Sea Level Change
a) Describe the processes of isostatic and eustatic sea level change
Both isostatic and eustatic changes relate to oscillations of sea level on local, regional or global scale - Sea Level Change introduction. Eustatic changes are the result of an increase or decrease in global water volume and isostatic changes involve changes in the height of the land relative to the sea.
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A “hot spot” for isostatic and eustatic change is during ice ages. During the last ice age, approximately 18,000 years BP, it was thought that sea levels were up to 150 metres below what they are at today meaning that it was possible to walk to France or Ireland. This was due to glacio-eustaticy, when water is frozen and turned into ice caps which cover land mass, causing a massive drop in sea levels. This is followed by the melting of these ice caps during interglacial periods, where sea level rises again.
Tectonic activity is the main cause of an isostatic change, and the most important tectonic activity is named isostatic readjustment. Isostatic readjustment is where the land naturally rises or falls due to over or underlying forces.
During the last ice age, an ice sheet covered Scotland and a lot of northern England. The pressure from the ice sheet caused the land to depress and so as the ice age ended, isostatic rebound occurred and Scotland began to rise again. This is known as glacio-isostacy and occurred all around the world during the last ice age. It’s thought that Greenland is still isostatically depressed by up to 1000 metres.
In Europe, complex changes in sea level occurred because as ice sheets melted, the sea level rose but at the same time isostatic rebound occurred, meaning both land and sea levels rose together.
Another cause of isostatic readjustment is tectonic activity. For example, in Papua New Guinea, coral terraces were formed when uplift from the plates, caused a drop in the relative sea level, exposing what were once, submerged coral reefs.
Climate changes are a secondary influence on sea level change, but as we can see from current affairs this type of influence is becoming more and more controversial, thanks to global warming and the greenhouse effect. In terms of the greenhouse effect, an increase in global temperatures will cause the thermal expansion of water and an increase in the rate of melting of ice caps also leading to an increase in sea level.
b) Using named examples, explain how isostatic and eustatic sea level changes give rise to distinctive coastal landforms.
Landforms occur whenever an isostatic or eustatic change takes place, which is expected just by looking at the landforms that occur as a result of “everyday” coastal functions.
The first landforms to note are the raised beaches found on the Gower in Swansea. These are beaches formed during a previous time period where the sea level was higher than it is today or where a beach has risen out of the water due to isostatic rebound.
Rias (drowned river vallies) are also landforms created by eustatic change. During the ice age, the south of England was not cover in ice and there were river valleys formed in areas such as Salcombe. However, as the ice caps melted and sea level rose, these river vallies were flooded causing a ria to form. A before and after diagram of a Ria is feature below.
The same thing can occur with glacial valleys which were formed during the ice age further north. In areas such as Scandinavia, fjords are very common and these are simply flooded glacial troughs.
Other landforms include deposition features such as barrier beaches, spits and dunes which retreat in land as eustatic change takes place. As sea level rises, barrier beaches, dunes and tombolos can be submerged if the rise is big enough. These landforms become sand bars such as Pole Sands. These sand bars then provide sediment to build another landform onshore. For dunes, the slacks separating different successions of dunes can flood so that the fore dunes form a barrier beach, cut of from the dune behind. As a result the dunes behind are now the fore dunes and so a succession begins again as the climax vegetation retreats inland as the dune system builds again.