The Solar System and the Planet Earth 311
of the prevailing magnetic field lines, just as iron filings are aligned by a
bar magnet. When the rock congeals the alignment of iron atoms are
frozen into the rock. The rock as a consequence, has a residual
magnetism indicating the original direction of the magnetic field during
its molten state just prior to its solidification. By examining basalts, the
rocks formed by volcanoes, or escaping from oceanic ridges one can
determine the direction of the Earth’s magnetic field at the time of
their formation. Sedimentary rocks are also frequently magnetized.
Sedimentary rock is formed from deposits of rock grains, which fall to
the bottom of the sea. If these rock grains are magnetized then they will
align themselves according to the direction of the prevailing magnetic
field. When the sedimentary rock forms as a result of the pressure from
above these alignments are frozen in and a record of the magnetic field is
made.
Evidence for the reversal of the Earth’s magnetic field was found in
piles of lava flows in which the polarity of the lava changed as one
examined deeper and older levels of the lava. These lava piles have been
found in different parts of the world. By dating the various levels, using
radioactivity techniques, it was found that the polarities of the lava flows
from different locales are correlated. The Earth has reversed its magnetic
field 25 times in the past 4 million years. The variations have been
irregular with millions of years passing between some reversals and only
a few thousand years in other cases.
One of the interesting effects of the reversal of the magnetic field is
its effect on life. A correlation between the reversal of the magnetic field
and the extinction of certain species has been noted. Recent speculation
suggests that perhaps the magnetic fields affect the weather and the
reversals cause climatic changes, which in turn can cause extinction of
certain species. Still another proposal suggests that perhaps the extinct
species were directly sensitive to the Earth’s magnetic field like certain
snails, flatworms and fruit flies living today, and that the reversal of the
magnetic field disturbed their life cycle causing extinction.
The mechanism, which causes the reversal of the Earth’s magnetic
field is not known and is presently a subject of much speculation as is the
origin of the Earth’s magnetic field. The only theory, which presently
can provide a plausible explanation of the Earth’s magnetic field and
its property of reversing direction, is the dynamo theory. Attributing
the Earth’s magnetism to the residual magnetism of the rocky mantle
could never explain the strength of the Earth’s field. Rock lying greater