Page 66 - Global Tectonics

THE INTERIOR OF THE EARTH 53

difficult to accomplish. The drilling of a borehole neces- flow values are thus low over the Precambrian shields
sitates the use of fluid lubricants that disturb the thermal and much higher over regions affected by Cenozoic

regime of the borehole so that it has to be left for orogenesis. Within the oceans the heat fl ow decreases
several months to allow the disturbance to dissipate. with the age of the lithosphere (Section 6.5), with high
Porous strata have to be avoided as pore water acts as a values over the oceanic ridge system and active mar-
heat sink and distorts the normal thermal gradients. ginal seas and low values over the deep ocean basins
Consequently, it is rarely possible to utilize boreholes and inactive marginal seas.

sunk for the purposes of hydrocarbon or hydrogeologic The average heat flux in continental areas is
−2
−2
exploration. In many areas readings may only be under- 65 mW m , and in oceanic areas 101 mW m , of which
taken at depths below about 200 m so as to avoid the about 30% is contributed by hydrothermal activity at
transient thermal effects of glaciations. the mid-oceanic ridge system (Pollack et al., 1993). As
Heat flow measurements are considerably easier to 60% of the Earth’s surface is underlain by oceanic

accomplish at sea. The bottom temperatures in the crust, about 70% of the geothermal energy is lost
oceans remain essentially constant and so no complica- through oceanic crust, and 30% through continental
tions arise because of transient thermal perturbations. crust.
A temperature probe is dropped into the upper soft
sediment layer of the seabed and, after a few minutes’
stabilization, the temperature gradient is measured by FURTHER READING
a series of thermistor probes. A corer associated with
the probe collects a sediment sample for thermal con-
ductivity measurements; alternatively, the role of one
Anderson, D.L. (2007) New Theory of the Earth, 2nd edn. Cam-
of the thermistors can be changed to provide a source
bridge University Press, Cambridge, UK.
of heat. The change in the temperature of this probe
Bott, M.H.P. (1982) The Interior of the Earth, its Structure, Constitu-
with time depends on the rate at which heat is con- tion and Evolution, 2nd edn. Edward Arnold, London.
ducted away from it, and this enables a direct, in situ Condie, K.C. (2005) Earth as an Evolving Planetary System. Elsevier,
measurement of the thermal conductivity of the sedi- Amsterdam.
ment to be made. Fowler, C.M.R. (2005) The Solid Earth: an introduction to global
A large proportion of geothermal energy escapes geophysics, 2nd edn. Cambridge University Press, Cambridge.
Jacobs, J.A. (1991) The Deep Interior of the Earth. Chapman & Hall,
from the surface by conduction through the solid Earth.
London.
In the region of the oceanic ridge system, however, the
Nicolas, A. (1989) Structure of Ophiolites and Dynamics of Oceanic
circulation of seawater plays a major role in transport- Lithosphere. Kluwer Academic Publishers, Dordrecht.
ing heat to the surface and about 25% of the geother- Park, R.G. (1988) Geological Structures and Moving Plates. Blackie,

mal energy flux at the Earth’s surface is lost in this London and Glasgow.
way. Ranalli, G. (1995) Rheology of the Earth, 2nd edn. Chapman & Hall,

The pattern of heat flow provinces on the Earth’s London.
Stein, S. & Wysession, M. (2003) An Introduction to Seismology,
surface broadly correlates with major physiographic
Earthquakes, and Earth Structure. Blackwell Publishing,
and geologic subdivisions. On continents the magni- Oxford.

tude of heat flow generally decreases from the time of Twiss, R.J. & Moores, E.M. (2006) Structural Geology, 2nd edn.
the last major tectonic event (Sclater et al., 1980). Heat W.H. Freeman, New York.