Geoscience Reference
In-Depth Information
Part VII
Energetics
By 2025, a population of 8.2 billion
would require an energy use of
55 TW!
WISE News Communique November 27, 1992
Kelvin could have concluded that an Earth cooled
by conduction was tens to hundreds of times
older than he published, even without internal
heat sources. A convecting and cooling Earth also
can account for the present heat flow, in the time
available, even without radioactivity, or, alter-
natively, with radioactivity but without secular
cooling. Clearly, the problem is ill-posed and non-
unique without additional constraints. Radioac-
tivity, minor heat sources and
secular cool-
ing (fossil heat)
contribute to the observed
heat flow. Currently, there is controversy about
how to interpret the observed heat flow, and how
it should be partitioned as to its source. Present
conclusions regarding Earth's thermal history are
no less dependent on assumptions, mathemat-
ics, physics and information from other disci-
plines, including meteoritics, than was the case
in the time of Kelvin, Darwin and Hutton. The
initial conditions of Earth's formation are impor-
tant, even if the Earth has forgotten its initial
temperature. Gravitational stratification, differ-
entiation and degassing control the subsequent
evolution.
Some geodynamicists claim that their under-
standing of the thermal
evolution of the
Earth is in disagreement with geo-
chemical data
, such as estimates of radioac-
tive heating. Geochemists believe that the stan-
dard
Overview
Lord Kelvin assumed that the Earth started as
a molten ball and calculated that it cooled to
its present condition by thermal conduction. The
still molten part was kept uniform by convection
and the frozen bits sank to the center. Kelvin
knew that Earth's temperature increased down-
ward into deep mines and guessed that the Earth
began as molten rock at 7000
◦
F. By solving
Fourier's equation, Kelvin found that it would
take a hundred million years for the Earth's tem-
perature gradient to level out to one degree every
50 feet. In numbers haughty for their implied
plus or minus nothing, Kelvin's final estimate,
in 1897, for the age of the Earth was 24 million
years. This calculation established that the Earth
had a finite age rather than the prevailing geo-
logic wisdom that there was '
no vestige of a
beginning, no prospect of an end
' or that
Earth's age was 'incomprehensibly vast'.
On hindsight we know that uncertainties in
the assumptions and parameters are such that
model
of
mantle
geochemistry
is now
