Sea level Changes Flashcards
What is eustatic change?
Eustatic sea level change is caused by a change in the volume of water in the sea, or by a chnage in the shape of the ocean basins.
The effects are always global and the main causes are:
1) Changes in climate. Different changes affect sea level in different ways:
> An increase of temperature causes melting of ice sheets, which increases sea level. It also causes water to expand, which increases sea level further.
> A decrease in tmeperature causes more precipitation to fall as snow. This increases the volume of water stored in glaciers and so reduces the volume of the sea, which decreases sea level.
2) Tectonic movements of the Earths crust that alter the shape of ocean basins. e.g. sea floor spreading increases the volume of the basin and so decreases sea level.
What is Isostatic change?
Isostatic sea level change is caused by vertical movements of the land relative to sea.
The effcts are always local and the main causes are:
1) Uplift or depression of the earths crust due to melting of ice sheets. Slow uplift of land can continue for thousands of years after the weight of retraeting glacier has gone.
2) Subsidence of land due to shrinkage after abstraction of groundwater, e.g. drainage of marshland.
3) Tectonic processes e.g. as one plate is forced beneath another at a plate margin.
How has sea level changed over the past 10,000 years?
During the last glacial period, roughly 12000 years ago, water was stored in ice sheets, so sea level was lower than present.
As tmeperatures atrted to increase, ice sheets melted and sea level increased rapidly. It reached its present level about 4000 years ago.
Over the last 4000 years, sea level has fluctuated around its present value.
Since about 1930, sea level has been rising.
How has climate change caused changes in sea level?
> Over the last century, global temperature has increased rapidly. This is called global warming. There’s been a sharp rise in temperature by 1.08 celsius from 1900-2016.
> Scientists believe the cause of climate change is due to new human activities such as deforestation and the burning of fossil fuels.
> These activities increase the concentration of greenhouse gases in the atmosphere, greenhouse gases absorb outgoing long-wave radiation, so less is lost in space.
> Increases in temperature are likely to cause increases in sea level, through the melting of ice sheets and thermal expansion of water in oceans.
> Global sea level is currently rising by 2mm each year.
How does sea level rise have major impacts on coastal areas?
1)
> Flooding of low-lying areas has increased with sea level rise.
> Submergence of low-lying islands. A lot of low-lying islands are at risk of disappearance
> Changes in the coastline. As the sea level rises the coastline changes- islands are created and the area of land is decreased.
> Contamination of water sources and farmland. Salt water may enter bodies of fresh water near the coast, damaging ecosystems and making the water unsuitable for loss of use.
Saltwater entering soils may damage crops and make lands impossible to farm.
2)
> Sea level rise and increased storminess will increase coastal erosion, putting ecosystems, homes and businesses at risk.
How are raised beaches formed?
> Raised beaches are formed when the fall in sea level leaves beaches above the high tide mark. Over time, beach sediment becomes vegetated and turns into soil.
> Sea level also exposes wave-cut platforms, raising them above their former level.
> The sea no longer erodes the cliffs above the raised beach and slowly gets covered by vegetation. They are called relict cliffs. Many caves, arches, stacks and stumps are found with these relict cliffs.
How is a Rias formed?
Rias are formed where river valleys are partially submerged. Rias have a long gentle and cross profile. They’re wide and deep at the mouth, becoming narrower and shallower the further inland they reach.
How are fjords formed?
Fjords are a lot like rias, but they are drowned glacial valleys rather than drowned river valleys. They are relatively straight and narrow, with very steep sides.
They have a shallow mouth caused by a raised bit of ground (threshold) formed by the deposition of material by the glacier. They’re very deep and further inland.
How are Dalmatian coastlines formed?
In areas where valleys lie parallel to the coast, an increase in sea level can form a dalmatian coastline. Valleys are flooded, leaving islands parallel to the coastline. It’s named after the dalmatian coastline in Croatia.
How do processes create and alter landforms/landscapes over time?
> Individual landforms (spirits, arches) combine to form landscapes- coastal landscapes can be dominated by processes such as erosion and deposition, but most are formed by both.
> Processes operating in coastal systems can create new landforms or change existing landforms.
This means that coastal landscapes change over time.
For example:
- A change in wind direction might increase deposition and therefore change a landscape dominated by erosive landforms to one dominated by depositional landforms.
- Relict landforms can still experience coastal processes e.g. it may be weathered by freeze-thaw weathering.
