Paleomagnetism
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Paleomagnetism(occasionallypalaeomagnetism) is the study of prehistoricEarth's magnetic fieldsrecorded in rocks, sediment, or archeological materials.Geophysicistswho specialize in paleomagnetism are calledpaleomagnetists.
Certain magneticmineralsinrockscan record the direction and intensity of Earth's magnetic field at the time they formed. This record provides information on the past behavior of the geomagnetic field and the past location oftectonic plates.The record ofgeomagnetic reversalspreserved involcanicandsedimentary rocksequences (magnetostratigraphy) provides a time-scale that is used as ageochronologictool.
Evidence from paleomagnetism led to the revival of thecontinental drifthypothesis and its transformation into the modern theory of plate tectonics.Apparent polar wanderpaths provided the first clear geophysical evidence for continental drift, while marinemagnetic anomaliesdid the same forseafloor spreading.Paleomagnetic data continues to extend the history of plate tectonics back in time, constraining the ancient position and movement of continents and continental fragments (terranes).
The field of paleomagnetism also encompasses equivalent measurements of samples from other Solar System bodies, such asMoon rocksandmeteorites,where it is used to investigate the ancient magnetic fields of those bodies anddynamo theory.Paleomagnetism relies on developments inrock magnetismand overlaps withbiomagnetism,magnetic fabrics (used as strain indicators in rocks and soils), andenvironmental magnetism.
History
[edit]As early as the 18th century, it was noticed that compass needles deviated near strongly magnetizedoutcrops.In 1797,Alexander von Humboldtattributed thismagnetizationtolightning strikes(and lightning strikes do often magnetize surface rocks).[2][3][4]19th century studies of the direction of magnetization in rocks showed that some recent lavas were magnetized parallel toEarth's magnetic field.Early in the 20th century, work by David,Bernard BrunhesandPaul Louis Mercantonshowed that many rocks were magnetized antiparallel to the field. Japanese geophysicistMotonori Matuyamashowed in the late 1920s that Earth's magnetic field reversed in the mid-Quaternary,a reversal now known as theBrunhes–Matuyama reversal.[5][3]
British physicistP.M.S. Blackettprovided a major impetus to paleomagnetism by inventing a sensitive astaticmagnetometerin 1956. His intent was to test his theory that thegeomagnetic fieldwas related toEarth's rotation,a theory that he ultimately rejected; but the astatic magnetometer became the basic tool of paleomagnetism and led to a revival of the theory of continental drift.
Alfred Wegenerfirst proposed in 1915 that continents had once been joined together and had since moved apart.[6][7]Although he produced an abundance of circumstantial evidence, his theory met with little acceptance for two reasons: (1) no mechanism for continental drift was known, and (2) there was no way to reconstruct the movements of the continents over time.Keith Runcorn[8]andEdward A. Irving[9]constructedapparent polar wanderpaths for Europe and North America. These curves diverged but could be reconciled if it was assumed that the continents had been in contact up to 200 million years ago. This provided the first clear geophysical evidence for continental drift. Then in 1963,Morley, Vine and Matthewsshowed that marinemagnetic anomaliesprovided evidence forseafloor spreading.
Fields
[edit]Paleomagnetism is studied on a number of scales:
- Geomagnetic secular variationis the small-scale changes in the direction and intensity of Earth's magnetic field. Themagnetic north poleis constantly shifting relative to the axis of rotation of Earth. Magnetism is a vector and so magnetic field variation is studied by palaeodirectional measurements ofmagnetic declinationandmagnetic inclinationand palaeointensity measurements.
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- Magnetostratigraphyuses thepolarityreversal history of Earth's magnetic field recorded in rocks to determine the age of those rocks.Reversalshave occurred at irregular intervals throughoutEarth's history.The age and pattern of these reversals is known from the study of sea floor spreading zones and the dating ofvolcanic rocks.
Principles
[edit]The study of paleomagnetism is possible becauseiron-bearing minerals such asmagnetitemay record past polarity of Earth's magnetic field. Magnetic signatures in rocks can be recorded by several different mechanisms.
Thermoremanent magnetization
[edit]Iron-titanium oxide minerals inbasaltand otherigneous rocksmay preserve the direction of Earth's magnetic field when the rocks cool through theCurie temperaturesof those minerals. The Curie temperature of magnetite, aspinel-groupiron oxide,is about 580 °C, whereas most basalt andgabbroare completely crystallized at temperatures below 900 °C. Hence, the mineral grains are not rotated physically to align with Earth's magnetic field, but rather they may record the orientation of that field. The record so preserved is called athermoremanent magnetization(TRM).
Because complexoxidation reactionsmay occur as igneous rocks cool after crystallization, the orientations of Earth's magnetic field are not always accurately recorded, nor is the record necessarily maintained. Nonetheless, the record has been preserved well enough in basalts ofoceanic crustto have been critical in the development of theories of sea floor spreading related to plate tectonics.
TRM can also be recorded inpotterykilns,hearths, and burned adobe buildings. The discipline based on the study of thermoremanent magnetisation in archaeological materials is calledarchaeomagnetic dating.[10]Although theMāori peopleofNew Zealanddo not make pottery, their 700- to 800-year-old steam ovens, orhāngī,provide adequate archaeomagnetic material.[11]
Detrital remanent magnetization
[edit]In a completely different process, magnetic grains in sediments may align with the magnetic field during or soon after deposition; this is known asdetrital remanent magnetization.If the magnetization is acquired as the grains are deposited, the result is a depositional detrital remanent magnetization; if it is acquired soon after deposition, it is a post-depositional detrital remanent magnetization.[12]
Chemical remanent magnetization
[edit]In a third process, magnetic grains grow during chemical reactions and record the direction of the magnetic field at the time of their formation. The field is said to be recorded bychemical remanent magnetization(CRM). A common form is held by the mineralhematite,anotheriron oxide.Hematite forms through chemical oxidation reactions of other minerals in the rock including magnetite.Red beds,clasticsedimentary rocks (such assandstones) are red because of hematite that formed during sedimentarydiagenesis.The CRM signatures in red beds can be quite useful, and they are common targets in magnetostratigraphy studies.[13]
Isothermal remanent magnetization
[edit]Remanencethat is acquired at a fixed temperature is calledisothermal remanent magnetization(IRM). Remanence of this sort is not useful for paleomagnetism, but it can be acquired as a result of lightning strikes.Lightning-induced remanent magnetizationcan be distinguished by its high intensity and rapid variation in direction over scales of centimeters.[14][13]
IRM is often induced indrill coresby the magnetic field of the steel core barrel. This contaminant is generally parallel to the barrel, and most of it can be removed by heating up to about 400 °C or demagnetizing in a small alternating field. In the laboratory, IRM is induced by applying fields of various strengths and is used for many purposes inrock magnetism.
Viscous remanent magnetization
[edit]Viscous remanent magnetizationis remanence that is acquired byferromagneticmaterials influenced by a magnetic field for some time. In rocks, this remanence is typically aligned in the direction of the modern-day geomagnetic field. The fraction of a rock’s overall magnetization that is a viscous remanent magnetization is dependent on the magnetic mineralogy.
Sampling
[edit]The oldest rocks on the ocean floor are 200 Ma: very young when compared with the oldest continental rocks which date from 3.8 Ga. In order to collect paleomagnetic data dating beyond 200 Ma, scientists turn to magnetite-bearing samples on land to reconstruct Earth's ancient field orientation. Paleomagnetists, like many geologists, gravitate towards outcrops because layers of rock are exposed.Road cutsare a convenient man-made source of outcrops.
- "And everywhere, in profusion along this half mile of [roadcut], there are small, neatly cored holes... appears to be a Hilton for wrens and purple martins."[15]
There are two main goals of sampling:
- Retrieve samples with accurate orientations, and
- Reduce statistical uncertainty.
One way to achieve the first goal is to use a rock coring drill that has an auger tipped with diamond bits. The drill cuts a cylindrical space around some rock. Into this space is inserted a pipe with acompassandinclinometerattached. These provide the orientations. Before this device is removed, a mark is scratched on the sample. After the sample is broken off, the mark can be augmented for clarity.[16]
Applications
[edit]Paleomagnetic evidence of both reversals and polar wandering data was instrumental in verifying the theories of continental drift and plate tectonics in the 1960s and 1970s. Some applications of paleomagnetic evidence to reconstruct histories ofterraneshave continued to arouse controversies. Paleomagnetic evidence is also used in constraining possible ages for rocks and processes and in reconstructions of the deformational histories of parts of the crust.[4]
Reversal magnetostratigraphy is often used to estimate the age of sites bearing fossils andhomininremains.[17]Conversely, for a fossil of known age, the paleomagnetic data can fix the latitude at which the fossil was laid down. Such apaleolatitudeprovides information about the geological environment at the time of deposition. Paleomagnetic studies are combined withgeochronologicalmethods to determine absolute ages for rocks in which the magnetic record is preserved. Forigneous rockssuch asbasalt,commonly used methods includepotassium–argonandargon–argongeochronology.
See also
[edit]- Geophysics– Physics of the Earth and its vicinity
- Magnetochemistry– Study of magnetic properties of chemical compounds
- Paleoclimatology– Study of changes in ancient climate
- Plate reconstruction– Process of reconstructing the positions of tectonic plates in the geological past
- Rock magnetism– The study of magnetism in rocks
Notes
[edit]- ^W. Jacquelyne, Kious;Robert I., Tilling(2001)."Developing the theory".This dynamic earth: the story of plate tectonics (online edition version 1.20).Washington, D.C.: U.S. Geological Survey.ISBN0-16-048220-8.Retrieved6 November2016.
- ^Humboldt, F.A. v. (1797)."Ueber die merkwürdige magnetische Polarität einer Gebirgskuppe von Serpentinstein"[About the strange magnetic polarity of a mountain peak of serpentine].Neues Journal der Physik(in German).4:136–140.On pp. 136-137 Humboldt found that a peak in the Oberpfalz mountains was magnetic. On p. 138, Humboldt noted that a mountain peak in the Harz Mountains — specifically, theSchuarcher(the snorer) — also showed magnetization. He attributed the magnetization to lightning strikes. From p. 138:"Bey den Schuarchern ist es […] nicht unwahrscheinlich, daß ein Blitzstrahl in dem Granit jenen magnetischen Streifen hervorgebracht habe, […]"(In the case of theSchuarcher[…] it's not improbable that a bolt of lightning produced in the granite that magnetic streak, […] ) Humboldt thought that this explanation was even more likely in the case of peak in the Oberpfalz because even fragments of the rock were magnetized:"Nicht bloß das anstehende Gestein, sondern auch jedes noch so klein abgeschlagene Stück hat seine beiden Pole, seine eigene magnetische Achse."(Not just the outcrop but also every chip, however small, has both [magnetic] poles, its own magnetic axis.)
- ^abGlen 1982,p.[page needed].
- ^abMcElhinny & McFadden 2000
- ^Matuyama, Motonori (1929)."On the Direction of Magnetization of Basalt in Japan, Tyōsen [Korea] and Manchuria".Proceedings of the Imperial Academy of Japan.5:203–205.doi:10.2183/pjab1912.5.203.
- ^Wegener, Alfred (1915).Die Entstehung der Kontinente und Ozeane[The Origin of Continents and Oceans] (in German). Braunschweig, Germany: Vieweg.
- ^Glen 1982,pp.4–5.
- ^Runcorn, S. K. (1956). "Paleomagnetic comparisons between Europe and North America".Proc. Geol. Assoc. Canada.8:77–85.
- ^Irving, E. (1956). "Paleomagnetic and palaeoclimatological aspects of polar wandering".Geofis. Pura. Appl.33(1): 23–41.Bibcode:1956GeoPA..33...23I.doi:10.1007/BF02629944.S2CID129781412.
- ^Herries, A. I. R.; Adams, J. W.; Kuykendall, K. L.; Shaw, J. (2006). "Speleology and magnetobiostratigraphic chronology of the GD 2 locality of the Gondolin hominin-bearing paleocave deposits, North West Province, South Africa".Journal of Human Evolution.51(6): 617–31.doi:10.1016/j.jhevol.2006.07.007.PMID16949648.
- ^Amos, Jonathan (7 December 2012)."Maori stones hold magnetic clues".BBC News.Retrieved7 December2012.
- ^"Detrital Remanent Magnetization (DRM)".MagWiki: A Magnetic Wiki for Earth Scientists.Retrieved11 November2011.
- ^abTauxe, Lisa (24 May 2016)."Chemical remanent magnetization".Essentials of Paleomagnetism: Web Edition 3.0.Retrieved18 September2017.
- ^Dunlop & Özdemir 1997
- ^McPhee 1998,pp. 21–22
- ^Tauxe 1998
- ^Herries, A. I. R.; Kovacheva, M.; Kostadinova, M.; Shaw, J. (2007). "Archaeo-directional and -intensity data from burnt structures at the Thracian site of Halka Bunar (Bulgaria): The effect of magnetic mineralogy, temperature and atmosphere of heating in antiquity".Physics of the Earth and Planetary Interiors.162(3–4): 199–216.Bibcode:2007PEPI..162..199H.doi:10.1016/j.pepi.2007.04.006.
References
[edit]- Dunlop, David J.; Özdemir, Özden (1997).Rock Magnetism: Fundamentals and Frontiers.Cambridge Univ. Press.ISBN0-521-32514-5.
- Glen, William (1982).The Road to Jaramillo: Critical Years of the Revolution in Earth Science.Stanford University Press.ISBN0-8047-1119-4.
- McElhinny, Michael W.; McFadden, Phillip L. (2000).Paleomagnetism: Continents and Oceans.Academic Press.ISBN0-12-483355-1.
- McPhee, John(1998).Annals of the Former World.Farrar, Straus and Giroux.ISBN0-374-10520-0.
- Tauxe, Lisa (1998).Paleomagnetic Principles and Practice.Kluwer.ISBN0-7923-5258-0.
Further reading
[edit]- Butler, Robert F. (1992).Paleomagnetism: Magnetic Domains to Geologic Terranes.Blackwell.ISBN0-86542-070-X.Archived fromthe originalon 18 February 1999.
- Tauxe, Lisa (2010).Essentials of Paleomagnetism.University of California Press.ISBN978-0-520-26031-3.
External links
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