Unit 3: Surficial Processes and Geomorphology

Weathering and Erosion

Origin (Definitions)

• Weathering: The in-situ (in-place) physical disintegration and chemical decomposition of rock at or near the Earth's surface. It breaks rock down.
• Erosion: The transportation or movement of weathered material (sediment) from one place to another by agents like water, wind, or ice.

Types of Weathering

• Physical (or Mechanical) Weathering: Breaking rock into smaller pieces (clasts) without changing its chemical composition.
  • Frost Wedging: Water seeps into cracks, freezes, expands, and breaks the rock.
  • Exfoliation (Pressure Release): Deeply buried rock (like granite) expands and "sheets" off in layers when the overlying rock is eroded away.
  • Thermal Expansion/Contraction: Daily heating and cooling (e.g., in deserts) can cause rock to crack.
• Chemical Weathering: Decomposing rock by chemical reactions, forming new, more stable minerals.
  • Hydrolysis: The most important process. Water (H₂O) reacts with silicate minerals. (e.g., Feldspar + water → Clay Minerals).
  • Oxidation: Reaction with oxygen, essentially "rusting." (e.g., Iron-bearing minerals → Hematite/Limonite (iron oxides)).
  • Dissolution: Some minerals simply dissolve in water, especially acidic water (e.g., Calcite in limestone dissolving to form caves).

Geological Work of Wind (Aeolian)

This is the dominant geological process in arid (desert) environments.

• Erosional Features:
  • Abrasion: The "sandblasting" effect of wind-blown sand, which carves rock.
  • Mushroom Rock (or Gara): A rock that has been eroded more at its base than its top, forming a mushroom shape.
  • Yardangs: Streamlined, parallel ridges of rock carved by wind.
• Depositional Features:
  • Sand Dunes: Mounds of sand deposited by wind. (e.g., Barchan dunes, transverse dunes).
  • Loess: Thick, widespread deposits of very fine, wind-blown silt.

Geological Work of Running Water (Fluvial)

This is the dominant agent of erosion on Earth's surface. It creates river systems (fluvial systems).

• Erosional Features:
  • V-Shaped Valleys: Rivers cut downwards, and gravity pulls the slopes in, creating a V-shape.
  • Gorges and Canyons: Very deep, steep-sided valleys cut by rivers.
  • Waterfalls: Occur where a river flows over a layer of hard rock onto softer rock.
• Depositional Features:
  • Alluvial Fans: Fan-shaped deposits where a fast river emerges from a steep mountain onto a flat plain.
  • Floodplains: The flat, fertile land on either side of a river, built up by sediment during floods.
  • Meanders & Ox-bow Lakes: S-shaped curves in a mature river. An ox-bow lake is a "cut-off" meander.
  • Deltas: A fan-shaped deposit of sediment where a river flows into a standing body of water (like a lake or ocean).

Geological Work of Glacier (Glacial)

A glacier is a large, long-lasting body of ice that moves slowly over land. It is a powerful agent of erosion, common in high mountains and polar regions.

• Erosional Features:
  • U-Shaped Valleys: Glaciers are wide and thick, scouring the entire valley floor and creating a characteristic U-shape.
  • Cirque: An armchair-shaped hollow at the head of a glacial valley.
  • Arête: A sharp, knife-like ridge formed between two cirques.
  • Horn: A sharp, pyramidal peak formed by three or more cirques (e.g., The Matterhorn).
• Depositional Features:
  • Moraine: A ridge of till (unsorted sediment) deposited by a glacier. (e.g., Terminal, Lateral, Medial moraines).

Geological Work of Volcanoes

Volcanism is an endogenetic process (Unit II) that creates major exogenetic (surface) landforms. A volcano is a vent or opening where molten rock (magma), gases, and ash erupt onto the surface.

• Lava Flows: Sheets of molten rock (lava) that flow over the land.
• Volcanic Cones: The "mountain" built up by successive eruptions.
  • Shield Volcanoes: Broad, gentle slopes, built from fluid lava flows (e.g., Mauna Loa, Hawaii).
  • Composite Volcanoes (Stratovolcanoes): Steep, conical, built from alternating layers of lava and ash (e.g., Mount Fuji).
• Caldera: A large, circular depression formed when a volcano collapses after a massive eruption.

Physiographic Sub-divisions of India

Physiography is the study of landforms (geomorphology). India is divided into several major physiographic units, each with its own distinct geology and landforms.

1. The Himalayan Mountains: A young, tectonically active range of folded mountains. Formed by the collision of the Indian and Eurasian plates.
2. The Northern Plains (Indo-Gangetic Plains): A vast, flat, alluvial plain formed by the deposition of sediment from the Indus, Ganga, and Brahmaputra river systems.
3. The Peninsular Plateau: A large, old, and stable triangular plateau. It is composed of ancient crystalline rocks.
   • Key features: Deccan Traps (a massive volcanic lava flow) and the Chota Nagpur Plateau (rich in minerals).
4. The Indian Desert (Thar Desert): A large, arid region in the northwest, dominated by sandy plains and sand dunes (aeolian landforms).
5. The Coastal Plains: Narrow strips of flat land bordering the Peninsular Plateau.
   • Western Coastal Plains: Narrow, between the Western Ghats and the Arabian Sea.
   • Eastern Coastal Plains: Broader, between the Eastern Ghats and the Bay of Bengal, with large deltas.
6. The Islands:
   • Andaman & Nicobar Islands: A volcanic island arc.
   • Lakshadweep Islands: A group of coral atolls.