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Paleomagnetism is the study of Earth’s magnetic history with the help of an ancient magnetic record found in rocks. It is a key tool in providing evidence for many fundamental concepts in geology.
During WWII, highly sensitive magnetometers were created to detect enemy submarines. Later, geologists updated these devices and trailed them behind research vessels to measure the local magnetic fields created by magnetized rocks on the seabed.
Scientists were amazed when they detected repeated patterns in the strength of the nearby magnetic field.
The magnetic field lines were nearly symmetrical around the top of mid-ocean ridges. The discovery of these structures was a breakthrough that verified the theory of seafloor spreading and inspired the idea of plate tectonics. It even enabled scientists to trace plate movements in geological history.
If we suspend a compass needle freely, the needle’s alignment would shift continually from one point to another. Additionally, if we measure the force on the magnetic needle that causes it to take its preferred alignment, we would discover that the intensity of this force, which depends on the intensity of the magnetic field, fluctuates constantly with its position in space.
This is the idea behind paleomagnetism. It is the investigation of the history of Earth’s magnetic field using the magnetic fields preserved in rocks.
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The Geodynamo
The geodynamo theory suggests a method by which the Earth creates its magnetic field. According to scientists, heat moving from the Earth’s core induces convection, which produces and preserves the magnetic field. Convection is the procedure of transferring heat through the movement of hot fluids.
Imagine how surprised scientists must have been half a century ago when they discovered geological evidence of the reversal of direction of the magnetic field. These magnetic reversals happen in unpredictable periods. The reasons are not fully understood, although computer simulations of the geodynamo show rare reversals happening in the absence of external forces, implying that they are caused solely by interactions within Earth’s core. The period between reversals has varied greatly, but averages around 300,000 years, with the most recent one occurring 780,000 years ago.
The Rock Record Of Magnetic Reversals
Magnetic anomalies show that the Earth’s magnetic field is not consistent throughout time. Geologists discovered around the 1960s that stratified layers of volcanic lava can provide a detailed record of this unusual behavior. As lavas rich in iron oxides solidify, they are magnetized in the direction of the Earth’s magnetic field. Since the cool lava “recognizes” the magnetization, even after the magnetic field has shifted, this phenomenon is known as thermoremanent magnetization.
Paleomagnetism As Evidence For Seafloor Spreading
Scientists were perplexed by the banded formations of magnetism seen on the seafloor until 1963, when two researchers, F. J. Vine and D. H. Mathews, and, independently, L. Morley and A. Larochelle, presented a stunning proposal. They reasoned that the magnetic intensity patterns on the seabed linked to areas of rock magnetized during previous regular and reversed magnetism episodes.
As the tectonic plates separate at the mid-ocean ridge, lava gushes from the Earth’s core and streams into the fissure, where it cools, hardens, and magnetizes in the direction of the Earth’s magnetic field at that time. When the seafloor moves away from the ridge, about half of the newly magnetic material flows to one side and half flows to the other, resulting in two symmetrical magnetized bands. Younger material covers the empty space, allowing the process to continue. In this respect, the seafloor serves as a magnetic recorder, collecting the archive of the evolution of the ocean.
Polar Wandering
Apparent Polar Wandering (APW) is a method of showing magnetic pole location. It assumes the continent stayed in a stable place and registers a chart tracing the location of magnetic poles at different times. Paleomagnetic investigations revealed that the APW trajectories differed between continents. Hence, the APW not only showed that continental drift did indeed happen, but it also gave a technique for estimating the movement. APW routes have been utilized to understand movements, collisions, and continental breakups.
Conclusion
Geologists employed paleomagnetism in conjunction with isotope dating techniques to determine the timeline of magnetic reversals during the past 170 million years. This information is utilized to date fresh rock formations. Paleomagnetic stratigraphy is valuable to archaeologists, anthropologists, and geologists alike. It is a valuable tool in the study of earth sciences and provides the basis for many fundamental theories.
