GLY150: INTRODUCTION TO OCEANOGRAPHY

University of North Carolina Wilmington

I. Theory of Continental Drift

II. Mantle Convection

Proposed by Sir Arthur Holmes in 1931
- Heat from radioactive decay of elements in the interior of the Earth drives convection within the mantle.
- Driving force of continental drift

III. Global Seismic Network

Showed locations of earthquakes throughout the world
Showed the velocity of seismic waves varied through the interior of the Earth, physical layers
- Lithosphere
- Asthenosphere
- Liquid Outer Core
- Solid Inner Core

IV.Theory of Sea Floor Spreading

Proposed by Harry Hess in 1960
- Based on new knowledge about the physiography of the seafloor
- The ocean floors are spreading apart, propelled by convection currents in the mantle
- As the sea floor moves away from the ridge crest, newly formed crust replaces it.
- The older portions of the sea floor are gradually consumed as they descend into the mantle at the trenches

V. Earth's Magnetic Field and Paleomagnetism

Magnetic field generated in liquid outer core
Resembles that of a bar magnetic with its axis inclined 11 degrees from the Earth's geographic axis
Reversals occur on average every 500,000 years from normal (present-day) to reversed polarity
Sea floor basalts record the sequence of reversals over last 160 m.y.
Fred Vine and D. H. Matthews linked the alternating high- and low- intensity stripes of magnetism measured over the sea floor to the concept of sea floor spreading.
By determining ages of seafloor basalts one can determine sea floor spreading rates.
- Mid-Atlantic Ridge 2-3 cm/yr.
- East Pacific Rise 10-16 cm/yr.

VI.Theory of Plate Tectonics

Proposed by J. Tuzo Wilson in 1965
The outer, rigid shell (lithosphere) of the Earth is divided into approximately 13 large plates and numerous small plates.
These plates are in motion with respect to one another (relative motion) and with respect to the underlying asthenosphere (absolute motion).
Further evidence:
- Earthquakes
- Ocean drilling to determine sediment ages and thickness
- Measuring distances between widely separated points on different lithospheric plates
- Hot spots

VII. Plate Boundaries

Divergent
- Two plates spreading apart
- Upwelling molten rock from mantle, new oceanic lithosphere is created
  - e.g. Mid-Atlantic Ridge
Convergent (Click here to see a virtual flyover of a convergent plate boundary (Mpeg file 6.6 MB))
- Two plates moving together, one sliding beneath the other
  - Large magnitude, deep earthquakes
  - Volcanic arcs
  - Trenches
- Oceanic-Oceanic
  - e.g. subduction of the N. America plate beneath the Caribbean plate, Lesser Antilles
- Oceanic-Continental
  - e.g. subduction of the Nazca plate beneath the South America plate, Chile
- Continental-Continental
  - e.g. subduction of the India plate beneath the Eurasia plate, Himalayas
Transform
- Two plates sliding past one another
- Lithosphere is neither created nor destroyed
  - e.g. San Andreas Fault

VIII. Plate Reconstructions