3. Rocks and Weathering Flashcards
Continental crust
35-70km thick
Very old
Lighter
Granitic
Oceanic crust
6-10km
Very young
Heavier
Mainly basaltic
The layers of the Earth
Surface Lithosphere Asthenosphere Mesosphere Outer core Solid inner core
Lithosphere
cool, rigid, brittle
crust and upper mantle
70km deep
Crust
2 types, continental and oceanic
Evidence for plate tectonics
Earthquakes
Changes in the magnetic field of the Earth
Paleomagnetism, the different magnetising of emerging magma
Fossil remains
Rock distribution
Fit of the continents
Types of plate of boundary
conservative, convergent, divergent
Conservative plate boundary
Plate boundary sliding past each other
Divergent plate boundary
New crust is formed from the moving away of two plates
Often in the middle of oceans
Convergent plate boundary/collision
When a continental and a oceanic plate converge, one plate will be subducted.
Forms fold mountains when 2 continental plates collide
Earthquakes
Why do plates move?
Convection currents of rising and falling magma because of radioactive decay in the core
Plates are dragged by their edges which have become cold and heavy
What is sea floor spreading
The movement of continents in response to the growth of oceanic crust between them
Mid-ocean ridges
Giant submarine mountain chains. New lithosphere is created here. Formed at divergent boundaries
Slab pull
Slab pull is the pulling force exerted by a cold, dense oceanic plate plunging into the mantle due to its own weight. The theory is that because the oceanic plate is denser than the hotter mantle beneath it, this contrast in density causes the plate to sink into the mantle.
Volcanic island arcs
Magma rises up out of the oceanic crust and forms volcanic island arcs parallel to the ocean trench. Formed when oceanic lithosphere is subducted under oceanic lithosphere
Ocean trenches
Formed at convergent boundaries by slabpull and subduction
Subduction zones
When oceanic lithosphere collides with another plate.
It is easily pushed down into the upper mantle. Subducted oceanic plate is cooler and denser so continues to go deeper. The older the crust the steeper it dips.
Benioff zone
Earthquakes occur here
How are mountains built by plate tectonics
They are built at convergent plate boundaries. Sedimentary rocks are folded up into an accretionary wedge
Weathering
the decomposition and disintegration of rocks in situ
Physical weathering
the disintegration of rock into smaller, angular fragments of the same rock, such as scree
Chemical weathering
the decomposition of rock, creating altered rock substances, such as kaolinite from granite
Biological weathering
when plants and animals chemically alter rocks and physically break them thorough their growth and movement
Freeze-thaw/frost shattering
when water in joints or cracks freezes at 0°C and expands by 10% exerting pressure on the rock
Granular disintegration
alternate expansion and contraction of minerals in rocks causes the rock to break down into small pieces
Block disintegration
when rocks split along joints into large rectangular shaped blocks
Disintegration
found in hot desert areas where there is a large diurnal temperature range.
Exfoliation
a type of disintegration where the outer layers of rock peel off due to stresses caused by heat
Wetting and drying
Physical weathering process where rocks are mechanically disintegrated by the accumulation of successive layers of water molecules in between the mineral grains of a rock
Salt crystallisation
the decomposition of rock by solutions of salt
Pressure release
overlying rocks are removed by erosion causing underlying rocks to expand and fracture
Hydrolosis
occurs on rocks with orthoclase feldspar
Hydration
the process whereby certain minerals absorb water expand and change
Oxidation
when iron compounds react with oxygen to produce a reddish brown coating
Humic acids
acids derived from the decomposition of vegetation
Chelation
the process in which plant roots can absorb relatively insoluble minerals
Limestone
a rock consisting of calcium carbonate, therefore susceptible to carbonation-solution
Carbonation
occurs on rocks with calcium carbonate e.g chalk or limestone
Granite
a rock prone to hydrolysis because of the presence of feldspar
Regolith
superficial and unconsolidated material found at the Earth’s surface.
Prone to downslope movement
Clay-rich regolith is particularly unstable because of their ability to retain water
Aspect
the direction in which a slope faces
Mass movement
any large-scale movements of the Earth’s surface that is not accompanied by a moving agent such as a river, glacier or ocean wave
Shear strength
the internal resistance of the slope
Shear stress
the forces attempting to pull a mass downslope
Heave/creep
a slow, small-scale process of mass movement that occurs mostly in winter
Talus creep
the slow movement of fragments on a scree slope
Falls
mass movement that occurs on steep slopes, especially bare rock faces where joints are exposed
Slides
mass movement that occurs when an entire mass of material moves along a slip plane
Van’t Hoff’s Law
The rate of chemical weathering increases 2-3 times for every increase of temperature of 10 degrees to a maximum of 60
What influences the rate and type of weathering
Joints and bedding planes, nature of the sedimentary rocks, chemical composition
What will increase chemical weathering
Increase in moisture and heat
What influences freeze thaw, salt crystallisation and insolation
critical temperature changes
frequency of cycles
diurnal and seasonal variations in temperature
What types of weathering are common and occur in Glacial/Periglacial regions
Frost is important in disintegration. Slow chemical weathering, Hydration weathering common from high moisture
What types of weathering are common and occur in Temperate regions
Fluctuations in precipitation and evaporation. Both mechanical and chemical weathering occur. H
What types of weathering are common and occur in Arid/Semi arid regions
Evaporation exceeds precipitation. High temperatures. High amounts of mechanical weathering, disintegration
What types of weathering are common and occur in Humid/Tropical regions
High seasonal rainfall. High amounts of moisture. Intense deep weathering
What causes rates and types of weathering to vary
Climate
Characteristics of Limestone
Permeability
Solubility
characteristics of granite
Igneous
Resistant to erosion
Susceptible to freeze-thaw and hydrolysis
Impermeable
Exogenic factors affecting slope processes
Climate Temperature Precipitation Vegetation Weathering Runoff
Soil
Part of the regolitih. Clay soils can hold more water than sandy soils
How are slopes open systems
Recieve inputs such as energy, mass, water and sediment
Produce outputs: energy(reradiated heat) and mass
Slopes also store kinetic energy which is converted into mass movement and erosion
Slope form
the shape of the slope in cross section
Slope processes
Activities acting on the slope
Slope evolution
The changes in slope over time
Types of mass movement
very slow: soil creep
Fast: avalanches
dry movement: rockfalls
Fluid: mudflows
What causes mass movement
gravity
slope angle
pore pressure
How does gravity cause mass movement
It moves the material (slide) and sticks its.
How does water cause mass movement
water lubricates the particles and fills the empty space between them
What causes slope failure
reduction in shear strength of the slope or increase in shear stress
Shear stress
increase in the forces trying to pull a mass downslope
shear strength
the internal resistance of the slope
Factors that contribute to increased shear stress
removal of lateral support
Removal of underlying support
Slope loading
Lateral Pressure
How might lateral support be removed
undercutting by rivers, wave action, faulting, rockfalls
What is slope loading
weight of water, vegetation, accumulation of debris
what is lateral pressure
water freezing, swelling of clays, pressure release
Factors that contribute to reduced to reduced shear strength
weathering effects: disentegration, hydration, dissolution of cementing minerals
Changes in pore pressure: saturation
changes in structure: fissures in clay
organic effects: burrowing of animals
how might water weaken a slope
Increase shear stress and decrease shear resistance
Increase the weight of a mobile mass
reduces the cohesion between particles
How can downslope movement be opposed
Friction
cohesive forces
pivoting
vegetation
What is surface wash
when the soil’s infiltration capacity is exceeded, and water drains across saturated or frozen ground following heavy downpours or melting of snow
Sheetwash
unchanneled flow of water over a soil surface
Types of heaves
soil creep
solifluction
Soil creep/solifluction
soil particles are heaved to the syrface by heating/wetting/freezing of water. Freeze thaw causes the soil the soil to expand and srink. the soil “rolls”
Types of slides
rotational slip
slumping
landslides
What causes slumping
when a mass of regolith detaches from it’s substrate along a spoon-shaped sliding surface. Regolith driven. Lubricated by a fluid
Angle of repose
maximum angle that a slope is stable
Rotational slip
slump mass is permable and is lubricated by liquid. slump mass is unconsolidated regolith. Slump mass detaches from headscarp and moves downslope
Types of falls
avalanches
rockfall
debris fall
What are falls
When material falls under the influence of gravity after being detached
talus
apron of debris sloping away from the cliff
what causes rockfalls
undercutting
Types of flows
mudflows lahar debris flow debris avalanche landslide
Mudflows
caused by heavy rain and flooding. required a 25 degrees+ gradient. mixture of water, mud and loose rocks
Lahar
when volcanic ash mixes with water/snow.
debris flow
mudflow mixed with large rock fragments
what causes flows
regolith sliding off bedding plain
Negative human impacts on slope stability
undercutting slopes, increasing slope angle
removal of vegetation
loading of slopes
addition of water
Preventative measures against mass movement
identification of regions at risk use netting barriers pinning terracing
Slope stabilisation
adding vegetation
peforated pipes to allow water to pass through and flow out
afforestation
retaining walls
Rainsplash erosion
erosive effect raindrops have on hillslopes. compact and dislodge particles in all directions
