Is Basalt Magnetic?

Basalt, a type of igneous rock, is known for its magnetic properties due to the presence of specific magnetic minerals within its composition. This comprehensive guide delves into the technical details and scientific principles behind the magnetism of basalt, providing a valuable resource for physics students and enthusiasts.

Magnetic Minerals in Basalt

The primary reason for basalt’s magnetic properties is the presence of two key magnetic minerals: magnetite (Fe3O4) and titanomagnetite (Fe2TiO4). These minerals contain ferromagnetic domains that align with the Earth’s magnetic field during the cooling process of the basalt, preserving a record of the field’s direction and intensity.

Magnetite (Fe3O4)

Magnetite is an iron oxide mineral that is strongly magnetic. It has a cubic crystal structure and is the most magnetic of all the naturally occurring minerals on Earth. Magnetite is a key component in the magnetic properties of basalt, as it can make up a significant portion of the rock’s mineral composition.

The magnetic properties of magnetite can be described by the following equation:

M = Ms * tanh(H/Hc)

Where:
– M is the magnetization of the material
– Ms is the saturation magnetization
– H is the applied magnetic field
– Hc is the coercivity of the material

Magnetite has a high saturation magnetization (Ms) of around 92 emu/g, making it a highly magnetic mineral.

Titanomagnetite (Fe2TiO4)

Titanomagnetite is a solid solution of magnetite (Fe3O4) and ulvöspinel (Fe2TiO4), another magnetic mineral. It has a similar crystal structure to magnetite and also exhibits ferromagnetic properties.

The magnetic behavior of titanomagnetite can be described by the following equation:

M = Ms * (1 - x) * tanh(H/Hc)

Where:
– M is the magnetization of the material
– Ms is the saturation magnetization of pure magnetite
– x is the mole fraction of ulvöspinel in the solid solution
– H is the applied magnetic field
– Hc is the coercivity of the material

The presence of titanomagnetite, along with magnetite, contributes to the overall magnetic properties of basalt.

Magnetic Properties of Basalt

The magnetic properties of basalt can be quantified through various measurements, including absolute paleointensity (API), magnetic susceptibility, and the concentration of magnetic minerals.

Absolute Paleointensity (API)

Absolute paleointensity (API) is a measure of the intensity of the Earth’s magnetic field recorded in the magnetic minerals of basalt during its formation. This information is valuable for understanding the evolution of the Earth’s magnetic field over geological time.

In a study of basaltic rocks from Baengnyeong Island, Korea, the Tsunakawa–Shaw (TS) method yielded 12 qualified API estimates with a mean value of 13.1 μT and a standard deviation of 1.7 μT. This indicates that the average field intensity during the Early Pliocene was around 13.1 (± 1.7) μT. The Thellier method of the IZZI protocol, combined with a bootstrap approach, gave a 95% confidence interval of 6.6‒19.7 μT for its mean API estimate, supporting the reliability of the TS-derived API mean estimate.

Magnetic Susceptibility

Magnetic susceptibility is a measure of the degree to which a material can be magnetized in an external magnetic field. Basalt has a larger magnetic susceptibility compared to other common rocks, such as andesite, volcanic breccia, and granite. This is due to the higher concentrations of magnetic minerals, primarily magnetite and titanomagnetite, present in basalt.

The magnetic susceptibility of basalt can be expressed using the following equation:

χ = M/H

Where:
– χ is the magnetic susceptibility
– M is the magnetization of the material
– H is the applied magnetic field

The higher the magnetic susceptibility of a material, the more easily it can be magnetized in an external magnetic field.

Concentration of Magnetic Minerals

The concentration of magnetic minerals, such as magnetite and titanomagnetite, within the basalt is a key factor in determining its magnetic properties. Basalt with a higher concentration of these magnetic minerals will exhibit stronger magnetic characteristics compared to basalt with lower concentrations.

The concentration of magnetic minerals can be determined through various analytical techniques, such as X-ray diffraction (XRD) or electron microprobe analysis (EMPA). These methods can provide quantitative data on the mineral composition of the basalt, including the relative abundance of magnetite and titanomagnetite.

Factors Affecting Basalt Magnetism

Several factors can influence the magnetic properties of basalt, including the cooling rate, the presence of other minerals, and the effects of shock and pressure.

Cooling Rate

The cooling rate of basalt can affect the size and distribution of the magnetic minerals within the rock. Faster cooling rates can lead to the formation of smaller, more uniformly distributed magnetite and titanomagnetite grains, which can enhance the overall magnetic properties of the basalt.

Presence of Other Minerals

The presence of other minerals in the basalt, such as hematite (Fe2O3) or pyrrhotite (Fe7S8), can also influence the magnetic characteristics of the rock. These minerals can either enhance or diminish the overall magnetic properties, depending on their concentration and magnetic behavior.

Effects of Shock and Pressure

High-pressure and shock events, such as meteorite impacts or tectonic processes, can also affect the magnetic properties of basalt. These events can cause changes in the crystal structure and magnetic behavior of the minerals, leading to alterations in the rock’s magnetic characteristics.

Applications and Implications

The magnetic properties of basalt have various applications and implications in different fields, including:

1. Paleomagnetism: The magnetic record preserved in basalt can provide valuable information about the Earth’s magnetic field history, which is crucial for understanding the planet’s geodynamics and evolution.

2. Planetary Geology: The magnetic properties of basalt on other planetary bodies, such as Mars, can be used to infer the presence and characteristics of their magnetic fields, which can provide insights into the formation and evolution of these celestial bodies.

3. Mineral Exploration: The magnetic properties of basalt can be used in geophysical surveys to aid in the exploration and identification of mineral deposits, as the presence of magnetic minerals can create detectable anomalies.

4. Archaeological Prospecting: The magnetic properties of basalt can be used in archaeological investigations to locate and study ancient structures, settlements, and other buried artifacts.

5. Material Science: The magnetic properties of basalt and its constituent minerals can be studied for potential applications in materials science, such as the development of novel magnetic materials or the optimization of existing magnetic devices.

By understanding the technical details and scientific principles behind the magnetism of basalt, physics students and enthusiasts can gain a deeper appreciation for the complex and fascinating world of geophysics and materials science.

References:

1. Paleomagnetic study of basaltic rocks from Baengnyeong Island, Korea: New absolute paleointensity data and rock magnetic analyses. Earth, Planets and Space, 2019.
2. Application of Magnetic Susceptibility Parameters in Petrology and Tectonics. IOPscience, 2017.
3. The effects of 10 to >160 GPa shock on the magnetic properties of minerals and rocks. Geochemistry, Geophysics, Geosystems, 2016.
4. Chapter 2. Magnetic Properties of Oceanic Basalts. ScienceDirect, 2008.
5. Magnetic and petrologic characterization of synthetic Martian basalts. Journal of Geophysical Research, 2009.