Geomagnetism and Paleomagnetism
The Earth generates a magnetic field in its molten core, and the record of that field frozen into rocks preserves a history of reversals and continental motion that underpins plate tectonics.
Definition
Geomagnetism is the study of the Earth's magnetic field, its spatial structure, temporal variation, and origin in the geodynamo, while paleomagnetism is the study of the fossil magnetization recorded in rocks, used to reconstruct past field behavior and the motion of tectonic plates.
Scope
This area covers the Earth's magnetic field and its history: the present geomagnetic field and its description as a geocentric dipole plus higher harmonics, its secular variation, and its generation by the geodynamo in the liquid outer core. It treats paleomagnetism, the recording of past field directions and polarities in rocks, the geomagnetic reversal timescale, and rock and environmental magnetism. The emphasis is on the field's origin in core dynamics and on extracting a paleomagnetic record from magnetic minerals.
Sub-topics
Core questions
- How is the geomagnetic field described and how does it change with time?
- How does fluid motion in the liquid core generate and sustain the field?
- How do rocks record the ancient magnetic field, and how reliable is that record?
- What do polarity reversals and apparent polar wander reveal about Earth history and plate motion?
Key concepts
- Geocentric axial dipole and the geomagnetic field components
- Secular variation and westward drift
- Geodynamo and magnetohydrodynamics of the core
- Geomagnetic polarity reversals and the polarity timescale
- Remanent magnetization and rock magnetism
Key theories
- Geodynamo theory
- The geomagnetic field is sustained by self-exciting dynamo action: convective and rotational motion of the electrically conducting liquid iron in the outer core induces currents that regenerate the field against ohmic decay.
- Paleomagnetic recording and reversals
- Magnetic minerals lock in the direction of the ambient field as rocks form, so ordered sequences of normal and reversed magnetization reveal a global reversal history and, through apparent polar wander, the past positions of continents.
Clinical relevance
Geomagnetism provides the polarity-reversal timescale that calibrates seafloor spreading and plate motions, supports magnetic surveying and navigation, and constrains models of core dynamics; environmental magnetism aids paleoclimate and pollution studies.
History
Gilbert argued in 1600 that the Earth itself is a magnet, Gauss put the field's mathematical description on a rigorous footing in the 1830s, Brunhes discovered reversely magnetized rocks in 1906, and mid-twentieth-century dynamo theory and the paleomagnetic reversal timescale became central evidence for plate tectonics.
Key figures
- William Gilbert
- Carl Friedrich Gauss
- Bernard Brunhes
- Edward Bullard
Related topics
Seminal works
- merrill1996
- butler1992
- fowler2005
Frequently asked questions
- Where does the Earth's magnetic field come from?
- It is generated by the geodynamo: convecting, electrically conducting molten iron in the outer core, organized by the Earth's rotation, drives electric currents that sustain the field, much as a self-exciting dynamo maintains its own magnetism.
- How does the magnetism of rocks help reconstruct plate motions?
- When rocks form they record the direction of the magnetic field, including its polarity; comparing these fossil directions across continents and the symmetric magnetic stripes on the seafloor reveals how plates and continents have moved over geological time.