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Theory of Plate Tectonics

Surface of the Earth is covered by aseries of plates

Creation of seafloor at mid-ocean ridgesis compensated by destruction at

subduction zones

Continents never subduct

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What drives plate motion?

Crust travels from spreading centerwhere it created to subuction zone

where it is destroyed.

What closes the loop?

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Deformation of Solids

Elastic deformation (reversible) Fracture (irreversible) Ductile deformation (irreversible) Rocks are brittle at low pressure, low

temperature, fast stresses

Rocks can flow at high pressures andtemperatures and when stress is appliedslowly

Can solids flow?

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Can solids flow?

http://www.youtube.com/watch?v=njTjfJcAsBg

Can solids flow?

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Does ductile flow in mantle connect downwelling atsubduction zones and crust formation at ridges?

Evidence for flow in themantle

Isostatic compensation of continentsand ocean crust

Isostatic rebound from glaciation

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Archimedes principle

The upward bouyant force on an object isequal to the weight of the displaced fluid

Fb= mbg (balance of forces)

mfg = mbg (Archimedes)

!fhfAbg= !bhbAbghf/hb= !b/!f

Fb

mbg hbhf

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Archimedes principle

The upward bouyant force on an object isequal to the weight of the displaced fluid

!keeping the pressure at depth constant

Water has a density of 1 g cm-2. Wood

has a density of .5 g cm-2. What fraction

of a wood block floats below the surface

of the water?

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Water has a density of 1 g cm-2. Ice has

a density of .9 g cm-2. What fraction of an

iceberg floats below the surface of the

water?

The mantle has a density of 3.3 g cm-2.

Continental crust has a density of 2.6 g

cm-2. If the continent were floating on the

mantle what fraction would be beneath

the surface?

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Continental Crust

How do we know continentshave roots?

Fg= GMm/r2

For which case will we measure a

stronger gravitational pull?

!=2.6

!=3.3

!=2.6

!=3.3

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Oceanic Crust

Viscosity of peanut butter 105x greater than waterViscosity of the mantle 1024x greater than water

How easily can the mantle flow?

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Is there convection in the

mantle?

What drives convection?

density contrast in a fluid (heating from below)

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Mantle Convection

Heat Source = Radioactive decay

What inhibits convection?

If a fluid is too viscous the heat will betransferred by conduction, not

convection

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Rayleigh Number

" = coefficient of thermal expansiong= gravitational constant

#T= temperature difference between the top and bottom

h= height

$= viscosity

%= thermal conductivityRa> 2000 Convection will occur

Ra of mantle O(106)

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All else being equal which of the following

properties do not encourage convection in

a fluid that is heated from below?1. Low viscosity2. Low thermal conductivity3. Thick layer of fluid4. Low coefficient of thermal expansion

Do plate boundaries

correspond to mantleconvection cells?

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Where is subduction?Ridges must be migrating away from each other

Are rising and descedingbranches at the same spot?

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Shallow (100s km) upwelling

at ridges

Driven by gap created by spreading. Not convection.

.. One part of the picture iscorrect

The slab descends to the core mantle boundary

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Does slab pullhelp drive plate motion?

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Where is rising motion?

Intra-plate (hot spot)volcanism

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Mantle plumes extend to the core-mantle boundary

Active Mantle plumes

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Which of the following statements is true?

1. Mantle convection is unrelated to platetectonics

2. Subduction zones correspond to thedowndrafts in mantle convection

3. Spreading centers correspond to theupdrafts in mantle covection

4. Hotspots correspond to the downdrafts inmantle convection