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8/12/2019 Lecture 11 Mantle Convection
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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