PALEOMAGNETISM
ANALYSES

Technique

   

Paleomagnetism, defined as the ancient magnetism preserved in rocks, soil, and ceramics, is useful for age- determinations and for measuring deformation. As a dating tool, paleomagnetism is frequently applied to sedimentary sections where magnetic-polarity stratigraphy can be correlated with the geomagnetic polarity time scale. This dating method is calibrated most accurately for rocks ranging in age from 0.5 m.y. to 5 m.y., although magnetic correlations involving Mesozoic and Paleozoic rocks are also possible. Fine-scale variations of magnetic-field direction and intensity make possible age-correlation of volcanic rocks and sediments younger than 30,000 years. This method is particularly useful for estimating recurrence rates of volcanic eruptions in hazard assessment. Paleointensity records for the time period from 50,000 to 10,000 years B.P. are critical for adjusting ages derived from carbon-14 and beryllium-10, because the geomagnetic-field strength modulates production of cosmogenic nuclides in the upper atmosphere.

Natural remanent magnetization, when averaged over periods exceeding 10,000 years, determines positions of the geomagnetic poles. This spatial reference frame allows estimation of deformation on scales from motion of continents down to rotation of rock outcrops.

Environmental magnetism is the study of magnetic properties in ecological systems. Magnetic susceptibility, iron-oxide chemistry, and granulometry are known to be sensitive to climatic change. Magnetic particles carried in sediment, aerosols, and organisms are useful tracers in ecological studies, such as erosion and sedimentation in watersheds, airborne pollution, or point-source contamination. When used in conjunction with traditional chemical analyses, magnetic measurements provide a quick, less-expensive means for extending ecological surveys.

The paleomagnetics staff welcome inquiries concerning possible applications of magnetic methods to your projects.

   


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URL http://geology.cr.usgs.gov/capabilities/paleom/tech.html
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