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.