Geoscience Reference
In-Depth Information
requirement that the core be a good electrical
conductor.
Although the outer core behaves as a fluid, it
does not necessarily follow that temperatures are
above the liquidus throughout. It would behave
as a fluid even if it contained 30% or more of sus-
pended particles. All we know for sure is that at
least part of the outer core is above the solidus
or eutectic temperature and that the outer core,
on average, has a very low rigidity and low viscos-
ity. Because of the effect of pressure on the liq-
uidus temperature, a homogenous core can only
be adiabatic if it is above the liquidus through-
out. An initially homogenous core with an adi-
abatic temperature profile that lies between the
solidus and liquidus will contain suspended par-
ticles that will tend to rise or sink, depending on
their density. The resulting core will be on the liq-
uidus throughout and will have a radial gradient
in iron content. The core will be stably stratified
if the iron content increases with depth.
core pressures, particularly if it is metallic. Some
metals have Poisson's ratios of 0.43 to 0.46 even
under laboratory conditions.
The solid inner core is the most remote and
enigmatic part of our planet, and except for
the crust, is the smallest 'official' subdivision of
Earth's interior. Only a few seismic waves ever
reach it and return to the surface. The inner
core is a small target for seismologists and seis-
mic waves are distorted by passing though the
entire Earth before reaching it. The inner core is
isolated from the rest of Earth by the low vis-
cosity fluid outer core and it can rotate, nod,
wobble, precess, oscillate, and even flip over, only
loosely constrained by the surrounding shells.
Its existence, size and properties constrain the
temperature and mineralogy near the center of
the Earth. Among its anomalous characteristics
are low rigidity and viscosity (compared to other
solids), bulk attenuation, extreme anisotropy and
super-rotation (or deformation).
The inner core has a radius of 1222 km and
a density about 13 g/cm
3
. Because of its small
size, it is difficult to determine a more accurate
value for density. It represents about 1.7% of the
mass of the Earth. The density and velocity jumps
at the inner-core--outer-core boundary are large
enough, and the boundary is sharp enough, so
that the boundary is a good reflector of short-
period seismic energy. The inner core is seismi-
cally anisotropic; compressional wave speeds are
3--4% faster along the Earth's spin axis.
The main constraint on composition and
structure is the compressional velocity and
anisotropy. From seismic velocities and cosmic
abundances we know that it is mainly com-
posed of iron--nickel crystals, and the crystals
must exhibit a large degree of common orien-
tation. The inner core is predicted to have very
high thermal and electrical conductivity, a non-
spherical shape, frequency-dependent properties
and it may be partially molten. It may be essen-
tial for the existence of the magnetic field and
for polarity reversals of this field. Freezing of the
inner core and expulsion of impurities is likely
responsible for powering the geodynamo.
Within the uncertainties the inner core may
be simply a frozen version of the outer core,
Fe
2
OorFeNiO,pureironoraniron--nickelalloy.
The inner core
In 1936 Inge Lehmann
used seismic data from
the
core shadow
to infer the presence of a higher
velocity inner core. Although no waves have yet
been identified that have traversed the inner
core unambiguously as shear waves, indirect evi-
dence indicates that the inner core is solid (Birch,
1952). Julian and others (1972) reported evidence
for PKJKP, a compressional wave in the man-
tle and outer core that traverses the inner core
as a shear wave.
Observation of PKJKP
is
difficult and claimed observations are contro-
versial.
Early free-oscillation models gave very low
shear velocities for the inner core, 2 to 3 km/s;
more recent models give shear velocities in the
inner core ranging from 3.46 to 3.7 km/s, in
the range of crustal values. The boundary of
the inner core is also extremely sharp. The Q
of the inner core is relatively low, and appears
to increase with depth. The high Poissons ratio
of the inner core, 0.44, has been used to argue
that it is not a crystalline solid, or that it is
near the melting point or partially molten or that
it involves an electronic phase change. However,
Poissons ratio increases with both temperature
and pressure and is expected to be high at inner
