paleomagnetism

 

study of the intensity and orientation of the earth's magnetic field as preserved in the magnetic orientation of certain minerals found in rocks formed throughout geologic time. Paleomagnetic studies of rocks and ocean sediment have demonstrated that the orientation of the earth's magnetic field has frequently alternated over geologic time. Periods of “normal” polarity (i.e., when the north-seeking end of the compass needle points toward the present north magnetic pole, as it does today) have alternated with periods of “reversed” polarity (when the north-seeking end of the compass needle points southward). The cause of these magnetic “flip-flops” is not clearly understood. Ideas of paleomagnetism began in the late 1920s, when French physicist Mercanton, suggested that because today's magnetic field is close to the earth's rotational axis, continental drift could be tested by ascertaining the magnetic characteristics of ancient rocks; however, it was not until after World War II that rock paleomagnetism data was gathered. Paleomagnetism is possible because some of the minerals that make up rocks—notably magnetite—become permanently magnetized parallel to the earth's magnetic field at the time of their formation. Rocks from hot liquid magma (see lava), or even minerals made up of crystals that grow at low temperatures, can acquire magnetization. Also, when magnetized minerals become disaggregated from their parent rocks by erosion and are carried into a basin, they will tend to align themselves parallel to the earth's magnetic field as they settle in still water. When the deposit into which they settle hardens into rock, the magnetization will be fixed. Geophysicists have been able to trace changes in the orientation of the earth's magnetic field through geologic time by carefully collecting rock specimens of different ages and determining the alignment of their magnetic fields. That technique has provided a timetable for periods of normal and reversed polarity, showing 171 reversals in the earth's magnetic field in the past 76 million years. Paleomagnetic studies of the ocean floor have been of decisive importance in establishing the modern theories of continental drift and seafloor spreading.

Paleomagnetism refers to the study of the record of the Earth's magnetic field preserved in various magnetic minerals through time. The study of paleomagnetism has demonstrated that the Earth's magnetic field varies substantially in both orientation and intensity through time. Paleomagnetists study the ancient magnetic field by measuring the orientation of magnetic minerals in rocks and sediments, then using similar methods to geomagnetism determine what configuration of the Earth's magnetic field may have resulted in the observed orientation.

Paleomagnetism can be divided into two fields:

• Polar wandering: the magnetic north pole is constantly shifting relative to the axis of rotation. This is responsible for the shifting magnetic declination required for compass work and orienteering.
• Magnetic polarity reversals: periodically, the Earth's magnetic field reverses polarity. The reversals have occurred at irregular intervals throughout the Earth's history. This method of identifying polar reversals was discovered by Frederick John Vine.

The study of paleomagnetism is possible because iron-bearing minerals such as magnetite may record past directions of the Earth's magnetic field. Paleomagnetic signatures in rocks can be recorded by three different mechanisms.

First, iron-titanium oxide minerals in basalt and other igneous rocks may preserve the direction of the Earth's magnetic field when the rocks cool through the Curie temperatures of those minerals. The Curie temperature of magnetite, a spinel-group iron oxide, is about 580°C, whereas most basalt and gabbro are completely crystallized at temperatures above 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 Thermal Remanent Magnetization (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 kilns, hearths, and burned adobe buildings (archaeomagnetism).

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 Detrial Remanent Magnetization (dDRM); if it is acquired soon after deposition, it is a post-depositional Detrital Remanent Magnetization (pDRM).

In a third process, magnetic grains may be deposited 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 Chemical Remanent Magnetization (CRM). 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.

Ages may be determined for rocks in which the magnetic record is preserved. For igneous rocks such as basalt, commonly used methods include potassium-argon and argon-argon geochronology.

Paleomagnetic evidence, both reversals and polar wandering data, was instrumental in verifying the theories of continental drift and plate tectonics in the 1960s and 70s. Some applications of paleomagnetic evidence to reconstructing histories of terranes have continued to arouse controversies. Paleomagnetic evidence also is used in constraining possible ages for rocks and processes and in reconstructions of the deformational histories of parts of the crust.

One of the pioneering scientists who studied paleomagnetism was the British physicist P.M.S. Blackett.

Because palaeomagnetism normally required whole rock samples, the oldest fields that we could measure were approximately 250 Ma ago (the oldest oceanic crust). This is no longer the case, and cutting edge research using 'Silicate Inclusions' (i.e. iron bearing minerals which have been exsolved from parent minerals such as plagioclase feldspar or pyroxene) can be used to provide field information for whatever age the host crystal is. This allows dating of rocks as old as 4 Ga, which would give scientists data about the strength of the Earth's magnetic field over a range of time much larger than currently available.

See also

• Geophysics
• Edward A. Irving, Canadian paleomagnetism specialist

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