paleomagnetism aging the ocean floor. paleomagnetism paleomagnetism, or ‘ancient magnetism,’ is...
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Paleomagnetism
Aging the Ocean Floor
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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.
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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.
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• 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.
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Oldest Ocean Crust
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Magnetic North• In the asthenosphere there is a mineral
called magnetite, which emerges in lava at mid-oceanic ridges.
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• 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.
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• 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.
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• The further from the ridge the ocean floor is, the older it is.
• The oldest ocean floor is in trenches.
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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.
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