Marine magnetic anomalies

Magnetic surveying is easily accomplished, and measurements have been carried out from survey vessels since the mid 1950s both on specific surveys and routinely on passage to the locations of other oceano-graphic investigations.

A most significant magnetic anomaly map (Fig. 4.1) was constructed after detailed surveys off the western seaboard of North America (Mason & Raff, 1961; Raff & Mason, 1961). The magnetic field was shown to be anything but uniform, and revealed an unexpected pattern of stripes defined by steep gradients separating linear regions of high amplitude positive and negative anomalies. These magnetic lineations are remarkably persistent, and can be traced for many hundreds ofkilometers. Their continuity, however, is interrupted atmajor oceanic fracture zones, where the individual anomalies are offset laterally by distances of up to 1100 km.

Subsequent surveys have shown that magnetic linea-tions are present in virtually all oceanic areas. They are generally 10-20 km wide and characterized by a peak-to-peak amplitude of 500-1000 nT. They run parallel to the crests of the mid-ocean ridge system (Chapter 6), and are symmetrical about the ridge axes (Fig. 4.2).

The source of these linear magnetic anomalies cannot be oceanic layer 1, which is made up of nonmagnetic sediments. They cannot originate at a depth corresponding to layer 3 as sources solely within this layer would be too deep to generate the steep anomaly gradients. The source of the anomalies must therefore be, at least in part, in oceanic layer 2. This conclusion is consistent with the basaltic composition of layer 2 determined by dredging and drilling (Section 2.4.6), since basalt is known to contain a relatively high proportion of magnetic minerals. The magnetic

Marine Magnetic Anomalies

Figure 4.1 Magnetic anomaly lineations in the northeastern Pacific Ocean. Positive anomalies in black; also shown are the oceanic fracture zones at which the lineations are offset (after Menard, 1964, with permission from the estate of the late Professor H. William Menard).

lineations therefore confirm that layer 2 is everywhere composed of this rock type.

If magnetic lineations are generated by a layer of homogeneous composition, how do the magnetic contrasts originate that are responsible for the juxtaposition of large positive and negative magnetic anomalies? The shape of a magnetic anomaly is determined by both the geometric form of the source and the orientation of its magnetization vector. Oceanic layer 2 maintains a relatively constant depth and thickness. Any anomalies arising because of rugged topography on the top of the layer would attenuate too rapidly to account for the amplitude of the anomalies observed on the surface 3-7 km above the seabed. Consequently, the lineations must arise because adjacent blocks of layer 2 are magnetized in different directions. Figure 4.3 shows an interpretation of magnetic anomalies observed over the Juan de Fuca Ridge in the northeastern Pacific. Layer 2

has been divided into a series of blocks running parallel to the ridge crest which have been assigned magnetization vectors which are either in the direction of the ambient geomagnetic field or in the reversed direction. The interpretation shows that the observed anomalies are simulated by a model in which the intensities of the magnetization vary, and that relatively high values of some 10 A m-1 are required to produce the necessary contrasts.

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