Geoscience Reference
In-Depth Information
are present, the b.c.c. phase is more stable than the h.c.p. phase under inner-
core conditions, which is opposite to the results for pure iron under inner-core
conditions. There is some chemical but not physical evidence in favour of nickel
in the inner core: nickel has no effect on density; nickel alloys easily with iron;
Fe-Ni phases are observed in meteorites; and nickel in the core would balance
its depletion in the mantle compared with cosmic abundances.
At low temperatures, FeO is non-metallic and forms an immiscible liquid with
Fe.Inthe past this led to doubts about the presence of oxygen in the core. High-
pressure and -temperature experiments on iron oxide, however, have shown that it
becomes metallic at pressures greater than 70 GPa and temperatures greater than
1000 K. This means that oxygen can alloy with iron in the core and suggests that
oxygen is very probably a constituent of the core. Since oxygen raises the melting
temperature of iron, the presence of oxygen elevates estimates of core tempera-
tures. Experiments at temperatures and pressures appropriate for the core-mantle
boundary have shown that liquid iron and iron alloys react vigorously with solid
oxides and solid silicates. Thus an iron-rich core would react chemically with
the silicate mantle. This may well be the explanation for the seismic complex-
ity of the core-mantle boundary (Section 8.1.4): the mantle and core are not in
chemical equilibrium; rather this is the most chemically active part of the Earth.
It is probable that the core contains oxygen and the outer core may contain as
much oxygen as sulphur. It is, however, not yet experimentally or computationally
possible to establish the concentrations of the various lighter alloying elements
present in the core.
8.2 Convection in the mantle
8.2.1 Rayleigh-B enard convection
Convection in liquids occurs when the density distribution deviates from equilib-
rium. When this occurs, buoyancy forces cause the liquid to flow until it returns
to equilibrium. Within the Earth convection occurs in the mantle and the outer
core. Density disturbances in the Earth could be due to chemical stratification or
to temperature differences. Chemical stratification is the main cause of convec-
tion in the outer core, but in the mantle the convection is of thermal origin. The
simplest illustration of thermal convection is probably a saucepan of water, or
soup heating on the stove.
For a
Newtonian viscous fluid
, stress is proportional to strain rate, with
the constant of proportionality being the dynamic viscosity of the fluid.
5
5
strain rate
∝
stress
=
dynamic viscosity
×
stress
For materials with a power-law relationship between strain rate and stress
strain rate
∝
(stress)
n
Thus, when the stress increases by a factor of ten, the stress increases by 10
n
.ANewtonian fluid
has
n
=
1. Higher values of
n
are used in modelling some Earth behaviour.
