3.3- Sea Level Change Flashcards
There are two ideas relating to sea level change:
- eustatic change
- isostatic change
What is eustatic change?
A global change in sea level resulting from an actual fall or rise in the level of the sea itself- one clear example of this is is that as temperatures increase, water expands meaning that sea levels rise when sea temperatures are warmer
What is isostatic change?
Local changes in sea level resulting from the land rising or falling relative to the sea- one of the most obvious examples of this is that continents collide and fold mountains form as the land is forced to rise up out of the sea
Sea level does not remain
Constant- it has changed throughout Earth’s history and continues to do so
Changes in sea level create
Distinct landforms of emergence (falling sea level relative to land) and submergence (rising sea level relative to land)
Describe the history behind sea level change
- at last glacial maximum 18,000 years ago, sea levels were on average 110m below their current level which meant that the UK was joined to mainland Europe and Scandinavia by a large land bridge where the North Sea is now called Doggerland
- by 10,000 years ago the northern North Sea was flooded and the coastline of Europe was beginning to look like it does today but Doggerland still linked with what is now the UK to what is now the Netherlands and Denmark
- this was followed by a rapid increase in which sea levels rose by 10m in just 500 years. Prior to the industrial revolution,sea levels had been fairly stable for about 3000 years but from the mid-19th century they started increasing exponentially
Current sea levels are rising
At 3.2 mm per year, largely due to thermal expansion
It is thought that a 4-6m rise in sea level is likely by
2100= which could have huge effects, although this could be greater depending on how the Greenland and Antarctic ice sheets respond to warming temperatures
What are the 3 causes of isostatic change?
1- tectonic activity
2- glaciation
3- post-glacial readjustment
Explain how tectonic activity can lead to isostatic change
- land can move up or down as a result of tectonic activity
- this could be a few metres as a result of one event
- alternatively, this could be on a much larger scales fossils in the Andes, Alps and Himalayas all show that the rocks found in these fold mountains were once on the ocean floor, but have been forced up thousands of metres as part of the formation of mountains
Explain how glaciation leads to isostatic change
- during an ice age, glaciers and eventually ice sheets form over the land in areas cold enough
- this causes the land surface to become pressed into the upper layer of the mantle (asthenosphere) and therefore relative sea level rises
Explain how post-glacial readjustment causes isostatic change
following the melting of ice sheets and glaciers on the land, the land is able to rise back up as the asthenosphere rebounds = called isostatic recovery and is responsible for falling sea levels in Scotland today as during the last ice age Scotland was the part of the UK covered with the thickest ice and it melted last
What are the causes of eustatic change?
- thermal expansion of water
- changes in ice sheet extent
- tectonic activity
Explain how thermal expansion of water causes eustatic change
As water is heated it expands, so even if the mass of the ocean (total amount of water) stays the same, the warmer water has a larger volume so takes up more space and sea level therefore rises
Explain how changes in ice sheet extent cause isostatic change
- where ice forms on a land during a glacial period it removes water from other stores including the ocean so sea levels fall
- when ice on the land melts there is less water locked up in ice on land so more available to other stores including the ocean = sea levels rise
Explain how tectonic activity leads to isostatic change
- where new land is being formed under the ocean, usually at mid-ocean ridges, this new rock/crust takes up space in the ocean basins, which would have been occupied by water, so the water is displaced and sea levels rise
- times of higher amounts of volcanic activity at mid ocean ridges have higher eustatic sea levels and times of lower amounts of volcanic activity have lower eustatic sea levels
What is total sea level change?
Balance between isostatic and eustatic change
Explain the total sea level change in Scotland
In Scotland today, isostatic recovery is causing the level of land to rise faster than thermal expansion and melting ice sheets are causing sea level to rise so the overall effect is that sea level is falling in Scotland and landforms of emergence are forming
What are examples of emergent features?
- raised beaches
- wave cut (marine) platforms
Explain how raised beaches form
- form where the sea level drops relative to the land so what was a beach, becomes stranded above the high water mark
- these are common on the west coast of Scotland
- other marine features such as cliffs and cliff profile features like caves may also be left above the high tide level in which case they are called relict features e.g. relict cliffs
- raised beaches are subject to vegetation succession once above high tide level and therefore may appear as flat, grassy terraces
Wave-cut (marine) platforms may also be left exposed
Above the level of current wave cut platform however these are often less visible as they may well be covered in beach sediment from the current beach
What are examples of submergent features?
- rias
- fjords
- dalmatian coasts
What are rias?
- rise are flooded river mouth/valleys
- where sea level rises relative to the land, river valleys flood and create coastal inlets
- mouth of river Exe in Exmouth is a good example of this as they make excellent natural harbours
What are fjords?
- fjords are similar to rias in that they are flooded valleys, however they are flooded glacial rather than river valleys
- when glaciers erode they create steep sided U-shaped valleys so fjords have very steep sides, often near vertical
- they can be very long over 200km
Where do dalmatian coasts occur?
- where river systems which once ran parallel to the coast (generally due to concordant geology) are flooded resulting in a series of long islands which would once have been the hills along the valley sides parallel to the coastline
What’s the key difference between rias and fjords and dalmatian coasts?
Rias and fjords run perpendicular to the coastline, dalmatian coast islands run parallel to it
Globally, sea level is increasing eustatically as
Climate change causes thermal expansion and the melting of ice sheets (some coastlines may continue to experience localised sea level fall due to isostatic factors, but globally, sea levels are increasing)- how much sea levels increase by varies according to predictions of how much warming is likely to occur and how the Antarctic and Greenland ice sheets are likely to respond
Summarise the impacts of sea level change:
- major centres of UK population including London, Hull and Middlesbrough are at risk from inundation
- lots of UK agriculture land could be lost through both flooding and increased coastal erosion- along the east coast is valuable arable land
- numerous power stations and gas and oil terminals which are often built along the coast risk flooding
- groundwater may become contaminated with sea water resulting in it becoming unusable
- loss of costal habitats such as mangroves
- sub-mergence of low lying islands such as Maldives- Maldives is the lowest country in the world, with maximum natural ground levels of only 2.4 metres above sea level
- migration away from coastal areas
What are environmental refugees?
If an island becomes submerged, its population becomes ‘environmental refugees’ = people forced to migrate as a result of changes in the environment
Explain how processes and time interact to form distinct coastal landforms and landscapes
- coastal systems are the product of a series of processes acting at a range of scales over a range of timescales from hourly changes in tide levels to geological-time scale changes in continental movement
- any coastline has a range of features formed by past processes but present processes are also acting upon these
- for example, raised marine terraces in Scotland are still being affected by present-day erosion at high tide
- current systems are responsible for more recent features but they are more likely to be modifying relict (previous) ones
