Transcript Plate Tectonics - Crescent School

Continental Drift
and
Plate Tectonics
The Theory of Continental Drift
What you will learn about the theory of
Continental Drift.
• Alfred Wegener and the theory of
Continental Drift (PANGEA!)
• Pieces of evidence found to support the
theory of continental drift.
• The theory of Plate Tectonics (how
continental drift happened)
Close examination of a globe often results
in the observation that most of the
continents seem to fit together like a
puzzle: the west African coastline seems to
snuggle nicely into the east coast of South
America and the Caribbean sea; and a
similar fit appears across the Pacific.
In 1912 Alfred Wegener (1880-1930) noticed
the same thing and proposed that the
continents were once compressed into a
single large continent which he called
Pangaea (meaning "all lands"), and over
time they have drifted apart into their current
position. He believed that Pangaea was
intact until about 300 million years ago, when
it began to break up and drift apart.
In the next few slides we will discuss
the evidences that Alfred Wegner and
his followers used to prove that the
Continents indeed drifted apart.
Four Main Pieces of Evidence
1. Wegener
noted the jigsaw
fit of South
America and
Africa, especially,
but also
elsewhere.
Making Connections: Canada’s Geography. Clark & Wallace.
Prentice Hall Ginn, 1999.
2. He found
fossils that were
the same on
both continents.
Making Connections: Canada’s Geography. Clark &
Wallace. Prentice Hall Ginn, 1999.
3. He found that on both sides of the Atlantic,
mountains were the same both in terms of
age and structure.
Making Connections: Canada’s Geography. Clark & Wallace.
Prentice Hall Ginn, 1999.
4. He found that ice sheets covered parts of
Africa, India, Australia and South America 250
million years ago. How could this happen in
places that are so warm today?
Making Connections: Canada’s Geography. Clark & Wallace.
Prentice Hall Ginn, 1999.
Problem? Wegener's hypothesis of
continental drift lacked a geological
mechanism to explain how the continents
could drift across the earth's surface.
….thus, the theory of PLATE TECTONICS!
It wasn’t until the the 1960s that the theory of
plate tectonics was advanced to explain how
the continents could separate.
The main features of plate tectonics are:
• The Earth's crust is broken into a series of
plates or pieces.
• The plates are continually moving.
http://www.ngdc.noaa.gov/mgg/ima
ge/mggd.gif
The Surface of the Earth with no water!
A map of the PLATES!
So let’s explain what’s happening!
• the outer surface of the Earth is a thin
crust of fragile rock, fractured like the
cracked shell of an egg
• the pieces of the shell are Earth's tectonic
plates -- there are 12 major ones -- and they
float along on top of a layer called the
asthenosphere
The plates have three basic
movements…
1. The plates move apart, new magma
wells up to the surface, forming new
crust. (volcanoes occur). New land is
being created at this point. This type of
movement is called Divergent.
2. When two plates collide, one plate may go
underneath (subduction) the other creating
huge valleys or oceanic trenches. These
trenches are as deep as 35,000 feet below
the ocean surface, are long and narrow,
and run parallel to and near the shoreline of
continents. They are associated with and
parallel to large continental mountain
ranges as well. This type of plate movement
is called Convergent.
When one plate goes underneath the other.
The plate that goes under will begin to melt
as it approaches the mantle. This new hot
molten rock will rise and if it breaks through
the lithosphere it will create a volcano. It is
important to note that since part of the
lithosphere is being lost at this point, there
must be some other point on the earth where
new land is being created, remember
divergent zones!
3. In some parts of the world plates are
rubbing up against each other creating
tremendous friction. When these plates
actually slip (move) they release this
energy in the form of earthquakes. Eg.
The San Andreas fault. This type of
movement is called Transform.
Notice that the trench is parallel to
the shoreline and the mountain
range
The three basic movements of plates.
The MidAtlantic Ridge
is a divergent
zone.
Iceland: On the Mid-Atlantic Ridge – GeoThermal Energy
Indian Plate collides
with Eurasian Plate.
This is an example
of a Convergent
Zone.
Mountains!
The result: the
Himalayas and
Mt. Everest
Earthquakes and Volcanoes! The Pacific Ring
of Fire
Transform
plate margins:
where two
plates slip past
one another.
Earthquakes!
The San
Andreas Fault,
California
http://sts.gsc.nrcan.gc.ca/page1/geoh/quake/figures.htm
Tectonic setting of western British Columbia and
Washington state. The oceanic Juan de Fuca plate is
moving beneath the continental North America plate at a
rate of about 4 cm/year. Great earthquakes occur along
part of the boundary between the two plates.
Summary
• Theory of continental drift and evidence to support it.
(Pangea!)
• Different movements of plates, geologic processes
and associated landforms
–
–
–
–
–
Diverging: ridges/volcanoes
Converging: trenches, mountains, island arcs
Subducting: same as above
Slipping/Transform: faults and earthquakes
Earthquakes and Volcanoes: along major tectonic plate
boundaries
The End!