Revision:Changes in Sea Level
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Changes in Sea Level
- During times of maximum glaciation large volumes of water were stored on the land as ice this meant that there was a eustatic fall in sea level. Ice accumulated and its weight pressed down on that part of crust beneath it causing isostatic changes in sea level.
- Eustatic Change = Water.
- Isostatic Change = land.
- Positive Change = Flood and therefore a rise in the base level.
- Negative Change = Water draining off the land and therefore a fall in the base level.
The Order of Events
- Formation of glaciers and ice sheets this gave a eustatic fall in the sea level giving a negative change in the base level.
- Further growth of ice sheets left an isostatic depression of the land under the ice producing a positive change in the base level.
- The ice sheets melted giving a eustatic rise in the sea level and a positive change in the base level.
- There was a decline of ice sheets and glaciers producing an isostatic uplift of the land leaving a negative change in the base level. There was a continuing eustatic rise in the sea level.
Tectonic Activity
- Tectonic changes have resulted in uplift (orgeny) of new mountains. Some seashells have been found high in the rocks of the Alps, Andes, and Himalayas which all are fold mountains.
- Tilting (epeirogeny) of the land has led to submergence of several ancient ports in the Mediterranean. Southeast England and the Netherlands have increased risk of marine flooding. The emergence of mid Atlantic ridge could have resulted in a eustatic rise in the sea level.
Changes in Sea Level Affect
- Shape of the coastlines and formation of new features by increased erosion and deposition.
- The balance between erosion and deposition by rivers resulting in drowning of lower sections of valleys and rejuvenation of rivers.
- Migration of plants, animals, and humans.
Landforms Resulting From Submergence
- Rias - these are drowned river valleys, which are sheltered, winding inlets, and occur in the southwest of Britain at the river Fal. Here the sea goes in 18km to Turo and then it deposits sediment due to balance rejuvenation, E.g. The Loe was a ria before a bar blocked it across its mouth.
- Dalmatian Coastlines - these are drowned river valleys, which are parallel to the coast, E.g. The Dalmatian Coastline, Yugoslavia.
- Fjords - these are drowned glacial valleys which are deep, long, narrow inlets with steep and often straight sides E.g. Sogne Fjord which is 100km long and the mouth is a shallow seaward entrance known as a Threshold.
The threshold causes many things to happen
Like:
- When the glacier began to float it stopped eroding.
- The glacier had a thin snout and this was less erosive.
- Large terminal moraine.
- Some thresholds are solid rock and some are morangle (deposited material.)
- Fjards - these occur when low land pre-glacial areas are flooded and they become shallower, wider, and they have less branches, E.g. Oslo, Norway, The Boltic Coast, Sweden, and Strangford Lough, Northern Ireland
Landforms Resulting From Emergence
- Following the global rise in sea level and still occurring in several parts of the world today there was an isostatic uplift of land as the weight of the ice sheets decreased. Landforms created as a result of land rising relative to the sea are raised beaches and erosion surfaces, E.g. Dyfed the Gower Peninsula, South Wales, Cornwall. All of these places had flat upland surfaces normally between 45-200m above the sea level.
- Raised Beaches / Shorelines - as the land rose former wave cut platforms and beaches were raised above the waves, E.g. In west Scotland it degraded cliffs and wave cut platforms, old cliff line which contained arches and stacks.
- E.g. At Kings cave, Arran, 8m of raised beach with former notches and caves. Isostatic adjustment is not constant and is in fact slowing down due to it nearly finishing rebounding. The amount of uplift is decreased from the centre of the ice caps.
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