Download Why is the oldest ocean crust only ~180 Ma?

Plate Tectonics, and the Wilson Cycle
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So, new ocean lithosphere is created at MOR. What
happens to old ocean lithosphere? A.  Is is conserved, thus the surface area of the Earth is
increasing.
B.  It forms mountains where ocean lithosphere collides
with continental lithosphere
C.  It sinks back into Earth’s asthenosphere as it gets older
away from the MOR. 2
Why is the oldest ocean crust only ~180 Ma?
•  Lithosphere cools with time
as it spreads away from the
MOR
•  Old lithosphere is thicker,
and colder (thus denser)
than young lithosphere
•  So, Old lithosphere sinks
deeper into the
asthenosphere
•  Thus, ocean floor becomes
deeper away from the MOR
•  ‘Older, colder, denser,
deeper 3
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So what happens to old lithosphere?
• 
Hess’ solution: Recycle lithosphere to the
asthenosphere
• 
He had no EVIDENCE! - but he was correct, in part
–  Downwelling limb of asthenospheric convection cell
–  Located at trenches/deeps/troughs
–  Old, cold, dense lithosphere sinks into the asthenosphere
at subduction zones
–  Subduction zones are not necessarily at margins of
continents, they can be in ocean basins (e.g., Marianas
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Trench)
Evidence for Subduction:
•  Wadati-Benioff zones
–  Dipping plane of EQs
•  Types of EQs
–  Tension/bending at top
–  Compression/stubbing
toe at base
•  Note the Tomography
The interpretation of Wadati-Benioff Zones: They
mark the location where Lithosphere is sinking into
the asthenosphere
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Wadati- Benioff Zones are only at trenches
•  Trenches are long, deep and
narrow troughs found in the
oceans
•  Notice that ocean lithosphere
subducts beneath continents, not
vise versa.
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What happens when two ocean plates ‘collide’?
A.  The faster moving plate
subducts.
B.  The slower moving plate
subducts.
C.  The older plate subducts
D.  The younger plate subducts
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Theory of Plate Tectonics:
•  Combined Seafloor Spreading and Subduction
–  Lithosphere is created at mid-ocean spreading ridges and consumed at
subduction zones
–  Continents are along for the ride (i.e. continents are part of a larger
lithospheric plate)
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Transform Plate Boundaries
• 
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Very common in the oceans
These link other plate
boundaries to one another Why do they exist?
–  Spreading is perpendicular to
MOR boundary
–  Since Earth’s surface is
curved, spreading requires a
‘tear’ to accommodate
motion perpendicular to the
rift.
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Oceanic transforms provide the proof of plate tectonics
Tuzo Wilson showed the following…
animation
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Why are Mid-Ocean Ridges Shallow?
A.  Because they are volcanic mountains
B.  Because the hot asthenosphere rising beneath the ridge lifts the lithosphere
C.  Because the plates are grinding against each other at the MOR, causing ‘crumpling’
of the ridge
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Why are Trenches Deep?
A.  Because the old ocean lithosphere sinks into the asthenosphere at trenches
B.  Because the continent pushes the subducting lithosphere deep into the asthenosphere.
C.  Because the convecting asthenosphere drags the lithosphere down, causing a deep
place in the oceans.
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Plate Tectonic Mechanism 1: Slab Sinking
• 
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Subducting slab pulls lithosphere along
Problems:
• 
Note: this, and the following mechanisms are simply a form of
convection!
–  Translate stress across slab
–  No Atlantic subduction (next slide)
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Problem: no Atlantic Subduction
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Plate Tectonic Mechanism 2:
o.
Lith
Lith
o.
asthenosphere
• 
Ridge Push: Sloped density contrast is unstable (Lith vs. Asth)
–  Horizontal force works to flatten this slope toward a stable configuration
–  The sloped asthenosphere pushes the overlying lithosphere
• 
Problem: Earthquakes at MOR are tensional (pulling) not compressional (pushing).
Demo: putty with plates
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Plate Tectonics mechanism 3:
•  Asthenospheric
convection:
–  Regional flow of
asthenosphere shears
overlying plate
–  Forces/drags the plate
along
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If ridge-push and regional mantle flow influence plate motion, what would you
expect plate motion to be like in figure b (on the right)?
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A. 
B. 
C. 
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Plate 1 moves faster to the right compared to plate 2 that moves to the left.
Both Plates 1 and 2 move to the right, at the same velocity.
Both plates 1 and 2 move to the right, but 1 moves faster than 2.
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Probably all three mechanisms work together • 
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Further complication: motion of
one plate influences all the others
So, motion of an individual plate
may be in response to slab pull on
another plate
E.g. why does Antarctica move?
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Analogy to tectonics
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Hydro-Tectonic Cycle: Water transferred between Surface-lithosphereasthenosphere-mesosphere
Recall the
Hydrologic
Cycle?
• Transfer of
water between
surface
reservoirs. • Powered by
the sun
What is the
energy source
that powers
this cycle?
Earth’s
internal heat!
Water fluxes between reservoirs:
• 
Volcanism at the mid-ocean ridge and the volcanic arcs releases water from the
asthenosphere to the surface
• 
Volcanism at hot-spots releases water from mesosphere and asthenosphere to the surface
• 
Geologic processes on the continent and ocean floor add water to the lithosphere
• 
Subduction transfers water from oceanic lithosphere to the asthenosphere and mesosphere
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Let’s Make Pangaea: look at oceans
Wilson/supercontinent Cycle: Oceans are
ephemeral and Continents Grow over time!
movie
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Pangaea! Panthalassic & Tethys
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Open Tethys
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Close Tethys:
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Today’s configuration:
Wilson/Supercontinent cycle:
Powered by Earth’s internal heat
movie
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Review Questions
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Why is subduction necessary?
What is the evidence for subduction? Where do we find subduction zones?
What are the three types of plate boundaries, and what happens at
each?
How do continents collide and what happens when they do?
Wilson showed that tectonics must operate on Earth. How did he do
this?
Describe how the three mechanisms for tectonics operate.
What is the hydrotectonic cycle? What are the reservoirs of water in
this cycle and how does water cycle between the reservoirs?
Consider the Wilson Cycle and its implication for today’s oceans.
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