Marine Science Notes 1

Earth Structure and Plate Tectonics

I. Introduction

  • The Earth has distinct layers.

  • Classification is based on:

    • Chemical composition

    • Physical characteristics

II. Layered Earth

A. Chemical Composition

  1. Crust

    • Thin, low-density outermost material

    • Types of crust:

      • Oceanic crust: Higher density, composition similar to basalt

      • Continental crust: Lower density, composition similar to granite

  2. Mantle

    • Composition: Oxygen (O), Manganese (Mn), Silicon (Si) in the ratio of 4:2:1

    • Knowledge obtained from studies of:

      • Ophiolites: Magma that has risen through ocean floor

      • Xenoliths: Magma brought to Earth's surface by volcanic eruptions

  3. Core

    • Composition: Predominantly Iron (Fe) and Nickel (Ni)

    • Theory based on studying meteorites and seismic wave behavior

B. Physical Characteristics

  1. Lithosphere

    • Cool, rigid outer layer

    • Includes crust and uppermost mantle

    • Thickness: ranges from 0 km to 280 km

    • Broken into tectonic plates

  2. Asthenosphere

    • Upper mantle below lithosphere

    • Thin, hot, flows slowly due to convection currents

  3. Mesosphere

    • Lower to middle mantle

    • Rigid and immobile due to high pressures and temperatures

    • Depth: Approximately 410 km - 660 km

  4. Outer Core

    • Entirely liquid layer

    • Discovered by geophysicist Inge Lehmann through seismic studies

    • Responsible for Earth's magnetic field

    • Approximately 2,300 km thick

  5. Inner Core

    • Solid due to immense pressure

    • Pressure prevents minerals from vaporizing

III. Isostatic Equilibrium

  • The lithosphere "floats" on the asthenosphere, which behaves like a liquid.

  • Heavier pieces sink deeper into the asthenosphere, while lighter pieces float higher.

  • Example: Basalt (denser) sinks beneath the ocean while granite (less dense) rises above.

IV. Seismic Studies

A. Seismic Waves

  • Studying seismic waves reveals the layers of Earth.

  • Seismic waves are produced by earthquakes and detected with seismographs.

  • Two types of seismic waves:

    1. P waves (Primary Seismic Waves):

    • Can pass through both solid and liquid.

    • Travel nearly twice as fast as S waves.

    • Behavior similar to sound waves.

    1. S waves (Secondary Seismic Waves):

    • Cannot travel through liquids.

    • Do not pass through the core.

B. Seismic Wave Behavior

  • P waves exhibit refraction when passing through varying densities.

  • S waves create a shadow zone and are absorbed at the core boundary.

V. Heat and the Asthenosphere

  • Question: What maintains the asthenosphere's pliability?

  • Answer: Internal heat from the Earth's core, sourced from:

    • Decay of radioactive elements (Potassium (K), Uranium (U), Thorium (Th))

    • Heat transfer mechanisms include:

    • Conduction: Through solid layers

    • Convection: Through the asthenosphere

VI. Historical Theories

A. Gondwana Theory

  • Proposed by Edward Seuss in the 1800s.

    • Suggested a single landmass, Gondwana, surrounded by one ocean (Tethys Ocean).

    • Cited fossil similarities as evidence that continents were once connected.

    • Did not endorse the idea of continental drift.

B. Continental Drift Theory

  • Proposed in 1912 by Alfred Wegener.

  • Claimed that all continents were once part of a supercontinent named Pangaea, surrounded by the Panthalassa ocean.

C. Seafloor Spreading

  • Proposed by Harry Hess in 1962, observed via sonar maps of the Pacific during WWII.

    • Theory that convection currents drive continental motion.

    • This leads to:

      • Spreading centers: Mid-ocean ridges where new seafloor forms.

      • Subduction Zones: Trenches where older seafloor is destroyed.

VII. Plate Tectonics

  • The lithosphere is divided into tectonic plates that float on the asthenosphere.

  • Plate Boundaries include:

    1. Divergent Boundaries: Plates move apart, forming ridges and rift valleys.

    2. Convergent Boundaries: Plates move towards each other, forming trenches and causing subduction.

    3. Transform Boundaries: Plates slide past each other, causing faulting.

VIII. Divergent Zones

  • Features of divergent zones include:

    • New seafloor formation at spreading centers, resulting in ocean ridges and rift valleys.

    • Presence of black smokers at mid-ocean ridges.

IX. Convergent Zones

  • Characteristics:

    • Old seafloor destruction

    • Formation of features like trenches

  • Example: Peru-Chile Trench and associated mountain ranges.

  • Involves collision of different types of lithosphere:

    • Granite (continental) and Basalt (oceanic).

    • Basalt being denser slides beneath granite in convergence scenarios.

X. Transform Zones

  • Plates slide past each other, creating shear stress.

  • Example: San Andreas Fault in California.

XI. Evidence for Plate Tectonics

  1. Paleomagnetism:

    • Basalt contains iron; minerals align with the magnetic field when molten.

    • This historical record aids in understanding tectonic movement and plate boundaries.

  2. Sediment Age and Thickness:

    • Age and thickness of oceanic sediments increase with distance from spreading centers.

  3. Identical Fossils:

    • Fossils found on separate continents suggest prior connection.

  4. Hot Spots:

    • Create island chains tracing plate movement.

  5. Puzzle Piece Formation:

    • Geographical shape of continents suggests they were once joined.

XII. Hot Spots

  • Extremely hot mantle areas that cause localized melting, leading to volcanic activity and the formation of features like the Hawaiian Islands.

XIII. Conclusion

  • The understanding of Earth's structure and dynamics is essential for investigating geological processes, including earthquakes, volcanic activity, and continental drift.

  • Mastery of plate tectonics is crucial for comprehending Earth's past and predicting future geological phenomena.