Geoscience Reference
In-Depth Information
Table 8.3 Physical properties of the core
Outer core
Inner core
Density (kg m −3 )
9900-1216
1276-1308
Volume (10 18 m 3 )
169
8
Mass (10 21 kg)
1841
102
Viscosity a
10 −3
10 13±3
(Pa s)
Seismic quality factor a
Q p (at 1 Hz)
6 × 10 4
250-600
Q s (at 10 −3 Hz)
100-400
Electrical conductivity a
(10 5 S −1 m −1 )
6 ± 3
6 ± 3
Specific heat a
(10 3 Jkg −1 K −1 )
0.5 ± 0.3
0.5 ± 0.3
Coefficient of thermal expansion a
(10 −6 K −1 )
8 ± 6
7 ± 4
Thermal diffusivity a
(10 −5 m 2 s −1 )
1.5 ± 1
1.5 ± 1
a
After Jeanloz (1990).
velocities are orders of magnitude greater than velocities of flow in the mantle.
The convection in the outer core is not completely separate from the convection
in the mantle; the two convection systems are weakly coupled. The difference
in viscosity across the CMB is so great that viscous coupling (via shear forces
across the CMB) is not important, but there is some thermal coupling. This
means that convection cells in the outer core tend to become aligned with con-
vection cells in the mantle, with upwelling in the outer core beneath hot regions
in the mantle and downwelling in the outer core beneath cold regions in the
mantle. The heat flux from the core is a major driving force for convection in the
mantle.
The changes in density resulting from convection in the outer core are much
less than density variations in the mantle and cannot be imaged by seismic meth-
ods. Changes in the magnetic field, secular variation, can, however, be used to
make estimates of the fluid flow at the surface of the core just below the CMB.
The steady toroidal flow is fairly well determined at a maximum of 20 km yr 1
with two main cells: a strong westward flow along the equator extends from the
Indian Ocean to the Americas and then diverges, with the return flows extending
north and south to high latitudes (Fig. 8.28). The toroidal flow beneath the Pacific
region is very small. This flow is the probable cause of the 'westward drift' of the
magnetic field. If the angular momentum of the core is constant, this westward
flow at shallow levels in the outer core must be balanced by eastward movements
at depth - the dfferential rotation of the inner core seems to satisfy that require-
ment. In contrast to the toroidal flow, estimates of the poloidal flow and the time
dependence of the flow are poor.
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