Geoscience Reference
In-Depth Information
Chapter 27
The thermal history of the Earth
Man grows cold faster than the planet
he inhabits.
mantle dynamics. The migration of ridges and
trenches is an important aspect of thermal his-
tory and geodynamics.
The heat budget of the Earth cannot be
treated as an instantaneous one-dimensional
heat-flow problem, or one that involves a homo-
genous mantle with uniform and static boundary
conditions. Both the radial and lateral structure
of the Earth must be considered. Viscosity of
the mantle is temperature, pressure, composi-
tion, location and time-dependent.
Magma oceans, plate tectonics, heat pipes
and stagnant-lid convection have transferred heat
out of the interior at various times. The cur-
rent Earth approximates stagnant-lid conditions
in one hemisphere and a lid that fully partic-
ipates in internal convection in the other. On
early Earth, the surface may have been cov-
ered with thick accumulations of unsubductable
basalt, penetrated by pipes of magma, as appears
to be the case on Io. In such a scenario the rate
of heat loss is regulated by the stress and density
of the basalt pile as much as by the viscosity of
the interior. The interior cools by depression of
the cold surface layer and delamination, which
displace hot material upwards. Even on today's
planet, eruption and mantle cooling depend on
the stress state of the overlying plate.
Albert Einstein
Starting with Kelvin there have been many con-
troversies and paradoxes associated with the evo-
lution of the Earth. These are not faith-based
controversies, in the ordinary sense; they are
based on calculations and assumptions -- which
areaformoffaith.Present-dayheatflowis
determined by the amount and distribution of
radioactive elements, their secular decline with
time, the delay between heat production and its
appearance at the surface, secular cooling of the
interior, and a variety of minor sources of heating
which are usually overlooked. Chemical stratifica-
tion of the mantle slows down the cooling of the
Earth; the upward concentration of radioactive
elements reduces the time between heat genera-
tion and surface heat flow. The initial conditions
of the Earth cannot be ignored; they have not
been forgotten.
Continents divert mantle heat to the ocean
basins and, in addition, tend to move toward
cold downwellings, thereby protecting their cold
keels. Surface conditions -- including the thick-
ness of plates -- and mantle viscosity control con-
vective vigor and cooling history of the mantle. A
global accounting of the heat lost from the inte-
rior by hydrothermal processes at the surface is
a lingering issue. A chemically layered mantle
with upward concentrations of the radioactive
elements, shallow return flow, and a low-viscosity
asthenosphere is the most plausible model of
Kelvin
Lord Kelvin assumed that the Earth started as
a molten ball and calculated that it cooled to
its
present
condition
by
thermal
conduction.
