Wednesday, 30 November 2011

Changing sea levels

Isostatic and eustatic sea-level changes

Changes in sea level due to local subsidence or uplift are referred to as Isostatic. For example, Scotland is rising as it is rebounding now that it is free of several kilometres of ice which has weighed it down since it was deposited during the last ice age. In contrast , the south east of England is dropping by almost a millimetre each year.

Changes in sea level due to changes in the volume of water in the oceans are called eustatic. This can be caused        
by melting of the polar icecaps releasing more water into the oceans. When the ice advances and more water is held in the ice caps then sea level falls. Eustatic change is also caused by rapid rate of sea floor spreading, causing the MOR to swell with magma. This causes sea level to rise.

Measuring past sea levels

  • Using seismic evidence to find unconformities where transgressing seas have resulted in the sea laying down younger beds. When the sea retreats regressions can also be identified as unconformities. 
  • Using exposed geology to estimate the area of flooded continents through time. When sea levels falls there will be raised beaches and cliff lines. When sea levels rises there will be submerged and marine organisms can be found in younger sediment. 
  • Using oxygen isotopes ratios to assess the past temperatures and therefore the amount of ice. 

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