science

Natural remanent magnetization

Natural remanent magnetization is a 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 Natural remanent magnetization rather than just read about it. In short: Natural remanent magnetization is the permanent magnetism of a rock or sediment. This preserves a record of the Earth's magnetic field at the time the mineral was laid down as sediment or crystallized in magma and also the tectonic movement of the rock over millions of years from its original position.

Key takeaways

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

Reference excerpt

Natural remanent magnetization is the permanent magnetism of a rock or sediment. This preserves a record of the Earth's magnetic field at the time the mineral was laid down as sediment or crystallized in magma and also the tectonic movement of the rock over millions of years from its original position. Natural remanent magnetization forms the basis of paleomagnetism and magnetostratigraphy.

Igneous rocks Natural remanent magnetism is important when studying igneous rocks and the majority of the studies are based on. This is because these rocks contain a magnetic field at the time when the rock was formed. By being able measure the angle difference between the current magnetic field and the direction of the rocks the inclination can be discovered as well as seeing at how much the magnetic fields have moved. This is also the most common method used to get the remanence direction and strength. The main difficulty that arises is if the rocks has significant weathering or are overlaid with thick layers of sediments. (Shuang Liu, 2018) Brunhes in 1906 discovered in the Pliocene lavas in France that showed various directions making the magnetic fields that usually pointed north and down point in south and down. He was able to demonstrate that the baked igneous rocks were magnetized with similar polarity to the other igneous rocks. This created the baked contact test that was able to find relative ages in the areas of igneous rocks. (Neil Opdyke, 1996)

Types There are several kinds of natural remnant magnetism that can occur in a sample. Many samples have more than one kind superimposed. Thermoremanent magnetization (TRM) is acquired during cooling through the Curie temperature of the magnetic minerals and is the best source of information on the past Earth's field. Magnetization formed by phase change, chemical action or growth of crystals at low temperature is called chemical remanent magnetization. Sediments acquire a depositional remanent magnetization during their formation or a post-depositional remanent magnetization afterwards. Some kinds of remanence are undesirable and must be removed before the useful remanence is measured. One is isothermal remanent magnetization, which as a component of natural remnant magnetism induced through exposing a particle to a large magnetic field, causing the field to flip its lower coercivity magnetic moments to a field-favored direction. A commonly cited mechanism of isothermal remanent magnetization acquisition is through lightning strikes. Another is viscous remanent magnetization, a remanence acquired when the rock sits in the Earth's field for long periods. The most important component of remanence is acquired when a rock is formed. This is called its primary component or characteristic remanent magnetization. Any later component is called a secondary component. To separate these components, the natural remnant magnetism is stripped away in a stepwise manner using thermal or alternating field demagnetization techniques to reveal the characteristic magnetic component. But not "all magnetic changes resulting from mechanical shock can be removed by AF demagnetization". Marine oil-bearing sandstones are physically unstable mineralogies whose low-field susceptibility and isothermal remanent magnetization increase irreversibly, even after weak mechanical shocks and an AF demagnetization in 100 mT peak alternating fields.

Chemical remnant magnetization in magnetite

Magnetite is used for measuring the chemical remnant magnetization. Since it is grown in a magnetic field then after a certain the field is blocked hence acquiring chemical remnant magnetization. However this concept and behavior is still not well understood.(Pick, 1991) A study was also conducted exploring when magnetite went under low-temperature oxidation to a maghemite. The results showed that this was not a truly effective method die to the separation between the chemical remnant magnetization and viscous remnant magnetization that was formed in the chosen field direction was not as effective.(Gapeev,1991)

Uses Remnant magnetism specifically measures how much magnetism is left when removed from a magnetic field. This is used to get information on the "consetration, mineralogy, and grain size of the magnetic material". This provided data on the minerals that add to magnetic signal. This provided information on the minerals and where they come from, occurrence in soils, and their magnetic behavior. (Singer, 2013)

Worked examples

Example 1 — a first encounter with Natural remanent magnetization

Start with the simplest possible case. Write down what Natural remanent magnetization claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In 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 Natural remanent magnetization 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 Natural remanent magnetization 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 Natural remanent magnetization

In research: Natural remanent magnetization appears in 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 Natural remanent magnetization 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: Natural remanent magnetization is common in secondary-school and first-year university syllabi. It links to neighbouring topics Ferromagnetism, Stratigraphy, so understanding it makes those chapters shorter.
In everyday life: Look for Natural remanent magnetization 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 Natural remanent magnetization in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Natural remanent magnetization 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 Natural remanent magnetization out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Natural remanent magnetization in simple terms?

Why does Natural remanent magnetization matter?

Because it connects several 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 Natural remanent magnetization?

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 Natural remanent magnetization.