Geology > Geophysics > Paleomagnetism

Paleomagnetism is a specialized area within geophysics, itself a subdiscipline of geology. This field studies the record of the Earth’s magnetic field in rocks, sediment, and archaeological materials. The primary goal of paleomagnetism is to understand geological and geomagnetic processes over geologic time scales.

The Earth’s Magnetic Field

The Earth can be approximated as a giant dipole magnet, with magnetic field lines emanating from the magnetic north to the magnetic south pole. This magnetic field is generated by the movement of molten iron and nickel in the outer core through a process known as the geodynamo. Because this field affects various geological processes, its record provides invaluable information about the history of the Earth.

Natural Remanent Magnetization (NRM)

The main concept in paleomagnetism is natural remanent magnetization (NRM). Rocks and minerals that contain ferromagnetic minerals, such as magnetite (\(\mathrm{Fe_3O_4}\)), can record the Earth’s magnetic field at the time they were formed. The types of NRM include:

• Thermoremanent Magnetization (TRM): Acquired by igneous and some metamorphic rocks as they cool from high temperatures. When temperatures fall below the Curie point, magnetic minerals lock in a record of the direction and strength of the Earth’s magnetic field.
• Detrital Remanent Magnetization (DRM): Acquired by sedimentary rocks when magnetic minerals settle out of water and align with the Earth’s magnetic field.
• Chemical Remanent Magnetization (CRM): Occurs when magnetic minerals form during chemical reactions, aligning according to the prevailing magnetic field.

Paleomagnetic Measurements and Techniques

Researchers extract paleomagnetic data through laboratory analyses, involving the measurement of the magnetic properties of rock samples. Main techniques include:

• Magnetometers: Instruments like the spinner magnetometer measure the magnetic moment of the samples.
• Demagnetization Procedures: Techniques like thermal and alternating-field demagnetization help isolate the primary remanent magnetization from secondary magnetic overprints acquired after formation.

Applications of Paleomagnetism

Paleomagnetism has several fundamental applications:

• Plate Tectonics: Paleomagnetic data provide evidence for the theory of plate tectonics by showing how continents have moved over geologic time. This is often visualized through polar wander paths.
• Geochronology: It aids in dating rock sequences by correlating them with known geomagnetic reversals.
• Environmental and Climate Studies: By analyzing sediment cores, paleomagnetism can help reconstruct past climatic conditions and environmental changes.

Mathematical Representation

For more quantitative aspects, the Earth’s magnetic field (\(\mathbf{B}\)) at a point can be expressed using:

\[
\mathbf{B} = \mu_0 ( \mathbf{H} + \mathbf{M} )
\]

where \(\mu_0\) is the magnetic permeability of free space, \(\mathbf{H}\) is the magnetic field intensity, and \(\mathbf{M}\) is the magnetization of the material.

Conclusion

Paleomagnetism serves as a critical tool in understanding a broad range of geological phenomena. Through the analysis of magnetic signatures preserved in rocks, scientists can infer patterns of past geomagnetic field behavior and, by extension, gain insights into the dynamic processes of the Earth’s interior and surface over millions of years.