Mantle upwelling and ocean islandsrallen.berkeley.edu/teaching/F04_GEO302_PhysChemEa… · ·...
Transcript of Mantle upwelling and ocean islandsrallen.berkeley.edu/teaching/F04_GEO302_PhysChemEa… · ·...
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Physics and chemistry of the Earth’s interior – Mantle upwelling
Mantle upwellingand ocean islands
Reading:Fowler p 249-254, 335-338
Physics and chemistry of the Earth’s interior – Mantle upwelling
Mantle convection?
What is mantle convection?
Why do we believe there is convection in the mantle?
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Physics and chemistry of the Earth’s interior – Mantle upwelling
Convection
In a fluid:• Occurs when density distribution
deviates from equilibrium• Fluid may then flow to achieve
equilibrium again
In a viscous solid heated from below:• Initially heat is transported by conduction into the fluid at the base• Increased temperature reduces the density making the material at the
base less dense than fluid above• Once the buoyancy force due to the density contrast overcomes the
inertia of the fluid convection begins
Physics and chemistry of the Earth’s interior – Mantle upwelling
Rayleigh number
κηρα
ττπ
32 TagRa
a
c ==
Non-dimensional number which described the nature of heat transfer
g - gravity ρ - densityα - thermal expansion coefficient T - temperature variationa - length scale: thickness of fluid layer κ - thermal diffusivityν - viscosity
τc – conductive time constantτa – advective time constant
The critical Rayleigh number:• the point at which convection initiates• approximately 103 (dependent on geometry)• By knowing the material properties and physical geometry we can
determine if there will be convection and the nature of that convection
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Physics and chemistry of the Earth’s interior – Mantle upwelling
Rayleigh number and convective mode
103
104
105
106
Ray
l eig
h n
um
ber
Convection plan view
Rayleigh number of the mantle:
Upper mantle (thickness 700 km):
106
Lower mantle (thickness 2000 km):
3x107
Whole mantle (thickness 2700 km):
108
Physics and chemistry of the Earth’s interior – Mantle upwelling
Global convection models
Heat sources: from core
plus internal heat within
mantle from radioactive
decay
Heat sink:cooling of the surface of the
Earth
Phase transitions:at 410 and 660 km impedes mass transport across boundaries
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Physics and chemistry of the Earth’s interior – Mantle upwelling
What about the plates?
One layer or two?
Subduction ? Downwelling
Ridges ? Upwelling
Cast your votes please…
Physics and chemistry of the Earth’s interior – Mantle upwelling
Downwelling = subductionLow temperature high density slabs observed extending through the entire mantle …more next time
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Physics and chemistry of the Earth’s interior – Mantle upwelling
Does upwelling = ridges?Do ridges represent the upwelling portion of mantle convection?
Tomography:There is no evidence for any mantle anomaly more than 200 km beneath MORs
Physics and chemistry of the Earth’s interior – Mantle upwelling
Does upwelling = ridges?Do ridges represent the upwelling portion of mantle convection?
Gravity:Deep sources would produce long wavelength gravity anomalies – not the case for MORs
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Physics and chemistry of the Earth’s interior – Mantle upwelling
Does upwelling = ridges? …NoMORs are shallow passive features• Mantle upwells in response to plates separating• The melt source rock is shallow, passive mantle - MORB
Physics and chemistry of the Earth’s interior – Mantle upwelling
The mantle plume
Proposed as the source of surface hotspot by Morgan (1972)
How many?
1972 Morgan 191973 Wilson 661980 Crough & Jurdy 421981 Vogt 117
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Physics and chemistry of the Earth’s interior – Mantle upwelling
Models of mantle plumes
Davis Quicktime movie
Some time for anomaly
to develop
Rapid rise through mantle
(~1m per year)
Resistance through
transition zone
Rapid accent through upper
mantle
Flattening beneath
lithosphere
Viscosity
Low
High
Higher
Physics and chemistry of the Earth’s interior – Mantle upwelling
Seismic evidencefor whole-mantle plumes is weak
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Physics and chemistry of the Earth’s interior – Mantle upwelling
Super-plumesThe Pacific and African
Physics and chemistry of the Earth’s interior – Mantle upwelling
What about Iceland?
The plume beneath the Iceland hotspot
How deep?Remains an open question
The model
Iceland 3D model viewer available at: http://www.geology.wisc.edu/~rallen/ICELAND/index.html
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Physics and chemistry of the Earth’s interior – Mantle upwelling
HawaiiCrustal structure
P-velocity
DensityNafe-Drake curve plus gravity
Interpretation
Observations:• Crust thickens to ~18 km• Relative high density and velocity• Flexure of the lithosphere
Interpretations:• Constant thickness crust
extends beneath islands• High density intrusive core
results from sheeted dyke intrusions
• Lower density extrusive flows at the surface
Physics and chemistry of the Earth’s interior – Mantle upwelling
HawaiiCrustal plumbing
Beneath Kilauea
Narrow conduit at depth
Broad rift near surface
Earthquake foci
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Physics and chemistry of the Earth’s interior – Mantle upwelling
IcelandCrustal thickness
Observations:• The crust thickens from 15 km at
the coast to 45 km in central Iceland
• Flexure of the lithosphere
Interpretation:
• The higher temperatures of the mantle plume combine with passive rifting at a MOR to produce voluminous melting
Physics and chemistry of the Earth’s interior – Mantle upwelling
IcelandCrustal plumbing
• Low velocities represent melt pathways through the crust
• No deep earthquake due to higher temperatures of ridge environment