The Paleomagnetic Significance of Aeromagnetic Surveys of the Hawaiian Islands!

Aeromagnetic surveys of the Hawaiian Islands have revealed that the pr imary magnetic anomalies associated with the islands are dipole anomalies caused by the intrusive rocks of the volcanic centers and rift zones. Comparisons of the direction of magn etization indicated by the dipole anomalies with results of laboratory measurements on iavas show that in many cases the lavas possess reverse polarization while the intrusive rocks are normally polarized. These results must be taken into account when interpreting the magnetic field of subm erged marine volcanic features such as seamounts and when establishing periods of reversal in the earth's magnetic field. AN AEROMAGNETIC SURVEY covering the ma jor islands at the southern end of the Hawaiian chain has recently been completed with flight lines approximately 1 mile apart. The structural and geologic implications of this survey are discussed in derailby Malahoff and W oollard (in a forthcoming issue of Pacific Science ) , to which pap er the reader is referred for examination of the actual anomaly contour maps. The dominant magnetic anomalies observed over the islands were found to be positive-negative anomaly pairs-typical dipole anomalies. Such anomalies might be expected to result from bodies with near vertical sides magnetized parallel to the present earth's field, which, in this area, has an inclina tion of 30-40°. From a comparison of the location of the dip ole anomalies with the geologic and gravimetric data on the islands, it is apparent that they are caused by the intrusive rocks associated with the volcanic centers and major rift zones on the islands. Such a result is in agreement with laboratory determinations of susceptibility and intensity of remnant magnetization of some Hawaiian rocks made by the authors. These measurements show that the intensity of remnant magnetization is much great er in most intrusive rocks of the Hawaiian Islands than in the lavas. In both types of rocks the intensity of remnant magnetization greatly exceeded that of induced 1 H awaii Inst itute of Geophysics Contribution No. 100 . magnetization-by a factor of 1:10 in olivine poor samples. Several model computations showed that .it is possible to explain the observed aeromagnetic anomalies by assuming that the intrusive rocks were either normally or inversely polariz ed in a direction nearly parallel to the present earth's field. This is in agreement with measurements by Tarling (1963) of direction of remnant magnetization carried Out on surface samples, primarily lavas, which also indicated directions of remn ant magnetization nearly parallel to the present earth's field. Because the magnetic anomalies caused by the remnant magnetization of the intrusive rocks are dip ole anomalies, it is possible to determine by inspection whether the intrusive rocks are normally or inversely polarized. This gross direction of remnant magnetization ( normal or reverse ) is given in Table 1, along with the results obtained by McDougall and Tarling ( 1963) , and Doell and Cox ( 1963), and measurements made by the writ ers on surface samples of both extrusive and intrusive rocks. The paleomagnetic and age dating results obtained by McDougall and Tarling ( 1963 ) from Hawaiian lavas have been utilized by Cox, Doell, and Dalrymple ( 1964) in conjunction with data from other areas to establish alternating periods of norm ality and reversal in the earth's magnetic field during the last four million years. The reality of these reversals, their leng th (i f they exist ) , and the