20:
814:"When I last did a magnetic experiment (about 1909) we were warned against careless handling of permanent magnets, and the magnetism was liable to change without much carelessness. In studying the magnetism of rocks the specimen has to be broken off with a geological hammer and then carried to the laboratory. It is supposed that in the process its magnetism does not change to any important extent, and though I have often asked how this comes to be the case I have never received any answer.
184:, 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
409:
370:
304:
629:. 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.
637:
Magnetic grains in sediments may align with the magnetic field during or soon after deposition; this is known as detrital remanent 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
454:
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
361:. 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.
624:
Magnetic grains may precipitate 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
101:
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
496:. 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
585:
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
149:
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
492:
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
459:(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.
614:
552:
521:
188:, showed that a single cleaning step was inadequate, and paleomagnetists began to routinely use stepwise demagnetization to strip away the remanence in small bits.
419:, 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
396:
identified four types of temperature dependence, one of which involves a reversal of the magnetization. This phenomenon played a role in controversies over marine
201:
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 (
70:). An understanding of remanence helps paleomagnetists to develop methods for measuring the ancient magnetic field and correct for effects like sediment
260:
is negative. This effect is weak but independent of temperature. A substance whose only magnetic response is diamagnetism is called a diamagnet.
213:. The more important minerals for rock magnetism are the minerals that can be magnetically ordered, at least at some temperatures. These are the
279:. Paramagnetism occurs in certain kinds of iron-bearing minerals because the iron contains an unpaired electron in one of their shells (see
949:
McCabe, C.; Elmore, R. D. (1989). "The occurrence and origin of Late
Paleozoic remagnetization in the sedimentary rocks of North America".
291:); others are magnetically ordered below a critical temperature and the susceptibility increases as it approaches that temperature (see
114:
and so on. With such methods, rock magnetists can measure the effects of past climate change and human impacts on the mineralogy (see
1026:
939:
870:
616:. The magnetization remains in the same state as the rock is cooled to room temperature and becomes a thermoremanent magnetization.
485:). This remanence can also be very stable, lasting without significant change for millions of years. TRM is the main reason that
712:
165:) that, if fulfilled, would allow the determination of the intensity of the ancient magnetic field to be determined using the
980:
Néel, Louis (1949). "Théorie du traînage magnétique des ferromagnétiques en grains fins avec application aux terres cuites".
675:
651:
456:
78:. Rock magnetic methods are used to get a more detailed picture of the source of the distinctive striped pattern in marine
24:
468:
158:
880:
Hunt, Christopher P.; Moskowitz, Bruce P. (1995). "Magnetic properties of rocks and minerals". In Ahrens, T. J. (ed.).
173:
developed a theory that explained these observations, showed that the
Thellier laws were satisfied by certain kinds of
481:
from the Earth's field. TRM can be much larger than it would be if exposed to the same field at room temperature (see
573:, responding reversibly to changes in the magnetic field. For remanence to be possible there must be a strong enough
157:. By heating rocks and archeological materials to high temperatures in a magnetic field, they gave the materials a
161:(TRM), and they investigated the properties of this magnetization. Thellier developed a series of conditions (the
389:
354:
332:
107:
315:. However, this magnetism can arise as the result of more than one kind of magnetic order. In the strict sense,
174:
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can be altered by this process. To remove this component, some form of stepwise demagnetization must be used.
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have two sublattices with opposing moments. One sublattice has a larger moment, so there is a net unbalance.
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384:, the most important of the magnetic minerals, is a ferrimagnet. Ferrimagnets often behave like
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in their response to a changing magnetic field. Most importantly for rock magnetism, they have
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118:). In sediments, a lot of the magnetic remanence is carried by minerals that were created by
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is a magnetic response shared by all substances. In response to an applied magnetic field,
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Irving, E. (1956). "Paleomagnetic and palaeoclimatological aspects of polar wandering".
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Runcorn, S. K. (1956). "Paleomagnetic comparisons between Europe and North
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427:), resulting in a moment nearly at right angles to the moments of the sublattices.
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refers to magnetic ordering where neighboring electron spins are aligned by the
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256:. Thus, the moment produced is in the opposite direction to the field and the
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are able to deduce the direction and magnitude of the ancient Earth's field.
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is carried by minerals, particularly certain strongly magnetic minerals like
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884:. Vol. 3. Washington, DC: American Geophysical Union. pp. 189–204.
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225:. These minerals have a much stronger response to the field and can have a
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does not occur widely in its pure form. It is usually incorporated into
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but is not immediately capable of carrying a remanence. Instead, it is
474:
554:, then cooling the rest of the way to room temperature in zero field.
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86:. They are also used to interpret terrestrial magnetic anomalies in
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Collectively, strongly magnetic materials are often referred to as
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When paleomagnetic work in the 1950s lent support to the theory of
882:
Rock
Physics and Phase Relations: A Handbook of Physical Constants
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The standard model for TRM is as follows. When a mineral such as
346:
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to understand how rocks record the Earth's magnetic field. This
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Schematic of alternating spin directions in an antiferromagnet.
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Schematic of unbalanced antiparallel moments in a ferrimagnet.
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122:, so rock magnetists have made significant contributions to
106:
measurements, temperature-dependent remanence measurements,
620:
Chemical (or crystallization) remanent magnetization (CRM)
577:
to keep the magnetization near a stable state; otherwise,
527:), applying a magnetic field and cooling to a temperature
928:
The earth: its origin, history, and physical constitution
283:). Some are paramagnetic down to absolute zero and their
640:
post-depositional detrital remanent magnetization (pDRM)
177:
magnets, and introduced the concept of blocking of TRM.
134:
Until the 20th century, the study of the Earth's field (
307:
Schematic of parallel spin directions in a ferromagnet.
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533:
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252:they act to shield the interior of a body from the
1049:UC Davis FORC Group, Introduction to FORC Diagrams
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287:is inversely proportional to the temperature (see
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759:
8:
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494:partial thermoremanent magnetization (pTRM)
859:Rock Magnetism: Fundamentals and Frontiers
659:, also known as viscous magnetization, is
388:, but the temperature dependence of their
327:. Below a critical temperature called the
209:) contribute a weak magnetism and have no
1015:The Physical Principles of Rock Magnetism
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82:that provides important information on
811:, had the following to say about it:
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423:. However, the moments can be tilted (
627:chemical remanent magnetization (CRM)
54:. The field arose out of the need in
7:
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657:Viscous remanent magnetization (VRM)
27:, a widely used tool for measuring
479:thermoremanent magnetization (TRM)
463:Thermoremanent magnetization (TRM)
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271:is a weak positive response to a
66:(the main source of magnetism in
713:Paleomagnetic secular variation
638:soon after deposition, it is a
16:The study of magnetism in rocks
807:, in his influential textbook
686:Applications of rock magnetism
676:natural remanent magnetization
652:Viscous remanent magnetization
646:Viscous remanent magnetization
589:and referred to by the symbol
457:natural remanent magnetization
1:
926:Jeffreys, Sir Harold (1959).
323:. The classic ferromagnet is
25:vibrating sample magnetometer
1044:Institute for Rock Magnetism
469:Thermoremanent magnetization
455:magnetic remanence, both as
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390:spontaneous magnetization
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952:Reviews of Geophysics
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674:for some time. The
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441:Magnetic mineralogy
435:Magnetic mineralogy
104:magnetic hysteresis
29:magnetic hysteresis
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911:10.1007/BF02629944
891:Geofis. Pura. Appl
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207:paramagnets
169:. In 1949,
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1058:Categories
850:References
394:Louis NĂ©el
337:hysteresis
250:Lenz's law
248:), and by
203:diamagnets
171:Louis NĂ©el
72:compaction
988:: 99–136.
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809:The Earth
771:NĂ©el 1949
661:remanence
581:make the
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452:remanence
450:Magnetic
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382:Magnetite
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242:electrons
227:remanence
211:remanence
186:Paleozoic
151:remanence
68:lodestone
64:magnetite
60:remanence
48:sediments
1019:Elsevier
1013:(1974).
1001:: 77–85.
668:minerals
473:When an
429:Hematite
359:sulfides
40:magnetic
961:Bibcode
899:Bibcode
475:igneous
130:History
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678:of an
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277:spins
52:soils
44:rocks
1023:ISBN
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357:and
347:Iron
325:iron
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