Environmental Engineering Reference
In-Depth Information
INTERNAL ENERGY AND HEAT FLOW
All planetary processes require energy, and Earth has five sources of energy intimately
linked with the formation and operation of our solar system. Three sources generate
thermal energy and two generate gravitational (potential) energy. The sun's role as the
principal
exogenetic
source of radiant energy is set out in Chapter 2 and its significance
for atmospheric, geomorphological and biospheric processes is explained in subsequent
chapters. Nucleosynthesis of helium from hydrogen in the sun is the essential energy
source of the solar system. Similar nuclear reactions in Earth's interior generate
endogenetic
heat by the continuing decay of radioactive isotopes of uranium
235
U,
238
U, to
lead,
206
Pb,
207
Pb, and of potassium,
40
K, to argon,
40
Ar, etc., primarily in continental
crust. The condensation of cosmic gases and compression of Earth's core, with a
corresponding decrease in volume, caused
adiabatic
heating similar to atmospheric
processes described in Chapter 4. Kinetic energy from planetesimal and meteorite
impacts also generates heat, supplemented by heat from fractionation and friction as core
and mantle materials segregated past each other.
The principal effects of endogenetic
thermogenesis
, or heat generation, are to
establish convection within the mantle, which drives plate tectonics, and to cause
geological phase transformation mobilizing rock between solid-liquid-gas states. This is
the key to continuing fractionation of rock material and the creation of
magma
, its
molten viscous state, essential to crustal evolution. Exogenetic heat powers the
geomorphological processes which ornament the continental crust, as we see in later
chapters, sharing this role in geological processes with
gravity
. Gravity is the force of
mutual attraction between two bodies and is a function of their masses and distance apart.
Earth's large mass centred around a dense core provides the primary, endogenetic source
of gravity for most geological processes but the gravitational fields of our sun and moon
influence astrogeological and some surface (especially tidal) processes.
Gravitational
energy
describes the potential energy of rock displaced away from Earth's core. This is a
further by-product of fractionation and an important consequence of tectonic uplift,
which drives surface geomorphological processes. Gravity adds a further twist, literally,
through a centrifugal component due to Earth's rotation, which slightly flattens its
spherical shape at the poles into an
oblate spheroid
.
Internal heat sources establish a
geothermal heat flow
from the core towards the cool
crust. With a core temperature calculated at 4,000° C and a mean surface temperature of
10° C, the average thermal gradient would be 0·62° C km
−1
. It is thought that core and
mesosphere gradients are slightly lower, owing to the slow release of heat stored from the
early accreting Earth, limited mostly to conduction in rigid rocks. However, near-surface
gradients observed in mines are up to sixty times greater at 20-40° C km
−1
, sufficient to
be tapped for geothermal power. This is due to crustal radioactive thermogenesis,
responsible for some 70 per cent of the continental crust flux, and to convection aided by
the viscous state of the asthenosphere.
Measured as a heat flux in milliwatts, rather than a thermal gradient, the mean surface
flux is 82 mW m
−2
or 0·082 W m
−2
. There are, however, several interesting variations.
Crustal heat flux diminishes over time and, in oceanic crust, with increasing distance
from mid-ocean ridges, where it may reach 200 mW m
−2
. The oceanic crust mean flux of
98 mW m
−2
is 75 per cent higher than the continental crust mean flux at 56 mW m
−2
,
despite the latter's radioactive source. On the other hand, ocean crust is virtually devoid
