ISOSTACY AND EUSTACY
ISOSTACY –
Isostatic Movement
CHANGES IN SEA LEVELS WHICH RESULT IN NEW
COASTAL LANDFORMS
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.
COASTLINE OF EMERGENCE
1. RAISED BEACHES
2. RAISED CLIFFS
3. RAISED PLATFORMS
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.
EUSTACY
COASTLINES OF SUBMERGENCE
Eustatic Movement = rise in sea levels
1. RIAS
2. FIORDS
3. DALMATION COASTS
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.
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.
Dalmatian Coastlines –
these are drowned river valleys, which are parallel to the
coast, E.g. The Dalmatian Coastline, Croatia
Found on CONCORDANT COASTLINES –
Features lie parallel to the coastline.
RIVER
REJUVINATION
ISOSTACY LEADS TO NEW LANDFORMS
ISOSTATIC MOVEMENT The change in landlevels relative to sea levels due to a
change in balance between erosion and deposition.
As the earth’s crust floats on molten rock in the mantle
changes in weight redistribute from highland to lowland
changing pre-existing features and also causing erosion
and deposition to increase after base level had been reached.
Rivers can change their GRADED PROFILE due to
isostatic or eustatic movement.
This leads to RIVER REJUVINATION.
Rejuvination - Young Again.
During rejuvination rivers get a new base level as land levels rise and sea
levels fall.
Land rises due to SPRING BACK effect after the last ice-age.
Rivers now erode again cutting new channels to the sea.
3 FEATURES OF REJUVINATION
1.
KNICKPOINTS
2.
INCISED MEANDERS
3.
PAIRED TERRACES
1.
KNICKPOINTS
Knickpoints are caused by a change in base level. When a river is
rejuvenated (that is, has a negative change in base level, the sea level
falls), the long profile is lengthened as land rises from the sea. This
increases the gradient of the long profile and causes the river to gain
more erosive energy.
A knickpoint marks the position of the former base level. The sudden
change in gradient forms waterfalls or rapids. Overtime a knickpoint
moves upstream.
KNICKPOINT DIAGRAM
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2. INCISED MEANDERS
Vertical erosion restarts on a rejuvinated river causing
an old meander to cut vertically down into the floodplain
while maintaining its winding course.
This creates incised meanders.
Example : River Nore
3.
PAIRED TERRACES
Once incised meanders form, they begin to erode laterally
To form a new floodplain far below the existing one.
The remnants of the old floodplain form terraces either
side of the river called paired terraces.
If rejuvination occurs many times to the same river
a series of paired terraces will form.
Paired terraces diagram
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