Geoscience Reference
In-Depth Information
is a hydrocarbon derivative of geological fractionates
where lithosphere, hydrosphere and atmosphere meet.
All three systems respond to heating,
photodissociation
or fractionation by sunlight, in addition to geological
processes, through biogeochemical reorganization.
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 tem-
perature 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
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 signifi-
cance for atmospheric, geomorphological and biospheric
processes is explained in subsequent chapters. Nucleo-
synthesis 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 con-
densation 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 meteor-
ite impacts also generates heat, supplemented by heat
from fractionation and friction as core and mantle
materials segregate 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 frac-
tionation and an important consequence of tectonic
uplift, which drives surface geomorphological processes.
Gravity adds a further twist, literally, through a centrifugal
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 of radioactive elements, so 95 per cent
of its heat flux must come from greater depth. Gradients
are steepest in oceanic lithosphere, which conducts heat
twice as efficiently, and continental lithosphere is cooler
than ocean lithosphere. Overall, oceanic crust accounts for
75 per cent of global geothermal heat flux by virtue of its
larger area and superior rate. Volcanoes and hot spots, not
surprisingly, experience the highest fluxes of 200-250
mW m
-2
. All of this indicates considerable thermal activity
in the shallow lithosphere. In particular, persistent
contrasts between 'hot' oceanic and 'cool' continental
lithosphere show that the sea floor holds the key to crustal
evolution via plate tectonics.
TECTONICS
From the great voyages of exploration in the Age of
Discovery after
AD
1450 it was noted that many conti-
nental coastlines appeared to fit together, particularly
those bordering the Atlantic Ocean, and seemed to have
become separated like the dispersed parts of a jigsaw
puzzle - perhaps by Noah's Flood! In 1912 Alfred Wegener
consolidated emerging theories of dispersal by
continental
drift
to propose the former existence of a single land mass,
