LECTURE NOTES:  The Continental Drift Theory and Plate Tectonics

The Continental Drift Theory was Proposed by Alfred Wegener, 1915 in his book,

“The Origins of Continents and Oceans”
“All the modern continents were united in the Paleozoic Era as a single supercontinent called Pangea”


Geologists Observed: Similar rock materials are found on continents now separated by deep oceans.

Evidence for Continental Drift…

Plant and animal fossils of the same species of were found on different continents which are now separated by an ocean.

Evidence for Continental Drift… 

Evidence from glacial ice movement indicated polar ice once flowed outward over a number of different continents.

Evidence for Continental Drift…

Edward Bullard & coworkers observed that the present day continents, actually the continental shelf regions, fit together much like a puzzle.

Exploration of the Sea Floor provided important evidence for Sea Floor Spreading: 
The Mid Atlantic Ridge

New Evidence: Apparent Polar Wander

Ocean crust basalts contain the mineral magnetite. Recall these rocks preserve the orientation of the Earth’s magnetic field when they cool. This magnetic signature is plotted to show changes through time.

When the "apparent pole" locations were examined N. America and Europe the "magnetic pole" locations were different. By retracing the paleolatitudes, N.A and Europe followed a similar wander path.

Sea Floor Spreading at Mid Ocean Ridge

Analysis of rock samples show a symmetric pattern of normal and reverse polarities parallel to the ridge AND increase in age away from the ridge.

    Age of Crustal Rocks on the Sea Floor

Transform Fault Boundaries

Modern Evidence of Plate Motions

Modern plate motions can be measured using data collected from satellites

Other Evidence: Location of Volcanoes

Other Evidence: Earthquake Foci Locations

Plate Boundaries Convergent Plate Boundaries

Crust is lost or recycled by subduction

  • A trench

  • Accretionary wedge

Ocean-ocean

  • Island arc volcanoes:

  • Japanese Islands

Ocean-continent type

  • Andesitic volcanoes

  • Cascades Mtns., Wa. Or

Continent-continent type

  • The Himalayan Mountains of India and China      

 

Summary of Evidence for Plate Tectonics

  1. Young age of ocean floor basalts

  2. Presence of repeated reversals of the Earth magnetic field in the geologic past

  3. Observations along midocean ridges – spreading of the sea floor and recycling of crust along subduction zones

  4. Precise documentation that the world's earthquake and volcanic activity is concentrated along oceanic trenches and submarine mountain ranges.

Wilson Cycle of Tectonic Plate Evolution

Phase I: Rifting of continental crust formation of a new ocean basin. Example: Red Sea 

Phase II:  The ocean basin widens and a mid-ocean ridge forms.. Example: Atlantic Ocean

Phase III: A subduction zone develops causing the ocean basin to closeDevelopment of Andesitic volcanoes within the Eastern continent. 

Phase IV: The ocean basin continues to close until both continents collide. The Himalayas  

Mantle Plumes and Hot Spots

Mantle heat can be concentrated beneath a lithospheric plate. This causes melting in the lower lithosphere as the plate migrates over the "hot spot". This leads to formation of volcanoes such as the Hawaiian Island Chain.

Let’s Think about it

  1. In which direction did the Pacific Plate move to produce the Emperor Seamounts?

  2. How long did this movement last?

  3. In which direction did the plate now move ?

  4. How fast did the plate move after 39.9 my ?

SUMMARY

  • Plate Tectonic theory defines plates as segments of continental and ocean crust including the upper mantle.

  • These plate lay on a partially molten mantle circulate heat which cause plate to move.

  • Many processes such as earthquakes, volcanism, mountain building, erosion just to name a few are effected by plate motions.

Theory of Plate Tectonics

Finally, recall that convection of heat in the asthenosphere is believed to provide the mechanism that Wegener’s Continental Drift theory lacked.  It’s this process that has been responsible for many mountain building processes and other volcanic and earthquake activity

 

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