past paper questions Flashcards
define ocean trench
large scale features that are long and relatively narrow depressions in the ocean floor (1) and mark the point where one tectonic plate subducts beneath another/at destructive margins either oceanic-oceanic or oceanic-continental (1)
define sea floor spreading
the moving apart (divergence) of ocean plates (1) as
a result of convection currents (1)
describe a conservative plate boundary
Plates pass sideways (1). One moving faster or in a different direction (1). Driven by convection currents (1). Friction builds up and is released sending shock waves (earthquakes) (1). No material is added or subtracted/no landforms formed (1)
Explain why slope processes occur at different rates
- Slope angle
- Geological structure
- Rock type
- Vegetation
- Amount of water/precipitation
- Removal/erosion of supporting material
- Human impact (traffic, excavation, building etc)
define pressure release (dilatation)
the reduction of pressure on rock by surface unloading (1) leads to the fracturing of the rock (creation of joints) (1).
define hydrolysis
chemical process involving water (1) reacting with certain rock minerals leading to the decomposition of the rock/producing soluble byproducts/hydroxides (1). The best example is the weathering of feldspar to clay (1).
Briefly explain the role of water in mass movement.
- Water increases lubrication which reduces the friction between the sliding surfaces.
- It may also increase the weight of the soil.
- Pressure from pore water is increased which reduces strength allowing movement to take place, e.g. mudflows.
- Wetting and drying of soils may lead to soil heave and creep.
- Freeze-thaw in joints on rock cliffs (free faces) may lead to rockfall.
Explain how temperature affects physical weathering processes
- fluctuating temperatures around 0 degrees Celsius enable physical weathering processes like freeze thaw to take place.
- Heating / cooling is relevant (exfoliation, granular disintegration) with the focus on the temperature regime which leads to the resulting weathering activity.
- Salt crystallisation is also relevant (evaporation produces salt crystals which increase in size; certain salts also expand on heating).
- Temperature helps to increase root action.
- Reference to the Peltier diagram should be credited where this helps to explain the answer. Diagrams should be credited where they help to explain an answer
‘Attempts to reduce mass movement are not always successful.’ With the aid of examples, how far do you agree?
Content may include a discussion of the success or otherwise of specific attempts to control mass movement (stabilisation of slopes by a variety of means, afforestation, drainage, slope regrading, netting, pinning, concreting, etc.) with an assessment as to why some schemes fail. Accept avalanches as mass movements.
describe the formation of scar/rock face
- the detachment of material from the rock face (1)
- by weathering (rockfall) or sliding (rock slide) (1)
- the accumulation of material on the lower slope (1)
- at the angle of repose, 30–35 (1)
Explain how water affects the movement of material on slopes.
- addition of weight to slope materials increasing downslope stress (the gravity effect)
- reduces internal strength by reducing cohesion by increased pore water pressures creating a greater fluidity such as in mudflows
- lubricating shear planes possibly leading to landslides
- water-induced weathering on rock faces leading to rockfalls
- freeze-thaw and wetting and drying in soils leading to heaves and downslope movement
- overland flow leading to sheetwash or rill/gully action
hydration
- Minerals absorbing water (1), leading to expansion (1) to create a hydrate / new mineral e.g. gypsum, anhydrate.(1)
carbonation
Rainwater absorbs carbon dioxide to produce slightly acidic water (carbonic acid) (1) which reacts with the calcium carbonate (e.g. limestone) resulting in soluble calcium bicarbonate (1).
Briefly describe how rock type affects the rate of physical weathering.
Reference to:
* the different rates of permeability (e.g. typically igneous rocks are less permeable than sedimentary rocks) (1)
* different rates of porosity (1)
* the blocky nature (jointing) such as limestone (1) or bedding planes such as a sandstone (1) strata etc. in some (1)
* different mineralogy affecting insolation weathering (1)
* types of physical weathering (freeze-thaw, heating/cooling, salt crystal growth, pressure release (dilatation)), and vegetation root action (1)
Describe and explain the formation of ocean trenches
- long narrow, trenches, approximately 10km deep, arc shaped, with steep sides. They are features of subduction zones
- subducting plate is dragged downwards and the movement leads to a downwarping of the non-subducting plate creating a deep trench
With the aid of examples, assess the extent to which mass movement on slopes can be reduced.
- syllabus lists pinning, netting, grading and afforestation but there are many others
- approach could be comparing successful attempts against unsuccessful attempts at reducing mass movements on slopes, or to look at one example more in depth and illustrating what has been done and the extent of the success
Netting
these can either drape (1) to help contain loose material (1) or be tensioned (1) restraining the movement of the rock face (1)
pinning
helps to reinforce the surface rock (1)
pin the larger block of rock (1)
tie together the rock face (1)
create anchors for other slope stability methods (1)
Explain the role of water in the movement of sediment on a hillslope
- rainsplash and sheetwash
- bare soil surfaces
- infiltration capacity overcome
- kinetic energy dislodges particle
- influenced by soil characteristics
pressure release (dilatation)
- material overlying the rock is removed (1) allowing the rock to expand (1) causing fractures/joints to be formed (1)
Freeze-thaw
physical weathering process (1) whereby water enters the cracks in a rock and freezes when the temperature is below 0oC (1) and expands by 9% of its volume (1).
Briefly explain why some rock types are more affected by the weathering process of carbonation
- weak carbonic acid reacts with calcium carbonate rich rocks
- rock types that have calcium carbonate as a constituent, such as chalk and limestone will be more affected
Explain how vegetation and relief affect the type of weathering
- vegetation: organic acids may increase the rate of weathering, microbes, such as fungi also affects the rate of weathering. Physical weathering by the presence of roots from vegetation exerting pressure
- relief:steeper removing moisture quicker
from soils, Shallow slopes will aid infiltration and chemical weathering of the bedrock. The aspect determine the dominant temperature regime
hydration
Hydration occurs when water enters into the mineral fabric of a rock (1) causing it to expand, putting pressure on the rock (1)
freeze-thaw
Freeze-thaw is the mechanical process whereby water enters the joints or pores of a rock (1) and expands and contracts on freezing and thawing (1)
Briefly describe how heating and cooling can result in the weathering of rocks
Heating and cooling leads to the expansion and contracting of a rock (1) which leads to stresses (1) causing the rock to disintegrate (1) (granular and block weathering)
Describe and explain the conditions that result in mass movements on slopes
- For mudflows, water content increasing the pore water pressure is crucial
- rockfalls, it might be freeze-thaw that is important
- soil creep, then wetting and drying and freeze-thaw will be important
- discussion of the influence of rock and material in determining which type of mass movement occurs
‘Rock type is the most important factor in determining the type and rate of weathering.’
- Rock type incorporates structure, texture and chemical composition
- Rocks might be affected by the same weathering process but rate of weathering may vary
- Both physical and chemical weathering processes
- Climate
- Other factors include relief, vegetation, human activity
Briefly describe the process of subduction at convergent (destructive) plate boundaries.
The convergent (moving together) of two tectonic plates of differing densities (1) by convection currents (1) whereby the denser plate is forced below the less dense plate (1).
Explain why the process of sea floor spreading only occurs at some tectonic plate boundaries.
- Sea floor spreading is present under the ocean, on margins which are diverging (1).
- This process occurs as the plates are drawn away from each other through the convection currents, forming new oceanic crust. Therefore it is the presence of two diverging oceanic plates which develops this process (1).
- Plate margins which draw plates together (convergent) (1) or allowing them to run side by side (conservative, such as the San Andreas fault) (1) will not have the process, as in convergent margins, for example, there is no separation or divergence of the plates to allow the molten material to be released in this way.
Explain how rainfall affects the type and rate of weathering
- supplies the moisture needed for certain chemical reactions to take place
- A lack of rainfall results in only very slight weathering, decreased rate
- As mean annual rainfall increases, the rate of weathering increases
- Physical weathering such as freeze-thaw and salt crystallisation may be explained
- Biological weathering, a result of the relation between rainfall and vegetation
- Secondary impacts may be explained such as vegetation protecting land surfaces and erosion by water leading to the exposure of rock leading to pressure release and renewed weathering at the rock surface
salt crystal growth
water entering cracks and pores in rock (1) evaporates,
leaving salts which expand, putting pressure on the rock (1).
hydration
: water entering minerals in rock (1) which causes expansion and leads to rock disintegration (1).
Briefly explain how vegetation root action can lead to the weathering of rocks.
The growth of roots in rock joints and crevices (1), leading to pressure on the rock (1) which can cause rock break up (1).
Explain how rock type affects the type and rate of chemical weathering.
- one of the main determinants of the type and rate of chemical weathering
- Some rock types are susceptible to specific processes
- granite is vulnerable to hydrolysis and calcareous rocks to carbonation
- Rock with joints and bedding planes tends to be more vulnerable because of the ease with which water can penetrate the rock
Briefly describe the process of subduction at convergent (destructive) plate boundaries.
The convergent (moving together) of two tectonic plates of differing densities (1) by convection currents (1) whereby the denser plate is forced below the less dense plate (1).
Briefly describe the process of subduction at convergent (destructive) plate boundaries.
The convergent (moving together) of two tectonic plates of differing densities
(1) by convection currents (1) whereby the denser plate is forced below the
less dense plate (1).
Explain why the process of sea floor spreading only occurs at some tectonic plate boundaries
- Sea floor spreading is present under the ocean, on margins which are diverging (1).
- This process occurs as the plates are drawn away from each other through the convection currents, forming new oceanic crust.
Therefore it is the presence of two diverging oceanic plates which develops this process (1). - Plate margins which draw plates together (convergent) (1) or allowing them to run side by side (conservative, such as the San Andreas fault) (1) will not have the process, as in convergent margins, for example, there is no separation or divergence of the plates to allow the molten material to be released in this way.
hydrolysis
Hydrolysis is a chemical process where hydrogen ions in water (1 mark) react with certain rock minerals to produce soluble hydroxides (1 mark). The best example is the weathering of feldspar to clay (1 mark).
vegetation root action.
Vegetation root action is the growth of roots in joints and crevices of rock (1 mark), forcing them apart as the roots develop (1 mark)
Briefly explain how salt crystal growth weathers rock
- salt-rich water can evaporate in the pores or crevices of rock
- leading to the growth of salt crystals
- repeated action and growth of crystals can exert pressure on the rock
- salt crystals can also expand on heating
Explain how ocean trenches and volcanic island arcs are formed
when one oceanic plate is subducted beneath another
oceanic plate. Ocean trenches can also be formed by the subduction of an oceanic plate below a continental plate
Briefly describe the weathering process of pressure release (dilatation)
- erosion of material above a rock
- reduction of pressure on the rock
- leading to dilation creating rock joints
- rock falls may also lead to pressure release
Explain how ocean trenches are formed
- converging of either an oceanic and a continental plate or two oceanic plates
- powered by convection currents
- subduction of the denser plate
- which creates the trench
Explain the movement of material on slopes
- mass movements or the movement of water and sediment (rainsplash, sheetwash, rills)
- water availability, slope steepness, soil and rock characteristics and nature of vegetation cove
salt crystal growth
water entering cracks and pores in rock (1) evaporates,
leaving salts which expand, putting pressure on the rock (1).
hydration
water entering minerals in rock (1) which causes expansion and leads to rock disintegration (1
Briefly explain how vegetation root action can lead to the weathering of rocks
The growth of roots in rock joints and crevices (1), leading to pressure on
the rock (1) which can cause rock break up (1)
Explain how rock type affects the type and rate of chemical weathering.
Rock type is one of the main determinants of the type and rate of chemical weathering. Some rock types are susceptible to specific weathering processes. Thus, granite is vulnerable to hydrolysis and calcareous rocks to carbonation. Rock with joints and bedding planes tends to be more vulnerable because of the ease with which water can penetrate the rock.
subduction
Subduction is the process where tectonic plates converge (1) with one plate being forced downwards under another plate (1) and is melted in the subduction (Benioff) zone (1)
conservative plate boundary
Conservative plate boundary is where plates pass sideways (1). One moving
faster in the same direction or in a different direction (1).
Briefly describe how fold mountains are formed
Tectonic plates converge (1), driven by convection currents (1), resulting in compression/buckling (1) and upfolding/uplift (1) of sediment (1) to produce the fold mountains
Explain the role of water in the surface movement of sediment on slopes.
- rainsplash will be in terms of the kinetic energy of the raindrops being sufficient to dislodge soil particles which, if there is a slope gradient, will tend to move downslope. Bare surfaces with loosely consolidated soil particles are needed
- For sheetwash, smooth surfaces are needed where infiltration capacity is exceeded and water moves over the surface as a sheet
- The processes are similar for rills but surface irregularities mean water flow becomes concentrated in channels rather than sheets
- Soil heave by wetting and drying, soil creep/talus creep, solifluction, surface mudflows are also relevant
Explain why mass movements have different rates of movement.
- Mass movements with a high water content, other things being equal, tend to be faster
- This is the result of lubrication of failure planes in the case of rock/debris slides
- Increase of pore water pressure in fine grained material, reducing internal cohesion for mud/debris flows
- Mass movements move faster on steeper slopes, in general
- Rapid increase of stress such as from external sources, for example earthquakes, can lead to sudden, fast movements
- Particle by particle movement, such as heave, is always going to be slower
- Rockfall will always be fast because of lack of support
- Vegetation influences mass movements
hydrolysis
Hydrolysis is a chemical process (1) where water reacts with minerals in a rock (1) leading to its decomposition (1).
pressure release (dilatation).
a physical weathering process (1) where rock becomes fractured (1) when weight (pressure) is removed from off it (1).
Briefly explain how a rock can be weathered by heating and cooling
Heating and cooling of a rock leads to expansion and contraction of the rock (1) and its constituent minerals, governed by the albedo of the rock and minerals (1). This stresses the rock which, over time, may lead to disintegration (block or granular weathering) of the rock (1).
Explain how the movement of tectonic plates leads to the formation of ocean trenches and ocean ridges
Ocean trenches are formed at converging plate boundaries by the subduction of one plate below the other, causing a deep-sea trench. Ocean ridges are formed at divergent plate boundaries, leading to the escape of magma which will produce ocean ridges.
