A geomagnetic reversal is a change in the Earth's dipole magnetic field such that the positions of magnetic north and magnetic south are interchanged (not to be confused with geographic north and geographic south). The Earth's magnetic field has alternated between periods of normal polarity, in which the predominant direction of the field was the same as the present direction, and reverse polarity, in which it was the opposite. These periods are called chrons.
Reversal occurrences appear to be statistically random. There have been at least 183 reversals over the last 83 million years (thus on average once every ~450,000 years). The latest, the Brunhes–Matuyama reversal, occurred 780,000 years ago with widely varying estimates of how quickly it happened. Some sources estimate that the four most recent reversals took on average 7,000 years to occur. Clement (2004) suggests that this duration is dependent on latitude, with shorter durations at low latitudes and longer durations at mid and high latitudes. Others estimate the duration of full reversals to vary from between 2,000 and 12,000 years, although some reversals may be as long as 70,000 years.
There have also been episodes in which the field inverted for only a few hundred years (such as the Laschamp excursion). These events are classified as excursions rather than full geomagnetic reversals. Stable polarity chrons often show large, rapid directional excursions, which occur more often than reversals, and could be seen as failed reversals. During such an excursion, the field reverses in the liquid outer core but not in the solid inner core. Diffusion in the outer core is on timescales of 500 years or less while that of the inner core is longer, around 3,000 years.
History In the early 20th century, geologists such as Bernard Brunhes first noticed that some volcanic rocks were magnetized opposite to the direction of the local Earth's field. The first systematic evidence for and time-scale estimate of the magnetic reversals were made by Motonori Matuyama in the late 1920s; he observed that rocks with reversed fields were all of early Pleistocene age or older. At the time, the Earth's polarity was poorly understood, and the possibility of reversal aroused little interest.
Three decades later, when Earth's magnetic field was better understood, theories were advanced suggesting that the Earth's field might have reversed in the remote past. Most paleomagnetic research in the late 1950s included an examination of the wandering of the poles and continental drift. Although it was discovered that some rocks would reverse their magnetic field while cooling, it became apparent that most magnetized volcanic rocks preserved traces of the Earth's magnetic field at the time the rocks had cooled through the Curie temperature. In the absence of reliable methods for obtaining absolute ages for rocks, it was thought that reversals occurred approximately every million years.
The next major advance in understanding reversals came when techniques for radiometric dating were improved in the 1950s. Allan Cox and Richard Doell, at the United States Geological Survey, wanted to know whether reversals occurred at regular intervals, and they invited geochronologist Brent Dalrymple to join their group. They produced the first magnetic-polarity time scale in 1959. As they accumulated data, they continued to refine this scale in competition with Don Tarling and Ian McDougall at the Australian National University. A group led by Neil Opdyke at the Lamont–Doherty Earth Observatory showed that the same pattern of reversals was recorded in sediments from deep-sea cores.
During the 1950s and 1960s information about variations in the Earth's magnetic field was gathered largely by means of research vessels, but the complex routes of ocean cruises rendered the association of navigational data with magnetometer readings difficult. Only when data were plotted on a map did it become apparent that remarkably regular and continuous magnetic stripes appeared on the ocean floors.
In 1963, Frederick Vine and Drummond Matthews provided a simple explanation by combining the seafloor spreading theory of Harry Hess with the known time scale of reversals: sea floor rock is magnetized in the direction of the field when it is formed. Thus, sea floor spreading from a central ridge will produce pairs of magnetic stripes parallel to the ridge. Canadian L. W. Morley independently proposed a similar explanation in January 1963, but his work was rejected by the scientific journals Nature and Journal of Geophysical Research, and remained unpublished until 1967, when it appeared in the literary magazine Saturday Review. The Morley–Vine–Matthews hypothesis was the first key scientific test of the seafloor spreading theory of continental drift.
Past field reversals are recorded in the solidified ferrimagnetic minerals of consolidated sedimentary deposits or cooled volcanic flows on land. Beginning in 1966, Lamont–Doherty Geological Observatory scientists found that the magnetic profiles across the Pacific-Antarctic Ridge were symmetrical and matched the pattern in the north Atlantic's Reykjanes ridge. The same magnetic anomalies were found over most of the world's oceans, which permitted estimates for when most of the oceanic crust had developed.
Observing past fields
Because no existing unsubducted sea floor (or sea floor thrust onto continental plates) is more than about (Ma) old, other methods are necessary for detecting older reversals. Most sedimentary rocks incorporate minute amounts of iron-rich minerals, whose orientation is influenced by the ambient magnetic field at the time at which they formed. These rocks can preserve a record of the field if it is not later erased by chemical, physical or biological change.
Because Earth's magnetic field is a global phenomenon, similar patterns of magnetic variations at different sites may be used to help calculate age in different locations. The past four decades of paleomagnetic data about seafloor ages (up to ~) has been useful in estimating the age of geologic sections elsewhere. While not an independent dating method, it depends on "absolute" age dating methods like radioisotopic systems to derive numeric ages. It has become especially useful when studying metamorphic and igneous rock formations where index fossils are seldom available.
Geomagnetic polarity time scale
Through analysis of seafloor magnetic anomalies and dating of reversal sequences on land, paleomagnetists have been developing a Geomagnetic Polarity Time Scale. The current time scale contains 184 polarity intervals in the last 83million years (and therefore 183 reversals).
TimespanNormal polarityName118–83 mya34n79.075–73.619 mya33n73.374–73.291 mya32r.1n73.004–71.587 mya32n.2n73.004–71.587 mya32n.2n71.338–71.071 mya32n.1n68.737–67.735 mya31n68.737–67.735 mya31n67.61–65.578 mya30n67.61–65.578 mya30n67.61–65.578 mya30n67.61–65.578 mya30n64.745–63.976 mya29n63.634–62.499 mya28n61.276–60.92 mya27n57.911–57.554 mya26n56.391–55.904 mya25n53.347–52.903 mya24n.3n52.801–52.757 mya24n.2n52.663–52.364 mya24n.1n51.743–51.047 mya23n.2n50.946–50.778 mya23n.1n49.714–49.037 mya22n47.906–46.264 mya21n43.789–42.536 mya20n41.521–41.257 mya19n40.13–39.631 mya18n.2n39.552–38.426 mya18n.1n38.113–37.92 mya17n.3n37.848–37.604 mya17n.2n37.473–36.618 mya17n.1n36.341–35.685 mya16n.2n36.341–35.685 mya16n.2n35.526–35.343 mya16n.1n34.94–34.655 mya15n33.545–33.058 mya13n30.939–30.479 mya12n30.098–29.765 mya11n.2n29.662–29.401 mya11n.1n28.745–28.578 mya10n.2n28.512–28.283 mya10n.1n27.972–27.027 mya9n26.554–25.992 mya8n.2n25.951–25.823 mya8n.1n25.648–25.496 mya7An25.183–24.835 mya7n.2n24.781–24.730 mya7n.1n24.118–23.999 mya6Cn.3n23.8–23.677 mya6Cn.2n23.535–23.353 mya6Cn.1n23.069–22.804 mya6Bn.2n22.75–22.588 mya6Bn.1n22.493–22.459 mya6AAr.2n22.248–22.151 mya6AAr.1n21.859–21.768 mya6AAn21.32–20.996 mya6An.2n20.725–20.518 mya6An.1n20.131–19.048 mya6n18.781–18.281 mya5En17.615–17.277 mya5Dn16.726–16.556 mya5Cn.3n16.488–16.327 mya5Cn.2n16.293–16.014 mya5Cn.1n15.155–15.034 mya5Bn.2n14.888–14.8 mya5Bn.1n14.612–14.178 mya5ADn14.076–13.703 mya5ACn13.51–13.302 mya5ABn13.139–12.991 mya5AAn12.819–12.775 mya5Ar.2n12.708–12.678 mya5Ar.1n12.401–12.184 mya5An.2n12.078–11.935 mya5An.1n11.531–11.476 mya5r.2n11.099–11.052 mya5r.1n10.949–9.92 mya5n.2n9.88–9.74 mya5n.1n9.642–9.58 mya4Ar.2n9.308–9.23 mya4Ar.1n9.025–8.699 mya4An8.257–8.225 mya4r.1n8.072–7.65 mya4n.2n7.562–7.432 mya4n.1n7.375–7.341 mya3Br.2n7.17–7.135 mya3Br.1n7.091–6.935 mya3Bn6.567–6.269 mya3An.2n6.137–5.894 mya3An.1n5.23–4.98 mya3n.4nThvera4.89–4.8 mya3n.3nSidufjall4.62–4.48 mya3n.2nNunivak4.29–4.18 mya3n.1nCochiti3.58–3.33 mya2An.3nGauss/Gilbert3.22–3.11 mya2An.2n3.04–2.581 mya2An.1nGauss–Matuyama2.15–2.14 mya2r.1n1.95–1.77 mya2nOlduvai1.07 mya – 990 kya1r.1nJaramillo780 kya – present1nBrunhes–Matuyama
Changing frequency over time The rate of reversals in the Earth's magnetic field has varied widely over time. Around , the field reversed 5 times in a million years. In a 4-million-year period centered on , there were 10 reversals; at around , 17 reversals took place in the span of 3million years. In a period of 3million years centering on , 13 reversals occurred. No fewer than 51 reversals occurred in a 12-million-year period, centering on . Two reversals occurred during a span of 50,000 years. These eras of frequent reversals have been counterbalanced by a few "superchrons": long periods when no reversals took place.
Superchrons A superchron is a polarity interval lasting at least 10million years. There are two well-established superchrons, the Cretaceous Normal Superchron and the Kiaman Superchron. A third candidate, the Moyero Superchron, is more controversial. The Jurassic Quiet Zone in ocean magnetic anomalies was once thought to represent a superchron but is now attributed to other causes.
The Cretaceous Normal Superchron (also called the Cretaceous Superchron or C34) lasted for 37million years, from about , including stages of the Cretaceous period from the Aptian through the Santonian. The frequency of magnetic reversals steadily decreased prior to the period, reaching its low point (no reversals) during the period. Between the Cretaceous Normal and the present, the frequency has generally increased slowly.
The Kiaman Reverse Superchron lasted from approximately the late Carboniferous to the late Permian, or for more than 50million years, from around . The magnetic field had reversed polarity. The name "Kiaman" derives from the Australian town of Kiama, where some of the first geological evidence of the superchron was found in 1925.
The Ordovician is suspected to have hosted another superchron, called the Moyero Reverse Superchron, lasting more than 20million years (485 to 463million years ago). Thus far, this possible superchron has only been found in the Moyero river section north of the polar circle in Siberia. Moreover, the best data from elsewhere in the world do not show evidence for this superchron. Certain regions of ocean floor, older than , have low-amplitude magnetic anomalies that are hard to interpret. They are found off the east coast of North America, the northwest coast of Africa, and the western Pacific. They were once thought to represent a superchron called the Jurassic Quiet Zone, but magnetic anomalies are found on land during this period. The geomagnetic field is known to have low intensity between about and , and these sections of ocean floor are especially deep, causing the geomagnetic signal to be attenuated between the seabed and the surface.
Statistical properties