Geoscience Reference
In-Depth Information
Figure 8.28.
Images show the radial component of the magnetic field (left) and the
fluid flow at the core-mantle boundary (right). Upper pair, numerical model of the
geodynamo with viscous stress-free boundary conditions at the rigid boundaries;
lower pair, Earth's field averaged over the years 1840-1990. Colour version Plate 17.
Reprinted with permission from
Nature
(Kuang and Bloxham,
Nature
,
389
, 371-4)
Copyright 1997 Macmillan Magazines Ltd.)
8.3.3 What drives convection in the outer core and how has
the core changed with time?
What drives or powers the dynamo? Estimates of the power needed to drive
the dynamo are 10
11
-10
12
W, a fraction of the
10
12
W heat flow through
the CMB. Any convection in the outer core must involve an inherent density
instability, with less dense material lying beneath denser material. Such a density
instability could be due to changes in the Earth's rotation or to heat (thermal
buoyancy), or it could result from chemical differences in the core. A number of
heat sources could contribute to thermally driven convection in the outer core.
One possible source is the radioactive isotopes
235
U and
40
K, which are present
in the crust and the mantle, and possibly in the core, but this can provide only part
of the energy needed to drive the dynamo. Another possible source of heat is the
primordial heat
: that heat which resulted from the formation of the Earth, which
the core is slowly losing. If the inner core is cooling, solidifying and separating
from the liquid outer core, then the latent heat of solidification (crystallization)
would also provide heat to help power the dynamo. However, it is likely that
chemical differences provide most of the energy needed to power the dynamo,
with density instabilities arising as the outer core crystallizes dense iron crystals.
These crystals, being denser than the liquid iron alloy of the outer core, fall
towards the inner core, and the less dense liquid rises. The gravitational energy
released in this process is then sufficient to drive the dynamo. Some energy is
∼
4
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