Sea Levels Changes (AQA A Level Geography)

• Sea level can change as a result of isostatic and eustatic change 
  • Eustatic change occurs when there is a greater volume of water in the ocean basins 
  • Isostatic change occurs when the height of the land changes relative to the water level 
• Isostatic change happens more slowly than eustatic change

Eustatic change

• Eustatic change happens due to changes in the amount of ice, as a result of thermal expansion and tectonics
  • This change will be global 
• Changing amounts of ice 
  • At the end of the last ice age 10,000 years ago global sea level rose rapidly due to melting ice creating well known waterways like the English Channel 
  • Sea levels may also decrease when ice forms, locking water away in the ice sheets and glaciers 
• Thermal expansion 
  • Thermal expansion occurs as water warms, warmer fluids expand to take up a greater volume 
• Tectonics
  • Magma rising to the surface lifts the crust and reduces the capacity of the oceans causing sea levels to rise

Isostatic change

• Isostatic change is a localised change and may be due to:
  • Post-glacial adjustment 
    • During a period of glaciation extremely heavy icy sheets weigh land down
    • When the glacial period ends and the ice melts the land will rebound to a higher level, lowering sea level in a process known as isostatic recovery or readjustment
  • Accretion
    • Within the sediment cell there are areas of net deposition causing land to build up 
  • Subsidence 
    • Caused by the lowering of the water table or increased deposition weighing down the sediment
  • Tectonics
    • The folding of the sedimentary rock 
    • Lava and ash from volcanoes increase the height of the land relative to the sea level
• The UK is still experiencing isostatic recovery from the end of the last ice age
  • Land in the north in Scotland is still rebounding and rising by approximately 1.5mm a year
  • Landsend in Cornwall is sinking by 1.1mm each year

Isostatic changes of the UK

Emergent coastline features

• Landforms that exist because of a reduction in sea level are called emergent landforms 
  • This happens during isostatic recovery 
• When land rebounds through isostatic recovery landforms which are the result of previous sea levels are revealed including:

Raised beaches 

• These are:
  • Beaches which are above high tide level
  • They are flat and covered by sand/pebbles
  • Experience succession 
• Raised beaches can be seen on the Scottish islands of Islay, Jura, Colonsay and Mull

Fossil cliff

• This is a:
  • Steep slope at the back of a raised beach
  • Wave-cut notches, caves and arches may be evident

Submergent coastline features

• Landforms that exist because of a rise in sea level are called submergent landforms
  • This happens when coastlines are flooded
• When coastal areas are flooded the lower course of river valleys can be flooded, creating the submergent landform called a ria or drowned river valley which:
  • Have a V-shaped cross-section
  • Are an estuarine coastline
  • Are the most common coastal landforms
• Glacial valleys are also flooded as a result of eustatic change creating a fjord
  • Fjords are deeper than rias, particularly inland, as they were originally the flat-bottomed U-shaped valley carved out by a powerful glacier
  • They have a relatively straight profile
  • Maybe deeper than the adjacent sea

The Milford Sound - a famous fjord in New Zealand 

• A Dalmatian coast is a submergent landform 
• Where a landscape of valleys and ridges are parallel to the coastline the low parts of the valleys will be flooded when sea level rises 
• The exposed tops of the ridges become small offshore islands parallel to the coast 
• The best known example of this is the Dalmatian Coast in Croatia that the landform is named after 

• The current levels of sea level stabilised around 3000 years ago
• Since the industrial revolution took flight in the nineteenth century global sea level has risen by about 235mm 
• This is enough to overcome some coastal defences and create coastal flooding through storm surges in some areas 
• As the world warms there will be increase eustatic sea level change and thermal expansion in ocean basins
• The IPCC predicts that sea levels by 2100 will be between 0.3 - 1.0m higher than they are now 
• This could inundate many coastal cities and settlements
• It could also salinise aquifers in low lying regions and increase coastal erosion 
• In many parts of the world, particularly low lying cities and coastal settlements that are particularly vulnerable climate change sea level planning has already started to take place 
• These places are likely to see land values drop as formerly valuable land is increasingly eroded, salinised or flooded by sea level rise 
• More than 1 billion people live on high risk coasts
• 75% of the world’s cities are coastal