Physical Geology 101
Paleomagnetism


Paleomagnetism

Paleo- =
Paleomagnetism -
Where is the Earth's magnetic field generated?

Magnetic Field Generation

Rotation of the planet causes the liquid iron to acts as a large electric dynamo - this generates a magnetic field.
Outer Core is liquid and convects due to heating from the Inner Core.
Convecting liquid iron also adds to this dynamo effect.

Magnetic Fields

Earth acts like a giant bar magnet, with magnetic force lines extending outward from the core.
Materials containing iron, or magnetized items will align themselves with the force lines if allowed to move freely.

Rock Magnetism

Rocks containing iron-rich minerals, such as magnetite, can also become magnetized.
During the formation process, minerals containing iron will become magnetized in the orientation of the Earth's magnetic force lines.
This magnetization is a direct record of the magnetic field's orientation and the latitude of the rock sample.

Magnetic Reversals

Earth's magnetic field reverses itself on average every 200,000 years.
Currently the Earth's magnetic field is in a "Normal" orientation - the force lines run from south to north
In a "Reverse" magnetic field, compass needles would point to the South Pole - the force lines would run from north to south.
Reversed magnetic fields are preserved in rock the same as normal magnetic fields.
Last reversal?

Magnetic Reversals

Reversal of magnetic field may be due to the disruption of flow within the Outer Core.
What causes this disruption is not understood.

Seafloors and Magnetism

How does paleomagnetism and magnetic reversals support seafloor spreading and continental drift?

Seafloor Age

One addition piece of data supports Seafloor Spreading:
Ocean floor close to the mid-ocean ridges is young in age - recently formed.
The oldest ocean floor lies furthest from the mid-ocean ridges.