C -> 1.6 - 2.2 Flashcards
Sognefjorden, Norway facts
- submergent coastline as a result of eustatic sea level rise since LGM
- largest fjord system in Norway - extends 200km inland from west coast
- alignment corresponds to fracture system in bedrock (exploited by rivers, then glaciers)
- main Fjord basin is at a depth of 800m below present sea levels (although deepest point is 1308m below)
- entrace at seaward end is shallower (100-150m) due to rock bar (threshold)
Eustatic SL Change - post-glacial:
- sea level rise approx. 125m since the last glacial maximum (LGM) - 21000 years ago
- meltwater pulse around 14k years ago - most ice melted by 6k years ago and sea level is now only slowly rising.
Eustatic SL Change - tectonic:
- faulting and folding of crust can create displacement of seabed (eg Sumatra since 2004 EQ, or Dalmatian coast in Croatia)
submergent landforms examples
Fjords
Rias
Dalmatian Coast
Fjords:
- Fjords are drowned glacial valleys - Norway. Glacial erosion often deep cut into landscape, so fjord is deeper than adjacent sea, they have a shallow entrance where there is a submerged lip formed by the ridge of terminal moraine
Rias:
- drowned valley due to river erosion, a section of river valley flooded by the sea, making it much wider than it would be based on river flowing into it. Common in periglacial areas, river eroded steep sided v shaped valleys into frozen landscape giving ria a v shaped cross section when valley flooded. Kingsbridge estuary on south Devon coast is a 6mile long ria.
fjords and rias being formed? (part 1)
- pre glacial rivers exploits fractures and faults in Norwegian bedrock, creating v-shaped river valleys
- ice sheets move forward during the Quarternary (ice age), and because the ground is permanently frozen, rivers can only erode downwards to cut steeper v-shaped valleys
- glacial erosional processes (plucking and abrasion), create deep, wide, steep U-shaped valleys.
fjords and rias being formed? (part 2)
- glacial ice advances, pushing river valley eroded material forward, depositing it near the sea entrance to the modern day fjord
- ice advances further down the river valley but not the coast, leaving a lip
- ice begins to retreat and sea levels begin to rise
fjords and rias being formed? (part 3)
- rock lip is breached by seawater and U shaped valleys floods
- u shaped valleys become over deepened; the main valley floor lies 800m below current sea levels
- sea levels continue to rise ~ 125m higher than LGM extent; leaving a large fjord system
thermal expansion definition
water particles vibrate faster, so the volume of water expands
louisiana:
- mississipi river flows into the gulf mexico
- alluvial, depositional coast - marshes swamps and barrier islands
- huge levees & engineering projects protect - New Orleans to the north
- Vulnerable to: storm surges. global sea level rise, isostatic tectonic uplift
eustatic (global sea level rise) - fall:
- happens during glacial periods
- ice sheets are formed on high latitude land
- evaporated water is locked up as ice
- less water in the sea
eustatic (global sea level rise) - rise:
- interglacial periods - ice sheets on land melt
- melted water returns to the sea, so sea level rises
- global temperatures also cause thermal expansion - increasing volume of ocean water, ie. global sea level rise
isostatic (localised) fall:
ice sheets melt, and land surface slowly rebounds upwards - lifting the land surface out of the sea - sea levels appear to fall.
north goes with the motion of northern glaciers
isostatic (localised) rise:
- during glacials: land is compressed by the weight of glaciers, causing sea level to appear to rise
- during interglacials: ice melts leads to sediment being deposited in large river deltas, leading to increased weight/ crustal sag, and delta subsidence - sea level appears to rise
submergent coasts come from
relative rise in sea level, changing the shape/ form of the coastline
relative fall in sea level results in
emergent coastline, eg. offshore bars, raised beaches, fossil cliffs
the other key factors are
wave types, topography, rock structure/ deposition
explain why Englands south coast is shrinking
isostatic sea level rise, land titles, perception that sea level in the south coast is higher
post glacial rebound in USA:
post glacial adjustment is the way in which the northern part of the US/Canada is rebounding upwards (relative sea level fall), whilst the southern of the USA is tilting downwards (relative sea level rise)
historical land loss in the USA
- prob because of eustatic SLR and isostatic tilt downwards in the southern USA. However, it could also be the case the global SLR is caused by thermal expansion
future land loss in the USA
- the Mississippi Delta is subsiding (sinking down) - partly because of the weight of the sediment pushing down on itself, and also the weight of building on that relatively flat land
how do deltas form?
because of deposition of river (fluvial) material.
this accumulation of material is called accretion
New Orleans:
the impact of the long term SL change requires management: levees are built on the edge of the river. the city is built ‘below sea level’
low pressure storm surges, particularly from hurricanes can raise local sea level over the levees
bedrock + superficial rock
- bedrock = older. rock, usually underneath superficial rock which is the younger, newly deposited sediments
north Antrim coast overview
- predominantly rocky coastline in the NI, UK
- history of volcanic - slow cooling molten basalt erupted 56-62 Million years ago - covers most of the Antrim coast
- has preserved softer sedimentary rocks underneath - laid down by warm shallow ocean
- subsequently covered by ice sheets during quarternary
unconsolidated material
weakest and most easily eroded (due to lack of strength and subsequent exploitation by water). they tend to slump when saturated. also consider if there are reactive minerals in the rock (eg. calcite)
sedimentary rocks are
composed of particles or pieces of other rocks eroded and laid down in layers over time, like chalk
igneous rocks are
formed as molten rock cools and solidifies, like basalt
metamorphic rocks are
rocks that have been subjected to intense heat/ pressure, like schist
age of rock:
there are exceptions to the general order of resistance to erosion, but we must consider age:
- ancient limestone have been compressed over millions of years so are more resistant
- recently erupted volcanic lava flows and tephra tend to be weaker and easier to erode
permeability:
permeability is important as groundwater flow can weaken rocks by exploiting joint structures and binding materials
- it can also create high water pressures within cliffs, reducing stability
- permeable rocks include sandstones and limestones
- impermeable rocks include clay, mudstone, as well as most igneous and metamorphic rocks
rock cycle - simple
ign rock from lava cools. eroded from weathering + biological processes, river carry unconsolidated sediment downstream and layer them. weight + water pressure cause them to be compressed - forming sedimentary rock. when Sed rock is subduct it experiences high heat and pressure, which causes it to morph and become metamorphic
geology:
- igneous is hardest
- met is next, crystalline harder, clastic weaker
- sedimentary is weakest
- oldest harder, young weaker
isostatic change
- rebound because ice melts
- crust is distorted, leaving anticlines and synclines
permeability
water runs through rock, BUT
removes cement that holds the sediment together, creating high water pressure and weight
joints -
brittle fracture surface in rocks along which little or no displacement has occurred
dips -
angle between horizontal plane and the inclined surface of the rock
fault -
fracture or rupture in the rock strata due to strain in which displacement is observable
folding -
isostatic rebound distorts and reshapes, leaving anticlines and synclines
porous rock -
has larger spaces between grains of rock, allowing it to hold more water
permeable rock -
have linked spaces between grains of rocks, allowing water to flow through
structure is
disposition of rock and its bedding planes,
this determines strength and surface area exposed to waves/ sub aerial erosion
- also is the joining of joints, which determines physical resistance of rocks to erosion
exposed -
glaciers/ rivers stripping away softer rock
igneous extrusive basalt -
volcanic lava, flowing over the top, eg, upper basalts
sedimentary - laterite, soil -
eroded and weathered from the basalt
igneous extrusive basalt -
thick volcanic lava, funnelled into valleys, eg Giants Causeway, cools by sea/ rivers = column joints
sedimentary - chalk -
chalk eroded by rivers into valley/ hills, SLR, dead microorganisms accumulated on sea floor
sedimentary - sandstone -
- continental drift - NI was at the equator
horizontal dip:
vertical or near vertical profile with notches reflecting strata that are more easily eroded, if these are eroded inwards, while harder rock strata remain unchanged, could cause instability and could lead to the top of the cliff collapsing down
seaward dip, high angle:
sloped, low angle profile with one rock facing the sea; vulnerable to rock slides down the dip slope.
Largely concordant, but at an angle, so the rock type will determine the rate of erosion and the angle will affect the amount of rock slides
seaward dip, low angle
profile may exceed 90o, prodding areas of overhanging rock, very vulnerable to rock falls
landward dip:
steep profiles of 70-80o producing a very stable cliff with reduced rock falls. very dependent on the rock type of the strata that are close to sea level, as this will determine the rate of coastal erosion
Types of rock and link to resistance
igneous, sedimentary and metamorphic have varying levels of resistance to each of these factors, as seen in the north Antrim coast.
Lithology factors
- consolidation of material
- rock type
- age of rock
- bedrock vs superficial
Isostatic change may be due to
May be due to post glacial adjustment, accretion (deposiiton = rise in local land level), tectonics.
Fall in local land level produces a rise in local sea level, due to:
post glacial adjustment, subsidence - deposition of sediment like alluvial deposits in large river deltas, mean weight of sediment deposition overcomes threshold and leads to very slow crustal sag, and delta subsidence - Mississippi, can also be caused by lowering of water table from human groundwater extraction or CC, can lead to land subsidence as pore water pressure removed.
Tectonics on eustatic =
rising magma at constructive plate margin/ hotspots lift overlying crust, reducing capacity of ocean and producing eustatic SLR, uplift of crustal plate reduced Indian Ocean capacity = causing 0.1mm eustatic rise in SL
Tectonics on isostatic
Folding = isostatic rise in SL for anticlines and fall for synclines. E.g. alpine folding at Eurasian - African destructive plate boundary produced an isostatic fall of 60cm around Croatia. Faulting can uplift, faulting can subside.
