Download Plate-Tectonics A review

Plate-Tectonics
A review
Imtiyaz A. Parvez
C-MMACS, Bangalore
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The Theory of Continental Drift
Alfred Wegener (1912) proposed:• A larger super-continent PANGEA split into smaller
fragements about 200-300 million years ago. These then
drifted apart to form the present arrangement of continents
• He had no satisfactory mechanism to offer, but appealed to a
less-dense continent “floating” and “drifting” over a more
dense oceanic crust (Like icebergs in the ocean).
Most Scientists were highly skeptical and the idea was NOT
widely accepted.
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This is what Wegener thought Pangea looked
like 200-300 million years ago.
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Fits of continents
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Various attempts to fit Africa and South America
Shoreline fit
(not great)
200 m. fit
(better)
Work done
by Carey in
1958
Continental
Slope 2000 m
(best)
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After fitting the continents of Africa and
South America, it is found that
Similar rock types extend from one continent to the other
The rocks are also of the same age
COCLUSIONS - perhaps the two continents were once part of a
single largest continent?
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Similar rock types extend across continents
Rocks in the Appalachians of North
America and the Caledonides of
Britain and Norway are very similar
and are also similar in age
When we fit Europe and North
America together, we find that the
Appalachians and Caledonides
form a single mountain chain.
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Distribution of Mesosausarus
Mesosaurus was a small reptile that lived about 250 million
years ago. Fossils of Mesosaurus are found in both
South America and Africa (green shaded areas)
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Distribution of other Reptiles and Plants
Similar fossils (reptils and plants) are found on the different continents.
How could they have crossed the oceans?
CONCLUSION- they did not, the continents were part of the same
landmass about 200-300 million years ago
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This is how Wegener thought the continents
moved over the last 200 million years:-
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Arguments Favoring Continental Drift
• Fits of continents
• Apparent discrepancy in inferred latitudes of ancient
rocks
• Rocks of same age and similar characteristics on
different continents
• Distribution of similar plants and animals on different
continents (how did they cross the ocean)
WEGENERS CONCLUSIONS:
The continents have drifted over the past 300 million years
to their present positions!
(not a very popular idea at the time!!!)
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Plate Tectonics
• The theory that describes the earth as being composed
of giant solid plates that move relative to each other
over the molten rock of the earth’s core.
PLATE TECTONICS
=
CONTINENTAL DRIFT
+
SEA-FLOOR SPREADING
+
SUBDUCTION
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Theory of Plate Tectonics
The theory of plate tectonics was a revolution in
the earth sciences that explained most of the
major geological features of the earth’s crust in a
single comprehensive theory
IT EXPLAINED
•The history of continents and ocean basins.
•Location of earthquake zones.
•Location of mountain ranges and mountain building.
•The location and origin of volcanoes.
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There are three basic types of plate
margins:-
1. SPREADING
2. CONVERGING
3. TRANSFORM
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SPREADING - plates move apart at
ocean ridges or split continents apart
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CONVERGING- Oceanic-continental
convergence
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CONVERGING- Oceanic-Oceanic
convergence
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CONVERGING- Continentalcontinental convergence
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Transform- Plates slide horizontally past
each other
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World-wide system of spreading ridges
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Three basic converging boundaries
Ocean - Continent Collision
Ocean - Ocean Collision
Continent - Continent Collision
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Ocean - Ocean Collision
Examples
Japan
Aleutian Islands
Indonesia
Tonga - Fiji
An oceanic plate is subducted beneath another oceanic plate,
resulting in the formation of an oceanic trench and island arc (Japan)
Earthquakes along the subducted slab
The sea behind the arc is Black-Arc Basin
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Ocean - Continent Collision
Dense oceanic crust is
subducted beneath lighter
Continental crust, resulting in the
formation of an oceanic trench
and a linear volcanic mountain
range.
Melting occurs both in the downgoing slab and in the crust
producing large diversity of
volcanic rocks. Also Earthquakes
along Bebioff Zone.
Examples- Cascades and Andes
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Continent - Continent Collision
Examples
Himalayas
European Alps
Mountain Chains
Earthquakes
No Volcanoes
Initiated as oceanic - continent subduction. Continental
crust on the subducted plate is too light to be subducted.
Consequently it crashes into the other continental crust,
Squeezing and folding the sediments between them to
produce a high mountain range.
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India collided into Eurasia
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History
of the
earth -
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Tectonic Plates
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Journey of the India
Landmass (Indian Plate)
before its collision with
Asia (Eurasian Plate)
about 40 to 50 million
years ago
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Seismicity around the world
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