Electric conductivity anomaly in the Earth's crust and mantle

Details Electric conductivity anomaly in the Earth's crust and mantle Electric conductivity anomaly in the Earth's crust and mantle

Feb 1971

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1971esrv....7...35r&link_type=abstract

Earth Science Reviews, Volume 7, Issue 1, p. 35-65.

Physics

Geophysics

5

Scientific paper

Much evidence that reflects anomalous distributions of electrical conductivity within the earth's crust and mantle has been put forward in recent years through geomagnetic and geoelectric observations. This anomaly is called the “CA” (conductivity anomaly), and the study of such heterogeneity has become one of the important topics of present-day geophysics. The existing CA's can be classified into two types. One is the “high-cut ΔZ type” which, as was first discovered at Mould Bay in the Canadian Arctic, is characterized by strong attenuation of the vertical magnetic variations of short period. The other is the “directional variation type” such as has been found in Japan. It seems likely that the former CA occurs over the surface of an unusually raised mantle layer of high conductivity and that the latter takes place over the areas where the top-surface of such a layer dips down into the deeper interior. Some of the undulations of the conducting layer are successfully correlated to those of high-temperature isotherms. It seems sometimes hard to ascribe the cause of a CA only to high underground temperatures. It is necessary in such cases, in order to be able to presume an extremely high conductivity, to assume partial melting, Fe-rich composition and hydration, although these points have not been worked out experimentally. The importance of electric currents, excited by a geomagnetic variation probably in a conductor such as an ocean, or in a water-saturated sedimentary layer has recently been pointed out. Such a “conduction effect type” mechanism may provide the third type of CA. The conduction effect seems to play an important role on the CA's found in north Germany, at Alert in the Canadian Arctic and at Eskdalemuir in Scotland. As has been the case in Japan, the western U.S.A. and Canada and so on, it is extremely useful to carry out multi-disciplinary geophysical studies over a CA area. Especially, studies of heat flow, seismic wave velocities and attenuation, and aeromagnetic and gravity surveys are useful for constructing a unified view of the underground structure.

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