Notes on Earth's Internal Structure and Plate Tectonics

Earth's Internal Structure

  • Layers of the Earth:
    • Crust:
    • Thin, rocky outer layer (16-40 km thick under oceans and 30-70 km under continents).
    • Composed of less dense rocks: granite (continental, 2.7 g/cm³) and basalt (oceanic, 3.0 g/cm³).
    • Mantle:
    • Divided into upper and lower mantles, dominant minerals are olivine and silicate minerals.
    • Upper mantle is cooler and more rigid, while lower mantle is hotter and capable of gradual flow.
    • Core:
    • Composed of outer (liquid, metallic iron, nickel) and inner core (solid iron).
    • Generates the Earth’s magnetic field through convective flow.

Plate Tectonics

  • Types of Plates:
    • Major plates: Pacific, North American, South American, Eurasian, African, Austral-Indian, Antarctica.
    • Minor plates: Nazca, Juan de Fuca, Philippine.

Evidence for Earth’s Internal Structure

  • Seismic Waves:
    • Utilized to explore internal structure; variations in wave speed due to different materials.
  • Kola Superdeep Borehole:
    • Deepest artificial point; reached 12.2 km depth, providing valuable information about the crust.

Plate Boundaries

  • Types of Boundaries:
    1. Divergent Plate Boundaries: (Constructive zones)
    • Plates move apart, new crust formed from magma rising.
    • Locations: Mid-Atlantic Ridge, East African Rift Valley.
    1. Convergent Plate Boundaries: (Destructive zones)
    • Plates collide, causing subduction (denser oceanic plates sink beneath continental plates).
    • Types:
      • Oceanic-continental: Causes volcanic mountain ranges (e.g., Andes).
      • Oceanic-oceanic: Forms volcanic islands (e.g., Aleutians).
      • Continental-continental: Forms mountain ranges (e.g., Himalayas).
    1. Transform Plate Boundaries:
    • Plates slide past each other, no new crust forms (e.g., San Andreas Fault).

Rates of Plate Movement

  • Average plate movement ranges from 2 cm/year to over 15 cm/year.
  • Example: East Pacific Rise has the fastest rates.

Historical Perspectives

  • Continental Drift: Proposed by Alfred Wegener; suggested that continents were once a single supercontinent (Pangaea) that drifted apart.
  • Convective Currents:
    • Proposed by Arthur Holmes; suggested movement of asthenosphere drives plate movements.
  • Developments in the 1940s and 1950s with ocean floor mapping led to recognition of mid-ocean ridges and symmetrical magnetic stripes.

Summary of Key Concepts

  • Learning about the Earth's internal structure helps explain geological phenomena like earthquakes, volcanoes, and mountain-building processes.
  • Understanding tectonic plate movement is crucial for predicting geological events and understanding the Earth's history.