Sea Levels Changes (AQA A Level Geography)

Eustatic & Isostatic Change

• Sea level can change as a result of isostatic and eustatic change 

  • Eustatic change occurs when there is a change in the 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

Emergent and Submergent Coastal Landforms

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

• 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 

Climate Change and Sea Levels

• 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