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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

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Physics and chemistry of the Earth’s interior – Mantle upwelling

Trac

ing

mel

t pa

thw

ays

bene

ath

Icel

and

Source depth unknown

Lateral flow of mantle material

below lithosphere

Melt fluxes through the crust and along

rift to south