Shaping of the Earth’s Surface Flashcards

The Dynamic Earth and the Theory of Plate Tectonics

Core Concepts and Shaping Forces: The Earth's surface is constantly being transformed by powerful internal (endogenic) and external (exogenic) forces.
Plate Tectonic Theory: Proposed by W.J. Morgan, this theory posits that the Earth's outermost layer is not a single unit but is broken into several large and small pieces called tectonic plates. These plates float and move slowly over the molten mantle below.
Lithosphere and Asthenosphere:
- Lithosphere: The rigid outer layer consisting of the crust and the upper mantle.
- Asthenosphere: A semi-molten, hot, mobile layer of partially molten rock located beneath the lithosphere that allows tectonic plates to move.
Plate Characteristics:
- Movement: Plates move at a rate of a few centimetres per year.
- Types of Plates:
  - Continental Plates: Carry continents.
  - Oceanic Plates: Carry ocean floors.
  - Mixed Plates: Carry both continents and oceans.
- Major Tectonic Plates:
  - Pacific Plate
  - Eurasian Plate
  - African Plate
  - North American Plate
  - South American Plate
  - Indo-Australian Plate
  - Antarctic Plate

The Earth's Internal Structure

Layer Depths and Composition:
- Crust: The outermost layer. Thickness varies from $30-40\,\text{km}$ under continents to $5-7\,\text{km}$ under oceans.
- Mantle: A mostly solid layer between the crust and outer core, approximately $2900\,\text{km}$ thick. It comprises the upper mantle ( $22\%$ ) and lower mantle ( $32\%$ ).
- Outer Core: A fluid layer mainly consisting of iron and nickel, approximately $2200\,\text{km}$ thick ( $22\%$ of interior).
- Inner Core: A solid, hot spinning metal ball; the densest part of the Earth ( $1250\,\text{km}$ radius).
- Total Radius: The Earth's approximate radius to the center is $6375\,\text{km}$ .
Convection Currents: Heat from the core causes molten material in the mantle to rise while cooler material sinks. This continuous movement creates convection currents that push and pull tectonic plates.

Plate Boundaries and Interactions

Convergent Boundary: Plates move toward each other.
- Continental-Continental: Forms fold mountains (e.g., the Himalaya).
- Oceanic-Continental: The oceanic plate sinks (subducts) beneath the continental plate, leading to volcanic activity and earthquakes.
Divergent Boundary: Plates move away from each other.
- Features: Magma rises to form new crust, creating mid-ocean ridges (e.g., the Mid-Atlantic Ridge).
Transform Boundary: Plates slide past each other horizontally.
- Effects: No crust is created or destroyed, but this movement causes major earthquakes (e.g., the San Andreas Fault in the United States).
The Ring of Fire: A major area in the basin of the Pacific Ocean where a large number of earthquakes and volcanic eruptions occur, corresponding to plate boundaries.

Weathering, Erosion, and Gradation

Landforms Definition: Natural features formed by processes such as weathering, erosion, deposition, and crustal movement.
Weathering: The breaking down of rocks into smaller pieces in situ (without movement).
- Physical Weathering: Caused by temperature changes, frost, wind, or ice expansion.
- Chemical Weathering: Reactions with water, air, or acids lead to new substances.
- Biological Weathering: Caused by plants (roots splitting rocks), animals, or micro-organisms.
Erosion: The wearing away and transportation of materials by natural agents.
- Agents: Water, wind, ice, and waves.
- Impact on Human Livelihoods:
  - Agriculture: Removes fertile topsoil, lowering crop yields.
  - Settlements: Washes away land, houses, and roads for those near coasts/rivers.
  - Industry: Destabilizes land for mining and construction; destroys beaches for tourism.
Agents of Gradation: Natural forces that level the Earth's surface by wearing down high areas and filling low areas.
- Running water (valleys/plains), Glaciers (U-shaped valleys), Wind (deserts), Waves (cliffs/beaches), Groundwater (caves).

Landforms Created by Running Water

Upper Course: Features steep gradients and strong erosive forces.
- V-shaped Valleys, Rapids, and Waterfalls: A waterfall forms where a river flows over a vertical drop. Hard rock resists erosion while soft rock below is worn away, creating a plunge pool.
Middle Course: The river begins to lose energy.
- Meander: A winding curve or bend. Erosion occurs on outer banks and deposition on inner banks. Meanders provide fertile soil for farming and influence settlement patterns.
- Oxbow Lakes: Formed when a meander is cut off from the main river.
Lower Course: Significant deposition occurs as the river slows.
- Delta: A triangular or fan-shaped area of land at the river mouth (e.g., Sundarbans delta). Rich in alluvial soil, ideal for rice and jute. Centers for trade and fishing, though prone to flooding.
- Levees and Alluvial Fans: Additional depositional features.

Coastal and Glacial Landforms

Coastal Features (Waves and Currents):
- Beaches: Depositional landforms of sand and pebbles. Act as natural barriers against erosion.
- Erosional Features:
  - Cliffs: Steep rock faces.
  - Sea Caves: Formed by wave erosion in weak rock.
  - Sea Arches: Formed when two caves meet through a headland.
  - Sea Stacks: Isolated rock pillars left when arches collapse.
Glacial Landforms:
- Erosional: U-shaped valleys, Cirques (bowl-shaped depressions), Aretes (sharp ridges), Hanging valleys, and Fjords (flooded glacial valleys).
- Depositional (Moraines): Landforms created by till (rocks and debris).
  - Lateral Moraines: Along glacier sides.
  - Terminal Moraines: At the end of the glacier's furthest advance.
  - Medial Moraines: Where two glaciers meet and join lateral moraines.

Wind and Underground Water (Karst) Landforms

Wind Landforms (Arid Regions):
- Erosional: Yardangs (streamlined ridges), Ventifacts (sandblasted rocks), and Deflation hollows (depressions that can form oases if they reach the water table).
- Depositional (Dunes):
  - Barchan Dunes: Crescent-shaped; single wind direction.
  - Longitudinal Dunes: Parallel to wind.
  - Star Dunes: Multiple arms from multi-directional winds.
  - Parabolic Dunes: U-shaped, often stabilized by vegetation.
Underground Water (Karst Topography): Chemically weathered limestone.
- Caves: Hollow spaces created by acidic water.
- Stalactites: Hang from ceilings.
- Stalagmites: Rise from floors.
- Sinkholes (Dolines): Surface depressions from ground collapse.

Historical and Traditional Contexts

Ancient Indian Earthquake Science: Earthquakes were termed 'bhūkampa'. In the Bṛihatsaṁhitā, Varāhamihira attributed earthquakes to four elemental forces: Vāyu (wind), Agni (fire), Indra (heaven/thunder), and Varuṇa (water), and linked them to animal behavior and planetary alignments.
Traditional Water and Soil Management:
- Sindhu-Sarasvatī Civilisation: Used contouring, bunding, terracing, dams, and canals. Documented in the Vedas, Kṛiṣhiparāśhara, and Kauṭilya’s Arthaśhāstra.
- Arthaśhāstra: Guidelines on land categorization by fertility.
- Zabo System (Nagaland): Uses earthen bunds on hillslopes and check dams to prevent erosion and recharge groundwater.

Landforms and Associated Disasters

Landslides: Caused by heavy rainfall, earthquakes, volcanic eruptions, steep slopes, deforestation, and road construction. Water reduces friction in soil/rocks.
Avalanches: Sudden instability of snow on steep slopes triggered by heavy snowfall, temperature rises (reducing friction), wind, or vibrations from human activity (skiing/construction).
Glacial Lake Outburst Floods (GLOFs): Sudden release of water from lakes dammed by ice or moraines. Driven by rapid glacial melting (climate change), earthquakes, or avalanches hitting the lake.
Dust Storms: Strong winds lifting dry, loose soil in arid regions. Factors include drought, sparse vegetation, and poor farming practices.
Specific Incident: A devastating flood struck Chamoli district, Uttarakhand in February 2021, causing loss of life and destruction of hydel projects.

Questions & Discussion

Ancient Signs: Varāhamihira noted changes in wind, rain, clouds, and animal behavior as earthquake signals.
Volcanic Observation: Students asked about grey powder in volcanic eruption photos (ash/tephra) and unique features like the mud volcano at Baratang Island (Andaman and Nicobar Islands) where mud bubbles due to gas pressure instead of lava.
Reflection on Hazards: Discussion regarding India's earthquake risk, specifically highlighting disasters like the 2001 Gujarat earthquake and the vulnerability of densely populated regions.
Geographical Barriers: The Himalayas served as protectors from invasion while Passes (e.g., Khyber) allowed cultural exchange. Rivers (Ganga, Nile, Indus) fostered agricultural civilizations.