Geology and Sea Level - Edexcel A-Level Geography
Sub-Aerial Processes (Weathering)
Definition: Weathering refers to the process that weakens a cliff in situ (in its original place without moving it). This process facilitates marine erosion by making it easier for the sea to remove rock debris.
Mechanical (Physical) Weathering: - Freeze-Thaw: This occurs when water enters cracks in the rock and freezes. Upon freezing, the water expands by . This expansion exerts significant outward pressure on the rock, which eventually shatters it. This process is most common in cold climates. - Salt Crystallisation: This involves seawater evaporating within the pores of the rock. As the water evaporates, salt crystals are left behind; these crystals grow and exert physical pressure on the rock structure. This is a common occurrence in the "splash zone" of the coastline.
Chemical Weathering: - Carbonation: Rainwater absorbs from the atmosphere to form a weak carbonic acid. This acid reacts chemically with the calcium carbonate found in rocks like Limestone and Chalk, causing the rock to dissolve. - Oxidation: This occurs when oxygen reacts with iron minerals within the rock, a process similar to "rusting." This chemical reaction causes the rock to become brittle and crumble.
Biological Weathering: - Plant Roots: Roots from plants grow into the joints of the rock and exert force as they expand, prying the rock apart. - Rock Borers: Certain animals, such as molluscs (specifically Piddocks) or sea urchins, drill into the rock to create shelter, physically breaking down the rock surface.
Mass Movement
Definition: Mass movement is the downslope movement of rock or soil under the direct influence of gravity. It is typically triggered by the saturation of the ground or the undercutting of cliff bases by marine action.
Rockfall: This is the rapid falling of rock fragments from a vertical cliff face. It is usually caused by freeze-thaw weathering and results in the formation of a talus slope at the base of the cliff.
Rotational Slumping: Recognized as the most common term used in exams. This involves material moving down a curved slip plane. It is frequently observed in clay-based geologies (e.g., the Holderness coast) when pore water pressure is high. This process creates a distinct "stepped" cliff profile.
Landslide: This refers to the rapid movement of a mass of material down a flat or linear slip plane. This often occurs in locations where rock layers dip toward the sea.
Mudflow: This happens when fine-grained sediment becomes heavily saturated and begins to flow downslope in a liquid-like manner.
Eustatic vs. Isostatic Change
Exam Tip: A mnemonic to remember the difference is Eustatic = Everywhere (Global).
Eustatic Change (Global): - Focus: A change in the total volume of water within the world's oceans. - Causes: The melting of ice caps, which causes sea levels to rise, or the thermal expansion of water as it warms.
Isostatic Change (Local): - Focus: A change in the level of the land relative to the sea. - Causes: During ice ages, the immense weight of ice sheets pushed the land down. Following the melting of the ice, the land "rebounded" or rose back up, a process known as Isostatic Rebound. - UK Context: The North and West of the United Kingdom are currently rising due to rebounding, while the South and East are sinking or "tilting" down.
Emergent and Submergent Coastlines
Emergent Coastlines (Falling Sea Levels): - Primary Cause: Isostatic rebound. - Raised Beaches: These are former beaches that are now stranded above the current high-tide mark. - Relict Cliffs: These are ancient cliffs located inland, which often still feature caves and other coastal erosional landforms.
Submergent Coastlines (Rising Sea Levels): - Primary Cause: Eustatic rise in sea level. - Rias: These are drowned river valleys that maintain a V-shape. They are common in areas like South Devon and Cornwall. - Fjords: These are drowned glacial valleys characterized by a U-shape and extreme depth. They are common in Norway. - Dalmatian Coasts: These occur where valleys that run parallel to the coast are flooded, resulting in a chain of offshore islands. This is associated with concordant geology.
Concordant and Discordant Coastlines
These classifications are based on Lithology (rock type) and Structure (the arrangement of rock layers).
Discordant Coastlines (Atlantic Type): - Structure: Rock layers run perpendicular (at a right angle) to the coastline. - Resulting Landforms: This arrangement creates Headlands and Bays. Softer rocks, such as clay and sand, erode more quickly to form bays, while harder rocks, like granite or limestone, resist erosion and stick out as headlands.
Concordant Coastlines (Pacific Type): - Structure: Rock layers run parallel to the coastline. - Resulting Landforms: This creates a relatively straight coastline unless the outer layer of hard rock is breached. If a breach occurs, the sea erodes the softer rock situated behind the hard layer to form Coves (e.g., Lulworth Cove).
Sand Dune Formation (Psammosere)
A sand dune system is defined as a "plant succession," which is the change in vegetation over time in a specific environment.
Requirements: Formation requires a large supply of sand, a large tidal range to allow the sand to dry out, and a consistent onshore wind.
Embryo Dunes: These are small bumps of sand that form around obstacles such as driftwood or stones. The conditions are characterized by very high salinity and dryness.
Fore/Yellow Dunes: Pioneer species, most notably Marram Grass, begin to grow. Marram Grass has long roots that help stabilize the sand. These dunes are termed "yellow" because there is still a significant amount of visible, bare sand.
Grey Dunes: As Marram Grass dies, it decomposes and adds humus (organic matter) to the sand. This turns the sand grey and makes it more soil-like. This improved substrate allows more diverse plants, such as gorse and heather, to grow.
Dune Slacks: These are depressions found between dunes where the depth reaches the water table. This allows moisture-loving plants to thrive.
Climax Community: This is the final stage of the succession where the soil is rich and stable enough to support large trees, such as Oak or Pine woodland.
Rivers as a Sediment Source
Rivers serve as a major input into the coastal Sediment Cell through Fluvial Deposition.
The Nile: Historically a prograding delta; the Aswan Dam now traps sediment, leading to coastal erosion of up to .
Estuaries and Salt Marshes (Haloseres)
Flocculation: A chemical process where saltwater causes river clay particles to clump together and sink.
Succession: Mudflats are colonized by salt-tolerant plants like Salicornia, trapping sediment to form a Salt Marsh.
Submergent Landforms: Rias
Formation: Drowned V-shaped river valleys caused by eustatic sea-level rise.
Description: Wide at the mouth and narrow inland; unlike a Fjord, a Ria is purely fluvial/river-shaped.
Example: Kingsbridge Estuary in South Devon.
Deltas
Condition: Formed when river sediment supply exceeds the rate of removal by waves and tides.
Arcuate Delta: Fan-shaped, such as The Nile.
Cuspate Delta: Tooth-shaped, such as The Tiber in Italy.
Bird’s Foot Delta: Sediment fingers extend outward, such as The Mississippi.
The Drainage Basin Link
Hjulström Curve: Explains how river velocity dictates the transport of sediment, from large boulders to fine silts.
Deforestation: Increases soil erosion and river sediment, causing Spits and Deltas to grow.
Dams: Lead to "sediment starvation" and rapid coastal recession downstream.
The hazard risk equation is as follows:
Risk (R) = (hazard (H) x vulnerability (V)) / capacity to cope
a disaster occurs when it actually causes an impact
Capacity to cope means a communities resilience in coping with a hazard
Factors that increase vulnerability:
inequality (unequal access to housing, education, health, and jobs)
Poor governance, corruption, lack of preparedness, planning, and regulations
Older, young people and women(?) are disproportionately affected by disasters
Highly populated areas
Rapid urbanisation
Low accessibility
Weak buildings and infrastructure (linking to preparedness)
PAR model:
root causes: limited access to power and resources, poor political and economic systems
Dynamic pressures: lack of skills, training, and investment , lack of free press, rapid population growth, urbanisation, and deforestation, high debt.
Unsafe conditions: fragile environment, economy and society, poor quality, low income housing
These two phenomena are part of the ENSO (El Niño Southern Oscillation) cycle.
1. Normal Conditions (The Baseline)
To understand the changes, you must first know the "normal" state of the Pacific:
• Winds: Strong Trade Winds blow East to West (from South America toward Australia).
• Water: These winds push warm surface water toward Australia/Indonesia, creating low pressure and heavy rain.
• Upwelling: In South America (Peru), cold, nutrient-rich water rises from the deep to replace the warm water pushed away. This creates high pressure and dry weather.
2. El Niño (The "Warm" Phase)
• The Change: Trade winds weaken or reverse direction (blowing West to East).
• The Shift: The pool of warm water shifts back toward South America.
• Impact on South America (Peru/Ecuador):
◦ Low pressure moves in, leading to extreme rainfall and flooding.
◦ The cold upwelling stops, killing off fish stocks (economic impact).
• Impact on Australia/South-East Asia:
◦ High pressure moves in, leading to severe drought and bushfires (e.g., Australia’s "Big Dry").
◦ Water Insecurity: Reservoirs dry up, and crop yields fail.
3. La Niña (The "Extreme Normal" Phase)
• The Change: Trade winds become exceptionally strong.
• The Shift: Warm water is pushed even further West, piling up around Australia/Indonesia.
• Impact on Australia/South-East Asia:
◦ Extreme low pressure leads to catastrophic flooding and tropical cyclones.
• Impact on South America:
◦ Extreme high pressure leads to even drier conditions and colder-than-usual sea temperatures.
4. Why it matters for Water Insecurity (A-Level Context)
• Teleconnections: ENSO events don't just affect the Pacific; they have global "knock-on" effects (e.g., El Niño can cause weaker monsoons in India and droughts in Ethiopia).
• Predictability: While we know the cycle happens every 3–7 years, its intensity is hard to predict, making it difficult for farmers to plan for water storage.
• Climate Change: Scientists are debating whether global warming is making El Niño events more frequent and more intense, which would lead to more "permanent" water insecurity in the Southern Hemisphere.