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 mantlechanges in weight redistribute from highland to lowlandchanging pre-existing features and also causing erosionand 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.

15.04.swf

KNICKPOINT DIAGRAM

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.

15.05.swf

Paired terraces diagram