310 The Poetry of Physics and The Physics of Poetry
iron and magnesium and contains a number of different layers. The
crust whose average thickness is 40 kilometers below the continents
and 9 kilometers below the oceans has a density of only 3 grams per
cubic centimeter. The crust consists mostly of granite, but layers of
sedimentary and volcanic rock are found in most locations as well. It is
only on the shields such as the Canadian Shield that the granite base of
the crust is exposed.
The Earth’s Magnetic Field
The magnetic field at the surface of the Earth is produced primarily by
agents within the interior of the planet. The exact causes are not known
although it is popularly believed that the liquid part of the metallic core
is responsible for this field. The geomagnetic field is a dipole field
identical to the one produced by a bar magnet. The axis of the Earth
dipole field makes an angle of 11° with the axis of rotation indicating the
magnetic field is associated with the rotation of the Earth.
The Earth’s magnetic field at the surface varies in magnitude both
with time and with the position on the Earth. The magnetic field strength
at the pole is 3 ½ times stronger than at the equator. This is a result of the
dipole nature of the internal magnetic field of the Earth.
There are many different types of temporal variations of the
geomagnetic field of approximately 1/2 of 1% due to the effects of
the Sun, the solar wind, the Van Allen radiation belts of ionized particles
and the upper atmosphere. There is also the variation due to the Sun
spot activity which also produces a daily variation and which comes in
an 11-year cycles. The records of magnetic measurements which have
been made at certain locales for the past 350 years indicate that the
magnetic field varies by as much as 20% in cycles whose periods are
approximately 100 years. Certain anomalous variations of the magnetic
field have been observed to drift towards the west at the rate of 1/5 of a
degree per year, which indicates a cycle of 2000 years, the length of the
time for this anomaly to propagate completely around the Earth. In
addition to these rather minor variations on a relatively short term scale
the residual magnetism of rocks indicate that the Earth actually reverses
the direction of its magnetic field on a time scale of millions of years.
Rocks are composed basically of silicon oxides. They also contain
iron oxides, however, which can become magnetized. When a rock is in
a molten stage the iron atoms are free to align themselves in the direction