Coastlines (DP IB Geography)

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

• 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

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