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Plate Tectonics
All images from
http://pubs.usgs.gov/publications/text/understan
ding.html#anchor5567033
unless otherwise noted.
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.
Wegener had
four main pieces
of evidence.
First he noted
the jigsaw fit of
South America
and Africa,
especially, but
also elsewhere.
Making Connections: Canada’s Geography. Clark & Wallace.
Prentice Hall Ginn, 1999.
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.
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.
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.
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 the 1960s that the
theory of plate tectonics was
advanced to explain how the continents
could separate..
http://www.pbs.org/wnet/savageearth/animations/hellscrust/index.html
The thin crust (20 km under the ocean)
and the very top of the mantle are called
the lithosphere. Beneath this is the
asthenosphere.
The main features of plate tectonics are:
•The Earth's crust is broken into a series
of plates or pieces.
•The ocean floors 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.
So, a few key concepts so far:
•The earth radiates energy from the
radioactive decay of elements in the
core.
•The energy released keeps the earthg’s
surface in constant motion and change,
albeit very slowly most of the time.
•Some geological change is abrupt:
volcanoes and earthquakes for example.
How do we know this?
“Advances in sonic depth recording during
World War II (SONAR) and the
subsequent development of the nuclear
resonance type magnometer led to detailed
mapping of the ocean floor. Among the
seafloor features that supported the seafloor spreading hypothesis were: midoceanic ridges, deep sea trenches, island
arcs, geomagnetic patterns, and fault
patterns.”
http://www.ucmp.berkeley.edu/geology/tecmech.html
The Surface of the Earth
http://www.ngdc.noaa.gov/mgg/ima
ge/mggd.gif
“The mid-oceanic ridges rise 3000
meters from the ocean floor and are
more than 2000 kilometers wide
surpassing the Himalayas in size. The
mapping of the seafloor also revealed
that these huge underwater mountain
ranges have a deep trench which bisects
the length of the ridges and in places is
more than 2000 meters deep.”
http://www.ucmp.berkeley.edu/geology/tecmech.html
http://www.ucmp.berkeley.edu/geology/tectonics.html
“Research into the heat flow from the
ocean floor during the early 1960s revealed
that the greatest heat flow was centered
at the crests of these mid-oceanic ridges.
Seismic studies show that the mid-oceanic
ridges experience an elevated number of
earthquakes. All these observations
indicate intense geological activity at the
mid-oceanic ridges.”
http://www.ucmp.berkeley.edu/geology/tecmech.html
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 oceans margins. They are associated
with and parallel to large continental
mountain ranges. There is also an parallel
association of trenches and island arcs. Like
the mid-oceanic ridges, the trenches are
geologically active, but unlike the ridges
they have low levels of heat flow.
http://www.ucmp.berkeley.edu/geology/tecmech.html
Scientists also 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.
It has also been found that the oldest
seafloor often ends in the deep-sea
trenches.
http://www.ucmp.berkeley.edu/geology/tecmech.html
So 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
convection currents in the asthenosphere
•the asthenosphere churns like a fluid
There are actually two types of crust:
•oceanic crust extends all over the earth
and is broken into the 12 large and many
smaller plates; and,
•continental crust which rides around on
top of the oceanic crustal plates
•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
•in a single year, earthquakes alone release
1026 ergs of energy, or the energy of
100,000 Hiroshima-sized nuclear bombs
•that is just one percent of the total
amount of energy that reaches the surface
from Earth's innards
http://www.pbs.org/wnet/savageearth/hellscrust/index.html
Convection currents rise up from the
radioactive core, carrying heat to the thin
crust of the earth.
Where the plates move apart, new
magma wells up to the surface, forming
new oceanic crust.
The main types of plate boundaries.
The MidAtlantic
Ridge: a
divergent
zone.
Iceland: On the Mid-Atlantic Ridge
Mid Ocean Convergence Zone
Convergence Zone: Oceanic Crust and
Continental Crust
Convergence Zone: Continental Crust to
Continental Crust
Indian Plate
collides with
Eurasian Plate
The result: the
Himalayas and
Mt. Everest
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.
Diagram illustrating
deformation associated
with a subduction thrust
fault. Top: elastic
deformation builds up
between earthquakes if
the thrust fault is locked;
the edge of the overriding
plate is dragged down
and a bulge forms farther
landward. Bottom: during
a large earthquake, the
leading edge of the plate
is uplifted and the bulge
collapses.
http://sts.gsc.nrcan.gc.ca/page1/geoh/quake/fig2.htm
http://www.pbs.org/wnet/savageearth/hellscrust/index.html
This map, which shows 20th-century earthquakes in red,
illustrates how they cluster on the edges of the major tectonic
plates (outlined in yellow).
http://www.pbs.org/wnet/savageearth/animations/hellscrust/index.html