Rock magnetism
Encyclopedia
Rock magnetism is the study of the magnetic properties of rocks
, sediments and soils. The field arose out of the need in paleomagnetism
to understand how rocks record the Earth's magnetic field. This remanence
is carried by minerals, particularly certain strongly magnetic minerals like magnetite
(the main source of magnetism in lodestone
). An understanding of remanence
helps paleomagnetists to develop methods for measuring the ancient magnetic field and correct for effects like sediment compaction
and metamorphism
. Rock magnetic methods are used to get a more detailed picture of the source of distinctive striped pattern in marine magnetic anomalies that provides important information on plate tectonics
. They are also used to interpret terrestrial magnetic anomalies in magnetic surveys as well as the strong crustal magnetism on Mars
.
Strongly magnetic minerals have properties that depend on the size, shape, defect structure and concentration of the minerals in a rock. Rock magnetism provides non-destructive methods for analyzing these minerals such as magnetic hysteresis measurements, temperature-dependent remanence
measurements, Mössbauer spectroscopy, ferromagnetic resonance
and so on. With such methods, rock magnetists can measure the effects of past climate change and human impacts on the mineralogy (see environmental magnetism
). In sediments, a lot of the magnetic remanence
is carried by minerals that were created by magnetotactic bacteria
, so rock magnetists have made significant contributions to biomagnetism
.
) and of magnetic materials (especially ferromagnetism
) developed separately.
Rock magnetism had its start when scientists brought these two fields together in the laboratory. Koenigsberger (1938), Thellier (1938) and Nagata (1943) investigated the origin of remanence
in igneous rock
s. By heating rocks and archeological materials to high temperatures in a magnetic field, they gave the materials a thermoremanent magnetization
(TRM), and they investigated the properties of this magnetization. Thellier developed a series of conditions (the Thellier laws) that, if fulfilled, would allow the determination of the intensity of the ancient magnetic field to be determined using the Thellier-Thellier method. In 1949, Louis Néel developed a theory that explained these observations, showed that the Thellier laws were satisfied by certain kinds of single-domain
magnets, and introduced the concept of blocking of TRM.
When paleomagnetic work in the 1950s lent support to the theory of continental drift
, skeptics were quick to question whether rocks could carry a stable remanence
for geological ages.
Rock magnetists were able to show that rocks could have more than one component of remanence, some soft (easily removed) and some very stable. To get at the stable part, they took to "cleaning" samples by heating them or exposing them to an alternating field. However, later events, particularly the recognition that many North American rocks had been pervasively remagnetized in the Paleozoic
, showed that a single cleaning step was inadequate, and paleomagnetists began to routinely use stepwise demagnetization to strip away the remanence in small bits.
. The more important minerals for rock magnetism are the minerals that can be magnetically ordered, at least at some temperatures. These are the ferromagnets, ferrimagnets and certain kinds of antiferromagnets. These minerals have a much stronger response to the field and can have a remanence
.
is a magnetic response shared by all substances. In response to an applied magnetic field, electrons precess (see Larmor precession
), and by Lenz's law
they act to shield the interior of a body from the magnetic field
. Thus, the moment produced is in the opposite direction to the field and the susceptibility
is negative. This effect is weak but independent of temperature. A substance whose only magnetic response is diamagnetism is called a diamagnet.
is a weak positive response to a magnetic field
due to rotation of electron spins
. Paramagnetism occurs in certain kinds of iron-bearing minerals because the iron contains an unpaired electron in one of their shells (see Hund's rules). Some are paramagnetic down to absolute zero and their susceptibility
is inversely proportional to the temperature (see Curie's law
); others are magnetically ordered below a critical temperature and the susceptibility increases as it approaches that temperature (see Curie-Weiss law).
refers to magnetic ordering where neighboring electron spins are aligned by the exchange interaction
. The classic ferromagnet is iron
. Below a critical temperature called the Curie temperature, ferromagnets have a spontaneous magnetization
and there is hysteresis
in their response to a changing magnetic field. Most importantly for rock magnetism, they have remanence
, so they can record the Earth's field.
Iron
does not occur widely in its pure form. It is usually incorporated into iron oxides, oxyhydroxides
and sulfides
. In these compounds, the iron atoms are not close enough for direct exchange, so they are coupled by indirect exchange or superexchange. The result is that the crystal lattice is divided into two or more sublattices with different moments.
, the most important of the magnetic minerals, is a ferrimagnet. Ferrimagnets often behave like ferromagnets, but the temperature dependence of their spontaneous magnetization
can be quite different. Louis Néel identified four types of temperature dependence, one of which involves a reversal of the magnetization. This phenomenon played a role in controversies over marine magnetic anomalies.
. However, the moments can be tilted (spin-canting), resulting in a moment nearly at right angles to the moments of the sublattices. Hematite
has this kind of magnetism.
is often identified with a particular kind of remanence that is obtained after exposing a magnet to a field at room temperature. However, the Earth's field is not large, and this kind of remanence would be weak and easily overwritten by later fields. A central part of rock magnetism is the study of magnetic remanence, both as natural remanent magnetization
(NRM) in rocks obtained from the field and remanence induced in the laboratory. Below are listed the important natural remanences and some artificially induced kinds.
If a rock is later re-heated (as a result of burial, for example), part or all of the TRM can be replaced by a new remanence. If it is only part of the remanence, it is known as partial thermoremanent magnetization (pTRM). Because numerous experiments have been done modeling different ways of acquiring remanence, pTRM can have other meanings. For example, it can also be acquired in the laboratory by cooling in zero field to a temperature
(below the Curie temperature), applying a magnetic field and cooling to a temperature 
, then cooling the rest of the way to room temperature in zero field.
The standard model for TRM is as follows. When a mineral such as magnetite
cools below the Curie temperature, it becomes ferromagnetic but is not immediately capable of carrying a remanence. Instead, it is superparamagnetic, responding reversibly to changes in the magnetic field. For remanence to be possible there must be a strong enough magnetic anisotropy
to keep the magnetization near a stable state; otherwise, thermal fluctuations
make the magnetic moment
wander randomly. As the rock continues to cool, there is a critical temperature at which the magnetic anisotropy becomes large enough to keep the moment from wandering: this temperature is called the blocking temperature and referred to by the symbol
. The magnetization remains in the same state as the rock is cooled to room temperature and becomes a thermoremanent magnetization.
that is acquired by ferromagnetic minerals by sitting in a magnetic field
for some time. The natural remanent magnetization
of an igneous rock
can be altered by this process. To remove this component, some form of stepwise demagnetization must be used.
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...
, sediments and soils. The field arose out of the need in 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...
to understand how rocks record the Earth's magnetic field. This remanence
Remanence
Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
is carried by minerals, particularly certain strongly magnetic minerals like magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
(the main source of magnetism in lodestone
Lodestone
A lodestone or loadstone is a naturally magnetized piece of the mineral magnetite. They are naturally occurring magnets, that attract pieces of iron. Ancient people first discovered the property of magnetism in lodestone...
). An understanding of remanence
Remanence
Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
helps paleomagnetists to develop methods for measuring the ancient magnetic field and correct for effects like sediment compaction
Compaction (geology)
Compaction refers to the process by which a sediment progressively loses its porosity due to the effects of loading. This forms part of the process of lithification. When a layer of sediment is originally deposited, it contains an open framework of particles with the pore space being usually...
and metamorphism
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
. Rock magnetic methods are used to get a more detailed picture of the source of distinctive striped pattern in marine magnetic anomalies that provides important information on plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
. They are also used to interpret terrestrial magnetic anomalies in magnetic surveys as well as the strong crustal magnetism on Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
.
Strongly magnetic minerals have properties that depend on the size, shape, defect structure and concentration of the minerals in a rock. Rock magnetism provides non-destructive methods for analyzing these minerals such as magnetic hysteresis measurements, temperature-dependent remanence
Remanence
Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
measurements, Mössbauer spectroscopy, ferromagnetic resonance
Ferromagnetic resonance
Ferromagnetic resonance, or FMR, is a spectroscopic technique to probe the magnetization of ferromagnetic materials. It is a standard tool for probing spin waves and spin dynamics...
and so on. With such methods, rock magnetists can measure the effects of past climate change and human impacts on the mineralogy (see environmental magnetism
Environmental magnetism
Environmental magnetism is the study of magnetism as it relates to the effects of climate, sediment transport, pollution and other environmental influences on magnetic minerals. It makes use of techniques from rock magnetism and magnetic mineralogy...
). In sediments, a lot of the magnetic remanence
Remanence
Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
is carried by minerals that were created by magnetotactic bacteria
Magnetotactic bacteria
Magnetotactic bacteria are a polyphyletic group of bacteria discovered by Richard P. Blakemore in 1975, that orient along the magnetic field lines of Earth's magnetic field. To perform this task, these bacteria have organelles called magnetosomes that contain magnetic crystals...
, so rock magnetists have made significant contributions to biomagnetism
Biomagnetism
Biomagnetism is the phenomenon of magnetic fields produced by living organisms; it is a subset of bioelectromagnetism. The study of the biological effects of magnetic fields is magnetobiology...
.
History
Until the 20th century, the study of the Earth's field (geomagnetism and paleomagnetismPaleomagnetism
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 of magnetic materials (especially ferromagnetism
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
) developed separately.
Rock magnetism had its start when scientists brought these two fields together in the laboratory. Koenigsberger (1938), Thellier (1938) and Nagata (1943) investigated the origin of remanence
Remanence
Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
in igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s. By heating rocks and archeological materials to high temperatures in a magnetic field, they gave the materials a thermoremanent magnetization
Thermoremanent 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), and they investigated the properties of this magnetization. Thellier developed a series of conditions (the Thellier laws) that, if fulfilled, would allow the determination of the intensity of the ancient magnetic field to be determined using the Thellier-Thellier method. In 1949, Louis Néel developed a theory that explained these observations, showed that the Thellier laws were satisfied by certain kinds of single-domain
Single domain (magnetic)
Single domain, in magnetism, refers to the state of a ferromagnet in which the magnetization does not vary across the magnet. A magnetic particle that stays in a single domain state for all magnetic fields is called a single domain particle . Such particles are very small...
magnets, and introduced the concept of blocking of TRM.
When paleomagnetic work in the 1950s lent support to the theory of continental drift
Continental drift
Continental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912...
, skeptics were quick to question whether rocks could carry a stable remanence
Remanence
Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
for geological ages.
Rock magnetists were able to show that rocks could have more than one component of remanence, some soft (easily removed) and some very stable. To get at the stable part, they took to "cleaning" samples by heating them or exposing them to an alternating field. However, later events, particularly the recognition that many North American rocks had been pervasively remagnetized in the Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
, showed that a single cleaning step was inadequate, and paleomagnetists began to routinely use stepwise demagnetization to strip away the remanence in small bits.
Types of magnetic order
The contribution of a mineral to the total magnetism of a rock depends strongly on the type of magnetic order or disorder. Magnetically disordered minerals (diamagnets and paramagnets) contribute a weak magnetism and have no remanenceRemanence
Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
. The more important minerals for rock magnetism are the minerals that can be magnetically ordered, at least at some temperatures. These are the ferromagnets, ferrimagnets and certain kinds of antiferromagnets. These minerals have a much stronger response to the field and can have a remanence
Remanence
Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
.
Diamagnetism
DiamagnetismDiamagnetism
Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the...
is a magnetic response shared by all substances. In response to an applied magnetic field, electrons precess (see Larmor precession
Larmor precession
In physics, Larmor precession is the precession of the magnetic moments of electrons, atomic nuclei, and atoms about an external magnetic field...
), and by Lenz's law
Lenz's law
Lenz's law is a common way of understanding how electromagnetic circuits must always obey Newton's third law and The Law of Conservation of Energy...
they act to shield the interior of a body from the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
. Thus, the moment produced is in the opposite direction to the field and the susceptibility
Magnetic susceptibility
In electromagnetism, the magnetic susceptibility \chi_m is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field...
is negative. This effect is weak but independent of temperature. A substance whose only magnetic response is diamagnetism is called a diamagnet.
Paramagnetism
ParamagnetismParamagnetism
Paramagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...
is a weak positive response to a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
due to rotation of electron spins
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
. Paramagnetism occurs in certain kinds of iron-bearing minerals because the iron contains an unpaired electron in one of their shells (see Hund's rules). Some are paramagnetic down to absolute zero and their susceptibility
Susceptibility
*In physics, the susceptibility of a material or substance describes its response to an applied field. There are many kinds of susceptibilities, for example:These two susceptibilities are particular examples of a linear response function;...
is inversely proportional to the temperature (see Curie's law
Curie's law
In a paramagnetic material the magnetization of the material is directly proportional to an applied magnetic field. However, if the material is heated, this proportionality is reduced: for a fixed value of the field, the magnetization is inversely proportional to temperature...
); others are magnetically ordered below a critical temperature and the susceptibility increases as it approaches that temperature (see Curie-Weiss law).
Ferromagnetism
Collectively, strongly magnetic materials are often referred to as ferromagnets. However, this magnetism can arise as the result of more than one kind of magnetic order. In the strict sense, ferromagnetismFerromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
refers to magnetic ordering where neighboring electron spins are aligned by the exchange interaction
Exchange interaction
In physics, the exchange interaction is a quantum mechanical effect without classical analog which increases or decreases the expectation value of the energy or distance between two or more identical particles when their wave functions overlap...
. The classic ferromagnet is iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
. Below a critical temperature called the Curie temperature, ferromagnets have a spontaneous magnetization
Spontaneous magnetization
Spontaneous magnetization is the term used to describe the appearance of an ordered spin state at zero applied magnetic field in a ferromagnetic or ferrimagnetic material below a critical point called the Curie temperature or .-Overview:...
and there is hysteresis
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
in their response to a changing magnetic field. Most importantly for rock magnetism, they have remanence
Remanence
Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
, so they can record the Earth's field.
Iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
does not occur widely in its pure form. It is usually incorporated into iron oxides, oxyhydroxides
Iron(III) oxide-hydroxide
A number of species are dubbed iron oxide-hydroxide. These chemicals are oxide-hydroxides of iron, and may occur in anhydrous or hydrated forms...
and sulfides
Iron sulfide
Iron sulfide or Iron sulphide refers to a chemical compound of iron and sulfur with a wide range of stoechiometric formulae and different crystalline structures.-Natural minerals:By increasing order of stability:...
. In these compounds, the iron atoms are not close enough for direct exchange, so they are coupled by indirect exchange or superexchange. The result is that the crystal lattice is divided into two or more sublattices with different moments.
Ferrimagnetism
Ferrimagnets have two sublattices with opposing moments. One sublattice has a larger moment, so there is a net unbalance. MagnetiteMagnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
, the most important of the magnetic minerals, is a ferrimagnet. Ferrimagnets often behave like ferromagnets, but the temperature dependence of their spontaneous magnetization
Spontaneous magnetization
Spontaneous magnetization is the term used to describe the appearance of an ordered spin state at zero applied magnetic field in a ferromagnetic or ferrimagnetic material below a critical point called the Curie temperature or .-Overview:...
can be quite different. Louis Néel identified four types of temperature dependence, one of which involves a reversal of the magnetization. This phenomenon played a role in controversies over marine magnetic anomalies.
Antiferromagnetism
Antiferromagnets, like ferrimagnets, have two sublattices with opposing moments, but now the moments are equal in magnitude. If the moments are exactly opposed, the magnet has no remanenceRemanence
Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
. However, the moments can be tilted (spin-canting), resulting in a moment nearly at right angles to the moments of the sublattices. Hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
has this kind of magnetism.
Types of remanence
Magnetic remanenceRemanence
Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
is often identified with a particular kind of remanence that is obtained after exposing a magnet to a field at room temperature. However, the Earth's field is not large, and this kind of remanence would be weak and easily overwritten by later fields. A central part of rock magnetism is the study of magnetic remanence, both as natural remanent magnetization
Natural Remanent Magnetization
Natural remanent magnetization 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...
(NRM) in rocks obtained from the field and remanence induced in the laboratory. Below are listed the important natural remanences and some artificially induced kinds.
Thermoremanent magnetization (TRM)
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 very stable, lasting without significant change for millions of years. TRM is the main reason that paleomagnetists are able to deduce the direction and magnitude of the ancient Earth's field.If a rock is later re-heated (as a result of burial, for example), part or all of the TRM can be replaced by a new remanence. If it is only part of the remanence, it is known as partial thermoremanent magnetization (pTRM). Because numerous experiments have been done modeling different ways of acquiring remanence, pTRM can have other meanings. For example, it can also be acquired in the laboratory by cooling in zero field to a temperature
(below the Curie temperature), applying a magnetic field and cooling to a temperature , then cooling the rest of the way to room temperature in zero field.The standard model for TRM is as follows. When a mineral such as magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
cools below the Curie temperature, it becomes ferromagnetic but is not immediately capable of carrying a remanence. Instead, it is superparamagnetic, responding reversibly to changes in the magnetic field. For remanence to be possible there must be a strong enough magnetic anisotropy
Magnetic anisotropy
Magnetic anisotropy is the direction dependence of a material's magnetic properties. In the absence of an applied magnetic field, a magnetically isotropic material has no preferential direction for its magnetic moment while a magnetically anisotropic material will align its moment with one of the...
to keep the magnetization near a stable state; otherwise, thermal fluctuations
Thermal fluctuations
In statistical mechanics, thermal fluctuations are random deviations of a system from its equilibrium. All thermal fluctuations become larger and more frequent as the temperature increases, and likewise they disappear altogether as temperature approaches absolute zero.Thermal fluctuations are a...
make the magnetic moment
Magnetic moment
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...
wander randomly. As the rock continues to cool, there is a critical temperature at which the magnetic anisotropy becomes large enough to keep the moment from wandering: this temperature is called the blocking temperature and referred to by the symbol
. The magnetization remains in the same state as the rock is cooled to room temperature and becomes a thermoremanent magnetization.Chemical (or crystalline) remanent magnetization (CRM)
Magnetic grains may be deposited from a circulating solution, or be formed during chemical reactions, and may record the direction of the magnetic field at the time of mineral formation. The field is said to be recorded by chemical remanent magnetization (CRM). The mineral recording the field commonly is hematite, another iron oxide. Redbeds, clastic sedimentary rocks (such as sandstones) that are red primarily because of hematite formation during or after sedimentary diagenesis, may have useful CRM signatures, and magnetostratigraphy can be based on such signatures.Depositional remanent magnetization (DRM)
Magnetic grains in sediments may align with the magnetic field during or soon after deposition; this is known as detrital remnant magnetization (DRM). If the magnetization is acquired as the grains are deposited, the result is a depositional detrital remanent magnetization (dDRM); if it is acquired soon after deposition, it is a post-depositional detrital remanent magnetization (pDRM).Viscous remanent magnetization
Viscous remanent magnetization (VRM), also known as viscous magnetization, is remanenceRemanence
Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
that is acquired by ferromagnetic minerals by sitting in a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
for some time. The natural remanent magnetization
Natural Remanent Magnetization
Natural remanent magnetization 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...
of an igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
can be altered by this process. To remove this component, some form of stepwise demagnetization must be used.
Applications of rock magnetism
- magnetic anomaliesMagnetic anomalyIn geophysics, a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying material...
- magnetostratigraphyMagnetostratigraphyMagnetostratigraphy 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...
- Paleomagnetic secular variationPaleomagnetic secular variationGeomagnetic secular variation refers to changes in the Earth's magnetic field on time scales of about a year or more. These changes mostly reflect changes in the Earth's interior, while more rapid changes mostly originate in the ionosphere or magnetosphere....
- paleointensity
- plate tectonicsPlate tectonicsPlate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
- biomagnetismBiomagnetismBiomagnetism is the phenomenon of magnetic fields produced by living organisms; it is a subset of bioelectromagnetism. The study of the biological effects of magnetic fields is magnetobiology...
- environmental magnetismEnvironmental magnetismEnvironmental magnetism is the study of magnetism as it relates to the effects of climate, sediment transport, pollution and other environmental influences on magnetic minerals. It makes use of techniques from rock magnetism and magnetic mineralogy...
- magnetic fabrics