Coastlines (DP IB Geography)

Revision Note

Advancing & Retreating Coastlines

  • Advancing and retreating coastlines are short-term processes

  • If erosion is higher than deposition, this will cause an eroding (retreating) coastline

  • If deposition is higher than erosion, this will cause an outbuilding (advancing) coastline 

  • Coasts can also advance and retreat as a result of long-term isostatic and eustatic change

Diagram based on Valentin’s Classification of Coasts

Diagram showing coastal evolution. Center has "Emergence" and "Submergence" labeled with arrows pointing outwards and inwards respectively. Surrounding labels: "Emerging Coast," "Advancing Coast (Land Gaining)," "Outbuilding Coast," "Submerging Coast," "Sea Gaining Retreating Coast," "Eroding Coast," and "Static Coastline." Arrows indicate transitions between these states.
Diagram based on Valentin’s Classification of Coasts
  • There are many factors affecting advancing and retreating coastlines: 

    • Erosion and weathering – wearing away the coastline

    • Wave type – destructive or constructive waves 

    • Climate change – increased storms and removal of coastal protection

    • Tides – carrying sediment that can erode or deposit

    • Geology – resistant or non-resistant rock type

Isostatic & Eustatic Processes

  • Isostatic and eustatic change refers to differences in sea level 

  • Sea level changes regularly and at smaller scales e.g. the tide

  • Sea level change can also occur on a bigger scale

  • Eustatic and isostatic change can occur simultaneously and coincide. Some coastlines flood whilst others, which were previously submerged, are now visible

  • Sea level change can produce emergent and submergent coastlines

Eustatic change

  • Eustatic sea level change is due to differences in ocean water volume 

  • This is a global process and the result of:

    • Changing climate

    • Thermal expansion

    • Tectonic activity

Changing climate

  • The hydrological cycle pauses during an ice age

  • Sea levels fall as the water freezes in ice or snow stores

  • When this ice melts, sea levels rise again

Thermal expansion 

  • As water warms, it expands

  • This increases water volume and raises sea levels

Tectonic activity 

  • Tectonic activity can change ocean topography e.g. ocean basins

  • Earthquakes or volcanic magma under the ocean can change the shape of ocean basins

  • If the ocean basin expands (and the amount of water stays the same) sea levels will sink and vice versa 

Isostatic change 

  • This is the change in sea level due to differences in land height:

    • If land height increases, sea levels will reduce and vice versa

  • Isostatic change is a more localised process

  • Post-glacial adjustment

    • During an ice age, the weight of the ice compresses into the earth’s crust, causing it to subside

    • After an ice age, the crust rebounds (isostatic rebound) and uplifts 

    • Isostatic rebound can occur quickly, like the removal of a plug 

    • It can also occur over a long period, as the mantle readjusts itself. This process is still occurring in some areas of the world after the last ice age

  • Tectonic activity 

    • Tectonic movement can also cause uplifting and subsidence

Isostatic change in the UK

Colored map of the UK showing vertical land movement in mm/yr. Scotland is rising (0.5-1.5 mm/yr), while most of England and Wales are subsiding (-0.5 to -1 mm/yr).
Isostatic change in the UK

Examiner Tip

Think about the future of isostatic and eustatic change, especially as climate change worsens. Will sea levels rise further? What could happen to our coastlines? 

Landforms Associated with Isostatic & Eustatic Processes

Emergent landforms

  • Emergent landforms form when isostatic change is faster than eustatic change

  • Sea levels sink, revealing emergent landforms

  • They sit above the current sea level (as they were formed when the sea level was higher than present)

Raised beaches

  • Beaches that are much higher than the current sea level 

  • They are flat and made of sand or rounded sediment

  • They are covered in vegetation due to plant succession

Relict cliffs

  • Steep slopes sit at the back of raised beaches, above sea level

  • Falling sea levels reveal other erosional landforms e.g. caves, arches, wave-cut notches 

  • A fall in sea level can leave whole coastlines of relict landforms behind

Submergent landforms 

  • Submergent landforms form when eustatic change is faster than isostatic change

  • As sea levels rise, they submerge areas of land, producing submergent landforms

  • Located in inland areas, flooded by rising sea levels

Fjords and Rias 

  • As the sea level rises, water inundates glacial valleys, forming fjords

  • Fjords mirror straight U-shaped valleys that glaciers produce

  • Fjords can be lower than sea level but are slowly getting shallower due to isostatic change

  • The mouth of the fjord meets the sea and is typically much shallower 

  • Rias are areas where water has flooded river valleys

  • Rias mirror the steep V-shaped valleys carved out by rivers

  • They twist and turn, just like a meandering river 

Labelled image of a Fjord

A fjord with steep-sided mountains, a narrow inlet, and a glaciated landscape. The glacial valley is flooded by the sea, with snowy peaks in the background.
Labelled image of a Fjord 

Examiner Tip

Make sure you know what emergent and submergent landforms look like. You might be given a picture like this in an exam to assess

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