Answer: Yes.
We know this from an examination of the geological record. When lavas
are deposited on the Earth’s surface, and subsequently freeze, and
when sediments are deposited on ocean and lake bottoms, and subsequently
solidify, they often preserve a signature of the ambient magnetic field
at the time of deposition. This type of magnetization is known as 'paleomagnetism'.
Careful measurements of oriented samples of faintly magnetized rocks taken
from many geographical sites allow scientists to work out the geological
history of the magnetic field. We can tell, for example, that the Earth
has had a magnetic field for at least 3.5 billion years, and that the
field has always exhibited a certain amount of time-dependence, part of
which is normal secular variation, like that which we observe today, and
part of which is an occasional reversal of polarity. Incredible as it
may seem, the magnetic field occasionally flips over! The geomagnetic
poles are currently roughly coincident with the geographic poles, because
the rotation of the Earth is an important dynamical force in the core,
where the main part of the field is generated. Occasionally, however,
the secular variation becomes sufficiently large such that the magnetic
poles end up being located rather distantly from the geographic poles;
we say that the poles have undergone an ‘excursion’ from their
preferred state. Now, we know from physics that the Earth’s dynamo
is just as capable of generating a magnetic field with a polarity like
that which we have today as it is capable of generating a field with the
opposite polarity. The dynamo has no preference for a particular polarity.
Therefore, after an excursional period of enhanced secular variation,
the magnetic field, upon returning to its usual state of rough alignment
with the Earth’s rotational axis, could just as easily have one
polarity as another. The consequences of polarity reversals for the compass
are dramatic. Nowadays, the compass points roughly north, or, more precisely,
the north end of the compass points roughly north at most geographical
locations. However, before the last reversal, which was about 780,000
years ago, the polarity was reversed compared to today's, and the compass
would have pointed roughly south, and before that reversed state the polarity
was like that which we have today, and the compass would have pointed
roughly north, and so on. The timings of reversals forms the so-called
'geomagnetic polarity timescale', shown here at the right. During a reversal,
between polarities, the geometry of the magnetic field is much more complicated
than it is now, and a compass could point in almost any direction depending
on one’s location on the Earth and the exact form of the mid-transitional
magnetic field. One of the things that is interesting about reversals
is that there is no apparent periodicity to their occurrence. Reversals
are random events. They can happen as often as every 10 thousand years
or so, and as infrequently as every 50 million years or more. Questions
about reversals are very popular with the general public, and further
information can be found in the references given in the Further
Reading page of the Geomagnetism website. |