Unit 6: Aeolian, coastal and karst processes - Subjective Questions

Aeolian processes are the geomorphic activities performed by wind, especially in arid, semi-arid, coastal, and sparsely vegetated regions.

The conditions required for effective wind action include:

• Limited vegetation: Sparse vegetation exposes loose sediment to wind.
• Abundant unconsolidated material: Fine sand, silt, and dust must be available for transport.
• Strong and persistent winds: Wind velocity must exceed the threshold required to move particles.
• Dry surfaces: Moisture binds particles together and reduces their mobility.
• Flat or gently sloping terrain: Such surfaces allow wind to travel over long distances.

Wind erodes through deflation, which removes loose particles, and abrasion, in which wind-driven particles strike and wear exposed surfaces. Deposition occurs when wind velocity decreases or when obstacles such as vegetation, rocks, or surface irregularities cause sediment to settle.

Aeolian sediment is transported in three principal ways:

• Surface creep: Coarse sand grains move by rolling or sliding along the ground when struck by moving particles.
• Saltation: Medium-sized sand grains are lifted briefly into the air and follow a series of short hops. This is the most important mode of sand transport.
• Suspension: Very fine particles, such as silt and clay, are carried high into the atmosphere by turbulent air currents and may travel long distances.

The mode of transport depends on wind velocity, particle size, particle density, and surface roughness. As wind speed increases, particles may shift from creep to saltation and then to suspension. Saltating grains also bombard the surface and initiate movement of other particles.

Deflation and abrasion are two different forms of wind erosion.

Deflation is mainly a process of removal, whereas abrasion is a process of mechanical wearing. Both processes commonly operate together in arid environments.

A sand dune is a mound or ridge of wind-deposited sand. Dunes form when wind loses energy and sand accumulates around an obstacle or in an area where surface roughness increases.

Important features include:

• A gentle windward slope facing the prevailing wind.
• A steep slip face, commonly close to the angle of repose of dry sand, about to .
• A crest separating the windward and leeward slopes.
• Continuous movement of sand up the windward side and down the slip face.

Common dune types are:

• Barchan dunes: Crescent-shaped dunes with horns pointing downwind; they form where sand supply is limited.
• Transverse dunes: Ridges at right angles to wind direction; they require abundant sand.
• Longitudinal or seif dunes: Long, narrow ridges aligned approximately parallel to the prevailing wind.
• Parabolic dunes: U-shaped dunes with arms pointing upwind, commonly stabilized by vegetation.
• Star dunes: Multi-armed dunes formed by winds from several directions.

Loess is a thick, blanket-like deposit of wind-blown silt, usually composed largely of quartz, feldspar, and clay minerals.

Formation involves the following stages:

• Fine sediment is produced by glacial grinding, desert weathering, or floodplain deposition.
• Dry, loose silt is picked up by strong winds.
• The particles are transported in suspension over considerable distances.
• Deposition occurs when wind velocity declines or when vegetation and surface irregularities trap the sediment.

Loess deposits are commonly yellowish-brown, porous, and usually lack distinct bedding. They are significant because:

• They form some of the world’s most fertile agricultural soils.
• Their fine texture allows rapid weathering and nutrient release.
• Their steep, vertical cliffs are vulnerable to erosion and collapse when saturated.
• They provide evidence of past climatic conditions and former glacial or arid environments.

Major coastal processes include wave erosion, weathering, mass movement, transportation, and deposition.

Wave erosion occurs through:

• Hydraulic action: Pressure of water and compressed air in cracks weakens cliffs.
• Abrasion or corrasion: Rock fragments carried by waves strike and wear the coast.
• Attrition: Rock fragments collide with one another and become smaller and rounder.
• Solution: Soluble minerals are dissolved by seawater.

Sediment is transported by:

• Longshore drift: Waves approach the beach at an angle, pushing sediment up the shore obliquely. Backwash returns sediment downslope under gravity, producing a zigzag movement.
• Swash and backwash: Swash moves material landward, while backwash moves it seaward.
• Tidal currents: Repeated tidal movement transfers sediment along and across the coast.

The direction and rate of sediment transport depend on wave energy, wave angle, tidal currents, sediment size, and beach slope.

A wave-cut platform is a gently sloping rock surface left at the base of a retreating sea cliff.

Its formation occurs as follows:

• Waves attack the cliff base, especially at the level of high-energy wave impact.
• Hydraulic action and abrasion enlarge weaknesses and produce a wave-cut notch.
• The notch becomes deeper and creates an unsupported overhang.
• The overhanging material collapses through mass movement.
• The cliff retreats landward, leaving a gently sloping rock surface exposed at low tide.
• Repeated erosion and collapse widen the platform.

The platform records the former position of the cliff base. Its width depends on rock resistance, tidal range, wave energy, weathering, and the duration of marine erosion. Features such as caves, arches, and stacks may develop when erosion exploits joints and faults in the cliff.

These erosional landforms commonly develop in resistant coastal headlands containing joints or faults.

• Sea cave: Waves enlarge a crack or weakness through hydraulic action and abrasion.
• Sea arch: Continued erosion causes the cave to break through the headland, forming an opening.
• Sea stack: The roof of the arch becomes unstable and collapses, leaving an isolated pillar of rock offshore.
• Sea stump: Continued wave erosion and weathering reduce the stack to a low remnant, often visible only at low tide.

The sequence can be represented as:

Joint or fault → Cave → Arch → Stack → Stump

The rate of development is influenced by rock type, joint spacing, wave energy, tidal conditions, and the presence of subaerial weathering. Softer rocks usually erode faster, while resistant rocks may form prominent headlands.

Coastal landforms may be classified according to whether marine processes remove or deposit material.

Erosional landforms:

• Sea cliffs
• Wave-cut platforms
• Caves
• Arches
• Stacks
• Stumps
• Headlands and bays

They form where wave energy is high and resistant rock is exposed to prolonged erosion.

Depositional landforms:

• Beaches
• Spits
• Bars
• Tombolos
• Sand dunes
• Mudflats and salt marshes

They form where wave energy or current velocity decreases and sediment accumulates. For example, a spit develops when longshore drift carries sediment beyond a bend in the coastline, whereas a stack forms through erosion of a rocky headland.

Thus, erosional landforms are mainly produced by the removal of material, while depositional landforms result from its accumulation.

Beaches form when sediment such as sand, gravel, or shingle accumulates along the shore due to constructive waves, longshore drift, or river supply.

A spit is a narrow ridge of sand or shingle attached to the land at one end. It forms when longshore drift carries sediment across a river mouth or beyond a change in coastline direction. The end may become recurved because of waves from another direction.

A bar forms when a spit extends across a bay and joins two sides of the coastline. It may partly or completely enclose a lagoon.

A tombolo is a depositional ridge that connects an island to the mainland or links two islands. It develops when wave refraction and diffraction reduce energy in the sheltered water behind the island, encouraging sediment deposition.

The formation of these features depends on sediment supply, wave direction, longshore drift, tidal currents, and coastal geometry.

Karst topography is a distinctive landscape produced mainly by the solution of soluble rocks, especially limestone, by naturally acidic water.

The main conditions required are:

• Soluble and permeable bedrock: Thick, jointed limestone or similar rocks allow water to enter and circulate.
• Carbonated water: Rainwater absorbs carbon dioxide from the atmosphere and soil, forming weak carbonic acid.
• Adequate rainfall: Water is needed to dissolve rock and transport dissolved material.
• Joints, bedding planes, and faults: These provide pathways for underground drainage.
• Sufficient relief and hydraulic gradient: These encourage groundwater movement.

The principal chemical reaction is:

Calcium bicarbonate is soluble and can be carried away in groundwater. Karst landscapes commonly contain sinkholes, caves, disappearing streams, and underground drainage systems.

Limestone is composed mainly of calcium carbonate, . Rainwater becomes mildly acidic when it absorbs carbon dioxide from the atmosphere and, more importantly, from soil respiration.

The reactions are:

Carbonic acid reacts with calcium carbonate:

Calcium bicarbonate, , is soluble in water and is transported through joints and bedding planes. When groundwater reaches a cave or emerges at the surface, pressure decreases and carbon dioxide may escape. This can cause calcium carbonate to precipitate:

Dissolution is enhanced by abundant rainfall, high soil carbon dioxide, warm temperatures, extensive jointing, and slow but continuous groundwater circulation.

A sinkhole, or doline, is a closed depression formed mainly by the dissolution or collapse of soluble bedrock.

Two principal types are recognized:

• Solution doline: Acidic water dissolves limestone gradually along joints and bedding planes, producing a surface depression.
• Collapse doline: Underground cavities enlarge until the roof becomes unstable and collapses suddenly or gradually.

Sinkholes may be circular, oval, or irregular in plan. Their sides may be gentle or steep, and their floors may contain soil, sediment, or temporary ponds. They commonly develop where limestone is close to the surface and groundwater drainage is well established.

Sinkholes are important indicators of karst drainage, but they may create hazards by damaging roads, buildings, pipelines, and agricultural land. They can also provide rapid pathways for pollutants to enter groundwater.

Caves develop when acidic groundwater enters limestone through joints, faults, and bedding planes. The water dissolves the rock and gradually enlarges these openings.

The process includes:

• Infiltration of rainwater through soil and fractured limestone.
• Chemical dissolution along underground pathways.
• Enlargement of passages by continued groundwater flow.
• Development of underground streams, chambers, and drainage networks.
• Possible abandonment of passages when the water table falls or the stream changes course.

A cave is an underground passage or cavity, while a cavern is usually a larger chamber or connected system of chambers. Karst drainage is often underground, so surface streams may disappear into swallow holes and reappear at resurgences or springs.

Cave development is controlled by rock purity, jointing, groundwater volume, hydraulic gradient, and changes in the water table.

These are depositional features formed inside caves when dissolved calcium carbonate precipitates from groundwater.

• Stalactites: Icicle-like deposits hanging from cave roofs. They form as mineral-rich water loses carbon dioxide and leaves calcium carbonate behind.
• Stalagmites: Mounds or cones growing upward from cave floors beneath dripping water.
• Columns or pillars: Structures formed when a stalactite and stalagmite join together.
• Flowstone: Sheet-like deposits formed when mineral-rich water flows over cave walls or floors.

A useful memory aid is: stalactites hold tight to the ceiling, while stalagmites might reach the ceiling. Growth is generally slow and depends on water supply, carbon dioxide concentration, evaporation, cave ventilation, and the availability of dissolved calcium carbonate.

Karst surface features develop through solution and underground drainage.

• Limestone pavement: A flat or gently sloping exposed limestone surface shaped by dissolution along joints and bedding planes.
• Clints: The blocks or slabs of limestone forming the pavement surface.
• Grikes: The enlarged fissures or grooves separating adjacent clints.
• Swallow holes: Openings through which surface streams disappear underground.
• Resurgences: Springs where underground streams reappear at the surface.

Rainwater enters grikes and widens them through carbonic-acid dissolution. Over time, the pavement becomes deeply dissected. Streams may be diverted underground through swallow holes, producing a dry surface valley or intermittent stream channel. They may later re-emerge at a resurgence where the underground passage meets an impermeable layer, a valley side, or the water table.

The three process groups differ in their dominant agents and mechanisms.

Aeolian processes are strongest in dry and sparsely vegetated areas. Coastal processes operate along shorelines where marine energy acts on land and sediment. Karst processes require soluble, permeable rocks and sufficient groundwater. Although their agents differ, all three systems involve erosion, transportation, deposition, and the interaction between climate, geology, topography, and available sediment.

Longshore drift is the movement of beach sediment parallel to the coastline due to obliquely approaching waves.

The main controlling factors are:

• Prevailing wind direction: Determines the usual direction of wave approach.
• Wave angle: A larger angle generally increases the alongshore component of sediment movement.
• Wave energy: High-energy waves transport larger quantities of sediment.
• Beach slope: Influences the strength of swash and backwash.
• Sediment size and shape: Fine, light particles are more easily transported than coarse material.
• Tidal currents and storms: These can reverse or intensify normal transport.
• Coastal orientation: Changes in shoreline direction can alter transport pathways.

Longshore drift redistributes sediment from eroding sections to depositional zones. It contributes to the formation of beaches, spits, bars, and tombolos. Human structures such as groynes may interrupt this movement, causing accretion on one side and erosion on the other.

Geomorphic processes are strongly controlled by environmental conditions.

• Climate: Arid climates encourage wind erosion because surfaces are dry and vegetation is sparse. Humid climates promote chemical weathering and karst development. Stormy climates increase coastal wave energy.
• Vegetation: Vegetation binds soil, reduces wind velocity, traps sediment, and protects coasts from erosion. Dense vegetation can therefore stabilize dunes and reduce runoff.
• Rock type: Soft coastal rocks erode quickly, while resistant rocks form headlands and stacks. Pure limestone is especially susceptible to solution.
• Rock structure: Joints, faults, bedding planes, and folds provide pathways for water and weaknesses for erosion. They influence the shape of caves, cliffs, arches, and karst depressions.
• Relief and drainage: Steep slopes increase runoff and mass movement, while permeable limestone encourages underground drainage.

The resulting landforms are therefore products of both process intensity and the resistance and structure of the underlying material.

Base level is the lowest level to which a stream or erosional system can lower its channel. Sea level is commonly regarded as the ultimate base level, although local or temporary base levels may occur.

In coastal geomorphology, changes in sea level influence cliff erosion, beach position, shoreline migration, marine terraces, and the development of estuaries. A rise in sea level may increase wave attack on the coast and drown existing valleys.

In karst geomorphology, the water table acts as an important base level for underground drainage. Caves often develop near the water table where groundwater flows laterally. If the water table falls, former caves may become dry and new passages may form at lower levels. Resurgences and underground streams are also controlled by local base levels and impermeable rock layers.

Thus, base level influences the direction, depth, and organization of both surface and underground erosion.