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Paleomagnetism
Aging the Ocean Floor
Paleomagnetism
Paleomagnetism, or ‘ancient
magnetism,’ is the study of the
changes in Earth’s magnetic field
over its history to determine how old
the ocean floor is in a particular spot.
Magnetic Field
• Earth’s magnetic field originates in
the core and it can reverse direction.
– Currently the magnetic field originates
in the north.
• We don’t know why it reverses, or
how to predict when it will occur.
• The field has reversed about 20 times
in the last 5 million years – the most
recent was 730 000 years ago.
• The oldest ocean floor is 200 MY, and
is in the northwest Pacific.
Oldest Ocean Crust
Magnetic North
• In the asthenosphere there is a mineral
called magnetite, which emerges in lava
at mid-oceanic ridges.
• It is magnetic, and
as the new crust
hardens, its
particles lock in
place pointing in
the direction of the
origin of the
magnetic field.
• Since the particles will align in
different directions depending on
where the magnetic origin is, the
result is an equal pattern on either
side of a ridge showing how many
times the field has shifted, and how
long each shift lasted.
• The further from
the ridge the
ocean floor is,
the older it is.
• The oldest
ocean floor is in
trenches.
Detecting the Field
• Earth’s magnetic field can’t be seen by the
naked eye, and it also can’t be detected by
a compass.
• Instead, we use a device called a
magnetometer which graphs out the
changes in the magnetic field.
• It is built into a ship or an airplane.