3. How do coastal landforms evolve over time as climate changes?

Question 1

What are eustatic changes?

Answer 1

Changes in the volume of water in the global ocean store, these changes are influenced by variations in mean global temperatures, affecting both the amount of water in the ocean store and its density
However, should be noted that sea level change is relative as it is also affected by changes in land level

Question 2

Climate change and sea level fall

Answer 2

• Decrease in global temperature leads to more precipitation being in the form of snow, eventually this snow turns to ice and so water is stored on the land in solid form rather than being returned to the ocean store as liquid
• Result is a reduction in the volume of water in the ocean store and a worldwide fall in sea level
• As temperatures fall, water molecules contract, leading to an increased density and a reduced volume, it is estimated that a 1 degree fall in mean global temperature causes sea level to fall approximately 2m
• About 130,000 years ago, during the Tyrrhenian inter-glacial period, global mean annual temperatures were almost 3 degrees high than today and sea level was about 20m above today’s. position
• Temperatures then fell during the onset of the Riss glacial period, reaching a minimum of about 7 degrees lower than today about 108,000 years ago
• As a result of this temperature decrease, less water was returned to the ocean store and sea levels dropped by over 100m, making them about 83m lower than the present day

Question 3

What are emergent landforms?

Answer 3

Landforms shaped by wave processes during times of high sea level are exposed when sea level falls, as a result they may be found well inland, some distance from the modern coastline

Question 4

Raised beaches

Answer 4

Areas of former shore platforms that are left at a higher level than the present sea level, they are often found a distance inland from the present coastline

Question 5

Abandoned cliffs

Answer 5

Behind the beach, along emergent coastlines, it is not uncommon to find abandoned cliffs with wave cut notches, caves and even arches and stacks

Question 6

Marine terraces

Answer 6

Larger scale of raised beach, tend to be smaller and localised, formed through marine erosion during previous period of higher sea level

Question 7

Example of raised beach

Answer 7

On southern tip of the Isle of Portland near Weymouth in Dorset, there is a distinct raised beach at a height of around 15m above the present day sea level
This is thought to be formed around 125,000 years ago during the Tyrrhenian inter-glacial period when sea levels were much higher than today’s
The Portland limestone here was eroded by hydraulic wave action, partly through the exploitation of the bedding plane weaknesses
Erosion rates at that time are estimated to have been as much as 1m/yr, other raised beaches at Portland are thought to date to about 210,000 years ago

Question 8

Modification of landforms

Answer 8

• After their emergence, these landforms were no longer affected by wave processes, however they continue to be affected by weathering and mass movement
• On top of the abandoned cliff on the Isle of Portland is a 1-1.5m layer of first-shattered limestone debris deposited when the area experienced periglacial conditions during the last glacial period
• At the same time, the cliff face itself was gradually degraded by frost weathering processes, leading to rock fall from the cliff face
• Evidence of other periglacial processes such as cryoturbation is also evident as contortions in fragmented limestone - they are the result of freezing and thawing of the permafrost in the subsoil during the late Pleistocene period, the final glacial phase
• In the post-glacial period, warmer and wetter conditions have led to the development of vegetation cover on many such exposures, often making them more difficult to recognise
• With further warming of the climate predicted for the upturn, continued degradation is likely to occur with chemical weathering perhaps becoming more influential, especially by carbonation of limestone cliffs and platforms
• Biological weathering on the raised beach may also become more significant with the colonisation of the surface by an increasing number of marine organisms such as limpets and whelks
• If temperatures increase sufficiently, the associated sea level rise could lead to these emergent landforms again being found much closer to or even at the coastline, they would then be subjected to wave processes once more

Question 9

Climate change and sea level rise

Answer 9

• An increase in global temperature leads to higher rates of melting of ice stored on the land in ice sheets, ice caps and valley glaciers
• As a consequence, there is a global increase in the volume of water in the ocean store and a consequent increase in the volume of water in the ocean store and a consequent rise in sea level
• As temperatures rise, water molecules expand and this also leads to an increased volume, the relationship between temperature and sea level is clear: a 1 degree rise i mean global temperature results in a sea level rise of approximately 2m
• Period of significant sea level rise since the Würm glacial period (25,000 years ago) is known as the Flandrian Transgression

Question 10

Key submergent landforms

Answer 10

Rias
Fjords
Shingle beaches

Question 11

What are rias?

Answer 11

• Submerged river valleys, formed as sea level rises
• The lowest part of the river’s course and the floodplains alongside the river may be completely drowned, but the higher land forming the tops of the valley sides and the middle and upper part of the river’s course remains exposed
• In cross section, rias have relatively shallow water, becoming increasingly deep towards the centre and the exposed valley sides are quite gently sloping
• In long section they exhibit a smooth profile and water of uniform depth
• In plan view they tend to be winding, reflecting the original route of the river and its valley, formed by fluvial erosion within the channel and sub-aerial processes on the valley sides
• Rias are typically underlain by alluvial deposits in buried channels that were eroded by the rivers that flowed down to the lower sea levels of Pleistocene glacial periods
• During interglacial periods, when sea levels rose, further deposition would have occurred as the rivers had less surplus energy for erosion
• Thus, in the Flandrian Transgression, significant infilling of these earlier channels occurred:
• In some places, such as at Padstow on the north coast of Cornwall, sand was washed in from the Atlantic Ocean, at low tide the River Camel flows between broad exposed sandbanks which are only exposed at high tide
• At Salcombe, the ria has remained relatively deep, bit it does have a shallow entrance or threshold due to a sand bar having been deposited in the low-energy environment at the mouth of the Kingsbridge estuary

Question 12

What are fjords?

Answer 12

• Submerged glacial valleys which have steep, almost cliff-like sides and the water is uniformly deep, often reaching over 1000m
• U-shaped cross section reflects the original shape of the glacial valley itself, they consist of a glacial rock basin with a shallower section at the end known as the threshold, his results from lower rates of erosion at the seaward end of the valley where the ice thinned in warmer conditions
• Due to the depth of water that occupied fjords during the Flandrian Transgression, marine erosion rates remained high and in some cases, the fjords were further deepened, in others, such as those on the west coast of South Island, New Zealand, there has been some infilling with sediments - deposited by let water from the glaciers of the Southern Alps, the volume of sediment has increased significantly in recent decades as glaciers have receded

Question 13

What are shingle beaches?

Answer 13

• When sea level falls as the volume of land-based ice grows, large areas of ‘new’ land emerge from the sea, sediment accumulates on this surface, deposited by rivers, meltwater streams and low-energy waves
• As sea levels rose at the end of the last glacial period, wave action pushed these sediments on-shore, in some places they beached at the base of former cliff lines, elsewhere the may form tombolos and bars
• The tombolo at Chesil Beach is thought to have formed in this way during the Flandrian Transgression, sediment carried into the English Channel by meltwater dung the Wurm glacial accumulated in locations such as Lyme Bay, as sea levels rose, the sediment was carried northwest by the impact of south-westerly prevailing winds and the resultant waves
• It moved a total of about 50km until it became attached to the Isle of Portland at one end and the mainland near Abbotsbury at the other, the beach now contains an estimated 100 million tones of shingle, varying in size by 1-2cm material to 5-7cm pebbles
• It was previously though that this tombolo was formed by the extension of a spit towards the Isle of Purbeck, but the lack of recurves and complex grading of pebbles suggest that it was not formed by longshore drift alone

Question 14

How are these landforms modified due to climate and sea level changes?

Answer 14

• Both rias and fjords may be modified by the wave processes acting on their sides and the present-day sea level, the valley sides may also be affected by the operation of sub-ariel processes in today’s climatic conditions or in any different climatic conditions of the future, this may eventually lead to a reduction in the steepness of the valley sides of fjords
• With sea levels predicted to rise by a further 0.6m in the next 100 years, water depth in rias and fjords will increase, marine rosion is also likely to increase due to stormier conditions and larger waves
• Shingle beaches, being composed of unconsolidated material, are especially vulnerable to modification - the tombolo at Chesil Beach has been significantly affected by present-day longshore drift processes and is likely to continue to be so in the future
• With further sea level rises predicted, shingle may well be moved even further to the north-east (currently moving at a rate of 17cm/yr) and a breach of the tombolo is highly likely in future storm events
• Recent storms have seen waves over-topping the beach, in 2009, 1m sized pieces of shelly clay were eroded from in front of the beach and washed up onto it at West Bexington in a winter storm, this sort of sediment addition will also become more common in future with higher sea levels and more storm events