Unit 6: Aeolian, coastal and karst processes

I. Orientation

Geomorphology explains how Earth’s surface is shaped by the interaction of processes, materials, energy and time. In this unit, wind, waves and moving water, and groundwater are the main agents. Their effects depend on environmental conditions such as climate, rock type, vegetation, relief, sea level and human activity.

• Process: A physical, chemical or biological action that modifies landforms; examples include abrasion, hydraulic action, solution and deposition.
• Geomorphic system: An interacting system of inputs, transfers, stores and outputs; sediment may be eroded from a cliff, transported by waves and deposited on a beach.
• Energy and competence: Moving air, water and groundwater require sufficient energy to entrain or transport particles; competence is the largest particle size an agent can move.
• Erosion, transportation and deposition: Erosion removes material, transportation moves it, and deposition occurs when energy falls below the level needed to carry sediment.
• Equilibrium and adjustment: Landforms often adjust toward a balance between sediment supply, transport and removal, although storms, floods and sea-level change can disrupt it.
• Scale and context: Processes operate from individual grains and joints to whole coastlines and drainage basins, over timescales from hours to millions of years.

II. Aeolian processes and landforms — wind-shaped environments

Aeolian geomorphology concerns the erosion, transport and deposition of sediment by wind. It is most effective where sediment is dry, loose and exposed, especially in deserts, beaches, semi-arid regions and areas with sparse vegetation.

A. Aeolian processes and landforms

Aeolian processes produce distinctive erosional and depositional forms through wind stress, sediment movement and spatial changes in wind speed.

• Wind erosion: Wind removes sediment by deflation and wears exposed surfaces by abrasion.
  • Deflation: Turbulent air lifts or rolls fine particles from the surface, producing hollows and desert pavement.
  • Abrasion: Sand grains strike rock surfaces, polishing or grooving them; ventifacts may develop faceted faces.
• Threshold velocity: Particles begin moving when wind shear exceeds the threshold needed to overcome gravity and friction; dry sand commonly moves at lower speeds than gravel because gravel is heavier.
• Transportation modes:
  • Suspension: Silt and clay remain airborne and may travel hundreds of kilometres.
  • Saltation: Sand grains follow short hops, commonly 0.1–1 metre high, striking other grains and initiating further movement.
  • Surface creep: Larger grains roll or slide when struck by saltating particles.
• Depositional landforms: Deposition occurs where wind velocity falls, surface roughness increases or obstacles trap sediment.
  • Dunes: Mounds of wind-blown sand have a gentle windward slipface and a steeper leeward slipface, commonly about 30–34°.
  • Barchans: Crescent-shaped dunes form where wind direction is consistent, sand supply is limited and the horns point downwind.
  • Linear and star dunes: Linear dunes develop under two dominant wind directions, while star dunes form where winds vary from several directions.
  • Loess: Wind-blown silt forms thick, porous deposits; China’s Loess Plateau contains deposits several tens of metres thick.
• Worked example: If a steady wind blows across a dry beach, saltation moves sand inland. Vegetation or a fence reduces wind speed, causing deposition and initiating a foredune; continued accumulation creates a larger dune.

B. Applications and limitations

The effects of wind are controlled by sediment availability, vegetation and surface moisture, so aeolian landforms cannot be interpreted from wind direction alone.

• Environmental significance: Dunes store sediment and protect inland areas from storm waves; loess soils can be highly fertile but are vulnerable to gullying and wind erosion.
• Human impacts: Overgrazing and cultivation remove vegetation, exposing soil to deflation and creating mobile dunes; windbreaks and replanting help stabilize surfaces.
• Interpretive limitation: Similar dune shapes may result from different wind regimes, and modern vegetation can obscure former aeolian activity.
• Measurement: Wind speed is recorded in metres per second, sediment size in millimetres, and dune migration in metres per year; these measurements distinguish active from stabilized systems.

III. Coastal processes and landforms — interaction of land and sea

Coastal geomorphology examines the constantly changing boundary between land and ocean. Waves, tides, currents, weathering, sediment supply, sea-level change and geological structure combine to produce erosional and depositional coasts.

A. Coastal processes and landforms

Coastal landforms arise from the transfer of wave energy, the movement of sediment alongshore and the balance between erosion and deposition.

• Wave generation and energy: Wind transfers energy to the sea; wave energy increases with wind speed, duration and fetch, the uninterrupted distance over which wind acts.
• Wave erosion:
  • Hydraulic action: Water and compressed air force open joints and cracks; repeated pressure can detach blocks.
  • Abrasion or corrasion: Rock fragments hurled against a cliff act as tools, especially at the wave base.
  • Attrition: Sediment collides and becomes smaller and rounder.
  • Solution: Chemically soluble rocks such as limestone dissolve in seawater.
• Erosional landforms: Wave attack enlarges weaknesses into caves; caves may develop into arches, arches collapse to form stacks, and stacks may be reduced to stumps. Headlands and bays result where resistant and weak rocks alternate.
• Wave refraction: Waves slow in shallow water near a headland and concentrate energy there, while energy is dispersed in bays. This encourages headland erosion and bay deposition.
• Hydraulic conditions: Destructive waves are steep, have strong backwash and remove beach material; constructive waves have strong swash and weaker backwash, building beaches.
• Sediment transport: Longshore drift moves sediment when waves approach obliquely. Swash carries material up the beach at the wave angle, while backwash returns it downslope.
• Depositional landforms:
  • Spits: Longshore drift extends sediment beyond a change in coastline direction; recurved ends may form where wind or currents vary.
  • Bars: A ridge of sediment may connect a headland or cross a bay, sometimes enclosing a lagoon.
  • Beaches: Constructive wave action deposits sand or shingle in the zone between normal low and high water.
• Tides and currents: Tidal range influences the vertical zone affected by waves, while tidal currents redistribute sediment through estuaries and tidal inlets.
• Worked example: Where longshore drift carries sand toward an estuary, a spit may grow across the estuary mouth. Reduced wave energy behind the spit encourages mudflat and salt-marsh deposition.

B. Applications and limitations

Coastal landscapes reflect both natural adjustment and human intervention, making sediment budgets essential for management.

• Sediment budget: A coast is in equilibrium when inputs approximately equal outputs. Dams, harbour walls and beach nourishment alter this balance.
• Engineering effects: Groynes trap sediment on their updrift side but may starve downdrift beaches; sea walls protect property but can increase reflected wave energy and beach scour.
• Sea-level change: Rising relative sea level promotes coastal inundation and erosion; falling sea level may expose former shorelines and produce raised beaches.
• Geological control: Resistant rocks form cliffs and headlands, while weak rocks erode into bays; jointing, bedding and fault lines determine the shape and rate of retreat.
• Hazard management: Managed realignment allows selected low-value land to flood and create intertidal habitat, whereas hard engineering is more suitable where infrastructure is concentrated.
• Interpretive limitation: A single storm can temporarily alter a beach profile, so long-term monitoring is needed to separate seasonal change from persistent shoreline retreat.

IV. Karst processes and landforms — limestone landscapes

Karst geomorphology develops mainly on soluble rocks, especially limestone, where groundwater enlarges fractures by carbonation and solution. It is strongest where rainfall is abundant, soil contains carbon dioxide and rock permeability allows water to circulate underground.

A. Karst processes and landforms

Karst landscapes result from the chemical removal of carbonate rock and the collapse or deposition of material in subterranean drainage systems.

• Carbonation and solution: Rainwater absorbs carbon dioxide from the atmosphere and soil, forming weak carbonic acid.

CO₂ + H₂O ⇌ H₂CO₃
CaCO₃ + H₂CO₃ ⇌ Ca(HCO₃)₂

Here, CO₂ is carbon dioxide, H₂O is water, H₂CO₃ is carbonic acid, CaCO₃ is calcium carbonate and Ca(HCO₃)₂ is soluble calcium bicarbonate.

• Permeability and infiltration: Joints, bedding planes and faults allow acidic water to enter limestone. Permeability controls the depth and speed of underground drainage.
• Surface landforms:
  • Limestone pavement: Solution widens joints called grykes, leaving blocks called clints.
  • Sinkholes or dolines: Closed depressions form through solution or collapse; their diameters range from a few metres to hundreds of metres.
  • Dry valleys: Surface streams disappear underground when swallow holes capture drainage.
  • Gorges: Collapse or renewed surface flow can produce steep-sided valleys.
• Underground landforms:
  • Caves and caverns: Enlarged joints and bedding planes form passages; large chambers develop where several routes intersect.
  • Stalactites and stalagmites: Carbonate precipitates when dissolved carbon dioxide escapes from dripping water. Stalactites hang from ceilings, while stalagmites rise from floors; a joined pair forms a column.
  • Underground rivers: Water follows conduits and may emerge as a spring or resurgence.
• Hydrological contrast: Karst drainage is rapid and subterranean rather than slow and surface-based; rivers may disappear abruptly at a swallow hole.
• Worked example: Water entering a joint dissolves limestone as it descends. The enlarged joint becomes a cave passage, and when water emerges at a spring, reduced pressure and carbon dioxide loss may deposit calcite as a stalactite.

B. Applications and limitations

Karst systems provide important water resources but are difficult to predict and highly sensitive to contamination and collapse.

• Water supply: Springs and underground aquifers can supply settlements, but conduit flow may transmit pollutants rapidly over several kilometres.
• Hazards: Sinkhole collapse threatens roads and buildings; underground erosion can remove support before surface subsidence becomes visible.
• Land-use effects: Quarrying removes limestone and caves, while impermeable surfaces increase rapid runoff toward swallow holes.
• Conservation: Vegetation and careful waste management reduce sediment and pollutants entering recharge zones; cave tourism requires control of lighting, access and physical damage.
• Climate and rock controls: Warm, wet conditions and carbon dioxide-rich soils accelerate solution, while pure, jointed limestone develops more extensive karst than impermeable or clay-rich limestone.
• Interpretive limitation: Surface drainage maps may fail to show the true basin because groundwater crosses topographic boundaries through underground conduits.