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GMEG - Geophysics Unit of Menlo Park, CA- (GUMP)

 

Geophysics Unit of Menlo Park, CA (GUMP)

U.S. Geological Survey - Western Region - Geology and Geophysics

USGS Rock- and Paleo-magnetics Laboratory -- What we do


paleomag building

 

The U.S. Geological Survey's Menlo Park Rock Magnetic Laboratory is a facility dedicated to the study of past variations of the Earth's magnetic field and the magnetic properties of rocks. Scientists affiliated with the laboratory work to gain a better understanding of the Earth's field by studying the magnetic record contained in rocks and archeologic materials. The knowledge gained can then be applied toward the solution of various geologic problems including the mapping of active volcanoes and correlation of ancient earthquake deposits to better understand the related hazards, aiding in the determination of young geologic materials, and deducing the tectonic histories of structurally complex regions.

One of the more intriguing aspects of the geomagnetic field is its ability to reverse its polarity. It has been found that the north and south magnetic poles have switched places repeatedly throughout Earth's history and many of these reversals have been accurately dated. Research in the laboratory led to the development of the geomagnetic polarity time scale, which documents these reversals. The time scale provides a means of establishing the age of young geologic materials, including volcanic rocks, sedimentary deposits, and deep-sea cores. It has been used to date changes in the marine fossil record, the on-land mammalian fossil record, and was one of the means by which ages of early man sites in Africa were determined. The time scale also was used to calibrate marine magnetic anomaly time scales and is a valuable tool for establishing correlations between geologic formations.

In addition to the long-term geomagnetic field behavior that produces the polarity signal, significant short-term variations also occur. This latter behavior, occurring on timescales of hundreds of years, is known as paleosecular variation (PSV). PSV is responsible for the large magnetic anomalies that exist in certain areas of the world and accounts for the gradual movement of the magnetic poles. The north magnetic pole, for instance, has moved more than 500 miles since its position was first located in 1831. The paleosecular variation record contained in rapidly deposited sediments serves as an important correlation tool. Rapid changes in magnetic field direction (less than 0.5 degrees to more than 20 degrees per century) allows the differentiation of individual lava flows and archeological artifacts with a resolution that is unattainable with radiometric dating. The secular variation signal provides a way to estimating recurrence interval and duration of volcanic eruptions, and is an important consideration in the assessment of volcanic hazards.

Although the magnetic poles do drift on a short-term basis due to secular variation, averaging their positions over a period of several thousand years yields a mean direction corresponding to the geographic poles (the Earth's spin axis). By determining the average magnetization of rock sequences on the stable parts of a continent for a given geologic period, an apparent polar wander (APW) path can be constructed for that continent. Magnetic directions in rocks from less stable parts of a landmass, such as the Pacific Northwest, can be compared with the appropriate reference paleomagnetic poles from the APW to determine their rotation and/or translation relative to the continent as a whole. Poineering research in the laboratory led to the realization that many rock assemblages ("suspect terranes") along the Pacific margin have been transported great distances from their place of origin. Because the Earth's spin axis changes little, if at all, an APW path also serves to document a continent's travels over geologic time. By comparing the APW paths from different continents, one can determine their past positions relative to one another.

The intensity of the geomagnetic field varies on time scales comparable to that of directional secular variation described above. Because the geomagnetic field provides shielding against incoming cosmic rays, its strength determines the amount of this radiation that reaches the upper atmosphere. Reactions with these cosmic rays produces radioactive isotopes of certain elements such as 10Be, 14C, 36Cl, 3He, and others that are useful for dating and correlating geologic materials. Fluctuations in magnetic field strength, however, determine the amount of nuclides produced at any given time and uncertainties in production rates are a major factor affecting the accuracy of age determinations. By accurately determining geomagnetic paleointensity through time, the production rates of the cosmogenic nuclides can be established more closely, thus enhancing the various dating methods.

A measure of how well a material acquires an induced magnetization in response to a magnetic field is known as that material's magnetic susceptibility. This property can be very useful for geologic correlations. In a sedimentary sequence or core, for example, magnetic susceptibility will vary largely as a consequence of the amount of magnetic material present. A record of the variation in magnetic susceptibility with depth in a core can then serve as a "fingerprint" that can be compared with other cores or sequences. This method can be very important in those materials that are too young to have recorded a polarity signature and where the secular variation signal may not be well expressed or is ambiguous. In certain areas, the magnetic susceptibility record has been found to be a proxy for the climatic record.

Anisotropy of magnetic susceptibility (AMS), where susceptibility of a rock varies according to the direction of the applied field, has applications for geologic studies. Although the susceptibility differences are quite small, they are easily measured. The minimum susceptibility direction has been shown to be perpendicular to the surface of lava flows and welded tuffs, and the maximum susceptibility direction to be parallel to the direction of flow. By determining the AMS at various sites within broadly distributed ash flow tuffs, researchers have been able to use the maximum susceptibility directions to determine the source areas of eruption. The same technique also can be used to determine current flow in sediments.

The Rock- and Paleo-magnetics Laboratory contains a wide variety of instruments used for the analysis of rock magnetism and provides an important resource for fulfilling many mission-related activities of the U.S. Geological Survey (USGS). The most recent addition to the laboratory is a magnetically-shielded room where experiments can be performed in an environment that blocks about 99.5% of the present-day field's intensity. This is the only magnetically-shielded room within the USGS and it allows researchers to work on many materials that previously were very difficult, if not impossible, to study. Within this room are a superconducting magnetometer capable of measuring extremely weak magnetic signals, and instruments to partially demagnetize samples using either alternating magnetic field or thermal methods. Elsewhere in the laboratory are a variety of instruments allowing the measurement of magnetic properties. Remanent magnetizations of igneous and sedimentary rocks can be measured on spinner magnetometers and partially demagnetized using alternating magnetic fields. A Rubens coil system provides dynamic shielding for a furnace array where rock samples can be heated either in very low fields (~0.2% of the intensity of the present-day geomagnetic field) or in field strengths selected by the researcher. This apparatus is one of the few instruments world-wide that can provide absolute values of geomagnetic paleointensity. Two large magnet systems are used to study induced magnetizations, and rock magnetic properties. Low-field magnetic susceptibility and anisotropy of magnetic susceptibility of rock samples also can be measured.

 

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