Paleomagnetism and Magnetic Reversals
Rocks lock in the direction of the magnetic field as they form, preserving a record of past field polarities and continental positions that established the reality of geomagnetic reversals and seafloor spreading.
Definition
Paleomagnetism is the study of the remanent magnetization recorded in rocks, used to determine the direction and polarity of the ancient geomagnetic field; magnetic reversals are the documented exchanges of the field's north and south magnetic poles that punctuate this record.
Scope
This topic covers the fossil magnetic record in rocks and what it reveals: how thermal, detrital, and chemical remanent magnetizations record the ancient field, the demonstration that the field has repeatedly reversed polarity, the construction of the geomagnetic polarity timescale, and the magnetic stripes of the seafloor. It treats apparent polar wander and its use in reconstructing plate motions and recognizing displaced terranes, along with field tests of paleomagnetic stability. The emphasis is on reading geological history from rock magnetization.
Core questions
- How do rocks acquire and retain a record of the ancient magnetic field?
- What evidence established that the geomagnetic field reverses polarity?
- How is the geomagnetic polarity timescale built and used to date rocks?
- How does apparent polar wander reconstruct the past motion of plates?
Key concepts
- Thermal, detrital, and chemical remanent magnetization
- Geomagnetic polarity reversals
- Geomagnetic polarity timescale
- Marine magnetic anomaly stripes
- Apparent polar wander paths and terrane analysis
Key theories
- Reversals and the polarity timescale
- Combining reversely magnetized rocks with radiometric dating showed that the geomagnetic field has repeatedly reversed polarity, allowing construction of a global polarity timescale that serves as a stratigraphic and dating framework.
- Vine-Matthews seafloor spreading record
- Vine and Matthews explained the symmetric magnetic stripes flanking ocean ridges as crust magnetized in alternating polarities as it formed and spread, directly tying paleomagnetism to seafloor spreading and plate tectonics.
Mechanisms
As igneous rock cools through the Curie temperature of its magnetic minerals, or as sediment settles and as new minerals grow, the minerals acquire a remanent magnetization aligned with the ambient field; if this magnetization is stable over geological time, the rock preserves the field's direction and polarity at formation, which can be measured and dated to reconstruct field and plate history.
Clinical relevance
The polarity timescale dates the ocean floor and calibrates plate motions and stratigraphy, apparent polar wander reconstructs paleogeography, and magnetostratigraphy provides high-resolution correlation for sedimentary basins of interest in resource exploration.
History
Brunhes reported reversely magnetized lavas in 1906; in the early 1960s Cox, Doell, and Dalrymple established the reversal timescale by dating polarity transitions, and Vine and Matthews's 1963 interpretation of marine magnetic stripes provided decisive evidence for seafloor spreading and plate tectonics.
Key figures
- Bernard Brunhes
- Allan Cox
- Fred Vine
- Drummond Matthews
Related topics
Seminal works
- vine1963
- cox1964
- butler1992
Frequently asked questions
- How do we know the Earth's magnetic field has reversed in the past?
- Many rocks are magnetized opposite to today's field, and by radiometrically dating sequences of normally and reversely magnetized rocks scientists found consistent, globally synchronous reversals, later confirmed by the symmetric magnetic stripe pattern of the seafloor.
- How does paleomagnetism show that continents have moved?
- Ancient rocks on a continent record magnetic directions pointing to apparent pole positions that differ from today's; tracing how these apparent poles changed through time, and comparing continents, reveals that the continents themselves moved across the Earth.