CONTINENTAL DRIFT AND PLATE TECTONICS

Continental drift concept Introduction and early concepts

Introduction – "did the continents originate at their present

locations or did they drift to where they are today– 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)

Initial ideas of continental drift • probably the most obvious feature which would lead

someone to believe the continents drifted apart is the map fits on 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

• 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

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

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

• convection cells in the Earth's interior are the force which split the lithosphere and are the driving force in continental separation

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).

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

Classification of plate boundaries 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

• 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

• includes plates with ocean basin to continental boundary ((page 440-A(490A)) (example--Cascade Mts.),

• ocean basin to ocean basin boundary (examples--Aleutian Islands and Japan—• continental to continental boundary (example--India and Asian boundary

• 3. Transform – Plates separated by horizontal plate boundary– Plates move in opposite directions along the plate boundary