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Continental Drift and
Plate Tectonics
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. The
fit is even more striking when the
submerged continental shelves are
compared rather than the coastlines.
In 1912 Alfred Wegener (1880-1930)
noticed the same thing and proposed that
the continents were once compressed into
a single protocontinent 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.
14_17.JPG
Wegener had four main pieces of evidence.
First he noted the jigsaw fit of South
America and Africa, especially, but also
elsewhere.
14_02a.jpg
Wegner also noted that fossils from South
America and Africa came from the same
extinct animal. Both continents back then had
the same climate and vegetation, today that is
not the case.
14_03.JPG
He found that on both sides of the Atlantic,
mountains were the same; both in terms of
age and structure.
14_02b.jpg
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?
14_04b.jpg
As technology progressed two other
evidences were added to Wegner’s
Theory. One is called Sea Floor
Spreading which will be talked about in
detail in the next few slides. 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!
Sea Floor Spreading:
“Advances in sonic depth recording during
World War II (SONAR) led to a detailed
mapping of the ocean floor. The Ocean Floor
in the Mid-Atlantic was found to be spreading
apart. Among the seafloor features that
supported the sea-floor spreading
hypothesis were: mid-oceanic ridges, deep
sea trenches and island arcs”
http://www.ucmp.berkeley.edu/geology/tecmech.html
The crust near the continental margins (the shoreline of the
continents today) is about 200 million years old. It gets
progressively younger toward the mid-Atlantic ridge, where
oceanic crust is forming today (red).
14_09.JPG
Scientists learned that the youngest regions of the ocean floor were along
the mid-oceanic ridges, and that the age of the ocean floor increased as
the distance from the ridges increased.
http://www.ucmp.berkeley.edu/geology/tecmech.html
Flash Corner:
See a neat flash movie on Continental Drift!
Wegener's hypothesis of continental
drift lacked a geological mechanism to
explain how the continents could drift
across the earth's surface.
It wasn’t until the 1960s that the theory of
Plate Tectonics was advanced to
explain how the continents could
separate. A Canadian by the name of
Tuzo Wilson played an important part in
the development of this theory.
What Tuzo Wilson did was change the way
scientists viewed the internal structure of the
earth.
Old
http://www.pbs.org/wnet/savageearth/animations/hellscrust/index.html
New
A simple look at the Earth’s Interior
A bit more
complicated
A closer look
A comparison of the thickness
In order for the theory of plate tectonics to be
possible. The crust of the earth called the
Lithosphere was subdivided. The upper
portion of the Lithosphere was called the
Earth’s crust.
The Crust had to adjust itself based on
density – the crust is composed of a dense
material mostly found at the bottom of
oceans called Oceanic Crust (basalt) and a
less dense material which we call the
Continental Crust (granite). But since there
is ‘more’ continental crust, it actually has
more weight over the mantle. Hopefully the
next diagrams will help!
Isostac(s)y
One interesting property of the continental
and oceanic crust is that these tectonic
plates have the ability to rise and sink. This
phenomenon, known as ISOSTACY, occurs
because the crust floats on top of the mantle
like ice cubes in water.
When the Earth's crust gains weight due
to mountain building or glaciation, it
deforms and sinks deeper into the
mantle. If the weight is removed, the
crust becomes more buoyant and floats
higher in the mantle.
This process explains recent changes in the
height of sea-level in coastal areas of
eastern and northern Canada and
Scandinavia. Some locations in these
regions of the world have seen sea-level rise
by as much as one meter over the last one
hundred years. This rise is caused by
isostatic rebound.
Both of these areas where covered by
massive glacial ice sheets about 10,000
years ago. The weight of the ice sheets
pushed the crust deeper into the mantle.
Now that the ice is gone, these areas are
slowly increasing in height to some new
equilibrium level. (PhysicalGeography.net)
The bottom portion of the Lithosphere was
renamed and called the Asthenosphere and
it has plastic characteristics, thus allowing
the plates of the earth to float on top and
move!
http://geog.ouc.bc.ca/physgeog/contents/10i.html
The main features of plate tectonics are:
• The Earth's crust is broken into a series of
plates (crustal plates) or pieces.
• These 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.
As mentioned before there are actually two
types of crust:
• Oceanic crust, which is thin and of course
found at the bottom of the oceans. It is a
compact, thin and heavy crust.
• Continental crust, since it has been
exposed to the atmosphere is bulkier (air)
and lighter than Oceanic crust.
http://geog.ouc.bc.ca/physgeog/contents/10i.html
Convection Currents power the plate
movements. Convection currents rise up
from the radioactive core, carrying heat to
the thin crust of the earth.
• The currents in the asthenosphere are
generated by heat rising to the earth’s
surface from the hot radioactive core
• At their boundaries, the plates spread
apart, converge, and slide past one
another
• This makes these areas the most
geologically active: earthquakes and
volcanoes and mountains
Earth’s Major Plates and their movement
Earthquakes and Volcanoes
http://www.ngdc.noaa.gov/mgg/ima
ge/mggd.gif
The Surface of the Earth without water
Click here to go to the actual site and zoom into
certain areas
There are four basic Plate movements or
boundaries:
1. Divergent: This is where the plates move
apart, new magma wells up to the surface,
forming new crust. The Mid-Atlantic ridge is a
prime example. New land is created
2. Convergent: Two plates come together.
Usually one of the plates subducts (goes under)
the other plate, creating a Subduction zone. The
crust at the leading edge of the subducting plate
melts back to magma. The Pacific Rim of Fire is
a good example. Land is destroyed – balance.
3. Transform Boundaries: This occurs
when two plates rub against each other.
This creates tremendous friction which is
eventually released in the form of
earthquakes. The San Andreas Fault is a
Transform boundary.
4. Isostacy(Rebound): Plates moving up
or down depending on the weight on the
plate. Glaciers and Mountains add weight.
Hot Spots – Hawaii – An area where
magma is being released and the
‘volcano’ is not depended on plate
movement – an ‘ever erupting volcano’.
The main types of plate movements.
Click here to see an excellent
‘Shockwave’ image of the movements of
the plates.
Iceland: On a Divergent Zone
What can
happen at a
Divergent
Zone.
Many things can happen at a Convergent
Zone:
Oceanic-Continental
Collision
Result - Volcanic mountains or arcs
Oceanic-Oceanic
Collision
Result - Island Arcs
Oceanic trenches, which are as deep as
35,000 feet below the ocean surface, are long
and narrow, and run parallel to and near the
shorelines of the continents. They are
associated with and parallel to large
continental mountain ranges. There is also a
parallel association of trenches and island
arcs.
http://www.ucmp.berkeley.edu/geology/tecmech.html
The Pacific Ring of Fire has many trenches
Another
look at the
famous
‘Rig of Fire’
14_10b.JPG
Continental-Continental
Collision
Result - Mountain Ranges
Transform Zone
Result - Earthquakes
The San
Andreas Fault,
California
Hot-Spot
http://sts.gsc.nrcan.gc.ca/page1/geoh/quake/figures.htm
Finally Canada’s role: The oceanic Juan de Fuca plate is
moving beneath the continental North America plate at a rate of
about 4 cm/year. Earthquakes occur along part of the boundary
between the two plates and Volcanoes occur as well. Mt. St.
Helens is a result.
Flash Corner:
1. Click here to see an excellent movie on Plate
Tectonics
2. Click here to see how plates move apart
3. Click here to see how plates move together
4. Click here to see how plates rub up against each
other
5. Click here to see how India collided
The End