3.b - submergent landscapes Flashcards
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. Salcombe = formed during post glacial sea level rise (Flandrian Transgression)
dendritic patter (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 there1
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 sea ward end of the valley where the ice thinned in warmer conditions
a good example is Milford Sound, New Zealand
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 and start bay are examples 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
