Magnetic Anomalies

deviations in values of a magnetic field on the surface of the earth from the normal values—that is, the values that characterize the geomagnetic field over an area substantially exceeding the area of the magnetic anomaly. Magnetic anomalies are depicted on maps by means of lines that connect points with equal values for some element of the earth’s magnetism (declination, by isogonic lines; inclination, by iso-clinic lines; and intensity of one of the components or a resultant vector, by isodynamic lines).

Magnetic anomalies are divided into continental, regional, and local, according to the size of the territory they cover. Continental anomalies spread over areas of 10,000-100,000 sq km. The normal field for them is the field of a uniformly magnetized sphere (a dipole field). According to current ideas, they are associated with characteristics of the movement of matter in the core of the earth (that is, they are part of the main geomagnetic field). The largest continental magnetic anomalies are in East Siberia and in the Sunda Islands.

Regional magnetic anomalies, which cover areas of 1,000-10,000 sq km, are caused by characteristics of the structure of the earth’s crust (primarily the crystalline foundation) and stand out against the background of the main geomagnetic field (dipole field + continental magnetic anomaly). They are known on the Siberian and East European platforms.

Local magnetic anomalies cover areas from a few square meters to hundreds of square kilometers. They are caused by irregularities in the structure of the upper parts of the earth’s crust or characteristics of magnetized rock (for example, as a result of a lightning strike). Local anomalies are often associated with mineral deposits; therefore, their study by means of magnetic prospecting is of great practical importance.

Magnetic anomalies are observed most intensively in areas where iron ores and other iron-containing rocks occur (for example, the Krivoi Rog and Kursk magnetic anomalies are defined by beds of ferruginous quartzites, and the magnetic anomalies in the region of Mount Magnitnaia in the Urals and Kiirunavaara in Sweden are related to deposits of magnetite).