Plumes, hotspots and the CMB

Lecture 6: GeodynamicsCarolina Lithgow-Bertelloni

Earth’s temperature profile

3000

2500

2000

1500

1000

500

0

DEPTH (KM)

50004000300020001000

TEMPERATURE (K)

Scales of Convection

[from Geoff Davies]

Plumes and Hotspots Hotspots

Island chains and age progressionImportance for plate motions, TPWFixityChemistryOrigin

PlumesDifference with large-scale upwellingsHeads and tails

Effects of viscosity on morphologyPlume initiation and flood basaltsHow much entrainmentComparison to geochemistry

Effects of compositionShape, heterogeneity, hotspot fixity

Relationship between large-scale upwellings and plumes?Capture by plate-scale flow

Consequences for heat flow

Where do they come from? Relationship to CMB structure

Hotspots

Concentrated volcanic activity. Linear volcanic chains in the interiors of the plates.Age progression along chain Chemistry of erupted lavas is significantly different than MOR or IASome hotspots have broad topographic swell ~ 1000 m

[Steinberger et al., 2004]

43-48 Ma

Hotspots and Plates

25-43 Ma

Hotspots, fixity and plate motions

Hotspot fixity and mantle wind

[Steinberger et al., 2004]

Chemistry

[Barfod et al., 1999]

Plumes and hotspots

Rayleigh-Taylor instabilityLarge head, thin tail

RheologyVigor of convectionCompositional vs thermal buoyancy

Ascent timesRheologyDeflection, capture by mantle windCompositional vs thermal buoyancy

[Griffiths and Campbell, 1990]

Rise time estimate

Ratio of buoyancy force to viscous forcesB = -4r3g/3=/r; R=r

Forces on the sphere balanced velocity constantB+R=0 V= -gr2/3c

If viscosity of sphere and surrounding differentc = /

c ~ 1-1.5

v

r

r = 500 km

V =80 km/My

Generating a mantle plume

QuickTime™ and aMotion JPEG A decompressor

are needed to see this picture.

[from Geoff Davies]

Large-scale upwellings and plumes

[Boschi and Dziewonski,1999]

[Ni et al.,2002]

Plume morphology: effects of viscosity

[Whitehead and Luther, 1975]

July 13, 2004 CIDER-KITP

Plumes heads and tails

0.3 m B C D

E

OUTLET

INLET

A

7.5

15.0

22.5

20

20

1 m

F-H

1 m

Screen N

Screen E

N

S

[Lithgow-Bertelloni et al., 2001]

Head & Tail Radii

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

10 5 10 6 10 7 10 8

Rao

[Lithgow-Bertelloni et al., 2001]

Thermochemical Plumes

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are needed to see this picture.

[Farnetani, 2004]

Entrainment and mixing

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are needed to see this picture.

Plumes and geochemical heterogeneity

[Samuel and Farnetani, 2003]

Evolution of heterogeneity

[LeBars and Davaille, 2004]

Plumes and large-scale upwellings

[Davaille, 2000]

Thermochemical plumes and fixity

[Jellinek and Manga, 2002]

Plume capture by large-scale flow

V and hot/int

No effect V<< 1 ~ 1Suppression V> 10 > 100Sweeping V intermediate f(

[Jellinek et al., 2002]

and Pe control BLT and Q ~ (Pe)1/3

Q ~ (Pe)1/2

Where do plumes originate?

[Davaille, 2000]

The plume source region: CMB

[Sidorin et al., 1999]

Plume Frequency3 Different Experiments

Episodic Crustal Production?

Condie, 1998