Natural remanent magnetization (NRM)
Natural remanent magnetization (NRM) refers to the inherent magnetism present in rocks when they are not influenced by external magnetic sources. Unlike induced magnetism, which occurs in the presence of an external magnetic field, NRM reflects the magnetic characteristics from the time of the rock's formation and the various magnetic influences it has experienced since. The magnetization process is closely linked to the atomic structure of the minerals in the rock, where the alignment of electrons can create a magnetic field.
Measuring NRM provides valuable insights for geologists, revealing information about the rock's history, including its original formation location and the tectonic movements of the Earth over millennia. Factors such as heat, chemical exposure, and other magnetic interactions can alter a rock's magnetism, resulting in multiple magnetization events throughout its life. By analyzing these magnetic signatures, scientists can reconstruct the dynamics of Earth’s tectonic plates and the shifts in its magnetic field.
Notably, NRM data has led to discoveries about historical shifts in Earth’s magnetic polarity, with significant changes occurring several times throughout geological history. This research enables a better understanding of both Earth's physical past and the processes that continue to shape its surface today.
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Natural remanent magnetization (NRM)
Natural remanent magnetism (NRM) is the amount of magnetism a rock has when it is not exposed to any other magnetic source. It is different from induced magnetism, which refers to the amount of magnetism a rock has when it is in the presence of an outside magnetic field. Measuring a rock's natural remanent magnetization can provide researchers with key information about where that rock was originally formed. This can help in determining how the surface of the Earth has changed over thousands of years.
Background
Rocks are made up of atoms. Each atom has electrons, which are negatively charged. These electrons spin around a nucleus in the center of the atom, generating an electrical charge. In most types of atoms, these electrons spin in different directions. In some atoms, however, they line up and spin in the same direction. This enables the atom to generate a type of energy known as magnetism. Since both types of atoms have a charge, they are both magnetic. However, when the electrons are not aligned and spinning in the same direction, the atom generates a very weak magnetic charge. Magnetism strong enough to exert a force on another object only occurs when the electrons in the object's atoms are circling the nucleus in the same direction.
This magnetism forms a field around the rock or other object that can attract and pull objects made of certain elements toward it. This force has both a positive and a negative charge. Opposite charges will attract and come together; however, placing two magnets with the same charge pointing toward each other will result in the magnets repelling each other.
Some elements are structured in a way that makes it easier for the electrons to align and spin in the same direction; others have a structure that is more conducive to random spinning. Rocks and other objects that have a structure that facilitates the development of a magnetic field are known as ferrous. The electrons in the atoms that make up these substances will always line up in a certain way, regardless of the placement of the rock.
These rocks generate a magnetic field that can be measured for strength. This is done with a magnetometer, sometimes called a gaussometer. Magnetometers are also used to measure Earth's magnetic field. The planet has a magnetic field because its core is largely made of iron, which is one of the most naturally magnetic elements. Other rocks both under and above Earth's surface are also magnetized. The extent to which each is magnetized depends in part on where it has been and what magnetic forces it has been exposed to since it was formed. Together, the effects of all these magnetic forces on a substance are known as its natural remanent magnetism.
Overview
A number of factors can affect the magnetism of an object and thus affect its natural remanent magnetism. Exposure to high heat, chemicals, other magnetic fields, and sediment from other magnetic substances are some of the things that can affect remanent magnetism. Through the effects of influences such as these, a rock can be magnetized several times in several different ways over the course of time. Depending on the elements from which it is made, a rock can gain magnetism, lose some of its magnetism, and even regain it.
The measurement of the total sum of all these changes is what is known as the rock's natural remanent magnetism. By studying this measurement and the makeup of the rock, scientists can learn a great deal about when, where, and how a particular rock was formed. If they then compare this to other rocks from other places in the world, researchers can learn about how Earth's tectonic plates and its magnetic field have shifted over the millennia.
Earth's tectonic plates are large, irregular chunks of rock that make up the outer shell of the planet. These rocks are made in several different ways. Igneous rocks were formed when the hot core of Earth cooled and hardened. Sedimentary rocks are made by layers of dirt and other substances that pile up until the layers on top compress the lower layers into a solid surface.
Increasing pressure from land and ocean waters, heat, upheaval from earthquakes and volcanoes, and other factors break up these rocks and often cause them to shift and move. Sometimes these forces result in the plates overlapping, and sometimes they break apart. Water flows between the plates, creating oceans and other bodies of water. Scientists are able to determine some of how this has happened in the past by measuring and comparing the natural remanent magnetism of rocks from various places.
They are able to do this because the rocks retain evidence of these changes, creating a trail for scientists to follow. For example, when a sedimentary rock is formed, any individual bits of soil and rock that pile up will each land with their magnetic poles lined up toward north on the Earth's magnetic field at that time. This will help researchers determine how the rock was positioned at the time it was formed.
Something similar happens when igneous rocks are formed; the magnetic particles in them will align with the direction that was north at the time the molten rock was solidifying. By taking this information and comparing it to other rocks formed before, after, and at the same time, scientists can develop a timeline of when and where certain changes occurred. It is through studies of these types of "memories" encoded in the rocks that scientists have been able to determine that the Earth's magnetic field has shifted polarity—reversed so that magnets point south instead of north—several times during its existence. The last such shift is believed to have happened 780,000 years ago, and some scientists theorize that another shift may happen in another thousand years or so.
Bibliography
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