Natural Remanent Magnetization
Encyclopedia
Natural remanent magnetization (NRM) is the permanent magnetism
of a rock
or sediment
. In some forms, it can preserve a record of the Earth's field and the tectonic movement of the rock over millions of years. Natural remanent magnetization forms the basis of paleomagnetism
and magnetostratigraphy
.
(TRM) is acquired during cooling through the Curie temperature of the magnetic minerals and is the best source of information on the past Earth's field. Magnetization formed by phase change, chemical action or growth of crystals at low temperature is called chemical remanent magnetization. Sediments acquire a depositional remanent magnetization during their formation or a post-depositional remanent magnetization afterwards.
Some kinds of remanence are undesirable and must be removed before the useful remanence is measured. One is isothermal remanent magnetization (IRM), which as a component of NRM generally means remanence induced by lightning strikes. Another is viscous remanent magnetization
(VRM), a remanence acquired when the rock sits in the Earth's field for long periods.
The most important component of remanence is that acquired when a rock formed. This is called its primary component or characteristic remanent magnetization (ChRM). Any later component is called a secondary component. To separate these components, the NRM is stripped away in a stepwise manner using thermal or alternating field demagnetization techniques to reveal the stable magnetic component.
Magnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
of a rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
or sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
. In some forms, it can preserve a record of the Earth's field and the tectonic movement of the rock over millions of years. Natural remanent magnetization forms the basis of paleomagnetism
Paleomagnetism
Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...
and magnetostratigraphy
Magnetostratigraphy
Magnetostratigraphy is a geophysical correlation technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout the section. The samples are analyzed to determine their characteristic remanent magnetization , that is, the...
.
Types of natural remanent magnetization
There are several kinds of NRM that can occur in a sample. Many samples have more than one kind superimposed. Thermoremanent magnetizationThermoremanent magnetization
When an igneous rock cools, it acquires a thermoremanent magnetization from the Earth's field. TRM can be much larger than it would be if exposed to the same field at room temperature . This remanence can also be very stable, lasting without significant change for millions of years...
(TRM) is acquired during cooling through the Curie temperature of the magnetic minerals and is the best source of information on the past Earth's field. Magnetization formed by phase change, chemical action or growth of crystals at low temperature is called chemical remanent magnetization. Sediments acquire a depositional remanent magnetization during their formation or a post-depositional remanent magnetization afterwards.
Some kinds of remanence are undesirable and must be removed before the useful remanence is measured. One is isothermal remanent magnetization (IRM), which as a component of NRM generally means remanence induced by lightning strikes. Another is viscous remanent magnetization
Viscous Remanent Magnetization
Viscous remanent magnetization , also known as viscous magnetization, is remanence that is acquired by ferromagnetic materials by sitting in a magnetic field for some time. The natural remanent magnetization of an igneous rock can be altered by this process...
(VRM), a remanence acquired when the rock sits in the Earth's field for long periods.
The most important component of remanence is that acquired when a rock formed. This is called its primary component or characteristic remanent magnetization (ChRM). Any later component is called a secondary component. To separate these components, the NRM is stripped away in a stepwise manner using thermal or alternating field demagnetization techniques to reveal the stable magnetic component.