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Paleomagnetism
Published: April 12, 2018
Updated: November 8, 2017
Author: Andrew P. Montry
Topic Editor: Michael Pidwirny
Topics:
Overview
Paleomagnetism is the study of the record of the Earth's magnetic field in rocks, sediment, or archeological materials. Certain minerals in rocks, sediment, or archeological materials lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field. This information has also been used to find the past location of tectonic plates. The record of geomagnetic reversals preserved in volcanic and sedimentary rock sequences provides a time-scale that is used as a geochronological tool.
The study of paleomagnetism led to the revival of the continental drift hypothesis and its transformation into plate tectonics. Apparent polar wander paths provided the first clear geophysical evidence for continental drift, while marine magnetic anomalies did the same for the process of seafloor spreading. Paleomagnetism continues to extend the history of plate tectonics back in time and are applied to the movement of continental fragments or terranes
History
As early as the 18th century, it was noticed that compass needles deviated near strongly magnetized outcrops. In 1797, Alexander Von Humboldt attributed this magnetization to lightning strikes (and lightning strikes do often magnetize surface rocks).[1][2] 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 in the opposite direction to the Earth's current magnetic field. Japanese geophysicist Motonori Matuyama showed that the Earth's magnetic field reversed in the mid-Quaternary, a reversal now known as the Brunhes-Matuyama reversal.[1]
The British physicist P.M.S. Blackett invented a sensitive astatic magnetometer in 1956 which revolutionized our knowledge on paleomagnetism and plate tectonics. Blackett's intent for this tool was to test his theory that the geomagnetic field was related to the Earth's rotation, a hypothesis that he ultimately rejected. The astatic magnetometer quickly became the basic tool of paleomagnetism research and led to a revival of the theory of continental drift. Alfred Wegener first proposed in 1915 that continents had once been joined together and had since moved apart.[1]
Although Alfred Wegener collected an abundance of circumstantial evidence, his theory of continental drift 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. In 1956, Keith Runcorn[3] and Edward A. Irving[4] published separate studies where they constructed apparent polar wander paths for Europe and North America. While these two curves showed diverging routes, the paths could be reconciled if it was assumed that the continents had been in contact up to 200 million years ago. These two studies provided the first clear geophysical evidence for continental drift. Then in 1963, Morley, Vine and Matthews showed that marine magnetic anomalies provided evidence for seafloor spreading and the mechanism that moves continents across the Earth's surface.
Fields of Paleomagnetism
- Secular variation studies look at small-scale changes in the direction and intensity of the Earth's magnetic field. The Magnetic North Pole is constantly shifting relative to the axis of rotation of the Earth. Magnetism is a vector and so magnetic field variation is made up of palaeodirectional measurements of magnetic declination and magnetic inclination and palaeointensity measurements.
- Magnetostratigraphy 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 history. The age and pattern of these reversals is best known from the study of sea floor spreading zones and the dating of volcanic rocks.
Sampling Procedure for Paleomagnetic Data
The oldest rocks on the ocean floor are about 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 collecting the data they require from outcrops because layers of rock are exposed and easy to sample. An important source source of newly exposed outcrops are human-made road cuts in bedrock. There are two main goals of sampling for palaeomagnetic data: 1) Retrieve samples with accurate orientations, and 2) 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 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.[5]
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 terranes have continued to arouse scientific 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.[2]
Reversal magnetostratigraphy is often used to estimate the age of sites bearing fossils and hominin remains.[6] 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 often combined with geochronological methods to determine absolute ages 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.
Further Reading
- Essentials of Paleomagnetism: Fourth Web Edition. Tauxe, 2016. https://earthref.org/MagIC/books/Tauxe/Essentials/
- "Paleomagnetism: Magnetic Domains to Geologic Terranes." Butler, 1992.
Author's Note
This article used some material from the Wikipedia article that was accessed on November 8, 2018. The Author(s) and Topic Editor(s) associated with this article have modified the content derived from Wikipedia with original content or with content drawn from other sources. All content from Wikipedia has been reviewed and approved by those Author(s) and Topic Editor(s), and is subject to the same peer review process as other content in the EoE. The current version of the Wikipedia article differs from the version that existed on the date of access. This article is licensed under the GNU Free Documentation License 1.2. See the EoE’s Policy on the Use of Content from Wikipedia for more information.
Citation
Montry, A. (2017, Nov 8). Paleomagnetism. Retrieved from Wikipedia
References
- ↑ 1.0 1.1 1.2 Glen, William (1982). The Road to Jaramillo: Critical Years of the Revolution in Earth Science.
- ↑ 2.0 2.1 McElhinny 2000
- ↑ 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.
- ↑ 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".
