Fluid, 90% iron solidified iron 20004000 km6000800010,00012,000 Mg(Fe) silicates phase changes...
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Transcript of Fluid, 90% iron solidified iron 20004000 km6000800010,00012,000 Mg(Fe) silicates phase changes...
fluid, 90% iron
solidified iron
2000 4000 km 6000 8000 10,000 12,000
Mg(Fe) silicates
phase changes
basaltic-granitic crust
chemical stratification and differentiation
upper mantle
outer core
inner core
D”, core-mantle boundary layer
2000 4000 km 6000 8000 10,000 12,000
lower mantle
core-mantle boundary
transition zone
crust
Structure of Earth as imaged by seismic waves
radius of earth = 6371 km
Seismic waves involve stress, strain, and density
Two important types of stresses and strains:
Pressure, P and volume change per unit volume, V/V
Shear stress and shear strain
Two types of elastic waves
Compressional or P waves involve volume change and shear
Shear or S waves involve only shear
P wave particle motions
S wave particle motions
Click on these links to see particle motions:
Elastic wave velocities determined by material properties
43
p
s
V
V
P wave velocity
S wave velocity
epicenter
expanding wavefront at some instant of time after earthquake occurrence
ray perpendicular to wavefront
seismograph station
Earth surface
Earthcenter
epicenter
ray
seismograph station
= epicentral distance in degrees
Earth surface
Earthcenter
tt() = total travel time along ray from earthquake to station
Globally recorded earthquakes during the past 40 years
earthquake depth 0-33 km 33-70 70-300 300-700
2,538,185 travel time observations from International Seismological Centre (ISC), for earthquakes with depths between “0” and 60 km.
These are the commonly reported phases as reported to the ISC from seismograph stations from around the world; see phase types on next page
distance, degrees
tim
e, m
inu
tes
These lines represent plus or minus one minute errors in reading arrival times
P
diffracted P
PKIKP
PKP
PcP
PP
PPP
ScS
SKS
S
PcS
SS
SSS
PS
PPSPKPPKP
PKKP
PKS
SKKS
PPP
surf
ace w
aves
wat
er w
aves
click on link to P and S phases in the earth
Nomenclature for seismic body phases
c = reflection at core mantle boundary
K
P or S
I or Ji = reflection at inner core-outer core boundary
P wave segments in blueS wave segments in red
inner core
outer core
mantle
2,538,185 travel time observations from International Seismological Centre (ISC), for earthquakes with depths between “0” and 60 km.
These are the commonly reported phases as reported to the ISC from seismograph stations from around the world; see phase types on next page
distance, degrees
tim
e, m
inu
tes
P
S
P diffracted around core
2,538,185 travel time observations from International Seismological Centre (ISC), for earthquakes with depths between “0” and 60 km.
These are the commonly reported phases as reported to the ISC from seismograph stations from around the world; see phase types on next page
distance, degrees
tim
e, m
inu
tes
PP
SS
2,538,185 travel time observations from International Seismological Centre (ISC), for earthquakes with depths between “0” and 60 km.
These are the commonly reported phases as reported to the ISC from seismograph stations from around the world; see phase types on next page
distance, degrees
tim
e, m
inu
tes
PcP
ScS
PcS
2,538,185 travel time observations from International Seismological Centre (ISC), for earthquakes with depths between “0” and 60 km.
These are the commonly reported phases as reported to the ISC from seismograph stations from around the world; see phase types on next page
distance, degrees
tim
e, m
inu
tes
PKIKP
PKP
SKSS in mantle, refracting and converting to P in outer core,
then refracting back out and
converting back to S in the
mantle
S
KS
SKKS
S in mantle, refracting and converting to P in outer core, P reflects once at
inner side of core-mantle
boundary, then refracting back out back with
conversion to S in the mantle
S
KS
Kreflection
2,538,185 travel time observations from International Seismological Centre (ISC), for earthquakes with depths between “0” and 60 km.
These are the commonly reported phases as reported to the ISC from seismograph stations from around the world; see phase types on next page
distance, degrees
tim
e, m
inu
tes
SKS
S
SKKS
outer core
inner core
lower mantle
upper mantlekm/sec
km
transition zone
D’’ layer
0
1000
2000
3000
4000
5000
6000
7000
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
dep
th
seismic wave velocity
Compressional (P) and Shear (S) wave velocities, Vp and Vs
outer core
inner core
lower mantle
upper mantlekm/sec
km
transition zone
D’’ layer
0
1000
2000
3000
4000
5000
6000
7000
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
dep
th
seismic wave velocity
Compressional (P) and Shear (S) wave velocities, Vp and Vs
No Shear waves in outer core!
43
p
s
V
V
modulus of incompressibility/
shear stressmodulus of rigidity
shear straindensity
(R) = "seismic parameter" derived from Vp(R) and Vs(R)
P
V V
2 24( )
3p sV V R
From Vp and Vs to seismic parameter
For self compression of homogeneous material
(R) = /
= - dP/(dV/V) = dP/(d/)
dP = - g dR
where R = radius to a point in the earth, andg = gravitational acceleration at that radiusg = GMR/R2
where MR = mass within sphere of radius R
d/dR = -/g
For self compression of homogeneous material
d/dR = -/g
This is the gradient in density determined by the seismic wave velocities. To obtain density, one must integrate by fixing the density, , and gravity, g, at the top of the layer and calculating both and g as one proceeds downwards.
The calculation assumes
a simple compression of material that does not change chemistry or phase.
the compression as one goes deeper produces an adiabatic temperature increase.
For self compression of homogeneous material
d/dR = -/g
The method is applied to the following layers:upper mantlelower mantleouter coreinner core
To determine the jumps in density between these layers, the following constraints are used:
Mass of earthMoment of Inertia of EarthPeriods of free oscillations of Earth
Density,
0
1000
2000
3000
4000
5000
6000
0 2000 4000 6000 8000 10000 12000 14000
core-mantle boundary
kg/m3
km
dep
th, k
m
Gravitational acceleration, g
0
1000
2000
3000
4000
5000
6000
0 2 4 6 8 10 12
core-mantle boundary
km
m/s2
dep
th, k
m
Pressure, P
core-mantle boundary
GPa.
km
0
1000
2000
3000
4000
5000
6000
0 50 100 150 200 250 300 350 400
dep
th, k
m
Density vrs pressure
0
2000
4000
6000
8000
10000
12000
14000
0 50 100 150 200 250 300 350 400GPa.
kg/m3
Density vrs pressure
0
2000
4000
6000
8000
10000
12000
14000
0 50 100 150 200 250 300 350 400GPa.
kg/m3
compression
compression
com
pos
itio
n
chan
ge
ph
ase
chan
ges liq
uid
to
solid
mantle density
crustal density
core-mantle boundary
Inner core/outer core boundary
1 mbar
fluid, 90% iron
solidified iron
2000 4000 km 6000 8000 10,000 12,000
Mg(Fe) silicates
phase changes
basaltic-granitic crust
Chemical stratification
upper mantle “Peridotite”:
65% olivine: (Mg,Fe)2SiO4
25% orthopyroxene (Mg,Fe)2Si2O6
10% clinopyroxene (Ca,Mg,Fe)2Si2O6
+ garnet (Mg,Fe)3AL2Si3O12
phase changes through transition zonelower mantle
85% Perovskite: (Mg0.9Fe0.1)SiO3
15% magnesiowustite (Mg0.9Fe0.1)O
+ Ca Perovskite ( Ca, Mg, Fe )SiO3
+ Corundum Al2O3outer core
90% Fe (Ni)
10% lighter alloy (FeO, S, Si, ?)
inner core
solid Fe + ?
oceanic crust continental crustMOHO
CMB
upper mantle
lower mantle
transition zone
outer coreCMB
D”
0 2000 4000 500030001000Temperature, degrees C
iron melting
Ad
iabatic g
radien
t
near surface thermal boundary layer = lithosphere
D” = Lower mantle thermo-chemical boundary layer
mantle convectionadvective heat flow
conductive heat flow
conductive heat flow
Temperature in mantle
?
man
tle meltin
g
cool, strong lithospheric boundary layer
slowly convecting mantle:plate tectonic engine
rapidly convectingouter core:
geomagnetic dynamo
solid inner core
subd
uctio
n
seafloor spreading
core-mantle thermo-chemical boundary layer
2000 4000 km 6000 8000 10,000 12,000
crust
Earth’s convective systems
inner core
mantle
The geomagnetic dynamo:• turbulent fluid convection• electrically conducting fluid• fluid flow-electromagnetic interactions• effects of rotation of earth
Generation of Earth’s magnetic field in the outer core
outer core