Magnetic surveys over the Hawaiian ridge and their geologic implications

Abstract

A geophysical and geological analysis is made of a total field magnetic survey of the major islands of Hawaii. It is established that the regional distortion of the earth's normal magnetic field due to the topographic mass of the Hawaiian Ridge rising in places to over 30,000 feet above the ocean floor seldom exceeds 150 gammas. On each island, local magnetic anomalies having the form of lenticular and circular dipoles are found. The lenticular dipole anomalies appear to be related to crustal rifts that have been invaded by magmatic material of mantle origin, and the circular dipole anomalies are associated with primary areas of volcanic eruption. Although the inferred crustal rifts have surface geologic expression in some areas, as the Koolau Mountains on Oahu, for the most part they do not. Furthermore, offshore magnetic data indicate these features extend beyond the islands and out into the adjacent, deep-water, oceanic area where they can be traced for one hundred miles or more. The most pronounced of these features is associated with the ocean floor Molokai Fracture Zone which magnetically extends across the Hawaiian Ridge without interruption for an unknown distance to the west. The circular dipole anomalies appear to represent the effect of intrusions in volcanic pipes rising from these crustal rifts which strike essentially east-west on the islands of Hawaii, Lanai, Maui, and Molokai, and WNW-ESE on Oahu, Kauai and Niihau. With two exceptions, all of the anomalies indicate normal polarization conformable with the present earth's field.