Paleomagnetism studies Earth's ancient magnetic field preserved in rocks. It reveals past field directions and intensities, helping scientists understand plate tectonics, continental drift, and geodynamo behavior.
This powerful tool aids in dating sedimentary rocks, reconstructing past continents, and correlating geological events globally. It's crucial for unraveling Earth's complex history and understanding its magnetic field evolution.
Paleomagnetism and Rock Magnetism
Principles of Paleomagnetism
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9.3 Earth’s Magnetic Field | Physical Geology View original
Paleomagnetism studies the Earth's magnetic field preserved in rocks, providing information about the direction and intensity of the Earth's magnetic field at the time the rocks formed or cooled below their Curie temperature
The primary types of natural remanent magnetization (NRM) include thermal remanent magnetization (TRM), chemical remanent magnetization (CRM), and detrital remanent magnetization (DRM)
TRM is acquired when magnetic minerals cool below their Curie temperature in the presence of an external magnetic field (igneous rocks, baked sedimentary rocks near igneous intrusions)
CRM is acquired when new magnetic minerals form or grow in the presence of an external magnetic field (diagenesis, metamorphism)
Natural Remanent Magnetization (NRM)
NRM is the permanent magnetization acquired by rocks during their formation or subsequent alteration, used to determine the direction and intensity of the Earth's magnetic field at that time
DRM is acquired when magnetic mineral grains align with the Earth's magnetic field as they settle through water and are deposited in sedimentary layers
The stability of NRM depends on factors such as the magnetic mineral composition, grain size, and the presence of secondary overprints due to later thermal or chemical events
Examples of rocks with stable NRM include basalts, volcanic tuffs, and well-preserved sedimentary rocks (red beds, limestones)
Apparent Polar Wander and Plate Tectonics
Apparent Polar Wander (APW)
APW refers to the observed movement of the Earth's magnetic poles relative to a particular continent or tectonic plate over geological time
APW paths are constructed by plotting the paleopoles (ancient magnetic pole positions) determined from paleomagnetic studies of rocks of different ages from a single continent or plate
The movement of the magnetic poles relative to a continent or plate is due to the movement of the continent or plate relative to the Earth's spin axis, not the actual movement of the poles themselves
Examples of APW paths include the North American APW path and the European APW path
Plate Tectonics and Paleomagnetism
APW paths provide evidence for the motion of tectonic plates over geological time, supporting the theory of plate tectonics
The consistency of APW paths across different continents for the same time periods provides evidence for the existence of past supercontinents (Pangaea, Rodinia)
Differences in APW paths between continents can be used to reconstruct the relative positions and movements of continents through time, aiding in the understanding of plate tectonic processes
Paleomagnetic data have been used to reconstruct the breakup of Pangaea and the subsequent drift of continents to their present-day positions
Paleomagnetic Data for Reconstruction and Geodynamo Studies
Continental Reconstruction
Paleomagnetic data can be used to determine the paleolatitude of a continent or tectonic plate at the time the rocks acquired their NRM, based on the inclination of the Earth's magnetic field recorded in the rocks
The declination of the NRM in rocks provides information about the rotation of a continent or plate relative to the Earth's magnetic north pole
By comparing the paleolatitudes and declinations of rocks from different continents of the same age, the relative positions of the continents at that time can be reconstructed
Paleomagnetic data have been used to reconstruct the configurations of past supercontinents (Pangaea, Gondwana, Laurasia)
Geodynamo Studies
Paleomagnetic data can be used to study the behavior of the Earth's geodynamo, which generates the Earth's magnetic field
Changes in the intensity and direction of the Earth's magnetic field over geological time, as recorded in rocks, provide insights into the dynamics of the Earth's outer core and the processes that drive the geodynamo
Paleomagnetic data have revealed the occurrence of magnetic field reversals, where the Earth's magnetic field flips its polarity, with magnetic north becoming magnetic south and vice versa
The intervals between magnetic field reversals range from tens of thousands to millions of years (Brunhes-Matuyama reversal, Jaramillo subchron)
Magnetic Stratigraphy for Dating and Correlation
Dating Sedimentary Sequences
Magnetic stratigraphy uses magnetic polarity reversals recorded in sedimentary rocks to establish a chronological framework for sedimentary sequences
During magnetic polarity reversals, the Earth's magnetic field switches between normal polarity (similar to the present-day field) and reversed polarity, which is recorded in sedimentary rocks as changes in the orientation of magnetic minerals
The pattern of magnetic polarity reversals in a sedimentary sequence can be compared to the geomagnetic polarity timescale (GPTS), a global record of magnetic polarity reversals dated using radiometric methods
By identifying the pattern of polarity reversals in a sedimentary sequence and matching it to the GPTS, the age of the sediments can be determined
Correlating Geological Events
Magnetic stratigraphy allows for the correlation of sedimentary sequences across different basins and even different continents, as the pattern of magnetic polarity reversals is global in nature
The correlation of sedimentary sequences using magnetic stratigraphy helps in understanding the timing and extent of geological events (mass extinctions, climate changes, sea-level fluctuations)
Magnetic stratigraphy is particularly useful for dating sedimentary sequences that lack other datable materials (fossils, volcanic ash layers) and can provide age constraints for these sequences
Examples of geological events studied using magnetic stratigraphy include the Cretaceous-Paleogene boundary, the Eocene-Oligocene transition, and the Messinian salinity crisis