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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.

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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

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So what happens to old lithosphere?

•  Hess’ solution: Recycle lithosphere to the asthenosphere

–  Downwelling limb of asthenospheric convection cell –  Located at trenches/deeps/troughs

•  He had no EVIDENCE! - but he was correct, in part –  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 Trench) 4

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 5

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) 8

Transform Plate Boundaries

•  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

animation

Tuzo Wilson showed the following…

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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

•  Subducting slab pulls lithosphere along •  Problems:

–  Translate stress across slab –  No Atlantic subduction (next slide)

•  Note: this, and the following mechanisms are simply a form of convection!

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Problem: no Atlantic Subduction

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Plate Tectonic Mechanism 2:

•  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

asthenosphere

Litho. Litho.

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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.  Plate 1 moves faster to the right compared to plate 2 that moves to the left. B.  Both Plates 1 and 2 move to the right, at the same velocity. C.  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

•  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-lithosphere-asthenosphere-mesosphere

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

What is the energy source that powers this cycle? Earth’s internal heat!

Recall the Hydrologic Cycle? • Transfer of water between surface reservoirs. • Powered by the sun

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Let’s Make Pangaea: look at oceans

Wilson/supercontinent Cycle: Oceans are ephemeral and Continents Grow over time!

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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 27

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Review Questions

•  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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