8.1. Marine Erosion, Sub-aerial Processes, and Marine Transportation and Deposition

Question 1

Marine erosion processes

Answer 1

caused by the action of waves

1) abrasion (corrasion)
2) wave Quarrying
3) cavitation (in Wave Quarrying)
4) solution (corrosion)
5) attrition

Question 2

Abrasion (corrasion)

Answer 2

where rock fragments are hurled at cliffs by breaking waves, gradually scraping away at the cliff face

Question 3

Wave Quarrying

Answer 3

• as waves break against the cliff face, the pressure
  of the breaking wave can compresses air in cracks.
• this compressed air gradually forces open the crack in the rock - as this process continues, the rock becomes increasingly weakened.

Question 4

Cavitation (in Wave Quarrying)

Answer 4

(the implosion or collapsing of air bubbles)

- is also part of this process; as the air bubbles collapse, they release jets of water which further weakens the rock.

Question 5

Solution (corrosion)

Answer 5

• this occurs where the salt water is able to dissolve some of the chemicals in rocks
• for example, limestone cliffs are gradually weakened as the salt water dissolves the
  calcium carbonate in the limestone.

Question 6

Attrition

Answer 6

this is where rock fragments carried by the waves hit

against each other and gradually wear down to form sand and silt

Question 7

Hydraulic Action (Wave Pounding)

Answer 7

• the sheer force (power) of waves hitting against the cliff face, remove or pull away (‘pluck’) loose rocks from the cliff face.
• these dislodged blocks of rock later abrade the
  cliff base.

Question 8

Sub-aerial processes

Answer 8

1) weathering

2) mass movement

Question 9

Types of sub-aerial weathering

Answer 9

1) Solution weathering
2) Salt crystallisation
3) Biological weathering
4) Freeze-thaw weathering
5) Slaking

Question 10

Solution weathering

Answer 10

• the change in the chemicals in rocks by acidic rainwater.
• in particular, limestone is weathered by rainwater
  containing dissolved CO2.

Question 11

Salt crystalisation

Answer 11

the growth of chloride crystals (from seawater) loosens rock fragments for erosional processes to work on.

Question 12

Biological weathering

Answer 12

molluscs in their search for food wear down rock surfaces.

Question 13

Freeze-thaw weathering

Answer 13

water enters the cracks in a rock and freezes; it expands and pushes the cracks further apart.

Question 14

Slaking

Answer 14

• Alternate wetting and drying of rocks; the accumulation of successive layers of water molecules in between mineral grains of a rock.
• The increasing thickness of the water pulls the rock grains apart with great tensional stress.

Question 15

Sub-aerial mass movements

Answer 15

soil creep, landslides, slumping and rock falls

Question 16

Factors affecting coastal erosion

Answer 16

1) rock type
2) rock structure
3) type of wave
4) shape of coastline

Question 17

How rock type affects coastal erosion

Answer 17

• some rock is easily eroded (e.g. clay and shale)
• these rocks tend to form wide beaches
• other rocks are resistant to erosion (e.g. limestone and chalk)
• these rocks tend to form steep steep cliffs or rocky outcrops (headlands)

Question 18

How rock structure affects coastal erosion

Answer 18

• where rocks are parallel to the coastline, the coastline is CONCORDANT and the amount of erosion will be determined by the rock type forming the coastline
• where the rocks outcrop at right angles to the coast, the coastline is known as DISCORDANT, and differential erosion may occur due to bands of hard and soft rock forming headlands and bays

Question 19

How rock type of wave affects coastal erosion

Answer 19

• the amount of energy a wave has will also determine the amount of erosion that will take place
• destructive waves have a steep angle of break and are high in energy
• they degrade the beach due to the scouring action of the strong backswash

Question 20

How shape of coastline affects coastal erosion

Answer 20

• on concordant coastlines, rocks are parallel to the wave front and therefore rates of erosion are similar along the coastline.
• on discordant coastlines, differential erosion may occur, where bands of hard and soft rock outcrop at right angles to the sea.
• consequently headlands and bays form along discordant coastlines and whilst headlands remain exposed to the force of the waves, bays are sheltered.

Question 21

Coastline receives sediment from where

Answer 21

• rivers flowing from the land to the sea
• erosion of the coast - e.g. rock falls and slumps
• material moved on to the coastline from the sea bed - mostly by constructive waves

Once the sediment arrives on the coastline, it can be transported in a variety of ways

Question 22

Movement of sediment up and down the beach

Answer 22

1) Suspension
2) Solution
3) Traction
4) Saltation

Question 23

Suspension

Answer 23

Fine sediment is carried as a suspension in the water, making it look muddy or murky

Question 24

Solution

Answer 24

Dissolved material is carried along in solution, so you can’t see it

Question 25

Traction

Answer 25

Larger pebbles and cobbles are rolled along the sea bed

Question 26

Saltation

Answer 26

Small pebbles are moved when one pebble hits another, causing it to bounce. This bouncing can set up a chain reaction

Question 27

Movement of Sediment Along the coastline: LONGSHORE DRIFT

Answer 27

• waves break on the beach at an oblique angle controlled by the prevailing wind direction
• material is moved up the beach in the swash
• material then moves straight back down the beach in backwash under the influence of gravity
• as the process continues material moves along the coast in a zig-zag movement

Question 28

Amount of Longshore Drift depends on:

Answer 28

• the strength of waves
• size and amount of material available for the movement
• incline (slope) of the beach

Question 29

Types of sediment

Answer 29

• coastal sediment comes in a range of sizes: cobbles, pebbles, shingle, sand and mud
• larger particles usually form a storm beach at the top of the beach while the smaller particles form the wider, more gently sloping lower beach
• all of this sediment can be moved along the beach by longshore drift

Question 30

Deposition of Coastal sediment

Answer 30

• takes place whenever the movement of coastal sediment slows down
• happens updrift of a groyne or in sheltered locations such as an estuary or bay
• deposition also occurs where the coastline changes direction abruptly - spits can develop as the beach is built out across the inlet, estuary or bay
• wave refraction around an island close to the coast can lead to the deposition of sediment between the island and coast - a cupsate tombolo

Question 31

Sediment Cell

Answer 31

• largely self-contained stretch of coastline by coastal processes
• they are regarded as closed systems as sediment is not usually transferred from one to the other
• each cell consists of a source (INPUTS), storage areas (beaches) and sinks (OUTPUTS)
• in reality, some sediment does get transferred between neighbouring cells

Question 32

Sediment Cell INPUTS

Answer 32

• coastal erosion (cliffs, sand dunes)
• the sediment transported by rivers and wind; rivers bring huge volumes of sediment into the coastal zone
• sea shells or offshore deposits (eg. those left on the sea shore) washing on to the beach by tides and currents)
• material transported (by longshore drift) by the erosion of other beaches; beach material can be moved through the beach and then moved along the coast (by longshore drift) to the next beach
• beach nourishment (sand taken from somewhere else and scattered on a beach)

Question 33

Sediment Cell OUTPUTS

Answer 33

• the sediment removed from the cell include sediments lost to the open sea during storm conditions - high energy waves are capable of transporting much material in just one storm
• a severe storm at the same time as a high tide can not only lower a beach but can also remove material such as sand dunes from inland
• material can also be blown onto sand dunes
• sediment can be removed by human interference (dredging and sand mining)
• longshore drift moving sediment into another cell

Question 34

Sediment Cell Boundaries

Answer 34

1) Longshore drift divides
- where the coastline abruptly changes direction such as at major headlands
- also where wave conditions causes a change in longshore drift direction
- since material is moved outwards from a drift divide there is a net output of sediment from the area. This results in a dominance of erosional processes and landforms e.g. eroding beaches and cliffs.

2) Sediment sinks
- where sediment transport paths meet so that sediment builds up in depositional environments
- occur in deep bays and estuaries, although spits may form sub-cell sinksk