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EQ3: Coastal erosion & sea level change alter physical characteristics & risk GapFill
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The sea level has naturally changed as a result of . After the last glacial maximum, around 20,000 years ago, the sea level was 120 metres lower than it is today. The shallow English Channel became land, connecting us to the rest of Europe. During ice ages, when the Earth’s climate is much cooler, precipitation on the land falls as snow. It doesn’t melt and so accumulates as ice – maybe four kilometres thick in some places. Water still slowly evaporates from the sea, but little returns to the sea until the ice melts as the climate .
While the sea level is lower, river valleys and glacial troughs will erode at lower levels. These features then become flooded when sea level rises again, and the ice melts and returns as water to the sea.
There are two types of change:
- : global sea level change
- : local change to the land level relative to the sea level
In the last ice age, Scotland, and much of Ireland and northern England, were covered by ice. During an ice age, the ice advances from the poles towards the equator, and later retreats back towards the poles.
Water, and therefore ice, is very heavy. That weight pushes the crust down into the below, causing a pivot effect. In the UK, Scotland sank, and the South Coast was raised up slightly.
After the ice melted, the pressure was unloaded and the opposite took place – Scotland rose and the South Coast . While much of the movement happened initially, we are still experiencing movement. It is a slow process, though – only a couple of millimetres per year.
This is a process called isostatic readjustment (or postglacial rebound). In areas which are rising, relic and features are found. In areas which are sinking, features are found as more of the coastline and its features are flooded.
Another way in which coastlines submerge is through . This is when sediment is deposited on the land, e.g. by rivers, and the extra load pushes down on and depresses Earth’s crust.
In some coastal environments, you will notice features that look as though they were once coastal features – such as a cliff, or flat areas that could have been a beach – but are no longer impacted by the sea. These are features – they were once eroded by the sea, either when global sea level was higher or when the local land level was lower. Sometimes, you will see a succession of features – caused, for example, by successive glacial or interglacial features. Now, these features are eroded through subaerial processes – weathering and mass movement.
There are two sets of features that you can look out for:
- features – inland (relic) cliffs, arches, stacks, etc.
- features – raised beaches and wave-cut platforms
are flooded upland river valleys caused by a rise in sea level or a fall in land level – they are submergent features. This means that they take the branching (dendritic) form of a river and its tributaries. You can see rias in the UK – for example, as estuaries in the South West. In areas where mountain ranges run parallel (in the same direction) to the coast, narrow strips are flooded. This is called Dalmatian coast – after the area of Croatia.
are flooded glacial troughs – another submergent feature. The glacial troughs were deepened during successive ice ages, when the sea levels were lower. They are quite spectacular features.
Humans are causing sea level to rise much faster than at any point for the past few thousands of years. Sea level is rising by about 3 mm each year.
This is caused by two mechanisms resulting from increased warming of the planet due to fossil fuel combustion, and the release of greenhouse gases from land-use change and agriculture (the greenhouse effect).
The two mechanisms are:
- melting land ice – glaciers and ice caps, Antarctica and Greenland, etc. – but NOT melting sea ice
- expansion of the water