Plumes, hotspots and the CMB Lecture 6: Geodynamics Carolina Lithgow- Bertelloni
Jan 04, 2016
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
QuickTime™ and aVideo decompressor
are needed to see this picture.
[Farnetani, 2004]
Entrainment and mixing
QuickTime™ and aVideo decompressor
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