The earths magnetic field and paleomagnetism the

The Earth's Magnetic Field and Paleomagnetism
The Earth has a magnetic
field that causes a compass
needle to always point
toward the North magnetic
pole, currently located near
the rotation pole. The
Earth's magnetic field is
what would be expected if
there were a large bar
magnet located at the
center of the Earth (we now
know that this is not what
causes the magnetic field,
but the analogy is still
good). The magnetic field
is composed of lines of
force as shown in the
diagram here.
A compass needle or a magnetic weight suspended from a string, points along these lines of
force. Note that the lines of force intersect the surface of the Earth at various angles that depend
on position on the Earth's surface. This angle is called the magnetic inclination . The
inclination is 0
o
at the magnetic equator and 90
o
at the magnetic poles. Thus, by measuring the
inclination and the angle to the magnetic pole, one can tell position on the Earth relative to the
magnetic poles.
In the 1950s it was
discovered that when
magnetic minerals cool
below a temperature
called the Curie
Temperature, domains
within the magnetic
mineral take on an
orientation parallel to
any external magnetic
field present at the time
they cooled below this
temperature.
At temperatures above the Curie Temperature, permanent magnetization of materials is not
possible. Since the magnetic minerals take on the orientation of the magnetic field present
during cooling, we can determine the orientation of the magnetic field present at the time the
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Plate Tectonics
8/26/2015

rock containing the mineral cooled below the Curie Temperature, and thus, be able to
determine the position of the magnetic pole at that time. This made possible the study of
Paleomagnetism (the history of the Earth's magnetic field). Magnetite is the most common
magnetic mineral in the Earth's crust and has a Curie Temperature of 580
o
C
Initial studies of the
how the position of
the Earth's magnetic
pole varied with time
were conducted in
Europe. These studies
showed that the
magnetic pole had
apparently moved
through time. When
similar measurements
were made on rocks
of various ages in
North America,
however, a different
path of the magnetic
pole was found.
This either suggested that (1) the Earth has had more than one magnetic pole at various times in
the past (not likely), or (2) that the different continents have moved relative to each other over
time. Studies of ancient pole positions for other continents confirmed the latter hypothesis, and
seemed to confirm the theory of Continental Drift.
Sea-Floor Spreading
During World War II, geologists employed by the military carried out studies of the sea floor, a
part of the Earth that had received little scientific study. The purpose of these studies was to
understand the topography of the sea floor to find hiding places for both Allied and enemy
submarines. The topographic studies involved measuring the depth to the sea floor. These