Unit 4.1 Landforms

Question 0

Explain how sea level relates to the boundary between oceanic and continental crust.

Answer 0

The boundary lies below sea level, at the continental rise.

The continental shelves, which account for about 25% of continental crust, lies below sea level.

Question 1

Describe how the Earth’s surface is distributed among altitudes.

Answer 1

The majority of the Earth’s surface lies in two bands of altitude:

• 4 to 5km below sea level: abyssal plains
• just below to 1km above sea level: continents and continental shelves

Question 2

Give the two internal sources of energy for geological processes.

Answer 2

• primordial heat, which remains from the time the Earth formed
• radioactive decay of elements such as thorium, potassium and uranium

Question 3

Give the layers of the Earth described by composition.

Answer 3

• inner core: solid, iron with 20% nickel
• outer core: liquid, iron with 5-15% other
• mantle: solid, silicates including peridotite
• crust: solid, silicates but less dense than mantle

Question 4

Give the layers of the Earth by mechanical properties.

Answer 4

• lithosphere: solid, comprising the crust and the upper mantle
• asthenosphere: weak, and acts as a liquid over long time periods, comprising the remainder of the mantle

The lithosphere is less dense than the asthenosphere, and so floats above it.

Question 5

Describe the difference between oceanic and continental crusts.

Answer 5

Oceanic: denser, formed of basalt, thinner (7km), younger (to 180ma)

Continental: formed of various rocks with a mean density lower than basalt, 20-80km thick, more complex structurally

Question 6

Explain the process of sea-floor spreading.

Answer 6

Divergent plate motion draws the asthenosphere upwards towards the plate boundary. As it rises the asthenosphere begins to melt, forming basaltic magma that creates new oceanic crust. The asthenosphere mantle cools as it approaches the surface and adheres to the plates on either side of the boundary, becoming part of their lithosphere.

This newly formed lithosphere is still relatively warm, making it slightly less dense and thus more buoyant than the older and cooler lithosphere that has been displaced further from the boundary. Divergent plate boundaries are therefore marked by elevated ridges on the ocean floor.

Question 7

Give the three types of convergent boundaries, and the factor which determines each type.

Answer 7

Convergence of oceanic and continental crust leads to the subduction of the denser oceanic crust. The point at which this plate turns downwards is marked by a trench. Magma rises from the subduction zone, creating volcanoes on the continental plate. E.g. the Andes.

Convergence of two oceanic plates sees the older (and therefore denser) oceanic crust subducted beneath the other. As above, magma rises to form volcanoes on the overriding plate, which in this case leads to an arc of volcanic islands. E.g. Japan.

Convergence between two continental plates leads to orogenesis, as the plates crumple and thicken, as neither can be E.g. Himalayas.

Question 8

Give the life cycle of an ocean.

Answer 8

A young ocean has only a divergent margin, e.g. the North Atlantic.

As it ages, the oceanic crust will cool and become more dense, until it is too dense to float above the asthenosphere, and a subduction zone forms.

If the rate of subduction is greater than divergence, the two boundaries will move together, and the divergent margin will become subducted.

Lacking a divergent margin, the oceanic crust will shrink and be destroyed as the continents either side of it collide.

This process lasts about 400-500 million years.

Question 9

Give the three types of plate boundary.

Answer 9

• divergent boundary
• convergent boundary
• transform fault boundary

Question 10

Explain how landforms are dynamic features dependent on uplift, denudation and subsidence.

Answer 10

Mountains are created as two continental plates collide, and both are crumpled, leading to uplift.

However, uplift is counted by two processes:

• denudation: weathering and transport of materials away
• subsidence: the thicker and heavier crust sinks slightly

Question 11

Why do tectonic plates move?

Answer 11

• the lithosphere floats on the asthenosphere because it is less dense
• density of oceanic crust is linked to heat, which in turn is linked with age: warmer crust is less dense, but crust cools with age
• as oceanic lithosphere cools, it becomes too dense to be supported by the asthenosphere, and so sinks/subducts, pulling the rest of the plate with it

(- subduction must be balanced by divergence, as the Earth does not shrink of grow)

Question 12

Explain how landforms are manifestations of processes in an environment and therefore, constitute a valuable and interpretable record of prevalent conditions.

Answer 12

Within the landform system, inputs and outputs are provided for by deposition and erosion, both of which are strongly linked to environmental conditions:

• weathering agents: physical, chemical or biological
• entrainment energy
• transport mediums: water, ice or wind
• depositional patterns

Question 13

Describe how landforms are systems that involve the input and output of materials.

Answer 13

Landforms can be viewed as systems in which material is moved: tectonic activity raises part of the surface, which is worn down by gravity, wind, water and ice; the resultant debris is transported to lower levels.

Within the landform system, initial input of material comes from tectonic uplift. Subsequent inputs and outputs are provided for by deposition and erosion, which remove material from one part and put it in another.

Question 14

List the different sources of energy for material transfer.

Answer 14

• internal heat from the Earth drives the tectonic activity which raises rocks, and imbues them with potential energy
• radiation from the Sun drives precipitation and wind which erodes rocks
• the gravitational pull of the Moon drives tides which erode along coastlines

Question 15

Give the four steps of erosion.

Answer 15

• detatchment of fragments through physical, chemical and biological weathering
• entrainment: the process that lifts particles off the surface and sets them in motion
• transport through solution, suspension, saltation (series of jumps) or traction
• deposition of material as conditions change

These stages occur at different fluid / wind speeds, due to the differing relative energy requirements. This is shown in a Hjulstrom diagram.

Question 16

Explain the role of geology in producing landforms.

Answer 16

Structural landforms are those controlled by the geology of the bedrock: folds and faults. For example, the East African Rift Valley is formed by a horst and graben structure.

Igneous landscapes result from intrusive or extrusive igneous activity: volcanoes, lava plateaux, bosses and tors, or intrusions which become exposed through the weathering of surrounding materials.

Question 17

Identify that specific landscapes can result in distinct landforms.

Answer 17

E.g. Tors in south west England caused by underlying igneous landforms.

E.g. rocky and barren Karst landscapes, formed where limestone is well drained and precipitation is heavy.

Question 18

Explain the role of potential energy in fluvial processes.

Answer 18

Water falling as precipitation carries potential energy. This is transferred to kinetic energy of flow, and while about 90% is dissipated as internal turbulence within the water, the remaining 10% is available to erode and transport rock particles.

Question 19

Describe the different types of load found in streams.

Answer 19

Load can be categorised as:

• dissolved load (predominantly ions)
• suspended load (predominantly small grains)
• bedload (larger grains moving by saltation and traction)

Question 20

Outline how we ‘measure’ the streams in a drainage basin.

Answer 20

Streams are ordered, from 1 (the smallest) upwards, using the Strahler stream ordering system.

The law of stream numbers holds that higher order streams are less abundant, to a (bifurcation) ratio of between 1:3 and 1:5.

The law of stream lengths holds that the cumulative mean of each order is 1:3 of the next highest order.

The law of drainage density determines “for every km^2 of basin there are x km of stream channels”.

Question 21

Recognise different stream patterns.

Answer 21

• dendritic
• parallel
• radial
• rectangular
• trellised
• annular
• centripetal
• deranged

Figure 4.7

Question 22

Explain how, when given enough time, a stream achieves equilibrium with its surroundings by becoming graded.

Answer 22

The tendency of all watercourses to become graded is related to the dynamics of fluid flow. The drag force of flowing water is related to gradient and water depth, and depth is strongly related to discharge. Expressed simply, shallow water on a steep slope can transport the same load as deep water on a shallow slope. As the discharge of a river generally increases from its source to the sea, a progressively lower gradient downstream is necessary to transport the stream load. An important feature of a graded stream is that discharge is just sufficient to transport the load provided by the drainage basin. In system terms, inputs match outputs and the stream is in equilibrium with its surroundings.

Question 23

Distinguish between erosional and depositional landforms.

Answer 23

Erosional

• valleys
• paired terraces

Depositional

• alluvial fans
• floodplains (and levees)
• unpaired terraces
• delta

Question 24

Recognise different types of stream channel.

Answer 24

Straight channels are unusual, associated with gentle gradients and cohesive banks of fine silt and clay.

Meandering channels …

Braided channels …

Question 25

Define ‘thalweg’.

Answer 25

The thalweg is the line of maximum depth in a stream and usually corresponds to the line of strongest current. Even in straight channels, the thalweg usually winds from one side to the other. When it approaches one bank, sand or mud deposits usually accumulate on the opposite side, and a regular series of these bars may be found alternating from one side to the other along the channel.

Question 26

Define ‘sinuosity’.

Answer 26

We call the measure of how straight or curved a steam is its sinuosity, which is expressed as follows:

Sinuosity = length of stream / straight distance

Question 27

Define the competence and capacity of a stream.

Answer 27

The size of the largest particle in the bedload reflects the competence of the stream. Generally, competence increases with water speed, but not in a simple linear fashion. The largest particle moved by the stream varies to the sixth power of the speed.

The capacity reflects the overall amount of material that is being transported. Capacity is measured as the volume of sediment passing a given point in a stream channel at any one time.

Question 28

Describe how alluvial fans are formed.

Answer 28

Alluvial fans are localised deposits that form where a stream loaded with sediment emerges from a confined mountain valley onto a flat unconfined plain. As the water flows out from the steep valley it spreads out and the hydraulic radius of the channel is reduced. The sudden decrease in gradient as the stream travels across the plain also causes a reduction in water speed. These factors combine to initiate the deposition of the steam’s load.

Alluvial fans have a semi-conical shape, with the apex of the cone pointing up the valley

Question 29

Describe how paired and unpaired terraces are formed.

Answer 29

Terraces are abandoned floodplains, formed when a stream become graded at a level above that of the present channel and floodplain; a change in the stream system, brought about by a drop in base level, causes a stream to erode further downwards through its deposits to a new stable level, where the next, lower, floodplain is created. This process can occur several times, each time producing a river terrace at a progressively lower level.

Paired terraces are assumed to be the same age and are the product of periodic episodes of vertical erosion. Unpaired terraces are different in age and indicate that a stream has simply shift from one side of the valley to the other as it was downcutting.

Question 30

Describe the types of delta, and the factors influencing form.

Answer 30

As the water of a steam meets the standing water of the sea or a lake, its speed is checked and it deposits its load to form a delta. As the water speed decreases, the load is deposited in order of decreasing mass. The bedload is deposited first and then the suspended particles settle out. A layer representing one depositional event is typically sorted, grading from coarse sediments at the stream mouth to finer sediments offshore. At times of high discharge, for example during floods, the top of the delta is eroded and redeposited both within and outside the channel. During successive floods the stream channel is breached and distributary channels created, which fan out giving e delta its triangular shape.

The shape of the delta is determined by the balance between discharge and wave/tidal forces

• high stream discharge and low wave and tide action give a river-dominated delta, with long sand fingers resembling a bird’s foot
• as coastal processes become more dominant the sediment deposited by the stream is spread out in both directions along the coastline, and a tooth-like (cuspate) wave-dominated delta is created
• tide-dominated deltas have funnel-shaped estuarine river mouths, produced by the ebb and flow of tidal currents

Question 31

Describe helical flow in a meandering river.

Answer 31

In a meandering steam the speed of the moving water is not the same everywhere. In a strait channel water moves fastest in mid-channel, but as water moves around a bend, the zone of high speed swings to the outside of the channel due to centrifugal forces.

The water at depth flows slower than that at the surface because of the frictional drag from the channel bed. The fast moving surface water builds up against the outside bank and generates a larger hydrostatic pressure than that caused by the slower moving water at depth. This pressure gradient causes water to sink and return to the inner bank along the river bed (fig 4.26).

Question 32

Describe the development of an ox-bow lake.

Answer 32

If the rate at which one meander moves downstream is greater than that of the one in front of it, then eventually only a narrow neck of land separates the two meanders. If migration continues or a period of high flow occurs, this neck can be breached leaving the old meander bend isolated. These abandoned meander bends are called ox-bow lakes (fig 4.28).

Question 33

Explain how waves and tides are formed.

Answer 33

Waves are generated by winds that blow across the surface of the sea: the friction between the air and the water transfers energy from the atmosphere to the ocean.

The ocean on the side of the Earth facing the Moon is attracted by its gravitational pull. This creates a tidal bulge on the side of the Earth facing the moon. As the Earth rotates, the tidal bulge is dragged around the globe and hits continents regularly.

Question 34

Describe how waves affect the coastline.

Answer 34

Physical weathering occurs through the impact of tons of water against solid rock. Wave action is aided by abrasion, hydraulic action and cavitation.

Chemical weathering occurs when seawater is forced into small cracks and crevices.

Due to wave refraction, the energy of waves (and erosion) is concentrated on headlands, whereas bays are areas of deposition.

Question 35

Distinguish between erosional and depositional coastal landforms.

Answer 35

Erosional: cave, arch, headland, stack, wave-cut platform (fig 5.8)

Depositional: tombolo, split, bay barrier, barrier islands (fig 5.14)

Question 36

Recognise that beaches are systems with inputs and outputs of material.

Answer 36

Input of material: longshore drift, sand washed onshore from sea floor, sand blown from hinterland cliffs and dunes, and river-transported sand.

Outputs of material: sand lost by longshore drift, sand lost offshore to the sea floor, sand blown inland, sand lost through in-situ weathering and sand swept upstream into an estuary or inlet.

The balance between inputs and outputs of material on a beach is reflected in the beach budget:

end mass = original mass + inputs - outputs

Question 37

Explain what a glacier is.

Answer 37

A glacier is a persistent body of ice which has, or has had in the past, the ability to flow.

Question 38

Recognise different types of glacier.

Answer 38

Ice sheets: continental sized, smothering almost all of the land surface within their perimeter.

Ice caps: smaller than ice sheets, but still covering mountain highlands.

Valley glaciers: ribbons of ice confined by the surrounding topography, which controls their shape and direction of movement.

Question 39

Describe how glaciers move.

Answer 39

When ice on a slope builds to a great enough thickness it will begin to deform and flow downslope under the pull of gravity.

Part of this flow is accomplished through movement within individual ice crystals. As the pressure of snow accumulating on the glacier increases, higher stress is placed on the glacial ice and deformation or creep takes place along internal planes within individual ice crystals.

Other processes are also at work within the ice. Ice crystals tend to melt and recrystallise at a small distance downslope.

The remaining flow is achieved in some glaciers through sliding along the glacier base. Dry-based glaciers are cold glaciers that are frozen to the ground beneath, and in these circumstances internal deformation is the major process. Wet-based glaciers have ice at their base that is at its melting point, and repeated melting and freezing at the glacier base helps the glacier to slide downslope.

Question 40

Outline how glaciers are systems with inputs and outputs.

Answer 40

Inputs: snow, which over time is converted into glacial ice.

Outputs: meltwater created when the ice melt, or escaping vapour created when it sublimes.

The glacier system can be divided into two zones: an upper zone, termed the zone of accumulation, where there is an area of net gain in mass and a lower zone, called the zone of ablation, where there is an area of net loss. These two zones are separated at the equilibrium line, where net loss of mass equals net gain. When balanced or steady state conditions exist, the equilibrium line is at a particular altitude midway between the start and terminus of the glacier.

Question 41

Distinguish between erosional and depositional glacial landforms.

Answer 41

Erosional features, such as cirques, arêtes, horns and glacial trough valleys indicate that glacial actions have removed material locally. Depositional features such as moraines, eskers and erratics indicate that material has accumulated locally.

Question 42

Explain how wind erodes, transports and deposits material.

Answer 42

Wind erosion involves two main processes: deflation and abrasion. Deflation involves the removal of unconsolidated material by lifting it into the air or rolling it along the ground. Deflation occurs whenever the ground surface consists of dry, loose particles and hence affects deserts, beaches, dry river beds and recent glacial deposits.

Abrasion occurs when wind propels particles at a cohesive rock and, eventually, wears it down. Ventifacts are aerodynamically shaped rocks that have been cut and sometimes polished by the wind.

Wind exerts similar force on material on the ground as water does on a stream bed - turbulence and forward motion combine to entrain particles. However, wind is approximately 1000 times less dense than water and is less able to entrain and transport relatively large particles.

Once entrained, some fine material is carried by wind in suspension, but the movement of sand-sized particles is usually by saltation.

Question 43

Outline how dunes are formed.

Answer 43

When wind meets and obstacle it sweeps over and around it and leaves a pocket of slower moving air immediately down wind of the obstacle. In these pockets of slower-moving air the eddies generated are weaker than the main flow, and the load there is deposited as two separate drifts, before coalescing to form a dune (fig 7.7).

Question 44

Describe how dunes have different scales, forms and complexities.

Answer 44

Scales:

• ripples
• dunes
• draas: large dunes with wavelengths measured in kilometres and heights in tens or hundreds of metres
• ergs: large tracts of shifting sand, over 30000km2

Form:

• crescentic: created with a single dominant wind direction
• linear: created by two wind directions
• star: wind blows from all directions
• reversing
• parabolic: presence of vegetation

Complex dunes are large dunes that have smaller dunes of different type and slip face orientation superimposed on them.

Question 45

Recognise the relationship between the size and duration of a landform.

Answer 45

The larger a landform is, the more material has to be eroded and rede posited to remove it. In addition, this material has to be transported for greater distances. Moving larger amounts of material further distances takes more time.

Question 46

Outline the transformation of fresh snow into glacial ice.

Answer 46

Fresh snow is a mass of delicate ice crystals with a density of about 5-30% that of water (50-300kg m-3), and is very porous. As the snow ages on the ground, air penetrates the pore spaces initiating cycles of sublimation (i.e. when ice is directly changed into water vapour) and refreezing. The original snowflakes are transformed into small round crystals. This partly melted, compressed snow is called névé. Névé has a higher density than fresh snow, exceeing 500kg m-3. After about a year, the fall of fresh snow has compacted the névé further into a yet denser form called firn, with fewer pore spaces. Further burial and compaction transforms firn into glacial ice, which has a density of about 850kg m-3. Overall, the transformation of freshly fallen snow into (glacial) ice may take 25 to 100 years.