Changing Sea Levels

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Eustatic and Isostatic Change

Eustatic Change:

  • The global rise/fall in sea levels linked to temperatures. 
  • Rise in temperature causes rising sea levels due a) melting ice caps and b) thermal expansion of water.
  • Fall in temperature causes a fall in sea level due to a) water thermally shrinks and b) more water is stored as ice.

Isostatic Change: 

  • Local rise/fall in sea levels created by isostatic change
  • This is where land is forced downwards by glaciers and then rebounds when the glacier on top melts.
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Stages of Isostatic Change

1. Climate gets colder, and precipitation falls as snow. Glaciers form, meaning less water returns to the sea, causing a fall in sea levels. 

2. The weight of ice causes the land to sink. Although the sea levels are low, they are fairly high in relation to the height of the land.

3. Temperature increases again, meaning ice masses melt, replenishing the main water store. Sea levels rise, and precipitation falls as rain. Submergent land features occur here such as rias and fjords.

4. As there is less weight on the land, isostatic rebound occurs, and the land moves up to previous levels. Sea levels remain high, altough they are quite low in relation to the land height. Emergent land features such as raised beaches form here.

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Submergent Landforms: Fjords

Description: Long narrow inlet with steep sides or cliffs, created in a valley carved by glacial activity.

Formed: Formed when a glacier cuts a U-shaped valley by ice segregation and abrasion of the bed rock, they are connected to the sea. Isostatic rebound when the glacier melts is faster than sea level rise, meaning the bed of the fjord is usually below sea level. The melt water and sea floods the valley forming a fjord. They consist of a glacial rock basin at the end, the threshold.

Example: Sogne Fjord, Norway (200km long)

Summary: U shaped, a drowned GLACIAL valley.

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Submergent Landforms: Rias

Description: A coastal inlet that was formerly a river valley, now partially submerged under water.

Formation: It is formed when the fast rising sea levels from melting glaciers floods the river valley or when coast levels fall due to tectonics. The floodplain is underwater, but the higher land remains exposed. Commonly forms estuaries.

Example: Georges River, Sydney

Summary: V shaped valley, a drowned RIVER valley

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Emergent Landforms: Relict Cliffs/Raised Beaches

Relict Cliffs: 

  • Cliff that was once fully or partially underwater. 
  • When sea levels fell, it exposed the cliff above water, and the face was moved further from the sea due to lack of water. It is not unusual to find old cliff lines with wave cut notches, caves, arches and stacks.
  • Example: Drumadoon, Isle of Arran, Scotland

Raised Beaches:

  • Areas of former wave cut platforms and their beaches which are at a higher level than the present sea level, raised above shoreline by a relative fall in sea level.
  • Example: Isle of Portland, Dorset, England.
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