Earth: Portrait of a Planet - Study Notes

EARTH: PORTRAIT OF A PLANET (SIXTH EDITION)

Author: Stephen Marshak

Chapter 3: Plate Tectonics - Early
  • Copyright: 2019 W. W. Norton & Company
Topographic Features of Earth
  • Distribution of Earth's surface features based on height and depth.
    • Trenches:
    • Deep geological structures indicating subduction zones.
    • Abyssal Plains: Flat areas on the ocean floor, typically located between the continental margins and mid-ocean ridges.
    • Mountain Ranges: Rise significantly above sea level; the highest mountain is Mount Everest at 8.85 km.
Heights and Depths of Earth's Features
  • Statistical Data:
    • Average height of land: 0.84extkm0.84 ext{ km}
    • Average depth of ocean: 3.69extkm3.69 ext{ km}
    • Relationships:
    • Highest mountain (Everest): 8.85extkm8.85 ext{ km}
    • Deepest trench (Mariana Trench): 11.04extkm11.04 ext{ km}
Early Theories of Plate Tectonics
  • Alfred Wegener (1915): Proposed the concept of the supercontinent Pangaea and the theory of continental drift.
  • Harry Hess (1960): Introduced seafloor spreading and subduction as central concepts.
  • Model Development: By 1968, a comprehensive model of plate tectonics was established.
Continental Drift Theory (Wegener's Concepts)
  • Key Propositions:
    • Continents are in motion and were once assembled into a single supercontinent called "Pangaea".
Evidence Supporting Continental Drift

  1. Fit of the Continents:

    • Continents align like puzzle pieces.
    • True edge is continental shelf instead of the shoreline (Bullard fit).

  2. Paleoclimate Belts:

    • Similar ancient climate zones observed across continents when aligned as Pangaea.

  3. Distribution of Fossils:

    • Fossils found in multiple continents that are currently separated.

  4. Matching Geological Units:

    • Similar rock types and mountain ranges detected in separate regions.
    • Example: Appalachian Mountains correspond with formations in Great Britain, Scandinavia, and Africa.
Opposition to Wegener's Theory
  • Fixist View:
    • Argued that no known forces could drive continental movement.
    • Criticized Wegener’s idea of continents plowing through ocean floor as implausible.
Transition to Mobilist View (Harry Hess)
  • Post-WWII understanding shifted due to advances in technology.
    • New evidence for moving crust emerged following the exploration of seafloor spreading.
Seafloor Spreading
  • Mechanism:
    • New ocean crust forms at mid-ocean ridges as the crust stretches apart.
    • Old crust descends and is destroyed at deep-sea trenches, causing seismic activity and distinct geological features.
Modern Understanding of Plate Tectonics
  • Paleomagnetism: Study of the Earth's magnetic field in rocks over geological time.
  • Wandering Poles: Historical movement of Earth's magnetic poles relative to the continents.
  • Magnetic Reversals: Occurrences where Earth's magnetic field flips, vital for understanding seafloor patterns.
  • Bathymetric Features: Rifts, trenches, and ocean sediments offer insights into tectonic plate movements.
  • Distribution of Earthquakes and Volcanism: Correlation with tectonic plate boundaries.
Earth's Magnetic Field
  • Magnetic Axis vs Spin Axis: Magnetic field is tilted approximately 10° to the spin axis of the Earth.
Paleomagnetism Concepts
  1. Paleomagnetic Declination (D): Difference between the magnetic north and geographic north.
  2. Paleomagnetic Inclination (I): Reflects latitude at which rocks formed, differing from the equatorial alignment.
  3. Dipole Model: Earth's magnetic field can be simplified to a dipole model represented by a bar magnet.
Magnetization Process in Rocks
  • As molten lava cools, magnetic dipoles align with Earth's magnetic field.
  • Once solidified, this alignment becomes locked in, preserving a record of Earth’s magnetic field at the time of formation.
Apparent Polar-Wander Path
  • Sequence of paleomagnetic poles illustrates the historical movement of Earth's continents.
Bathymetry and Geological Features Evidence
  • Seafloor Mapping: Sonar technology enables bathymetric profiles to be created for ocean floors.
  • Patterns observed support the theory of plate movements.
Oceanic Crust Age Distribution
  • Oceanic crust ages are younger at mid-ocean ridges and older as you move away from these boundaries.
  • Isochron Maps: Used to understand the ages of different seafloor parts through magnetic reversals data.
Key Questions for Study
  1. What were the main concepts proposed by the early theories of Continental Drift?
  2. What evidence supports the theory of sea-floor spreading?
  3. How do magnetic reversals provide evidence for sea-floor spreading?
  4. Why do oceanic rocks exhibit a distinct magnetic signature?
  5. How do oceanic crust ages vary in relation to divergent plate boundaries?
  6. Analyze the bathymetric features of spreading ridges in terms of their elevation and geological significance.
Miscellaneous
  • Geological Time Scale: Overview of significant epochs and periods relevant to the discussion of plate tectonics and paleomagnetism.
  • Macroscopic Observational Evidence: Connections to the geological past inferred from the physical formations present today.
End of Lecture Slide Set for Chapter 3
  • Contributors: Enhanced Art Slides prepared by Brian Zimmer, Appalachian State University