Physical processes 1 (Edexcel GCSE Geography A): Revision Notes
Physical processes 1
Introduction to coastal physical processes
Coastlines are continuously shaped through the interaction of three main physical processes: weathering, mass movement, and erosion. Understanding how each of these processes works and causes changes along coastlines is essential for comprehending coastal landscape formation.
These three physical processes work together as an interconnected system - weathering weakens the rocks, mass movement transports the weakened material, and erosion removes it entirely. This continuous cycle is what creates the diverse coastal landscapes we see today.
Weathering on the coast
Weathering is the process that breaks down rocks whilst leaving the broken-down material in the same place (in situ). This process helps to weaken coastal rocks and cliffs, making them more vulnerable to other coastal processes. There are three main types of weathering that operate along coastlines:
Chemical weathering occurs when rocks react with slightly acidic substances. A common example is limestone being dissolved through carbonation, where carbonic acid in rainwater reacts with calcium carbonate in the rock, gradually dissolving it away.
Biological weathering happens through the actions of living organisms. Plant roots can grow into cracks within rocks, gradually widening them as the roots expand. Similarly, animals such as piddocks (marine molluscs) can bore into rock surfaces, weakening the coastal structure over time.
Mechanical weathering involves physical forces breaking rocks apart without changing their chemical composition. Water enters cracks in rocks and when it freezes, it expands by approximately 9%, exerting pressure on the surrounding rock. Through repeated cycles of freezing and thawing, this process gradually causes rocks to split and break apart.
Worked Example: Freeze-Thaw Weathering
Step 1: Water enters cracks in coastal rocks during wet weather Step 2: Temperature drops below 0°C, causing water to freeze and expand by 9% Step 3: The expansion creates pressure that widens the existing cracks Step 4: Ice melts when temperature rises, and the process repeats Step 5: Over many freeze-thaw cycles, the rock eventually splits apart
Mass movement on the coast
Mass movement refers to the downhill transportation of weathered material under the influence of gravity. The rate and type of mass movement depends on several key factors:
- The type of material involved (clay, sand, rock fragments)
- The amount of water present in the material
- The specific type of movement occurring
Sliding represents one form of mass movement where loosened rocks and soil suddenly move down a slope. This typically occurs when material becomes unstable and blocks of rock or sediment slide along slip planes - these are surfaces of weakness within the cliff or slope structure.
Slumping is another important type of mass movement that frequently occurs on clay coastlines. During slumping, loose, water-saturated rocks move downward under gravitational force along curved slip planes. This process often creates a characteristic stepped appearance on cliff faces where multiple slump blocks have moved.
Mass movement is particularly active during periods of heavy rainfall when water saturates the cliff materials, adding weight and reducing the strength of the rock structure. This is why coastal landslides often occur during or shortly after storms.
How waves erode the coast
Wave erosion operates through four main mechanisms that work together to wear away coastal rocks and cliffs:
Abrasion occurs when breaking waves hurl sand, pebbles, and boulders against the coastal rocks during storms. This process acts like natural sandpaper, gradually wearing away the cliff face through repeated impact of these materials.
Hydraulic action involves the sheer force and impact of water hitting the coastline. This is particularly effective during storms when waves have greater energy. The water also compresses air trapped in joints and cracks within rocks, and when the wave retreats, this compressed air expands rapidly, helping to force the rocks apart.
Attrition happens when rocks and pebbles carried by waves collide with each other and break into progressively smaller pieces. Over time, this process reduces large boulders to smaller stones, then to pebbles, and eventually to sand-sized particles.
Solution represents the chemical action of seawater dissolving certain types of rocks. This process is particularly effective on limestone and chalk coastlines, where the slightly acidic seawater can gradually dissolve the calcium carbonate that forms these rocks.
These four erosional processes work simultaneously and are most powerful during storm conditions when waves have maximum energy. The combination of all four processes makes wave erosion one of the most effective agents of coastal change.
Understanding slumping in detail
Slumping represents a particularly important process on many British coastlines, especially those composed of clay materials. When rocks become saturated with water, they lose their structural integrity and become much heavier. The combination of increased weight and reduced strength means that gravity can overcome the material's resistance, causing it to move downward along curved failure surfaces called slip planes.
This process is especially common along clay coastlines because clay materials retain water easily and become very unstable when wet. The movement typically occurs as discrete blocks rather than individual particles, creating the characteristic stepped appearance often seen on unstable cliff faces.
Worked Example: Slumping Process on Clay Cliffs
Step 1: Heavy rainfall saturates clay materials in the cliff Step 2: Water increases the weight of the cliff material and reduces its strength Step 3: The saturated clay can no longer support its own weight Step 4: Material begins to move along a curved slip plane under gravity Step 5: A discrete block of cliff slides down, creating a stepped cliff profile Step 6: The process repeats as more material becomes saturated
Key Points to Remember:
- Weathering weakens rocks in place through chemical reactions, biological activity, and physical forces like freeze-thaw cycles
- Mass movement transports weathered material downhill under gravity, with sliding and slumping being key processes on unstable coastlines
- Wave erosion uses four main processes - abrasion (throwing debris), hydraulic action (water pressure), attrition (rocks hitting each other), and solution (chemical dissolution)
- Clay coastlines are particularly vulnerable to slumping because clay retains water and becomes unstable when saturated
- These processes work together to continuously shape and change coastal landscapes over time