1; movies
Topography of a fast spreading ridge (EPR)
Topography of a slow
spreading ridge (south
atlantic)
2; topography
Melt beneath a fast-spreading
ridge (East Pacific Rise)
Ophiolites
3; classic ophiolites
Oman ophiolite
Pillow lavas
Sheeted Dikes
Layered Gabbros
Stokes law
Vs is the particles' settling velocity (vertically downwards if ρp > ρf, upwards if ρp < ρf)
g is the acceleration due to gravity,
ρp is the density of the particles, and
ρf is the density of the fluid
4; settling
Massive gabbro
Banded harzburgite
Impregnated dunnite
Hot spot volcanism: a global phenomenon
5; Hawaii-emperor chain
The origin of hot
spot volcanoes
from melting of
plumes
Dynamic models of mantle convection
Plates going down
Plumes coming up
Rapid, small-cell convection on Io
Why does the mantle melt to produce hot
spot magmas?
• Isentropic
decompression
melting
• Fluxing by volatiles
• Heating of the
lithosphere by a hot
plume
• Unconventional heat
sources
Simple variations on the decompression
melting theme
• Variations in potential
temperature -- hotter mantle
produces deeper melting,
more magma
• Variations in the thickness of
the lithosphere -- controls
the depth at which melting
terminates
• Fractional vs. batch melting
• All of these can vary from hot
spot to hot spot and within a
single volcano, producing
distinctive chemical
signatures
Temperature variations near head of
plume
QuickTime™ and a
GIF decompressor
are needed to see this picture.
3D Model by Ribe and Christensen
Why does the mantle melt to produce hot
spot magmas?
• Isentropic
decompression
melting
• Fluxing by volatiles
• Heating of the
lithosphere by a hot
plume
• Unconventional heat
sources
Why does the mantle melt to produce hot
spot magmas?
• Isentropic
decompression
melting
• Fluxing by volatiles
• Heating of the
lithosphere by a hot
plume
• Unconventional heat
sources
cold
hot
plume
Hawaii (topography/bathymetry)
Qu i c k T i m e ™ a n d a
Ph o t o - J P EG d e c o m p re s s o r
a re n e e d e d to s e e th i s p i c t u re .
Geological map of the big island of Hawaii
Hilo
N
"Kea" trend
Loih i
0
20 k m
"Loa" trend
QuickTime™ and a
Animation decompressor
are needed to see this picture.
HSDP drilling in 1993 and 1999 into the
flank of Mauna Kea volcano
• >95% recovery, to
a total depth of 3.1
km below sea
level
• Penetration
through ~1 km of
subaerial lavas, ~2
km of submarine
deposits, both
hyaloclastites and
pillows
Mauna Kea
HSDP drill site
estimate of average subsidence rate
0
ML
MK
50 0
subaer ial
submarine
depth (mbsl)
1,0 00
r o t ar y d r illed
in t er vals
1,5 00
first intrusive
first pillow lava
2,0 00
2,5 00
3,0 00
0
0.1
0.2
0.3
0.4
modal abundance (volume fraction)
vesicle abundances
vesicles - ML
vesicles - MK
vesicles - intrusives
hyaloclastite formation -“prograding delta” volcano growth
volcanic sampling
of a zoned plume
a
b
c
d
e
Trace elements and isotopic ratios are generally
correlated with variations in SiO2 content (Kurz et al,
2003)
What if the length scales of compositional heterogeneities are
small?