Download Plate Tectonics

Continental Drift and
Plate Tectonics: A Closer
Look
In 1912 Alfred Wegener (1880-1930)
noticed that the continents fit
together like the pieces of a puzzle.
He proposed that the continents
were once compressed into a single
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.
1. He noted the jigsaw fit of South America
and Africa, especially, but also elsewhere.
14_02a.jpg
2. 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.
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3. He found that on both sides of the Atlantic,
mountains were the same; both in terms of
age and structure.
14_02b.jpg
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?
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As technology progressed, one other
evidence was added to Wegner’s Theory:
Sea Floor Spreading which will be
explained in detail in the next few
slides.
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).
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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.
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 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 4 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
There are 3 basic Plate movements or
boundaries:
1. Divergent: This is where the plates PULL
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 PUSH 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.
Another
look at the
famous
‘Ring of
Fire’
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The main types of plate movements.
Click here to see an excellent
‘Shockwave’ image of the movements of
the plates.
Transform Zone
Result - Earthquakes
The San
Andreas Fault,
California
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.