earth science

Paleomagnetism

Paleomagnetism is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Paleomagnetism rather than just read about it. In short: Paleomagnetism (occasionally palaeomagnetism) is the study of prehistoric Earth's magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called paleomagnetists.

Key takeaways

Paleomagnetism belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Paleomagnetism to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Paleomagnetism from memory before moving on to harder problems.

Reference excerpt

Paleomagnetism (occasionally palaeomagnetism) is the study of prehistoric Earth's magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called paleomagnetists. Certain magnetic minerals in rocks can record the direction and intensity of Earth's magnetic field at the time they formed. This record provides information on the past behavior of the geomagnetic field and the past location of tectonic plates. The record of geomagnetic reversals preserved in volcanic and sedimentary rock sequences (magnetostratigraphy) provides a time-scale that is used as a geochronologic tool. Evidence from paleomagnetism led to the revival of the continental drift hypothesis and its transformation into the modern theory of plate tectonics. Apparent polar wander paths provided the first clear geophysical evidence for continental drift, while marine magnetic anomalies did the same for seafloor spreading. 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). The field of paleomagnetism also encompasses equivalent measurements of samples from other Solar System bodies, such as Moon rocks and meteorites, where it is used to investigate the ancient magnetic fields of those bodies and dynamo theory. Paleomagnetism relies on developments in rock magnetism 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, Alexander von Humboldt attributed this magnetization to lightning strikes (and lightning strikes do often magnetize surface rocks). 19th century studies of the direction of magnetization in rocks showed that some recent lavas were magnetized parallel to Earth's magnetic field. Early in the 20th century, work by David, Bernard Brunhes and Paul Louis Mercanton showed that many rocks were magnetized antiparallel to the field. Japanese geophysicist Motonori Matuyama showed in the late 1920s that Earth's magnetic field reversed in the mid-Quaternary, a reversal now known as the Brunhes–Matuyama reversal. British physicist P.M.S. Blackett 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 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 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 and Edward A. Irving constructed apparent polar wander 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 provided evidence for seafloor spreading.

Fields Paleomagnetism is studied on a number of scales:

Geomagnetic secular variation is the small-scale changes in the direction and intensity of Earth's magnetic field. The magnetic north pole is constantly shifting relative to the axis of rotation of Earth. Magnetism is a vector and so magnetic field variation is studied by palaeodirectional measurements of magnetic declination and magnetic inclination and palaeointensity measurements.

Magnetostratigraphy uses the polarity reversal history of 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 age and pattern of these reversals is known from the study of sea floor spreading zones and the dating of volcanic rocks.

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

Thermoremanent magnetization

Iron-titanium oxide minerals in basalt and other igneous rocks may preserve the direction of 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 (1,076 °F), whereas most basalt and gabbro are completely crystallized at temperatures below 900 °C (1,650 °F). Hence, the mineral grains are not rotated physically to align with Earth's magnetic field, but rather they may record the orientation of that field. The record so preserved is called a thermoremanent magnetization (TRM). Because complex oxidation reactions may occur as igneous rocks cool after crystallization, the orientations of 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 oceanic 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. The discipline based on the study of thermoremanent magnetisation in archaeological materials is called archaeomagnetic dating. Although the Māori people of New Zealand do not make pottery, their 700- to 800-year-old steam ovens, or hāngī, 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. If the magnetization is acquired as the grains are deposited, the result is a depositional detrital remanent magnetization; if it is acquired soon after deposition, it is a post-depositional detrital remanent magnetization.

Chemical remanent magnetization

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Paleomagnetism

Start with the simplest possible case. Write down what Paleomagnetism claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Paleomagnetism before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Paleomagnetism ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Paleomagnetism

In research: Paleomagnetism appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Paleomagnetism in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Paleomagnetism is common in secondary-school and first-year university syllabi. It links to neighbouring topics Geochronological dating methods, Historical geology, Paleomagnetism, so understanding it makes those chapters shorter.
In everyday life: Look for Paleomagnetism outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Paleomagnetism in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Paleomagnetism means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Paleomagnetism out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Paleomagnetism in simple terms?

Why does Paleomagnetism matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Paleomagnetism?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Paleomagnetism.