Discover the structure of Earth's interior, including the crust, mantle, and core. Learn about their composition, properties, and significance in physical geography.

Interior of the Earth: Earthquakes & Volcanoes

• The Interior of the Earth
  • The interior of the earth can only be understood through indirect evidences, as no one can physically reach there.
  • The configuration of the earth's surface is shaped by processes operating inside the earth.
  • Processes involved:
    • Exogenic processes - External forces shaping the landscape.
    • Endogenic processes - Internal forces impacting landscape development.
  • Importance of Understanding:
    • Human life is influenced by the physiography of the region.
    • Understanding earthquakes, tsunamis, and landscape development requires knowledge of the earth's interior.
• Sources of Information About the Interior
  • Earth's Radius: 6,370 km.
  • Challenges: Direct access to the earth's core is impossible, so knowledge is based on estimates, inferences, and observations.
  • Direct Sources:
    • Surface Rocks: Easily accessible materials from the surface.
    • Mining:
      • Gold mines in South Africa reach depths of 3–4 km.
      • Beyond this depth, extreme heat prevents further exploration.
    • Drilling Projects:
      • Examples: Deep Ocean Drilling Project and Integrated Ocean Drilling Project.
      • Deepest drill at Kola (Arctic Ocean): 12 km depth.
    • Volcanic Eruptions: Magma analysis provides direct information but does not reveal the exact source depth.
  • Indirect Sources:
    • Temperature, Pressure, and Density:
      • These characteristics increase with depth.
      • Scientists estimate their rates of change and overall values based on the earth's thickness.
    • Meteors:
      • Provide material and structural information similar to the earth.
    • Gravitation:
      • Gravitational force (g) varies by latitude:
        • Greater at the poles, lesser at the equator.
        • Differences are influenced by the uneven mass distribution within the earth.
      • Gravity Anomalies: Differences from expected gravity readings help map mass distribution in the crust.
    • Magnetic Field: Magnetic surveys indicate the distribution of magnetic materials in the crust.
    • Seismic Activity: A crucial source of information about the earth's interior.

Earthquakes: P & S Waves

• Earthquake
  • An earthquake is the shaking of the earth, caused by the release of energy generating waves that travel in all directions.
  • Why does the earth shake?
    • Energy is released along a fault (a sharp break in crustal rocks).
    • Rocks along the fault move in opposite directions but are locked by friction.
    • Over time, friction is overcome, causing blocks to slide abruptly, releasing energy.
    • Key points:
      • Focus (Hypocentre): Point where energy is released.
      • Epicentre: Point on the surface directly above the focus, first to experience waves.
  • Earthquake Waves
    • All natural earthquakes occur in the lithosphere (up to 200 km depth).
    • Seismograph: Instrument that records earthquake waves.
    • Types of Waves:
      • Body Waves: Travel through the earth’s interior.
        • P-Waves: Primary waves, travel faster, and through solids, liquids, and gases.
        • S-Waves: Secondary waves, slower, and only travel through solids.
      • Surface Waves: Move along the surface, slower but cause more destruction.

      

    • Wave Propagation:
      • P-Waves vibrate parallel to wave direction, causing compression and expansion.
      • S-Waves vibrate perpendicular to wave direction, creating crests and troughs.
      • Velocity changes with material density; denser material means higher velocity.
  • Shadow Zone
    • Areas where certain waves are not recorded by seismographs.
    • Characteristics:

      

      • P-Waves: Shadow zone appears between 105° and 145° from the epicentre.
      • S-Waves: Cannot travel through liquids, creating a larger shadow zone covering over 40% of the earth's surface.

      

  • Types of Earthquakes
    • Tectonic Earthquakes: Most common, caused by rock movement along a fault.
    • Volcanic Earthquakes: Occur in areas with active volcanoes.
    • Collapse Earthquakes: Caused by underground mine collapses.
    • Explosion Earthquakes: Result from chemical or nuclear explosions.
    • Reservoir-Induced Earthquakes: Occur near large reservoirs.
  • Measuring Earthquakes
    • Magnitude: Measured on the Richter Scale (0–10), representing energy release.
    • Intensity: Measured on the Mercalli Scale (1–12), representing visible damage.
  • Effects of Earthquakes
    • Immediate hazardous effects:
      • Ground shaking
      • Differential ground settlement
      • Land and mudslides
      • Soil liquefaction
      • Ground lurching
      • Avalanches
      • Ground displacement
      • Floods (dam and levee failures)
      • Fires
      • Structural collapse
      • Falling objects
      • Tsunami (if epicentre is below ocean waters)
    • Tsunami: Waves generated by tremors, not earthquakes themselves. Effects are devastating if magnitude exceeds 5 on the Richter Scale.

    

  • Frequency of Earthquake Occurrences
    • Major tremors: Occur once every 1–2 years.
    • Minor tremors: Occur almost every minute globally.

Structure of the Earth: Crust, Mantle & Core

• Structure of the Earth
  • The Crust
    • The outermost solid part of the earth, brittle in nature.
    • Thickness:
      • Varies between oceanic and continental areas.
      • Oceanic Crust: Thinner, mean thickness of 5 km.
      • Continental Crust: Thicker, mean thickness of 30 km.
      • Thicker in regions of major mountain systems, up to 70 km in the Himalayas.
  • The Mantle
    • Located below the crust, extending from Moho’s Discontinuity to a depth of 2,900 km.
    • Upper Mantle:
      • Known as the Asthenosphere (meaning "weak").
      • Extends up to 400 km.
      • Main source of magma during volcanic eruptions.
    • Lithosphere:
      • Comprises the crust and the uppermost part of the mantle.
      • Thickness ranges from 10–200 km.
    • Lower Mantle: Extends beyond the asthenosphere and is in a solid state.
  • The Core
    • Identified through earthquake wave velocities.
    • Core-Mantle Boundary: Located at a depth of 2,900 km.
    • Outer Core: Liquid state.
    • Inner Core: Solid state.
    • Composed of heavy materials like nickel and iron (referred to as the nife layer).

Volcanoes: Types, Structure & Properties

• Volcanoes and Volcanic Landforms

  

  • Volcanoes

    A volcano is a place where gases, ashes, and/or molten rock material (lava) escape to the ground.

    • Active Volcano: Releases materials such as gases, ashes, or lava in the recent past.
    • Source: The mantle below the crust contains a weaker zone called the asthenosphere, where molten rock material (magma) originates.
    • Upon reaching the surface, magma is called lava.
    • Materials released during eruptions include:
      • Lava flows
      • Pyroclastic debris
      • Volcanic bombs
      • Ash and dust
      • Gases (nitrogen, sulfur compounds, chlorine, hydrogen, argon)
  • Types of Volcanoes
    • Shield Volcanoes:

      

      • Largest volcanoes on Earth, made mostly of basalt.
      • Basaltic lava is very fluid and non-explosive unless water enters the vent.
      • Examples: Hawaiian volcanoes.
    • Composite Volcanoes:

      

      • Characterized by cooler, viscous lavas and explosive eruptions.
      • Produce large quantities of pyroclastic material and ash.
      • Form layers near the vent, giving the volcano a composite appearance.
    • Caldera:
      • The most explosive volcanoes.
      • Often collapse into a depression after an eruption (called a caldera).
    • Flood Basalt Provinces:
      • Produce highly fluid lava that spreads over large areas.
      • Examples: Deccan Traps in India.
    • Mid-Ocean Ridge Volcanoes:
      • Occur along oceanic ridges spanning 70,000 km.
      • Characterized by frequent eruptions at the ridge’s central portion.
  • Volcanic Landforms

    

    • Intrusive Forms:
      • Formed when lava cools within the crust, creating igneous rocks.
      • Types:
        • Batholiths:
          • Large dome-shaped magmatic bodies.
          • Appear on the surface after erosion removes overlying material.
          • Made of granite and form the cooled portion of magma chambers.
        • Lacoliths:
          • Dome-shaped intrusive bodies with a level base.
          • Connected to a magma source below.
          • Example: Domal hills of the Karnataka Plateau.
        • Lopoliths: Saucer-shaped bodies concave to the sky.
        • Phacoliths: Wavy igneous intrusions found at syncline bases or anticline tops.
        • Sills: Horizontal, thick igneous intrusions.
        • Dykes:
          • Vertical intrusions formed when lava solidifies in cracks or fissures.
          • Common in the Deccan Traps of Maharashtra.