CONTINENTAL DRIFT AND PLATE TECTONICS

The diagram to the left shows a map fit of the continental masses--this is a piece of data used to confirm that the continents drifted-- the concept of continental drift initiated the rock plate concept

I. Continental drift concept

A. Introduction and early concepts

1. Introduction

• the big question is "did the continents originate at their present locations or did they drift to where they are today?"---even with much data indicating the continents drifted there are those who refute the continental drift idea--we will treat some data supporting the drift idea in I.B. in the outline below

• formulation of the drift concept supplied much information to the rock plate and plate tectonic concept

• our earlier studies on the Earth's interior structure should give us a better understanding of how the lithosphere is able to drift (solid upper Earth floating on the asthenosphere)

2. Initial ideas of continental drift

• probably the most obvious feature which would lead someone to believe the continents drifted apart is the jig saw puzzle or map fit of continents. Looking at a global map it would appear that the continents could be brought together to fit like a jig saw puzzle--this would seem to imply that the continents must have been together at one time and drifted apart

• click here to see the map fit of land masses

• who was the first to see the map fit phenomenon?---Alfred Wegener was the first to publish a summary of the ideas of the continental drift concept in 1912 and for this was given the title of "The Father of Continental Drift"

• some facts noted in the Wegener's publication were:
  • the (protocontinent) was named Pangaea
  • the southern portion was named Gondwana (Gondwanaland) and the northern portion, Laurasia
  • the protocontinent broke in pieces about 200 million years ago

B. Some important data confirming continental drift

1. Map fit (jig saw puzzle effect)--mentioned earlier

2. Mountain chains formed with longitudinal axes perpendicular to the movement of continents

• an example is the formation of the Himalayan Mountains--the trend of the axis is east-west and formed from the Indian subcontinent moving northward buckling up material while colliding with the Asian continent--some believe another example is the Rocky and Andes mountain chains (axes trending mostly N-S) formed in part from the western movement of the North and South American continents

3. Same fossils, rocks, mountain ranges, or glacial features located in areas on different continents representing prejoined positions prior to continental drift

• click here to see matching fossils
• click here to see matching mountain ranges
• click here to see  matching continental ice deposits

4. The presence and shape of the global ocean ridge

• the global ocean ridge is a raised region on the ocean basin which is believe to represent the prejoined or splitting area of separated continents--note the shape of the ridge contours the shape of the coast lines of the separated continents--this is most evident in the mid Atlantic ocean

• click here to see the extension of the global ocean ridge

• convection cells in the Earth's interior are the force which split the lithosphere and are the driving force in continental separation
• click here to see a pair of convection cells

5. Seafloor spreading

• the age of rocks located the same distance away from the center of the ridge on both sides of the ocean ridge are the same--rocks are youngest nearest the ridge center and progressively older away from the center--this indicates new rocks form at the ridge centers and older rocks are pushed away to make room for younger rocks resulting in spreading of the ocean floor and continental drifting

• also the magnetic intensities in the rocks are found to be the same on equivalent sides of the ocean ridge--on both sides of the ridge the magnetic intensity alternates between normal (high intensity) and abnormal (low intensity)
• click here to see magnetic intensities across the ocean ridge

click here for more information treated above and material below

click here for still another reference source for continental drift and plate tectonics

CLICK HERE TO SEE ANIMATIONS OF CONTINENTAL DRIFT PAST AND FUTURE

II. Lithospheric plates and plate tectonics

• if the ocean floor is spreading from ocean ridges and continents are drifting, where are the spreading sections going?-- the answer to this question was important in initiating the concept of rock plates--the question was answered with the discovery of subduction zones in the lithosphere--rock sections moving away from spreading zones are moving towards subduction zones

A. Rock plates

• ocean basin and continental sections float in the asthenosphere, some plates move towards each other, some away from each other, and some parallel to each other

B. Classification of plate boundaries

• click here to see the 3 types of plate boundaries, divergent (A), convergent (B), and transform (C)

• volcanism and seismic activity are concentrated at plate boundaries

1. Divergent

• includes all ridges and rifts
• tensional forces caused by convection cells drive plates apart

• kinds of divergent boundaries

  • includes ocean basin to ocean basin boundary or continental to continental boundary--examples are the mid-ocean ridge and African Rift Valley respectively--click here to see ocean basin to ocean basin boundary andclick here to see continental to continental boundary

2. Convergent

• includes all subduction boundaries
• compressional forces caused by convection cells are active to drive plates towards each other
• volcanic arcs are associated with convergent boundaries

• kinds of convergent boundaries
  • click here to see the 3 types of convergent boundaries

  • includes plates with ocean basin to continental boundary ((page 440-A(490A)) (example--Cascade Mts.), ocean basin to ocean basin boundary ((page 440-B(490B)) (examples--Aleutian Islands and Japan--click here to see these, continental to continental boundary ((page 440-C(490C)) (example--India and Asian boundary click here to see these.

3. Transform

• click here to see the transform fault boundary off the coast of California

C. Map of Major plates on Earth

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