168 EXPLORATION GEOPHYSICS 



20,000 amperes) give rise to relatively strong magnetic fields. Hence, 

 lightning discharges may offer an explanation for the irregular rock mag- 

 netization sometimes observed on exposed hill tops. 



Stresses. — Forces such as are active in mountain building, faulting and 

 folding affect the magnetization of rocks. Similar phenomena may be pre- 

 dicated on certain observed effects or reactions in rods or w^ires. For 

 example, stretching a rod increases its intensity of magnetization in a weak 

 field. A ferrous metal rod becomes magnetized when it is bent. There is a 

 change in intensity of magnetization in certain substances with volume 

 changes. These phenomena are examples of such reactions, and some of 

 them may be duplicated by geologic processes. Some unusual magnetic 

 anomalies which cannot be explained by differences in magnetic suscepti- 

 bilities of known formations may possibly be due to such magneto-mechan- 

 ical effects. 



Structural Movements. — Geologic activity sufficient to cause dis- 

 ruptive deformation and overturning of deposits or dikes may often pro- 

 duce abnormal magnetizations. There are cases where north magnetic poles 

 logically occur near the surface under conditions where south magnetic 

 poles should be present. This is shown after a magnetic map has been 

 contoured, when magnetic lows are found where magnetic highs would 

 normally be expected to appear. 



Disintegration. — Since trivalent iron is more magnetic than bivalent 

 iron, it follows that magnetization will be much reduced when magnetite 

 disintegrates to limonite or hematite. 



Metamorphism. — The reverse of the above is evident in contact and 

 dynamo metamorphic processes in sedimentaries and other rocks. The 

 bivalent iron is changed to the trivalent form so that accumulations of mag- 

 netic rocks are often found near intrusive bodies. 



Concentration.- — Concentration of magnetic minerals has been shown 

 to cause increase in magnetization in contact metamorphic zones and in the 

 black sand accumulations of placer deposits. Such concentrations may take 

 the form of dikes, fissure fillings, replacements, tabular veins, or irregular 

 masses. 



A study of the distribution of ferromagnetic minerals in one Keween- 

 awan lava flow in Wisconsin indicates that the magnetic minerals show a 

 concentration at definite horizons, or roughly parallel to definite planes. 

 This may well account for the linear magnetic disturbances which occur 

 parallel to the flows. As a consequence, any distortion or disruption of 

 these planes would cause a concomitant change in the magnetic anomalies 

 associated with them. 



In sedimentary rocks the concentration of ferromagnetic minerals into 

 definite horizons is also marked. In sandstone formations, magnetite-rich 



