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
·
a solid lithosphere floating on top of the asthenosphere gives an idea how plates can move
·
Accretion of continental material is also an important
during continental drift to help shape the continents today— see
page 322 in text
·
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
·
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:
o
the (protocontinent) was named
Pangaea
o
the southern portion was named Gondwana
(Gondwanaland) and the northern portion, Laurasia
o
the protocontinent broke in pieces
about 150-200 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 – see
page 59 in text for formation of a divergent boundary
·
includes all ridges and rifts
·
tensional forces caused by convection cells drive plates
apart
·
kinds of divergent boundaries
o 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 and click here to see continental to continental boundary
·
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
o
click here to see the 3 types of convergent boundaries
·
click here to see the transform fault
boundary off the coast of California
C. Map of Major plates on Earth
