Transcript Plate Boundary

Plate Tectonics:
Grand Unifying Theory
of everything...
for the Geosciences,
at least!
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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
•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
• 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
Convergent 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).
>
• 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 to the “Contracting Earth” model favored
by many geoscientists at the time
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 widely separated today, but
were not in the distant geological past.
The Gondwana
Ice
The
Gondwana
IceCap
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 Mid oceanic ridge system.
The mountain chain had a down-dropped
valley in the middle, called a rift valley.
could only be caused by pulling apart
• 1962: Captain Harry Hess (US Navy)
> proposed Sea-floor Spreading to account for
continental movement and size of oceans.
Magnetic reversals recorded in the rocks were
discovered later and verified Hess’ hypothesis
>
patterns were symmetrical with respect to the ridge axis.
Magnetic Data and the Mid Ocean Ridge Rift valley
an example of an Oceanic crust divergent boundary
• proof for sea floor spreading and a mechanism to move continents as
Part of the larger tectonic plates
Age of the Ocean Basins
Final verification of sea floor spreading.
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
oceanic-oceanic subduction;
>continental-oceanic subduction;
Continental-continental collision
.
• Divergent : where two plates spread apart at
a ridge or rift valley.
oceanic
>continental
• Transform : where two plates or plate segments
slide past each other.
> driven by convection cells within the mantle, the plates
interact with each other
> faults and earthquakes often occur along plate boundaries
Earthquakes
Occur along breaks in the Crust known as
faults
Energy stored along faults released as
rock strength and friction are
overcome releasing energy waves
Three kinds of energy waves
•
•
•
P-waves (fastest)-move through any substance
S-waves (next fastest)-move through solids ONLY
Surface waves (slowest)-move along the Earth’s
surface
Convection currents drive plate motion
Earthquake waves allow us to
determine what the internal
structure of Earth looks like
concentric 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!
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Convergent
Continental-Continental
Plate Boundary
Continental-Convergent
Plate Boundary
Continental
crust
Himalayas
Indian
subcontinent
Tibet
Plateau
Convergent Subduction (Oceanic-Continental) Plate Boundary
Western coast of South America
Convergent
subduction Oceanic-Oceanic
Plate Boundary
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
Continental
Divergent
boundary
East African
Rift Valley
East African
Rift Valley
･ Eastern Africa
appears to be
rifting away from
the rest of the
continent.
San Andreas Fault- a Transform Boundary
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Which type of plate boundary is
not commonly associated with
volcanoes?
A.Continent-Oceanic convergent
subduction
B. Divergent
C. Transform