Exogenetic processes and resultant landforms
Multiple Choice Questions (MCQ)
- Which of the following is the primary process by which rivers erode their beds and banks through the grinding action of transported materials?
(a) Corrosion
(b) Hydraulic action
(c) Corrasion
(d) Attrition
Answer: (c) Corrasion - In coastal areas, the erosion of weaker rock layers leading to the formation of protruding landmasses of resistant rock is known as:
(a) Bays
(b) Sea caves
(c) Headlands
(d) Wave-cut platforms
Answer: (c) Headlands - The process of wind picking up and transporting loose, dry particles of sand and dust is called:
(a) Abrasion
(b) Deflation
(c) Attrition
(d) Plucking
Answer: (b) Deflation - Bowl-shaped depressions found at the head of glacial valleys, formed by plucking and abrasion, are known as:
(a) U-shaped valleys
(b) Hanging valleys
(c) Cirques
(d) Roches moutonnées
Answer: (c) Cirques - Long, narrow, and deep coastal inlets that were originally U-shaped glacial valleys flooded by the sea are called:
(a) Sea caves
(b) Fiords
(c) Oxbow lakes
(d) Ventifacts
Answer: (b) Fiords
Short Answer Questions (SAQ)
- Differentiate between ‘corrosion’ and ‘corrasion’ as agents of river erosion.
Answer: ‘Corrosion’ (or solution) is the process where river water dissolves soluble rocks like limestone and chalk through chemical reactions, widening and deepening the channel. ‘Corrasion’ (or abrasion) is the mechanical wearing away of the riverbed and banks by the grinding action of rock fragments, pebbles, and sand carried by the flowing water. - Explain the formation of a sea arch.
Answer: Sea arches form when wave action erodes weaknesses, such as joints or faults, on both sides of a headland. This erosion creates two separate sea caves that gradually deepen and eventually meet, forming an arch-like structure through the headland. - What are ‘ventifacts’, and how are they formed?
Answer: Ventifacts are rocks that have been shaped and smoothed by wind abrasion. They are formed in arid and semi-arid regions where wind-borne sand particles blast against exposed rock surfaces, acting like sandpaper and eroding them over time, often creating flat, polished faces.
Long Answer Questions (LAQ)
1.Describe the different erosional processes carried out by rivers and explain the formation of any three distinct riverine landforms resulting from this erosion.
Answer: Rivers erode the landscape through four main processes:
- Corrasion (Abrasion): The grinding action of transported sediments against the riverbed and banks.
- Corrosion (Solution): The chemical dissolution of soluble rocks by river water.
- Hydraulic Action: The sheer force of flowing water dislodging loose rocks and sediments.
- Attrition: The breaking down of transported rock fragments into smaller, smoother particles through collisions.
Three distinct riverine landforms formed by these erosional processes are:
- V-shaped Valleys/Gorges: In the upper course, where the river has a steep gradient, vertical erosion is dominant. Corrasion and hydraulic action are particularly effective in cutting down into the bedrock, creating deep and narrow, V-shaped valleys. When the valley is very deep and narrow with steep sides, it is called a gorge.
- Waterfalls: Waterfalls form where a river flows over a layer of resistant rock followed by a softer layer. The softer rock erodes more rapidly due to corrasion and hydraulic action, creating a vertical drop over the resistant layer. The continuous undercutting of the harder rock eventually leads to the retreat of the waterfall upstream.
- Meanders: In the middle and lower courses, where the gradient is gentler, lateral erosion becomes more significant. Due to variations in flow velocity and channel depth, the river starts to develop bends called meanders. On the outer bank of a meander, the water flows faster, leading to increased erosion (corrasion and hydraulic action) and the formation of a steep river cliff.
2.Explain how glaciers erode the landscape and describe the formation of two characteristic glacial landforms.
Answer: Glaciers erode the landscape through two primary processes:
- Plucking (Quarrying): Meltwater from the glacier seeps into cracks and joints in the bedrock. When this water refreezes, it expands, exerting pressure and breaking off pieces of rock. These loosened rocks are then incorporated into the moving ice.
- Abrasion: Rocks and debris embedded within the glacier act like a giant abrasive tool, grinding and scraping against the underlying bedrock as the ice moves. This process polishes the rock surface and creates striations and grooves that indicate the direction of glacial movement.
Two characteristic glacial landforms formed by these erosional processes are:
- U-shaped Valleys: Unlike the V-shaped valleys carved by rivers, glaciers are massive bodies of ice with immense weight and erosive power. As a glacier moves down a pre-existing river valley, it erodes both the sides and the bottom of the valley. Plucking widens the valley walls, making them steeper and straighter, while abrasion deepens the valley floor and smooths it out. This results in a characteristic U-shaped valley with a wide, flat bottom and steep sides.
- Cirques: Cirques are bowl-shaped, amphitheater-like depressions found at the head of glacial valleys, typically in mountainous regions. They are formed by the combined erosive action of plucking and abrasion by a glacier occupying a high-altitude hollow. Freeze-thaw action on the headwall above the glacier also contributes to the erosion and steepening of the cirque walls. Once the glacier melts, a steep-walled, often lake-filled depression remains.