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Paleomagnetism (or palaeomagnetism
[see ]), is the study of magnetic fields recorded in rocks, sediment, or archeological materials.
Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.''
Certain magnetic
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
s in
rock
Rock most often refers to:
* Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids
* Rock music, a genre of popular music
Rock or Rocks may also refer to:
Places United Kingdom
* Rock, Caerphilly, a location in Wales ...
s can record the direction and intensity of
Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic ...
at the time they formed. This record provides information on the past behavior of the geomagnetic field and the past location of
tectonic plates
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
. The record of
geomagnetic reversals
A geomagnetic reversal is a change in a planet's 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 field has alternated b ...
preserved in
volcanic
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates a ...
and
sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
sequences (
magnetostratigraphy
Magnetostratigraphy 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 '' ...
) provides a time-scale that is used as a
geochronologic
Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is pr ...
tool.
Evidence from paleomagnetism led to the revival of the
continental drift
Continental drift is the hypothesis that the Earth's continents have moved over geologic time relative to each other, thus appearing to have "drifted" across the ocean bed. The idea of continental drift has been subsumed into the science of pl ...
hypothesis and its transformation into the modern theory of
plate tectonics
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
.
Apparent polar wander Apparent polar wander (APW) is the perceived movement of the Earth's paleo- magnetic poles relative to a continent while regarding the continent being studied as fixed in position. It is frequently displayed on the present latitude-longitude map as ...
paths provided the first clear geophysical evidence for continental drift, while marine
magnetic anomalies
In 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 ...
did the same for
seafloor spreading
Seafloor spreading or Seafloor spread is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.
History of study
Earlier theories by Alfred Wegener a ...
. 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
In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its own ...
).
The field of paleomagnetism also encompasses equivalent measurements of samples from other Solar System bodies, such as
Moon rocks
Moon rock or lunar rock is rock originating from Earth's Moon. This includes lunar material collected during the course of human exploration of the Moon, and rock that has been ejected naturally from the Moon's surface and landed on Earth as ...
and
meteorites, where it is used to investigate the ancient magnetic fields of those bodies and
dynamo theory
In physics, the dynamo theory proposes a mechanism by which a celestial body such as Earth or a star generates a magnetic field. The dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid can ...
. Paleomagnetism relies on developments in
rock magnetism
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 ...
, and overlaps with
biomagnetism, magnetic fabrics (used as strain indicators in rocks and soils), and
environmental magnetism.
History
As early as the 18th century, it was noticed that compass needles deviated near strongly magnetized outcrops. In 1797, Von Humboldt attributed this magnetization to lightning strikes (and lightning strikes do often magnetize surface rocks).
In the 19th century studies of the direction of magnetization in rocks showed that some recent lavas were magnetized parallel to the Earth's magnetic field. Early in the 20th century, work by David, Brunhes and Mercanton showed that many rocks were magnetized antiparallel to the field. Japanese geophysicist
Motonori Matuyama showed in the late 1920s that the Earth's magnetic field reversed in the mid-
Quaternary, a reversal now known as the
Brunhes–Matuyama reversal
The Brunhes–Matuyama reversal, named after Bernard Brunhes and Motonori Matuyama, was a geologic event, approximately 781,000 years ago, when the Earth's magnetic field last underwent reversal. Estimations vary as to the abruptness of the reve ...
.
The British physicist
P.M.S. Blackett
Patrick Maynard Stuart Blackett, Baron Blackett (18 November 1897 – 13 July 1974) was a British experimental physics, experimental physicist known for his work on cloud chambers, cosmic rays, and paleomagnetism, winning the Nobel Priz ...
provided a major impetus to paleomagnetism by inventing a sensitive astatic
magnetometer in 1956. His intent was to test his theory that the
geomagnetic field was related to the Earth'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 Wegener
Alfred Lothar Wegener (; ; 1 November 1880 – November 1930) was a German climatologist, geologist, geophysicist, meteorologist, and polar researcher.
During his lifetime he was primarily known for his achievements in meteorology and ...
first proposed in 1915 that continents had once been joined together and had since moved apart. 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
(Stanley) Keith Runcorn (19 November 1922 – 5 December 1995) was a British physicist whose paleomagnetic reconstruction of the relative motions of Europe and America revived the theory of continental drift and was a major contribution to plat ...
and
Edward A. Irving constructed
apparent polar wander Apparent polar wander (APW) is the perceived movement of the Earth's paleo- magnetic poles relative to a continent while regarding the continent being studied as fixed in position. It is frequently displayed on the present latitude-longitude map as ...
paths 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 Matthews showed that marine
magnetic anomalies
In 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 ...
provided evidence for
seafloor spreading
Seafloor spreading or Seafloor spread is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.
History of study
Earlier theories by Alfred Wegener a ...
.
Fields
Paleomagnetism is studied on a number of scales:
* ''
Geomagnetic secular variation
Geomagnetic 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 mag ...
'' is the small-scale changes in the direction and intensity of the Earth's magnetic field. The magnetic
north pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True North Pole to distingu ...
is constantly shifting relative to the axis of rotation of the Earth. Magnetism is a vector and so magnetic field variation is studied by palaeodirectional measurements of
magnetic declination
Magnetic declination, or magnetic variation, is the angle on the horizontal plane between magnetic north (the direction the north end of a magnetized compass needle points, corresponding to the direction of the Earth's magnetic field lines) an ...
and
magnetic inclination
Magnetic dip, dip angle, or magnetic inclination is the angle made with the horizontal by the Earth's magnetic field lines. This angle varies at different points on the Earth's surface. Positive values of inclination indicate that the magnetic fi ...
and palaeointensity measurements.
* ''
Magnetostratigraphy
Magnetostratigraphy 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 '' ...
'' uses the
polarity reversal history of the Earth's magnetic field recorded in rocks to determine the age of those rocks. ''
Reversals'' have occurred at irregular intervals throughout
Earth's history
The history of Earth concerns the development of planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to understanding of the main events of Earth's past, characterized by constant geologic ...
. The age and pattern of these reversals is known from the study of
sea floor spreading
Seafloor spreading or Seafloor spread is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.
History of study
Earlier theories by Alfred Wegener an ...
zones and the dating of volcanic rocks.
Principles of remanent magnetization
The study of paleomagnetism is possible because
iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
-bearing minerals such as
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...
may record past directions of the Earth's magnetic field. Magnetic signatures in rocks can be recorded by several different mechanisms.
Thermoremanent magnetization
Iron-titanium oxide minerals in
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
and other
igneous
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
rocks may preserve the direction of the Earth's magnetic field when the rocks cool through the
Curie temperature
In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cur ...
s of those minerals. The Curie temperature of magnetite, a
spinel-group
iron oxide, is about 580 °C, whereas most basalt and
gabbro
Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...
are completely crystallized at temperatures below 900 °C. Hence, the mineral grains are not rotated physically to align with the Earth's field, but rather they may record the orientation of that field. The record so preserved is called a
thermoremanent magnetization 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 ver ...
(TRM). Because complex oxidation reactions may occur as igneous rocks cool after crystallization, the orientations of the 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 of the ocean crust to have been critical in the development of theories of sea floor spreading related to plate tectonics. TRM can also be recorded in
pottery
Pottery is the process and the products of forming vessels and other objects with clay and other ceramic materials, which are fired at high temperatures to give them a hard and durable form. Major types include earthenware, stoneware and ...
kiln
A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or chemical changes. Kilns have been used for millennia to turn objects made from clay int ...
s, hearths, and burned adobe buildings. The discipline based on the study of thermoremanent magnetisation in archaeological materials is called
archaeomagnetic dating
Archaeomagnetic dating is the study and interpretation of the signatures of the Earth's magnetic field at past times recorded in archaeological materials. These paleomagnetic signatures are fixed when ferromagnetic materials such as magnetite cool ...
. Although the
Māori people
The Māori (, ) are the indigenous Polynesian people of mainland New Zealand (). Māori originated with settlers from East Polynesia, who arrived in New Zealand in several waves of canoe voyages between roughly 1320 and 1350. Over several c ...
of
New Zealand
New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...
do not make pottery, their 700- to 800-year-old steam ovens, or
hangi, provide adequate archaeomagnetic material.
Detrital remanent magnetization
In a completely different process, 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 soon after deposition, it is a post-depositional detrital remanent magnetization (pDRM).
Chemical remanent magnetization
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 by ''chemical remanent magnetization'' (CRM). A common form of chemical remanent magnetization is held by the mineral
hematite, another
iron oxide. Hematite forms through chemical oxidation reactions of other minerals in the rock including magnetite.
Redbed
Red beds (or redbeds) are sedimentary rocks, typically consisting of sandstone, siltstone, and shale, that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain th ...
s,
clastic sedimentary rocks (such as
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates ...
s) are red because of hematite that formed during sedimentary
diagenesis
Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...
. The CRM signatures in redbeds can be quite useful and they are common targets in
magnetostratigraphy
Magnetostratigraphy 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 '' ...
studies.
Isothermal remanent magnetization
Remanence that is acquired at a fixed temperature is called ''isothermal 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 magnetization can be distinguished by its high intensity and rapid variation in direction over scales of centimeters.
[
IRM is often induced in drill cores by 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 ℃ 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 in ]rock magnetism
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 ...
.
Viscous remanent magnetization
Viscous remanent magnetization is remanence that is acquired by ferromagnetic materials by sitting in 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.
Paleomagnetic procedure
Collecting samples on land
The oldest rocks on the ocean floor are 200 mya – very young when compared with the oldest continental rocks, which date from 3.8 billion years ago. In order to collect paleomagnetic data dating beyond 200 mya, scientists turn to magnetite-bearing samples on land to reconstruct the Earth's ancient field orientation.
Paleomagnetists, like many geologists, gravitate towards outcrops because layers of rock are exposed. Road cuts are a convenient man-made source of outcrops.
:"And everywhere, in profusion along this half mile of oadcut there are small, neatly cored holes ... appears to be a Hilton for wrens and purple martins."
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 a pipe tipped with diamond bits. The drill cuts a cylindrical space around some rock. This can be messy – the drill must be cooled with water, and the result is mud spewing out of the hole. Into this space is inserted another pipe with compass
A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself wit ...
and inclinometer attached. 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.
Applications
Paleomagnetic evidence, 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 of terrane
In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust (geology), crust fragment formed on a tectonic plate (or broken off from it) and Accretion (geology), accreted or "Suture (geology), sutured" to crust lying on another pla ...
s have 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.
Reversal magnetostratigraphy is often used to estimate the age of sites bearing fossils and hominin
The Hominini form a taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes the extant genera ''Homo'' (humans) and '' Pan'' (chimpanzees and bonobos) and in standard usage excludes the genus ''Gorilla'' (gorillas).
The ...
remains. Conversely, for a fossil of known age, the paleomagnetic data can fix the latitude at which the fossil was laid down. Such a ''paleolatitude'' provides information about the geological environment at the time of deposition.
Paleomagnetic studies are combined with geochronological methods to determine absolute ages for rocks in which the magnetic record is preserved. For igneous rock
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...
s such as basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
, commonly used methods include potassium–argon and argon–argon geochronology.
See also
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Notes and references
Notes
Citations
Sources
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Further reading
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External links
Geomagnetism & Paleomagnetism background material
Paleomagnetic Data from NGDC / WDC Boulder
Paleomagnetic database at the Scripps Institution of Oceanography (MagIC)
{{Authority control
Historical geology
Geochronological dating methods