Plate+Tectonics

Introduction to Plate Tectonics: The study of the movement of large plates that cover Earth's crust and the geologic activity caused by this movement, including earthquakes, volcanic activity, and mountain building.

Structure:
- Crust:
  - Continental Crust: Primarily composed of granite, thicker and older.
  - Oceanic Crust: Composed mainly of basalt, thinner and younger.
- Mantle: Mainly olivine, extends beneath the crust.
- Core: Comprised predominantly of iron, with some nickel.

Lithosphere: Strong and rigid outer shell of rock, including crust and upper mantle.
Asthenosphere: Weak and ductile layer beneath the lithosphere, allows for movement of the lithosphere above it.

Alfred Wegener (1920s) proposed continental drift, suggesting continents were once a supercontinent named Pangaea.
- Evidence for Drift:
  - Fitting shapes of continents (especially coastlines).
  - Similar fossils found on continents now separated by oceans.
  - Ancient climatic indicators aligned with Pangaea.
  - Glacial deposits found in warmer regions counterintuitive without land movement.
Resistance to Wegener's Theory: Lack of solid mechanisms to explain how continents moved; ideas of centrifugal effects and tidal forces were rejected.

Types of Boundaries:
- Divergent: Plates move apart, creating new seafloor (sea-floor spreading).
- Convergent: Plates collide, leading to subduction and mountain formation.
- Transform: Plates slide past each other, causing earthquakes, as seen with the San Andreas Fault.

Harry Hess: Proposed that new oceanic crust is created at mid-ocean ridges and subducted elsewhere.
Evidence:
- Bathymetry: Sea-floor mapping reveals ridges and trenches.
- Paleomagnetism: Shows symmetrical magnetic anomalies across mid-ocean ridges, indicating sea-floor spreading.
- Age of Oceanic Crust: Younger crust found at mid-ocean ridges, older crust farther away.

Earth’s Magnetic Field: Generated by movement in the liquid outer core, leads to declination and inclination in rocks.
Magnetic Reversals: Documented changes in Earth's magnetic field over time, preserved in volcanic rocks.

Hot-spot Mechanism: Result from mantle plumes, generating volcanoes as plates move over fixed hot spots (example: Hawaiian Islands).

Ridge Push: Gravitational pull from elevated mid-ocean ridges pushes plates apart.
Slab Pull: Denser, older oceanic lithosphere sinks back into the mantle, pulling the rest of the plate with it.

The theory of plate tectonics revolutionized our understanding of geological processes and the dynamic nature of Earth, explaining patterns of earthquakes, volcanoes, and the evolution of continents throughout geological history.