Creation of Magnetism in Rocks
- Sylvia Rose
- 3 days ago
- 4 min read
Rocks hold within them hidden forces. While not all rocks are obviously magnetic, most carry a subtle magnetic field. It comes from their igneous creation or long exposure to Earth's magnetism.
Magnetism comes from movement of electric charges. In atoms, electrons spin around the nucleus with "moments" of magnetism. When moments align in a specific direction, they add to the rock's overall magnetism.
Minerals in rocks can generate or respond to magnetic fields. Types of magnetism include ferromagnetism, paramagnetism, and diamagnetism.
1. Ferromagnetic Minerals
The most important minerals for rock magnetism are ferromagnetic, a class characterized by the ability to retain a permanent magnetic field even if the external magnetic field is removed. The most common include:
Magnetite (Fe3O4)
This iron oxide is the most strongly magnetic mineral found in rocks. Its cubic crystal structure enables alignment of electron spins, creating a strong magnetic moment.
Magnetite is among the most powerful magnetic minerals on Earth. With a high iron content, magnetite can retain its magnetic properties even after external magnetic fields are removed.
It is commonly found in igneous, sedimentary, and metamorphic rocks. About 10% of the Earth's crust is composed of magnetite. This mineral contributes to magnetic characteristics of many rocks.
Maghemite (γ-Fe2O3)
A closely related mineral to magnetite, maghemite is formed from magnetite through oxidation.
Pyrrhotite (Fe(1-x)S)
Pyrrhotite, an iron sulfide mineral, can show weak magnetic properties due to its unique crystal structure. Its magnetism varies based on composition, especially in volcanic or metamorphic rocks.
This mineral is golden or bronze and turns black on oxidization. It may contain gems like spinels. It's the second most common magnetic rock after magnetite.
Hematite (α-Fe2O3)
Hematite is also an iron oxide. While less magnetic than magnetite, hematite is common and can carry a weak magnetic signal especially in sedimentary rocks.
It can be magnetized under particular conditions, as during the cooling of magma or formation through biological processes. In sedimentary rocks hematite forms from oxidation of iron-rich minerals, describing ancient climate conditions.
2. Rock Magnetization
The way ferromagnetic minerals acquire magnetic properties depends on the rock's formation history and the environmental magnetic field present at the time.
Thermo-Remanent Magnetization (TRM)
This is the most important process for igneous rocks. As molten rock cools, the ferromagnetic minerals align themselves with Earth's magnetic field.
Below a specific temperature called the Curie temperature (around 578°C for magnetite), this alignment becomes "locked in." This preserves a record of the magnetic field's direction and intensity at the time of cooling.
Volcanic rocks which can retain their magnetic signatures for over 250 million years.
Chemical Remanent Magnetization (CRM)
This happens when ferromagnetic minerals crystallize from solution or through chemical alteration at relatively low temperatures. As the crystals grow, they align themselves with the ambient magnetic field.
They retain that magnetization even after the field changes. This is especially important in sedimentary rocks and products of weathering.
Detrital Remanent Magnetization (DRM)
Common in sedimentary rocks, DRM occurs when magnetic grains, like magnetite, are transported by water or wind and settle into place. As they settle, they align themselves with the Earth's magnetic field. Alignment may not be perfect due to turbulence and grain interactions.
Viscous Remanent Magnetization (VRM)
This is a weaker form of magnetization occurring over long periods of time due to thermal fluctuations at room temperature. It can slowly overprint the original magnetic signal, important when interpreting older rock samples.
3. Factors Influencing Rock Magnetism
Mineral Composition: The type and amount of ferromagnetic minerals present are the most significant factors. Rocks rich in magnetite tend to be more strongly magnetic than those containing only trace amounts.
Grain Size: Smaller grain sizes generally lead to more stable magnetization.
Temperature: High temperatures can demagnetize rocks, erasing the historical magnetic record.
Stress: Stress can also affect the magnetic alignment of grains, potentially altering or erasing the original magnetic signal.
Time: Over geological timescales, chemical alteration and other processes can affect the magnetic properties of rocks.
In addition to mineral composition and magnetization processes, geological activities significantly affect rock magnetism. Tectonic activity shapes and modifies rocks.
Plate Tectonics
The shifting of tectonic plates can change existing rocks, sometimes reorienting magnetic grains. When tectonic plates collide, they generate high pressure and temperature, triggering the metamorphic process.
Volcanic Activity
Volcanic eruptions, especially those with lava rich in basalt, create new rocks full of magnetic minerals. As the volcanic rocks cool, they capture the Earth’s magnetic field, giving insight into historical magnetic orientations.
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