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
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
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
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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