass into a new form. Material is mostly of a small size. Particle size involved is generally small
Flows speed
Mudflows are faster and more fluid than earthflows which tend to be thicker and deeper. A higher water content will enable material to flow across gentle angles
Common causes of flows
Earthflows and mudflows can occur on the saturated toe of a landslide or may form a distinctive type of mass movement. Small flows may develop locally whereas others may be larger and more rapid
Evidence of flows
In theory, mudflows give way to sediment-laden rivers but the distinction is very blurred
Rockslides, landslides and rotational slides description
Slides occur when an entire mass of material moves along a slope plane. These include rockslides and landslides of any material, rock or regolith and rotational slides which produce a series of massive steps or terraces. Landslides occur when the material moves downslope as a result of shear failure at the boundary of the moving mass. This may include a flowing movement as well as straightforward sliding
Rockslides, landslides and rotational slides speed
Slides range from small-scale slides close to roads to large-scale movements that kill thousands of people
Common causes of rockslides, landslides and rotational slides
Slides commonly occur where there is a combination of weak rocks, steep slopes and active undercutting. Slides are often caused by a change in the water content of a slope or by very cold conditions. As the mass moves along the slip plane it tends to retain its shape and structure until it hits the bottom of a slope. Slip planes occur at the junction of 2 layers, at a fault line, where there is a joint, along a bedding plane or at the point beneath the surface where the shear stress becomes greater than the shear strength. Weak rocks have little shear strength to start and are particularly vulnerable to the development of slip planes. The slip plane is typically a concave curve and as the slide occurs the mass will he rotated backwards. Loose rock, stones and soil have a tendency to move downslope. They will do so when the downward force exceeds the resistance produced by friction and cohesion. Landslides are very sensitive to water content which reducers the strength of the material by increasing the water pressure. This pushes particles apart weakening the links between them. Water adds weight to the mass increasing the downslope force
Evidence of rockslides, landslides and rotational slides
Slides can form new cracks or unusual bulges in the ground. The slip surface is deeper than that of other landslide types and not structurally controlled. They have a scarp and back-tilted bend or block at the top with limited internal deformation
Rockfalls description
Occur on steep slopes especially on bare rock faces where joints are exposed
Rockfalls speed
Maximum rockfall velocity occurs when rocks become airborne while rotating over the steepest area with a velocity up to 90 metres per second. Material rolling over gently sloping high friction surfaces will reduce velocity to 2 metres per second
Common causes of rockfalls
The initial cause of the fall may be weathering or erosion prising open the lines of weakness. Once the rocks are detached, they fall under gravity
Evidence of rockfalls
If the fall is short, it produces a relatively straight scree. If it is long it forms a concave scree. Falls are significant in producing the retreat of steep rock faces and in providing debris for scree slopes and talus slopes
Surface wash
Occurs when the soils infiltration capacity is exceeded. Common in winter as water drains across saturated or frozen ground following prolonged or heavy downpours or the melting of snow. Also common in arid and semi-arid regions where particle size limits percolation
Rainsplash erosion
Raindrops can erode hillslopes. On a 5 degree slope, 60% of movement is downslope. This is 95% on a 25 degree slope. The amount of erosion depends on rainfall intensity, velocity and distribution. Most effective on a slope between 33 and 45 degrees and at the start of rain when soil is still loose
Sheetwash
The unchannelled flow of water over a soil surface. On most slopes it breaks into areas of high velocity separated by areas of lower velocity. It can transport material dislodged by rainsplash. Sheetwash erosion of soil occurs through raindrop impact and transport of water overland rather than in channels. The result is a uniform layer of soil being eroded
Rills
A relatively shallow channel, less than 10’s of cm deep and carrying water and sediment for a short time. Common in agricultural areas after the removal of vegetation in harvest season and so the ground being left bare. They are also common in areas after deforestation or land-use changes. Ground compaction by machinery may also lead to the generation of rills during rainfall
Throughflow
Water moving down through the soil. It is channelled into natural pipes in the soil. This gives it enough energy to transport material and added to its solute load may amount to a considerable volume
Vegetation and land cover
Planting vegetation on slopes can stabilise soil and reduce erosion. The roots of plants bind the soil, preventing it moving downslope. Deforestation can increase the risk of mass movement by reducing the stabilising of plant roots
Terracing
Involves creating flat, horizontal areas on a slope using retaining walls or embankments. It helps reduce the slope angle and minimise mass movement potential
Grading and excavation
Can alter the natural slope of the land. Poorly planned or executed grading and increase the risk of landslides but proper grading can stabilise slopes
Surface water management
Poorly managed surface water (inadequate drainage or improper irrigation) can increase the weight of the slope and contribute to instability. Proper management mitigates this
Engineering structures
Retaining walls, rock bolts and other structures can be implemented to stabilise slopes and prevent mass movement
Earthworks and slope reinforcement
Adding reinforcement materials (geotextiles) or altering the composition of the slope through grading and filling can enhance slope stability
Land use planning
Can prevent construction on high-risk areas, minimising human-induced triggers
Monitoring and early warning systems
Monitoring systems such as sensors and early warning systems can help detect changes in slope stability allowing for timely evacuation or mitigation measures
Education and awareness
Educating local communities about the risks of mass movement and promoting responsible land use can prevent human activities that may exacerbate instability
Slopes in urban areas
Intensity of slope modification is high so buildings and roads need to be constructed safely with sound engineering. Almost all foundations cause modification to the natural slope. Modification tends to increase as construction moves to steeper slopes. To provide a horizontal base and access, a cut and fill technique is used creating a small level terrace with an over-steepened slope at both ends. The steep slopes are less stable than the natural slope and in intense rainfall are susceptible to small but harmful landslides
Terracing prevention method
Involves the shaping of slopes into steps
When mass movements occur the waste is captured and accumulates on the flat steps and impeded from sliding or flowing down the slope
Afforestation prevention method
Trees bind the soil from moving and can intercept or slow mass movement especially earthflows
Wedging or deflection walls prevention method
Wedges are strong walls or objects that redirect mass movements to other areas
The purpose of wedges is to protect a particular area from mass movements like mudflows or avalanches
Building retaining walls can stabilise slopes by providing structural support against gravity
These can be made from concrete, gabions or reinforced earth
Drainage controls prevention method
Water adds weight to the soil
By observing drainage patterns and routes, streams can be diverted to restrict water from vulnerable slope areas
Pipes can be inserted along the slope so water leaves the slope, preventing soil liquefaction
Artificial channels can direct water flows away from houses and storm drains can remove excess water from the surface
Relocation prevention method
Relocating settlements from unstable slopes is the best prevention
Implementing land use regulations that restrict development in high-risk areas and require setbacks from hazardous slopes can prevent exposure to mass movement hazards and minimise potential damage
Geotechnical monitoring prevention method
Implementing monitoring systems like inclinometers, piezometers and ground-based radar can provide real-time data on slope stability and deformation allowing for early detection of potential mass movements
Rockfall protection prevention method
In areas prone to rock falls, measures like catch fences, rockfall barriers or drapery systems can intercept falling debris and redirect it from vulnerable areas
Debris flow control protection method
Debris flow which is a mixture of water, sediment and debris can be mitigated with debris basins, check dams or diversion channels
These contain and dissipate the energy of flowing debris
Afforestation stabilisation method
Trees bind the soil and stabilise slope movement
Using vegetation to stabilise slopes is also eco-friendly
Geomat stabilisation method
Synthetic nets that are embedded on a surface to avoid excess movement and reduce the impact of rainsplash
Reinforcing soil with geosynthetic materials like geotextiles or geomeshes can improve strength and stability especially in areas with weak or erodible soils
Slope levelling stabilisation method
Involves flattening and grading slopes to reduce mass movement
Bolts or nails stabilisation method
Rocky slopes can be nailed along cracks or failure planes to stabilise movement
Installing rock bolts, anchors or soil nails into unstable rock can reinforce them
This is commonly used in steep slopes and rock faces
Cementing or shotcrete stabilisation method
Fractures along the slope can be plastered with cement to stabilise movement
