Editing Earth's magnetic field (section)

=== Magnetic field reversals ===
[[File:Geomagnetic polarity late Cenozoic.svg|thumb|upright|Geomagnetic polarity during the late [[Cenozoic Era]]. Dark areas denote periods where the polarity matches today's polarity, light areas denote periods where that polarity is reversed.]]
{{Main|Geomagnetic reversal}}

Although generally Earth's field is approximately dipolar, with an axis that is nearly aligned with the rotational axis, occasionally the North and South [[geomagnetic poles]] trade places. Evidence for these ''geomagnetic reversals'' can be found in [[basalt]]s, sediment cores taken from the ocean floors, and seafloor magnetic anomalies.<ref>{{cite book |last=Vacquier |first=Victor |title=Geomagnetism in marine geology |year=1972 |publisher=Elsevier Science |location=Amsterdam |isbn=978-0-08-087042-7 |page=38 |edition=2nd}}</ref> Reversals occur nearly [[Poisson distribution|randomly]] in time, with intervals between reversals ranging from less than 0.1 million years to as much as 50 million years. The most recent geomagnetic reversal, called the [[Brunhes–Matuyama reversal]], occurred about 780,000 years ago.<ref name="inconstant">{{cite news |url=https://science.nasa.gov/science-news/science-at-nasa/2003/29dec_magneticfield/ |title=Earth's Inconstant Magnetic Field |work=Science@Nasa |last=Phillips |first=Tony |date=29 December 2003 |access-date=27 December 2009 |archive-date=1 November 2022 |archive-url=https://web.archive.org/web/20221101165248/https://science.nasa.gov/science-news/science-at-nasa/2003/29dec_magneticfield/ |url-status=dead }}</ref><ref name="MMMch5">{{harvnb|Merrill|McElhinny|McFadden|1996|loc=Chapter 5}}</ref> A related phenomenon, a geomagnetic ''excursion'', takes the dipole axis across the equator and then back to the original polarity.<ref name="MMMexcursion">{{harvnb|Merrill|McElhinny|McFadden|1996|pp=148–155}}</ref><ref>{{cite journal |url=https://www.sciencedaily.com/releases/2012/10/121016084936.htm |title=Ice Age Polarity Reversal Was Global Event: Extremely Brief Reversal of Geomagnetic Field, Climate Variability, and Super Volcano |doi=10.1016/j.epsl.2012.06.050 |journal=Earth and Planetary Science Letters |date=16 October 2012 |access-date=21 March 2013|bibcode=2012E&PSL.351...54N |last1=Nowaczyk |first1=N. R. |last2=Arz |first2=H. W. |last3=Frank |first3=U. |last4=Kind |first4=J. |last5=Plessen |first5=B. |volume=351 |page=54 }}</ref> The [[Laschamp event]] is an example of an excursion, occurring during the last ice age (41,000 years ago).

The past magnetic field is recorded mostly by [[Magnetic mineralogy#Strongly magnetic minerals|strongly magnetic minerals]], particularly [[iron oxides]] such as [[magnetite]], that can carry a permanent magnetic moment. This [[remanent magnetization]], or ''remanence'', can be acquired in more than one way. In [[lava flow]]s, the direction of the field is "frozen" in small minerals as they cool, giving rise to a [[thermoremanent magnetization]]. In sediments, the orientation of magnetic particles acquires a slight bias towards the magnetic field as they are deposited on an ocean floor or lake bottom. This is called ''detrital remanent magnetization''.<ref name="McElhinny2000" />

Thermoremanent magnetization is the main source of the magnetic anomalies around mid-ocean ridges. As the seafloor spreads, [[magma]] wells up from the [[Mantle (geology)|mantle]], cools to form new basaltic crust on both sides of the ridge, and is carried away from it by seafloor spreading. As it cools, it records the direction of the Earth's field. When the Earth's field reverses, new basalt records the reversed direction. The result is a series of stripes that are symmetric about the ridge. A ship towing a magnetometer on the surface of the ocean can detect these stripes and infer the age of the ocean floor below. This provides information on the rate at which seafloor has spread in the past.<ref name="McElhinny2000" />

[[Radiometric dating]] of lava flows has been used to establish a ''geomagnetic polarity time scale'', part of which is shown in the image. This forms the basis of [[magnetostratigraphy]], a geophysical correlation technique that can be used to date both sedimentary and volcanic sequences as well as the seafloor magnetic anomalies.<ref name="McElhinny2000" />