Geoscience Reference
In-Depth Information
A molten iron-rich core appeared early in Earth
history, the evidence being in the remnant mag-
netic field and isotopic record of ancient rocks.
This in turn implies a short high temperature
accretion for the bulk of the Earth, with per-
haps a drawn out accretionary tail to bring
in noble gases and other volatile elements and
to salt the upper mantle with siderophile ele-
ments that would otherwise be in the core. The
long-standing controversy regarding a drawn-out
(100 milllion years) versus a rapid (
15
14
Ni
13
M
=
58.7
Fe
12
M
=
55.8
SOLID
MOLTEN
11
Fe
80.2
Si
19.8
M
49
−
1 Myr) ter-
restrial accretion appears to be resolving itself
in favor of the shorter time scales and a high-
temperature origin. The core is approximately
half the radius of the Earth and is about twice
as dense as the mantle. It represents 32% of the
mass of the Earth. A large dense core can be
inferred from the mean density and moment of
inertia of the Earth, and this calculation was per-
formed by
Emil Wiechert
in 1891. The exis-
tenceofstonymeteoritesandironmeteoriteshad
earlier led to the suggestion that the Earth may
have an iron core surrounded by a silicate man-
tle. The first seismic evidence for the existence
of a core was presented in
1906 by Oldham
,
although it was some time before it was real-
ized that the core does not transmit shear waves
and is therefore probably a fluid. It was recog-
nized that the velocity of compressional waves
dropped considerably at the core-mantle bound-
ary. Beno Gutenberg made the first accurate
determination of the depth of the core, 2900 km,
in 1912, and this is remarkably close to current
values. The core-mantle-boundary is referred to
as the
Gutenberg discontinuity
and as the
CMB.
Although the idea that the westward drift
of the magnetic field might be due to a liq-
uid core goes back 300 years, the fluidity of the
core was not established until
1926 when Jef-
freys
pointed out that tidal yielding required
a smaller rigidity for the Earth as a whole than
indicated by seismic waves for the mantle. It was
soon agreed by most that the transition from
mantle to core involves both a change in com-
position and a change in state. Subsequent work
has shown that the boundary is extremely sharp.
There is some evidence for variability in depth,
in addition to hydrostatic ellipticity. Variations in
∼
10
M
=
44.6
9
1.2
2
3
4
Pressure (M bar)
Fig. 10.1
Estimated densities of Fe, Ni and some Fe-rich
alloys compared with core densities. The estimated reduction
in density due to melting is shown (dashed line) for one of
the alloys.
12
V
P
11
SOLID
Fe
Fe
802
Si
19.8
V
φ
10
Ni
Fe
9
8
1.2
2
3
4
Pressure (M bar)
Fig. 10.2
Compressional wave velocities in the outer core
and compressional and bulk sound speeds in the inner core
compared to estimates for iron and nickel. Values are shown
for Poisson ratios in the inner core.
lower-mantle density and convection in the lower
mantle can cause at least several kilometers of
relief on the core-mantle boundary. The outer
core has extremely high Q and transmits P-waves
with very low attenuation. The elastic properties
and density of the core are consistent with an
iron-rich alloy (Figures 10.1 and 10.2). Evidence
that the outer core is mainly an iron-rich
fluid also comes from the magnetohydrodynamic
