4 Sea level change

Question 1

What is eustatic sea level change?

Answer 1

Global rise or fall in sea level/global ocean store associated with climatic change and variations in mean global temperatures

Question 2

What are the physical factors that can affect global temperature and volume of water in oceans?

Answer 2

• Variations in the Earth’s orbit around the Sun, typically every 400,000 years
• Variations in the amount of energy produced by the Sun, with a solar maximum every 11 years or so
• Changes in the composition of the atmosphere due to major volcanic eruptions which reduce incident solar radiation
• variations in the tilt of the Earth’s axis, occurring every 41,000 years

Question 3

How does a decrease in global temperature influence eustatic sea level change?

Answer 3

More snow
- If it does not melt it will accumulate until it becomes compressed into ice
- Water stored as glaciers and ice sheets rather than in oceans

Thermal contraction
- Molecules have less energy and stay closer together meaning the water becomes denser and occupies reduced volume (1C fall = 2m fall)

Question 4

How does an increase in global temperature influence eustatic sea level change?

Answer 4

• Melting ice sheets and glaciers cause more water to enter the Earth’s oceans
• Thermal expansion of warmer water

Question 5

Describe the evidence in history for sea level change

Answer 5

• 130,000 years ago, Tyrrhenian inter-glacial period, global mean annual temperatures were almost 3C higher and sea level 20m higher
• Temp fell during Riss glacial, 7C lower, therefore, less water returned to the ocean so 83m lower than today

Question 6

What is Earth’s natural climate variability?

Answer 6

Since Pleistocene Epoch, 2.4 million years ago, Earth has been in an Ice age with long, cold, glacial periods interspersed by shorter, warmer interglacial periods (current Holocene interglacial)

Question 7

Why does Earth’s climate experience natural variability?

Answer 7

Milankovitch cycle, volcanic eruptions, sun spots

Question 8

Describe how milankovitch cycles influences Earth’s climate

Answer 8

Cyclical and predictable variations in the shape of Earth’s orbit and the nature of the Earth’s axial tilt
- Eccentricity of orbit –> circular to more elliptical over 100000 years
- Tilt of axis –> varies 22.5-24.5 over 41000 year cycle
- Precession of Earth which varies on a 23000 cycle
Affects amount of insolation and global temps

Question 9

Describe how volcanic eruptions influences Earth’s climate

Answer 9

Aerosols from eruptions can reflect incoming solar radiation, reducing global temperature
No regularity or cycle to volcanic activity (unlike milankovitch or sun spots)

Question 10

Describe how sun spots influences Earth’s climate

Answer 10

11 year cycle in sun spots
Areas of intense solar activity activity on the sun’s surface
A peak sun spot activity receives more solar radiation

Question 11

What is isostatic sea level change?

Answer 11

Local relative rise or fall in sea level associated with an actual rise or fall in the level of the land

Question 12

Describe how isostatic sea level change works (not on syllabus)

Answer 12

Tectonic uplift lead to rapid rise of land
Post-glacial isostatic rebound (more gradual)
- Weight of ice sheet compresses land
- Ice age ended and sheet melted, weight lifted and the land rebounded upwards
- South-east England dips into the sea

Question 13

What are landforms of emergence?

Answer 13

Relative fall in sea level
Landforms shaped by wave processes during times of high sea level are left exposed when sea level falls
As a result, may be found inland away from the modern coastline

Question 14

What is a raised beach?

Answer 14

Former shore platforms left at a higher level than the present sea level
Often found distance inland from coastline

Question 15

What is an abandoned cliff?

Answer 15

Found behind the beach along emergent coastlines with wave-cut notches, caves and even arches and stacks

Question 16

Raised beach case study

Answer 16

Isle of Portland, Dorset
Height of 15m above present-day sea level
Formed 125000 BP during Tyrrhenian Interglacial period (sea levels higher)
Portland limestone eroded by hydraulic wave action (1m/year)

Question 17

Abandoned cliff case study

Answer 17

King’s Caves, Isle of Arran, Scotland
Raised beach backed by sandstone abandoned cliff
Extensive cave formation
Active coastline 6000BP, but post-glacial isostatic rebound has raised shoreline features 8m above current sea level

Question 18

What processes are landforms of emergence affected by?

Answer 18

Raised beaches and abandoned cliffs are no longer affected by wave processes and marine erosion
Still shaped by weathering and mass movement

Question 19

How is the abandoned cliff on Isle of Portland by modified?

Answer 19

1-1.5m layer of frost-shattered limestone debris was deposited during periglacial conditions in the Wurm glacial
Simultaneously, the cliff face experienced freeze-thaw weathering, leading to rockfall from the cliff face

Question 20

What has the change of conditions led to on landforms of emergence?

Answer 20

During the current Holocene interglacial, warmer and wetter conditions have led to the growth of vegetation on much of the exposed rock, making it difficult to identify features such as raised beaches

Question 21

What has weathering and mass movement led to on landforms of emergence?

Answer 21

Eg carbonation of the limestone abandoned cliffs due to carbonic acid in rain water, or biological weathering as plants colonise the raised beach
Degradation of features (softer and more blended)

Question 22

How could sea level rise affect landforms of emergence?

Answer 22

Present and future sea level rise could see these emergent features once more at sea level, at which time they would be subject to wave process, become active beaches, shore platforms and cliffs

Question 23

What is a fjord?

Answer 23

Submerged glacial valleys formed as sea level rises

Question 24

What are the features of a fjord?

Answer 24

• Steep, cliff-like valley sides and uniformly deep water, often reaching over 1000m
• U-shaped cross section reflects the original shape of the glacial valley
• Shallower section at mouth (threshold) and snout (end) as glacier thinner and less erosive and deposition of moraine (glacial debris) may have raised the land level
• Straight routes, glacial erosion create truncated spurs as they bulldozed through

Question 25

Due to their deepness, whatt can fjords support and what may this lead to?

Answer 25

High energy waves
Ongoing marine erosion may deepen them further
Or may start to be infilled will sediments
Milford Sound, New Zealand, meltwater river runoff contains high levels of glacially-eroded sediment from the Southern Alps, which its deposits in the fjord

Question 26

What is a ria?

Answer 26

Submerged river valleys, formed as sea level rise
Lower part of river’s course and floodplains completely drowned
Higher land (tops of valley sides and middle/upper course un-submerged)

Question 27

What are the features of a ria?

Answer 27

Relatively shallow cross section, deeper towards the centre
Gently sloping, asymmetrical valley sides - fluvial erosion and subaerial weathering and mass movement of the valley sides
Asymmetrical as common in valleys of meandering rivers
Winding plan view (reflect original course)
Long profile - smooth, concave with water of uniform depth

Question 28

What are beneath present day rias?

Answer 28

Buried channel of older river which flowed at lower levels, when sea levels were lower than today (Wurm Glacial Maximum)
Low sea levels so river had more gravitational potential energy as base level was lower so had more energy to erode deep, gorge-like channels

Question 29

What happens to rivers as sea level rose during the Flandrian transgression ?

Answer 29

Rivers lost this extra gravitational potential energy, more deposition filling in the deep channels
Could be accelerated by washing in of sand from the rising Atlantic Ocean - large sand bars still seen

Question 30

How do shingle beaches form?

Answer 30

When sea level falls as the volume of land-based ice grows, large areas of ‘new’ land emerge from the sea
Sediment accumulates on this deposited by rivers, meltwater streams and low-energy waves
As sea levels rose during the Flandrian Transgression, wave action pushed this sediment onshore. They beached at the base of former cliff lines or formed tombolos and bars.

Question 31

How did the tombolo at Chesil Beach form?

Answer 31

Sediment carried into the English Channel by meltwater during the Wurm glacial accumulated in locations such as Lyme Bay
As sea levels rose, the sediment was carried NE by SW prevailing winds and resultant waves
Became attached to Isle of Portland at one end and the mainland

Question 32

What processes modify fjords and rias?

Answer 32

• Wave processes acting on the shoreline
• Sub-aerial processes in todays climatic conditions on valley sides. Can trigger further processes eg mass movement and a reduction in steepness of valley sides
• More marine erosion due to stormier conditions and larger waves

Question 33

How will sea level impact landforms of submergence?

Answer 33

• Sea levels predicted to rise by 0.6m in the next 100 years, water depth in rias and fjords will increase

Question 34

How will shingle beaches be modified by processes?

Answer 34

Composed of unconsolidated material so are especially vulnerable
The tombolo at Chesil Beach has been affected by longshore drift processes and this will continue
- Shingle of Chesil Beach may be driven further NE (currently moving at 17cm/year)
- A breach of the tombolo is highly likely in future storm events
In 2009, 1m piece of clay eroded from in front of the beach and washed up onto it in a winter storm. This addition will be more common with higher sea levels and more storm events