Topic 4 - Plate Tectonics + Plate Theory
1. Introduction and Evolution of the Theory
The theory of plate tectonics developed from various historical observations:
1620, Francis Bacon: Noted that continents on opposite sides of the Atlantic Ocean fit together, suggesting they were once joined.
1750, Georges Buffon: Observed similar life forms in Europe and North America, supporting the hypothesis of a past connection.
1883, Edward Suess: Proposed that southern hemisphere continents formed a supercontinent called Gondwana.
1908-1912, Alfred Wegener: Formulated the hypothesis of continental drift, proposing continents move over time.
1950s-1960s: Ocean floor mapping and other discoveries led to the modern theory of plate tectonics.
2. Earth's Internal Structure
Layer | Composition | Approximate Thickness | Characteristics | Density (g/cm³) |
|---|---|---|---|---|
Crust | Low-density rocks (basalt in oceans, granite in continents) | 5–50 miles (8–80 km) | Cold, rigid; oceanic crust is thinner and denser, continental crust is thicker and less dense | 2.7–2.9 |
Mantle | Magnesium/iron silicate minerals | ~1800 miles (2866 km) | Largest volume, with plastic asthenosphere and rigid mesosphere layers | ~4.5 |
Outer Core | Iron and nickel | ~2200 miles (2258 km) | Liquid, slightly cooler than inner core | ~10.7 |
Inner Core | Iron and nickel | Radius ~1222 km (759 miles) | Solid due to intense pressure, similar size to the moon | Very high density |
Earth's Physical Layers
Lithosphere: Crust plus uppermost mantle, cold and rigid, brittle, forms tectonic plates.
Asthenosphere: Upper mantle layer below lithosphere, behaves plastically allowing plate movement.
Mesosphere: Deeper, rigid mantle layer.
Seismic Wave Evidence: P-waves can travel through solids and liquids; S-waves cannot travel through the liquid outer core, helping reveal Earth's interior.
3. Theory of Continental Drift
Alfred Wegener proposed that continents move over Earth's surface. Key evidence included:
Fit of continents like South America and Africa.
Matching rock formations and mountain ranges across continents.
Identical fossils found on widely separated continents.
Climatic evidence such as glacial deposits distributed across multiple continents.
4. Evidence for Sea Floor Spreading and Plate Movement
Harry Hess (1960): Proposed that new oceanic crust forms at mid-ocean ridges as tectonic plates separate.
Old ocean crust is destroyed in subduction zones at deep ocean trenches.
The movement is fueled by mantle convection cells.
Radiometric dating shows oceanic crust youngest near ridges and older farther away.
Earthquakes align along spreading centers and subduction zones.
Magnetic stripes on the ocean floor record Earth's magnetic field reversals, confirming sea floor spreading.
Example Location
Iceland is a unique site where a spreading center is above sea level, allowing direct observation of mid-ocean ridge processes.
5. Additional Concepts on Density and Earth’s Interior Study
Density measures mass per unit volume (g/cm³). Water's density is 1 g/cm³.
Earth's average density is about 5.51 g/cm³, indicating dense inner materials.
Seismic waves from earthquakes and nuclear tests give detailed information about Earth's internal structure.