Transcript Plate Tectonics

Plate Tectonics:
Grand Unifying Theory
of everything...
for the Geosciences,
at least!
The Earth’s lithosphere (crust) is broken into pieces
that move by means of mantle convection currents,
and in the process allow continents to move like boxes
on a conveyor belt
A movie showing plate tectonics ...
QuickTime™ and a
Cinepak Codec by Radius decompressor
are needed to see this picture.
Plate tectonics theory says that the Earth’s
core is broken up into pieces that move by
plowing through the other rocks on Earth’s
surface?
A. True
B. False
Introduction
• Plate Tectonics Theory
> explains location of and actions associated with....
- Volcanoes (distribution and composition)
- Earthquakes (distribution, strength, depth)
- Mountain Ranges (of all types)
- Natural resources (distribution and formation of
Coal, Mineral Resources, & Oil)
- Ocean basins ( age, shape, and size)
- biologic distribution and behavior
+ (fossils and modern animals)
- seemingly incompatible climate records
A theory that unified the geosciences by explaining
nearly all geological phenomena
Plate Tectonics and Continental drift are NOT
the same thing!
Continental drift was the hypothesis postulated
by Alfred Wegener that was rejected due to the
wrong explanation for how the continents were
moving
Plate tectonics built upon Wegener’s rejected
hypothesis and added a lot of new information
including...
•Sea floor spreading
•Paleomagnetism
•Age of sea floor along oceanic ridges
Plate Boundaries, Earthquakes and Volcanoes
•Red boxes show the location of the next three slides
A small shield volcano in Iceland
•thin lava is created at a DIVERGENT PLATE
BOUNDARY
•The low viscosity lava creates the shield volcano
Confirmation
of
Sea-Floor
Spreading
Confirmation of Sea-Floor Spreading
• Deep sea drilling supports the hypothesis that ocean crust is forming at
the ridges and moving away from the ridges.
Mt Rainier in Washington State- a classic example of a
stratovolcano
•The lava associated with this type of volcano is
created as oceanic crust melts and pushes through
continental crust along Convergent subduction plate
boundaries
Subduction (Oceanic-Continental) Plate Boundary
Denser oceanic
oceanic crust
crust subducted
subducted beneath
beneath continental
continental crust.
crust.
¥¥Denser
Andesitic volcanic
volcanic arcs
arcs form
form on
on the
the leading
leading edge
edge of
of the
the
¥¥Andesitic
continental plate.
plate.
continental
Andes Mountains,
Mountains, Cascades
Cascades
¥¥Andes
A couple of cinder cones and a lava flow in New Mexico
•This type of volcanic cone is often found associated with
other volcanoes, and commonly where plates have been
completely subducted and the melted material is in its
last gasps.
Alfred Wegener, Continental Drift Hypothesis
• 1912-1915: Alfred Wegener
proposed a hypothesis called "continental drift".
Continents were once together
> Supercontinent; Pangaea (Pan = all,Gaea = land).
>
His ideas would become the foundation for Plate tectonics
once additional data were collected!
• Based on a lot of geological evidence
> mountain ranges, fossils, continental fit, & similar rocks
• Wegener's theory lacked a scientifically valid
explanation for how the continents could move.
- Hypothesis was rejected for the above reason
An alternative model, “Contracting Earth”, was instead
favored by many geoscientists at the time in spite of
it’s many shortcomings
Continental Fit
• 1965: Edward
Bullard proposed
modification of
Wegener’s idea of
fitting continents by
using the 2,000 m
isobath, which
corresponds to the
continental slope
and the edge of
continental crust.
Similarity of Rock Sequences
• Identical Carboniferous rocks on Gondwana is tough to explain
if the continents were not joined at that time
• Various mountain ranges are currently
widespread. Rejoining separated continents
shows the linear nature of the mountain chains.
Tropical locations have geological evidence of Carboniferous
glaciation.
• The former parts of Gondwanaland are today widely
separated.
The Gondwana Ice Cap
• If the continents
are reassembled,
the glacial
evidence makes
sense.
• At the same time
most of Laurasia
was in the tropics.
Gondwanaland Animal and Plant Fossils
Sea-Floor Spreading
• Sea floor mapped during the 1950s
- revealed the ocean ridge system.
• Magnetic reversals recorded in the rocks
> symmetrical with respect to the ridge axis.
• 1962: Harry Hess
> proposed sea-floor spreading to account for
continental movement.
Magnetic Data and the Ridge Axis
• proof for sea floor spreading and a mechanism
to move plates
Age of the Ocean Basins
Which is not something that was
used to support Continental drift
hypothesis?
A. The fit of the continents
B. The alignment of mountains
C. The locations of fossils on various
continents
D. The arrangement of unique rocks
on continents
E. All of the above were used to
support Continental Drift
hypothesis
Plate Boundaries
• Convergent
: where two plates meet,
> with one being subducted or
> both being forced upward.
• Divergent : where two plates spread
apart at a ridge or rift.
• Transform : where two plates or plate
segments slide past each other.
driven by convection cells within the
mantle, the plates interact with each
other
Convection currents drive plate motion
Earthquakes allow us to
determine what the internal
structure of Earth looks likeconcentric spheres of material,
each layer with specific
characteristics that influence
seismic wave movement
3D maps of the earth’s convection currents based
on seismic wave analysis-complicated, but likely
more realistic!
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Continental-Convergent Plate Boundary
Continental
crust
Himalayas
Indian
subcontinent
Tibet
Plateau
Oceanic-Oceanic Plate Boundary
Volcanic island
island arcs
arcs form
form nearly
nearly parallel
parallel to
to the
the subduction
subduction
¥¥Volcanic
zone, which
which is
is expressed
expressed as
as aa deep
deep ocean
ocean trench.
trench.
zone,
Most modern
modern island
island arcs
arcs are
are in
in the
the Pacific
Pacific Ocean.
Ocean.
¥¥Most
Island Arc
N
Sea of Japan
To RoC
Pacific Ocean
East African Rift Valley
¥ Eastern Africa
appears to be
rifting away from
the rest of the
continent.
San Andreas Fault- a Transform Boundary
QuickTime™ and a
Cinepak Codec by Radius decompressor
are needed to see this picture.
Which type of plate boundary is
not commonly associated with
volcanoes?
A.Continent-Oceanic convergent
subduction
B. Divergent
C. Transform