Physics for Geologists, Second edition

xiv Preface
waves diminishes with depth. Indeed, the scuba diver at oceanic margins
will be unhappy with the definition of wave base in various American Geo-
logical Institute publications (Bates and Jackson 1987 for example): 'The
depth at which wave action no longer stirs the sediments; it is usually about
10 meters'. Sediment on continental shelves can be stirred by ocean swell at
much greater depths than 10 m - at least to 100 m - as we shall see. The def-
inition above would only be true of waves of short wavelength up to about
7or 8m.
Physics is also important in the definition of many terms, particularly in
groundwater studies, and these will be examined closely. Some of the defin-
itions found in glossaries and dictionaries are inconsistent with the physics
of the phenomenon in question.
The physics involved in geology is not confined to the scale of our own
perceptions. The climatic regions of the Earth are very largely determined
by the circulation of the atmosphere between the warm tropics over a large
area and the cold poles over a small area. Convection is a fairly straight-
forward topic of meteorology, but is far more difficult in the context of the
interior of the Earth. The mid-ocean ridges and the plates seem to require
convection within the Earth, but a full-scale plausible convection process
has not yet been developed and remains speculative to some extent at least.
At the other end of the scale, the so-called absolute age-determination of
rocks involves the decay of radioactive elements and some understanding of
particle physics.
Geologists cannot ignore extra-terrestrial influences. The seasons are the
result of our attitude in our orbit - the fact that our axes of rotation, about
the Sun and about the poles, do not coincide. Tides, which can give rise to
strong currents over large areas near land, are caused by the interaction of
the sun and the moon on the oceans. Meteorites light up our night skies by
the incandescence caused by atmospheric friction, and those that survive this
to reach the Earth show signs of this heat.
The principle of uniformitarianism insists that the processes were essen-
tially the same in the past as they are today. The seas, tides, waves, wind,
rain, rivers, all shaped the Earth in the past, as did earthquakes and volcanic
eruptions.
Data acquisition is an important aspect of geology, and we must frequently
do this indirectly by physical means in the interests of economy. For exam-
ple, we need to distinguish between natural gas, crude oil and water in the
rocks. This can most readily be achieved by measuring the resistivity or con-
ductivity of the rock - or its response to radioactive bombardment. Shales
and mudrocks can be distinguished (usually) from other lithologies by their
natural gamma radioactivity. We need to know the porosity of rocks that
contain fresh water, or crude oil or natural gas, and this can be obtained by
measuring the speed of sound through the rock because the larger the pore
volume, the slower the speed of sound through the rock. The analysis of
Copyright 2002 by Richard E. Chapman