3.A/B - emergent/submergent coastal landscapes Flashcards
when do emergent coastal landscapes form?
as sea level falls
what causes sea level change?
- global temp ↑ = thermal expansion and ice sheets/glaciers melting = greater volume
- climate change = extremes of weather
- decrease in global temp = more precipitation falls as snow which turns into ice = ↓ vol of water in ocean = thermal contraction
what landforms occur as a result of sea level fall?
emergent landforms
what landforms occur as a result of sea level rise?
submergent landforms
what are the 2 types of sea level change?
eustatic and isostatic
what can eustatic change lead to?
coastlines being drowned/emerging from the sea
what are 3 key factors coastlines are influenced by?
geology, relief and human activity
when did the ice age happen and where was sea level in this time?
- about 2 million years ago the world’s climate cooled and an ice age began that lasted until about 14,000yrs ago
- during this Ice Age sea level was at least 120m lower than it is today
how can earthquakes cause sea level change?
- earthquakes can cause vertical movements either side of faults; this can lead to an apparent rise or fall in sea level at the coast
how did the 2 different parts of Britain tilt?
- during the ice age 2mill to 14,000 yrs ago, much of the British Isles was covered with ice to a line just north of London and Bristol, known as the Thames Severn line.
- this additional weight on the crust pushed the northern part of the British Isles downwards (called compression), while the south, without the weight of the ice, rose slightly.
- at the end of the ice age the loss of weight in the north led to the British Isles tilting the other way: Scotland and the North rose (isostatic rebound) while the south sank.
- coastlines in scotland rose above sea level while coastlines in the south of england became submerged.
what are the potential positives of future coastal flooding?
new natural environment like salt marshes, which are important habitats for wildlife, being vital areas for migrating birds
what is eustatic change?
global changes in sea level due to changes in the atmosphere
what is isostatic change?
localised/regional changes in sea level due to movements of the Earth’s crust
what is a transgression?
advance in the sea/sea level rise
what is a regression?
retreat of the sea
what are the causes of transgressions/sea level rise?
- EUSTATIC CHANGE
> global
> ice caps melting
> happened between 10-6000 yrs ago
> will submerge existing landforms e.g. river/glacial valleys
> ↑ input of sediment
what are the causes of regressions/sea level fall?
- actually a rise in land level which seems like a fall in sea level
- ISOSTATIC CHANGE/rebound of land
- creates emergent landforms (e.g. relic cliffs)
- ice sheets sit on land = weight
- increased temperatures = ice sheets melt = weight removed = land rebounds faster than sea levels rise
what is happening in Southern Iceland?
- in Southern iceland we have an isostatic change in sea level because the ice sheets are reduced in weight and volume, and the land rebounds and rises up
- this relative drop in sea level happens in spite of a eustatic rise in sea level
what is the recent trend of global eustatic changes in sea level?
- over last 6000 yrs = quite a lot of stability which leads to civilisation progressing and development
- over the past 18,000 yrs change has been big - roughly 120m rise in sea level
- sea level has risen at a linear rate as the Devensian Ice melts
what is the recent trend of isostatic changes in sea level around the UK?
isostatic = changes in the level of land due to ice melting.
- localised e.g Scotland.
- from tectonic activity (eqs)
- from removal/addition of weight from land
emergent Scotland/north.
- becomes risen above sea level = DECOMPRESSION/ISOSTATIC REBOUND
South/kent starting to become submergent.
- 3.5mm/yr change difference
what is a long term natural reason for the changes in global temperature?
MILANKOVITCH CYCLES - most significant
- variations in the earth’s orbit and tilt
- long term climatic shifts (e.g. glacial cycles) are caused by astronomical events such as changes in the Earth’s axis and orbit
- affect the amount of solar radiation reaching the planet’s surface and its spatial/temporal distribution
- they operate on time scales that vary from 10,000-100,000 yrs with long glacial periods followed by shorter interglacials
what is the first short term natural reason for the changes in global temperature?
- CHANGES IN SOLAR OUTPUT= only in past 30 yrs have satellites been able to measure solar irradience accurately
- sun’s output varies over time
- there is a positive correlation between the number of sunspots and solar energy outputs
- solar output follows an 11 year cycle. the difference in energy output between max and min sunspot activity is only 0.1% - not enough to impact global climate significantly
- longer timescales = solar output is more variable
- difficult to isolate the impacts of this from other influences
what is the second short term natural reason for the changes in global temperature?
2: VOLCANIC ACTIVITY
- eruptions = huge amounts of volcanic ash and sulphur dioxide into stratosphere = potential to change the climate
- sulphur dioxide has a cooling effect and is more persistent. in atmosphere it is converted to sulphuric acid = sulphate aerosols
- these reflect solar radiaton back into space and lower temps in the troposphere
what are emergent features affected by?
- after their emergence, these landforms were no longer affected by wave processes
- however, they continue to be affected by weathering and mass movement
what is a raised beach?
- areas of former shore platforms that are left at a higher level than the present sea level
- they are often found a distance inland from the present coastline.
- behind the beach along emergent coastlines it is not uncommon to find abandoned cliffs w/ wave cut notches, caves and even arches and stacks.
- deposits are likely still sand and pebbles
what is a marine terrace?
- much larger scale landscape features than raised beaches which are quite small scale and localised at the base of relic cliffs
- terraces don’t necessarily have cliffs above them
- their formation is essentially the same as raised beaches - marine erosion during a previous period of higher sea level
- found as far as 50m higher up than raised beaches
what is a relict cliff?
- there is a sharp rise in the level of the land that marks the position of former coastal cliffs; now known as relic cliffs
- found behind the raised beach/marine terrace
what current processes could be modifying emergent landforms today?
- landforms aren’t affected by marine/wave processes yet are affected by weathering and mass movements
- cliffs arent undermined = slope will decrease in gradient and are far more stable
- people pressure and footpath erosion will alter landforms
- relic cliffs may become vegetated
what do emergent coastlines represent?
- coastlines that are now raised above present day sea-level.
- this means the coastline has experienced a relative fall in sea-level (marine regression)
where do emergent coastlines originate?
- such coastlines may have 2 origins:
- they may have formed at times when eustatic sea-level was higher, for example, during warmer interglacial periods
- they may have formed and become lifted by isostatic rebound. this is when there is a relative rise of the land above present day sea levels (e.g Scotland)
when do submergent coastlines occur?
- when sea level rises, submerging existing landforms
- due to climate change, thermal expansion of water, glaciers melt = ↑ volume of sea water
- eustatic change = global rise in sea level
what do submergent coastlines represent?
- coastlines that are now below present day sea level
- this means the coastline has experienced a relative rise in sea-level (marine transgression)
- they will have formed at times when eustatic sea-level was lower for example during colder glacial periods
what are the 3 examples of submergent features?
- glacial valley floods = fjords
- river valley floods = rias
- sediment “rolled” onshore by rising sea level = shingle beach
what is a ria?
- rias are drowned river valleys
- in hilly areas, e.g. cornwall and devon, river valleys that were previously at sea level become submerged
- the result is often a very large estuary at the mouth of a relatively insignificant river (or else sediment would quickly fill the ria)
what do rias look like?
- drowned river valley = formed as sea levels ↑
- lower part of river course may be drowned but higher land forming the tops of the valley side at middle/upper course remains exposed
- relatively shallow water becomes deep as you go towards centre because of an original river channel there
- valley sides are gently sloped
- can be found on the south coasts of Devon and Cornwall e.g. Helford Passage = formed during post glacial sea level rise (Flandrian Transgression)
- dendritic pattern (see pic)
- surrounding land used for farmland predominantly
how has sea level change (eustatic) changed rias?
- ↑ sea level = floods = flood plains created = makes centre deeper as original river channel was there
how are current geomorphic processes altering rias?
- rivers flooded = deposition at mouth - gradually filling up. causes sedimentation
- weathering = causes mass movement
- eustatic change
- coastal erosion
how can you tell a ria from a fjord?
ria:
- no hanging valleys/waterfalls
- dentritic pattern
- much smaller than fjords
- wave cut platforms, cliffs and mudflats
what marine/subaerial processes will be dominant in rias such as the Helford Passage?
- in upper course, lots of potential energy focused downwards (erosion concentrated on valley floor)
- abrasion, hydraulic action
- sea level ↑ is very sig.
- wave cut platforms and cliffs = marine processes are v influential
what is a fjord?
- drowned, U-shaped GLACIAL valleys
- originally formed by glacier eroding the valley sides and deepening the valley floor to create a glacial trough (u shaped valley)
- they are long inlets with very steep, straight valley sides
what do fjords look like?
- fjords are submerged glacial valleys.
- they have steep cliff like parabolic valley sides and the water is uniformly deep, often reaching over 1000m
- the Sogne Fjord is nearly 200km long (those in Scotland are less developed as the ice was not as thick during the glacial period)
- the U Shaped valley cross section reflects on the original shape of the valley itself
- they consist of a glacial rock basin w/ a shallower section at the end known as the threshold. this results from a lower rate of erosion at the seaward end of the valley where the ice thinned in warmer conditions
how has sea level change altered fjords?
due to the depth of the water that occupied fjords during the Flandrian Transgression, marine erosion rates remained high and the fjords were further deepened
how are current geomorphic processes altering fjords?
- weathering and sediment deposition
- weathering = mass movement = sides less steep
- rivers deposit sediment at end of fjord which will fill them in
- ongoing eustatic rise will continue to deepen them
- current coastal processes will cause a notch/beaches to form
what are shingle beaches?
- chesil beach is an example of shingle barrier beaches that have formed at time of lower sea levels and then rolled onshore as sea levels has risen
- the shingle refers to the size and shape of the material found on the beach (typically between 2 and 200mm and often rounded).
- it is likely the shingle is glacial in origin - glaciers bulldoze morraine and that is the sediment
what do shingle beaches look like?
- when sea level falls as the volume of land based ice grown, 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, wave action pushed these sediments onshore
- in some places, they beached at the base of former cliff lines, elsewhere they may form tombolos and bars
- occurs everywhere in the world
how has sea level change altered shingle beaches?
- eustatic change
- sea level rose at end of last glacial period which pushed deposited sediment onshore all over the world
how are current geomorphic processes altering shingle beaches?
- currents and LSD/prevailing wind action
- as they are composed of unconsolidated material they are especially vulnerable to modification
- with further sea level rises predicted, shingle may well be moved even further to NE at Chesil Beach
- destructive and constructive waves = causes erosion
