Natural remanent magnetization (NRM) is the permanent

magnetism Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particles ...

of a rock or

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...

. This preserves a record of the Earth's magnetic field at the time the mineral was laid down as sediment or crystallized in magma and also the

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...

movement of the rock over millions of years from its original position. Natural remanent magnetization forms the basis of

paleomagnetism Paleomagnetism (or palaeomagnetismsee ), is the study of magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.'' Certain magnetic minerals in roc ...

and

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 '' ...

Types

There are several kinds of NRM that can occur in a sample. Many samples have more than one kind superimposed.

Thermoremanent magnetization When an igneous rock cools, it acquires a thermoremanent magnetization (TRM) from the Earth's field. TRM can be much larger than it would be if exposed to the same field at room temperature (see isothermal remanence). This remanence can also be ver ...

(TRM) is acquired during cooling through the

Curie temperature In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cur ...

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 A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wit ...

. 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 induced through exposing a particle to a large magnetic field, causing the field to flip its lower coercivity magnetic moments to a field-favored direction. A commonly cited mechanism of IRM acquisition is through 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 acquired when a rock is 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 characteristic magnetic component. But not "all magnetic changes resulting from mechanical shock can be removed by AF demagnetization". Marine oil-bearing sandstones are physically unstable mineralogies whose low-field susceptibility and isothermal remanent magnetization increase irreversibly, even after weak mechanical shocks and an AF demagnetization in 100 mT peak alternating fields.

Notes

References

* * * * * {{Refend Ferromagnetism Stratigraphy

Types

See also

Notes

References