Download Plate Tectonics

Continental Drift
and
Plate Tectonics
All images from
http://pubs.usgs.gov/publications/text/understandi
ng.html#anchor5567033
unless otherwise noted.
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.
• This lead to the theory of Plate
Tectonics (how the plates moved)
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 distribution. He believed that
Pangaea was intact until about 300 million
years ago, when it began to break up and
drift apart.
Three 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.
After a certain
period, whoever,
the fossils begin
to evolve
differently on the
different
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 supporting sea-floor spreading.
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? Supporting
sea-floor spreading.
Making Connections: Canada’s Geography. Clark & Wallace.
Prentice Hall Ginn, 1999.
As technology progressed two other
evidences were added to Wegner’s
Theory. One is called Sea Floor
Spreading. This will be explained in class.
•Do not worry about understanding Magnetic
signature for your test.
The other is called Magnetic Signature. Rocks that are
formed in Polar regions take on a ‘Polar’ characteristic
and rocks formed near the Equator take on an
‘Equatorial’ signature. Huge rocks and mountains with
Equatorial signatures have been found in Polar regions
and vica versa!
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,
spreading from the center, sinking at the
edges, and being regenerated.
•Convection currents beneath the plates
move the crustal plates in different directions.
•The source of heat driving the convection
currents is radioactivity deep in the Earth's
mantle.
http://www.ngdc.noaa.gov/mgg/ima
ge/mggd.gif
The Surface of the Earth with no water!
A map of the 12 MAJOR
PLATES!
In order for the theory of plate tectonics to be
proven, the internal structure of the earth had
to be redrawn. The solid part of the earth, the
part that is exposed to the atmosphere or
covered by the hydrosphere (the oceans) is
known as the lithosphere (or the crust), had
to be broken up into two parts. The
lithosphere now is made up of the crust and
at the bottom of the lithosphere and at the
very top of the mantle we added a new
sphere called the asthenosphere.
The asthenosphere is not a solid and not a
liquid – it is plastic in nature.
OLD
http://www.pbs.org/wnet/savageearth/animations/hellscrust/index.html
New
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 vast set of convection
currents in the asthenosphere
• the asthenosphere churns like a fluid
• the plates on top of the asthenosphere
move according to the direction of the
currents.
Convection currents rise up from the
radioactive core, carrying heat to the
asthenosphere, the asthenosphere begins to
flow due to the heat. The crust of top (the
plates) move like a boat floating on top of
water. This boat can move in many
directions.
So what is happening on the
earth?…
• Some plates are moving apart (ridges of volcanoes)
• Some plates are colliding. If they collide they can
either buckle (mountains form) or some plates will
slide beneath each other (trenches form).
• Some plates are slipping beside each other (faults
occur, which create earthquakes)
• Often earthquakes and volcanoes occur at all of the
plate boundaries due to these processes.
• These processes form very interesting features both
within the ocean and on land.
The plates have three basic
movements…
1. Where the plates move apart, new
magma wells up to the surface, forming
new crust. (volcanoes occur
underwater). New land is being created
at this point. This type of movement is
called Divergent.
Scientists also learned that the youngest
regions of the ocean floor were along the
centre of the mid-oceanic ridges, and that the
age of the ocean floor increased as the
distance from the ridges increased. It was
this discovery that convinced everyone that
Continental Drift and Plate Tectonics were no
longer theories.
http://www.ucmp.berkeley.edu/geology/tectonics.html
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. There is also a parallel association
with island arcs. 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.
Terms
• Divergent zone: Where plates are moving
apart. Magma wells up and a ridge is formed.
• Converging zone: plates are moving towards
each other.
• Subduction: When a plate slips beneath
another plate in a converging zone.
• Slip Zone or Transform fault: Where two plates
rub up against each other.
The MidAtlantic Ridge
is a divergent
zone.
Iceland: On the Mid-Atlantic Ridge
Notice that the trench is parallel to
the Island arcs, and the shoreline of
the continent.
This is a Convergent (subduction) Zone.
Notice that one plate is covered by an
ocean and the other has some land that
is above the water. Also note the creation
of Island Arcs.
Another Convergence Zone. Notice that
one plate is covered by an ocean and the
other is not creating Volcanic arcs.
Yet another Convergence Zone, notice both
plates are exposed (no water) creating a
mountain range.
Indian Plate
collides with
Eurasian Plate
The result: the
Himalayas and
Mt. Everest
Earthquakes! The Pacific Ring of Fire
Transform
plate margins:
where two
plates slip past
one another.
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!)
• Plate tectonics: how plates move (convection currents)
• 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!