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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:
•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 proto-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.
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
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.
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)
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.
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
‘Ring 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.
Folding, Faulting and Denudation
Folding is the process that bends and
twists rocks, usually due to compression
Faulting is the process where rocks move
past each other along a fracture
http://www.geog.ouc.bc.ca/physgeog/
contents/10l.html
Plate boundaries
• There are three types of plate
boundaries:
– spreading zones,
– transform faults, and
– subduction zones
Spreading Zones
• At spreading zones, molten rock rises, pushing two
plates apart and adding new material at their edges.
• Most spreading zones are found in oceans; for example,
the North American and Eurasian plates are
spreading apart along the mid-Atlantic ridge.
• Spreading zones usually have earthquakes at shallow
depths (within 30 kilometers of the surface).
• This type of crustal deformation is called tension…
pulling apart.
• Tension extends the crust causing it to thin and
lengthen. Rifting, like that which created the Great
Rift Valley of Africa, is a result of tension
Transform Faults
• Transform faults are found where plates slide past one
another. An example of a transform-fault plate boundary
is the San Andreas fault, along the coast of California
and northwestern Mexico.
• Earthquakes at transform faults tend to occur at shallow
depths and form fairly straight linear patterns .
• When plates slide past one another in opposite directions
along transform plate boundaries, a shearing stress is
created.
• Shearing stress cuts the crust into parallel blocks
displacing them horizontally relative to one another.
• Shearing takes place along the San Andreas Fault where
the Pacific Plate is moving past the North American
Plate.
Fault
Normal
Transform Fault
Stike-Slip Fault
Reverse Fault
Thrust Fault
Normal
Fault
Reverse
Rift Fault or
Fault
Graben is
produced when
tensional stresses
result in lowering
or sinking of a
block of rock
Horst
Fault is
the
reverse of
a Graben
Subduction zones
• Subduction zones are found where one plate overrides,
or subducts, another, pushing it downward into the
mantle where it melts.
• An example of a subduction-zone plate boundary is found
along the northwest coast of the United States, western
Canada, and southern Alaska and the Aleutian Islands.
• Subduction zones are characterized by deep-ocean
trenches, shallow to deep earthquakes, and mountain
ranges containing active volcanoes.
• This type of crustal deformation is called compression.
Less common than Ocean Subduction
zones are C to C convergences