“Paleomagnetism is the study of the Earth’s past magnetic fields present in rocks and fine sediments like an actual record that can be enjoyed when played on the reverse.”
Research carried on in the ocean floor after World War II revealed that, the ocean floor is full of relief spanning from the highest mountain ranges like Tamu Massif, which is a giant underwater volcano off the coast of Japan and Earth’s deepest point like Challenger Deep (10,898 meters deep).
Magnetometers dragged by survey ships sailing above the rifts found that the magnetic strip like pattern on either side of the rift were mirror images of each other. The rocks present at equidistant locations from the crest of oceanic ridges were found to match in age, polarity as well as their constituent minerals.
Deciphering the History of Earth’s Magnetic Field
The ocean floor is mainly made up of basalt, which is formed as a result of underwater volcanic activity. Basalts contain magnetic minerals which get aligned in the direction of the magnetic field when the rock solidifies.
The volcanic and sedimentary rocks record the paleomagnetism at the time when that part of the ocean floor was created. This is known as magnetostratigraphy.
Paleomagnetists use this record to date rocks and to map the magnetic field that was present at the time of the formation of those magnetized rocks and sediments. By studying the past behavior of Earth’s magnetic field, we can also infer the past location of tectonic plates.
This data is also used to better understand the problems of regional and local tectonics, geodynamics, and thermal history of our planet.
Cause of Earth’s Magnetism
There are no satisfactory conditions or explanations for Earth’s Magnetism. The cause of its magnetism is not from inner Earth or from materials present in rocks but because such rocks will lose their magnetism when heated more than the Curie temperature.
The rapid change in the direction of the magnetic field is not due to permanently magnetized rocks. So the only possible explanation is the presence of electric currents in the liquid outer core of the Earth, which is composed of molten Ni-Fe.
The Earth thus acts as a gigantic dynamo. The rotation of the Earth causes the buoyant Ni-Fe rich fluid in the liquid outer core to rise in curved trajectories, which generates a mechanical convection current in the molten outer core.
It is due to this interaction of the electric currents in the liquid outer core and the mechanical current generated by Earth’s rotation that a self-sustaining magnetic field is produced. Whenever the directions of convection currents changes (180 degrees) the magnetic field reverse its direction.
However, almost all of the Earth’s magnetic energy remains confined within the core. The last magnetic reversal in the history of the Earth happened 780,000 years ago during the Stone Age, which means that modern humans have not experienced any magnetic reversal.
Principles of Remanent Magnetization
The study of paleomagnetism is possible because iron-bearing minerals such as magnetite act like magnetic signatures by recording the history of Earth’s magnetic field with the help of several different mechanisms.
Natural Remnant Magnetism (NRM)
Paleomagnetism which aims in reconstructing the direction and strength of magnetic field over geologic time is possible because rocks have Natural Remanent Magnetization i.e., magnetization retained in rocks for millions of years (unless disturbed thermally above Curie temperature) even if the Earth’s magnetic field changes or reverses its polarity.
- Thermoremanent Magnetization
It results when a magnetic material is cooled below the Curie point in the presence of an external field (usually the Earth’s field). Its direction depends on the direction of the field at the time and place where the rock cooled.
This record has been preserved well enough in basalts of the ocean crust, which is useful in the development of theories of seafloor spreading related to plate tectonics.
- Detrital Remanent Magnetization
It occurs during the slow setting of fine-grained sediments in the presence of an external field. Varied clays exhibit this type of remanence as the minerals may align with the magnetic field during or soon after deposition this is known as Detrital Remanent Magnetization (DRM).
- Chemical Remanent Magnetization
It takes place when magnetic grains increase in size or are changed from one form to another as a result of chemical action at moderate temperatures that is below the Curie point. This process may be significant in sedimentary and metamorphic rocks.
Chemical remanent magnetization is shown by the mineral hematite. Therefore chemical remanent magnetization signatures in red beds are quite useful, and they are common targets in magnetostratigraphy studies.
- Isothermal Remanent Magnetization
It is the remanence which is acquired at a fixed temperature. Remanence of this sort is the residual magnetism left following the removal of an external field. Lightning strikes produce isothermal remanent magnetization over small areas.
It can also be induced by applying fields of various strengths in the laboratory. Lightning-induced remanent magnetization is often induced in drill cores by the magnetic field of the steel core barrel.
- Viscous Remanent Magnetization
Viscous remanent magnetization is the remanence acquired by ferromagnetic materials due to long exposure to an external field. This rem remanence is quite stable and is more characteristic of fine-grained rocks as compared to coarse-grained rocks.
How to Collect the Evidence for Paleomagnetism?
The oldest ocean floor is not older than 200 million years, which is very young when compared to the oldest continental rocks of the Istaaq Gneissic Complex (Greenland) which dates 3.8 billion years ago. To reconstruct the Earth’s paleomagnetic field beyond 200 million years, scientists collect magnetite-bearing samples on land.
While sampling the scientists make sure to retrieve samples with accurate orientations, by using a rock coring drill and to reduce uncertainty in the data collected.
Utility of Paleomagnetism
Paleomagnetic studies have indicated that the orientation of the Earth’s magnetic field has flipped numerous times in the geologic past. Periods of “normal” polarity that is when the north needle of the compass points toward the present north magnetic pole, have alternated with periods of “reversed” polarity that is when the north needle points towards the south.
Polarity reversals – have occurred at irregular intervals usually measured in millions of years. However, the time for the poles to interchange is only a few thousand years.
The magma records the changes in Earth’s magnetic field. This results in an alternate pattern of normal and reversed magnetic polarity striping on the seafloor. Paleomagnetic rocks present on either side of the submarine ridges serve as evidence to the concept of Sea Floor Spreading.
The theory of seafloor spreading supported the idea of continental drift, which led to the groundbreaking concept of plate tectonics. On studying the magnetic signatures of the rocks on the ocean floor, scientists found that some ocean sediments which although perfectly laid out side by side recorded the reversed directions for magnetic field lines.
The ocean floor mainly comprises basalt. Basalt is an iron-rich substance, these iron-rich minerals get aligned with the magnetic field, thereby recording the paleomagnetism in ocean ridges.
The magnetic field has frequently alternated in the geologic past. These alternating periods of normal and reversed magnetism are known as periods of “normal” polarity and “reversed” polarity respectively.
When the new rock pushed the older rock, which formed millions of years ago when the magnetic field was reversed, farther away resulting in the magnetic striping.
The rocks which are at equal distance on either side of the crest of mid-oceanic ridges are truly similar. The age of the rocks keeps on increasing as one moves away from the crest. Rocks closer to the mid-oceanic ridges record the current polarity and are the youngest.
Magnetostratigraphy uses the magnetic record of the past to determine the age of those rocks. Reversals have occurred at irregular intervals throughout Earth history. The polarity reverses globally, in a time span of 103–104 years.
This has led to the development of a new type of stratigraphy known as magnetic polarity stratigraphy. Therefore by piecing together the history of Earth’s magnetic field, we can help in predicting its future behavior.
Magnetic reversals, as well as polar wandering data, was crucial as it largely contributed to the verification of the theories of continental drift and plate tectonics in the 1960s and 1970s.
The concept of apparent polar wander paths was evidence for continental drift as it helped determine the speed, direction, and rotation of continents whereas magnetic anomalies on the ocean floor helped in proving seafloor spreading.
The absolute ages of rocks which contain the magnetic record can be determined by paleomagnetism and geochronology by using several methods such as potassium-argon and argon-argon geochronology.