Exogenetic processes and resultant landforms

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.