sea level change

• When do eustatic and isostatic change occur?

  • 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
• Why does eustatic change occur?

  • Eustatic change happens due to changes in the amount of ice, as a result of thermal expansion and tectonics
  • This change will be global 
• eustatic change- 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 
• eustatic change- thermal expansion?

  • Thermal expansion occurs as water warms, warmer fluids expand to take up a greater volume 
• What is the last glacial maximum?

  the period when glaciers/ice sheets were at there highest levels

  this was 21,000 years ago
• eustatic change- tectonics?

  Magma rising to the surface lifts the crust and reduces the capacity of the oceans causing sea levels to rise
• How can isostatic change be caused by 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 the sea level in a process known as isostatic recovery or readjustment
• How can isostatic change be caused by acretion?

  • Within the sediment cell, there are areas of net deposition causing land to build up 
• How can subsidence cause isostatic change?

  • Caused by the lowering of the water table or increased deposition weighing down the sediment
• Sea level rise?

  the last 20,000 years have seen the fastest and largest change in sea level

  this period of time has been named the Flandrian Transgression
• How can isostatic change be caused by tectonics?

  • The folding of the sedimentary rock 
  • Lava and ash from volcanoes increase the height of the land relative to the sea level
• UK & isostatic change?


  • 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
• What is the biggest single factor responsible for eustatic change in the past 300 years?

  thermal expansion

  as water becomes warmer it occupies a larger volume
• What is the 2nd largest factor responsible for the increasing volume of the world's oceans?

  melting ice as water is redistributed from polar ice sheets and continental glaciers as they melt

  melting ice may become the most significant in the next century
• Define eustatic change?

  the global change in the sea level resulting from a change in the volume of water in the oceans
• What are features of an emergent coastline?


  • 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 and fossil cliffs
• What are raised beaches?


  • 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
• What are fossil cliffs?

  • A steep slope at the back of a raised beach
  • Wave-cut notches, caves, and arches may be evident
• Eustatic change- a fall in temperatures:

  • a decrease in temperature as a result for entering a glacial period, leads to more precipitation to fall as snow
  • when cold temperatures persist for a prolonged period of time, water is unable to evaporate as it will remain stored in the glacier/ice sheet and so is unable to return to the sea
  • this causes the hydrological cycle to slow down
  • as water freezes, particles become more compacted and globally, sea level will fall
• What is an inter-glacial period?


  the period of time between one glacial period and another
• How do tectonics affect water level (eustatic)?

  rising magma at a divergent plate margin/hot spots lifts the overlying crust, reducing the capacity of the ocean and producing sea level rise

  uplift of crustal plate reduced Indian Ocean capacity causing 0.1mm eustatic rise in global sea levels

  high rates of volcanism on the seafloor displace water out of the ocean basin producing higher sea levels
• Define isostatic change?


  a local change in the sea level resulting from the movement of the land relative to the sea
• What is post glacial isostatic adjustment?


  the crust floats on the mantle beneath and when weighted by an ice sheet it's forced into the mantle resulting in a rise in sea level

  if the weight is lifted e.g when an ice sheet melts, the crust 'rebounds' and rises- creating a fall in sea level relative to land movement?
• Isostatic sea level change caused by tectonics?


  a coastal region experiencing seismic activity may experience land being shifted upwards or downwards as geological pressures are released and rocks adjust either side of fault lines

  as a result, there can be a rapid change in relative sea-level, but over a relatively localised area
• Isostatic change: subsidence?


  excessive pumping (humans choose to remove water) of aquifer systems has resulted in permanent subsidence and related ground failures

  in some systems, when large amounts of water are pumped, the subsoil compacts, thus reducing in size and number the open pore spaces in the soil previously held water

  this causes the land to sink
• What is an aquifer?

  an undeground layer of water-bearing mateial, consisting of permeable or fractured rock, or of unconsolidated materials
• Isostatic change - acretion?


  this occurs when sediment is added to a landform e.g a river delta by deposition

  it can build up, to form new land, allowing a delta to grow out to sea

  it tends to be balanced by subsidence, caused by the weight of the newly deposited sediment

  in large river deltas, this impact is profound as the weight of sediment leads to slow crustal sag

  most deltas are already undergoing natural subsidence resulting in accelerated rates of sea level rise
• 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 landform
• Submergent coastlines?


  the rise of sea level allows the sea to submerge the low-lying land masses

  this is particularly common in existing river valleys and glacial valleys
• How are Rias formed?

  when river valleys are submerged by rising sea level

  they share the same long and cross profiles of a river

  e.g Dartmouth
• What are Rias submerged coastlines?

  drowned river valleys in unglaciated areas, caused by sea level rise flooding up to the river valley
• How are dalmation coastlines formed?

  the same way as Rias, but the rivers run parallel to the coast, leaving islands as they were submerged

  e.g Dalmation coastline in Croatia
• How are Fjords formed?

  formed when rising sea levels submerge a glacial valley/trough

  straight and narrow with very steep sides: U shaped

  they have a shallower part near the mouth where the glacier left the valley and deposited material

  many examples on the Western Coast of Norway
• Why is sea level change difficult to predict?

  The melting of polar ice caps and glaciers significantly contributes to sea level rise. Predicting the behaviour of ice sheets involves understanding complex processes such as the influence of ocean temperatures on ice melt.

  Local land movements, such as subsidence (sinking) or uplift, can influence relative sea level changes. These movements can be caused by natural processes like tectonic activity or human activities such as groundwater extraction or sediment deposition. Predicting these movements accurately on a global scale is challenging.
• What is isostatic rebound?

  Isostatic rebound refers to the slow adjustment of the Earth's crust following the melting of large ice sheets from past glacial periods. As the weight of ice is removed, the underlying crust slowly rebounds or uplifts over time.

  This process can cause localised changes in sea level, with areas previously depressed by the weight of ice experiencing gradual uplift and a decrease in relative sea level.
• he Devensian British-Irish Ice Sheet was a large mass of ice that covered approximately two thirds of Britain and Ireland around 27,000 years ago. All of Scotland and Ireland, most of Wales, and most of the north of England was underneath the ice sheet during the Last Glacial Maximum.
• Bangladesh (the world’s most densely populated country with an estimated population of about 169 million in 2015)