Rock magnetism

(geophysics) The natural remanent magnetization of igneous, metamorphic, and sedimentary rocks resulting from the presence of iron oxide minerals.

The permanent and induced magnetism of rocks and minerals on scales ranging from the atomic to the global, including applications to magnetic field anomalies and paleomagnetism. Natural compasses, concentrations of magnetite (Fe₃O₄) called lodestones, are one of humankind's oldest devices. W. Gilbert in 1600 discovered that the Earth itself is a giant magnet, and speculated that its magnetism might be due to subterranean lodestone deposits. Observations by B. Brunhes in 1906 that some rocks are magnetized reversely to the present Earth's magnetic field, and by M. Matuyama in 1929 that reversely and normally magnetized rocks correspond to different geological time periods, made it clear that geomagnetism is dynamic, with frequent reversals of north and south poles. Nevertheless, permanent magnetism of rocks remains important because it alone provides a memory of the intensity, direction, and polarity of the Earth's magnetic field in the geological past. From this magnetic record comes much of the evidence for continental drift, sea-floor spreading, and plate tectonics. See also Continental drift; Geomagnetism; Magnet; Magnetism; Paleomagnetism; Plate tectonics.

The magnetism of rocks arises from the ferromagnetism or ferrimagnetism of a few percent or less of minerals such as magnetite. The magnetic moments of neighboring atoms in such minerals are coupled parallel or antiparallel, creating a spontaneous magnetization M_S. All magnetic memory, including that of computers, permanent magnets, and rocks, is due to the spontaneous and permanent nature of this magnetism. Spontaneous magnetization requires no magnetic field to create it, and cannot be demagnetized.

The magnetism can be randomized on the scale of magnetic mineral grains because different regions of a crystal tend to have their M_S vectors in different directions. These regions are called magnetic domains. Grains so small that they contain only one domain (single-domain grains) are the most powerful and stable paleomagnetic recorders. Larger, multidomain grains can also preserve a paleomagnetic memory, through imbalance in the numbers, sizes, or directions of domains, but this memory is more easily altered by time and changing geological conditions. See also Ferrimagnetism; Ferromagnetism; Magnetism; Magnetite; Magnetization; Magnetometer.

Rock magnetism is the study of the magnetic properties of rocks. Associated with paleomagnetism, the purpose of rock magnetism is to understand how magnetic properties of rocks affect the paleomagnetic data recorded by those rocks. Hysteresis, magnetic susceptibility and FORC diagrams (First Order Reversal Curve) are examples of a few tools and properties used in rock magnetism.

External links

• UC Davis FORC Group, Introduction to FORC Diagrams

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