1.3 How do coastal landscapes evolve over time as climate changes? Flashcards
1.3 How do coastal landforms evolve over time as climate changes?
Key idea ⮕ Emergent coastal landscapes form as sea level falls.
Emergent landforms
Landforms shaped by wave processes during times of high sea level are left exposed when sea level falls. These landforms may be found inland, some distance from modern coastlines.
Raised beaches (Emergent landforms)
Areas of former shore platforms and their beaches which are at a level higher than the present sea level. Often found a distance inland from the present coastline.
Marine terraces (Emergent landforms)
Larger scale landscape features than raised beaches, which are quite small scale and localised at the base of relic cliffs. Terraces do not necessarily have have cliffs above them. Their formation, however, is essentially the same as raised beaches - marine erosion during a previous period of higher sea level.
Abandoned cliff (Emergent landforms)
Found behind a beach along emergent coastlines. They typically have wave-cut notches, caves and even arches and stacks.
Relic cliffs
An old cliff displaying features such. as caves, arches and stacks.
Modifications of emergent landforms
After the emergence of landforms, they are no longer affected by wave processes. They do continue to be affected by weathering and mass movement.
If temperatures increase sufficiently, the associated sea level rise could lead to these emergent landforms agains being found much closer to, or even at, the coastline. They would then be subjected to wave processes once more.
Raised beaches (emergent landform modification)
Biological weathering may also become more significant with the colonisation of the surface by an increasing number of marine organisms, such as limpets and whelks.
Abandoned cliffs (emergent landform modification)
On top of the abandoned cliff on the Isle of Portland is a 1-1.5 m layer of frost-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.
Periglaciation (Periglacial conditions)
Periglaciation describes geomorphic processes that result from seasonal thawing of snow in areas of permafrost, the runoff from which refreezes in ice wedges and other structures. “Periglacial” suggests an environment located on the margin of past glaciers.
Permafrost
Permanently frozen layer of soil beneath the surface of the ground.
Cryoturbation
The mixing of materials from various horizons of the soil down to the bedrock due to freezing and thawing.
It can push larger artefacts to the surface of a site.
Pleistocene
Pleistocene
Climate change and sea level rise
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 rise in sea level.
As the temperatures rise, water molecules expand and this also leads to an increased volume. The relationship between temperature and sea level is again clear: a 1ºC rise in mean global temperature results in a sea level rise of approximately 2m.
Submergent landforms
-Rias
-Fjords
-Shingle beaches
Rias (Submergent landforms)
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. The exposed valley sides are quite gently sloping.
Fjords (Submergent landforms)
Fjords are submerged glacial valleys. They have steep, almost cliff-like, valley sides and the water is uniformly deep, often reaching over 1,000 m.
The Sogne Fjord in Norway is nearly 200 km long although those in Scotland are less well developed as the ice was not as thick during the glacial period.
The U-shaped cross section reflects the original shape of the glacial valley itself.
Shingle beaches (Submergent landforms)
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 (Pleistocene), wave action pushed these sediments onshore. In some places they beached at the base of former cliff lines; elsewhere they may form tombolos and bar.
Holocene
The current interglaciation period, extending from 11,700 years ago to the present on the geologic time scale. Follows the Pleistocene.
Modifications of submergent landforms (Rias and Fjords)
Both rias and fjords may be modified by the wave processes acting on their sides at the present-day sea level. The valley sides may also be affected by the operation of sub-aerial 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 0.6 m in the next 100 years, water depth in rias and fjords will increase.
Modifications of submergent landforms (Shingle beaches)
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 be moved even further to the northeast (It is currently moving at a rate of about 17 cm/year) and a beach of the tombolo is highly likely in future storm events. Recent storms have seen waves over-topping the beach.
