Page 87 - Global Tectonics

74 CHAPTER 4

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 has been divided into a series of blocks running parallel
composed of this rock type. to the ridge crest which have been assigned magnetiza-
If magnetic lineations are generated by a layer of tion vectors which are either in the direction of the

homogeneous composition, how do the magnetic con- ambient geomagnetic field or in the reversed direction.
trasts originate that are responsible for the juxtaposition The interpretation shows that the observed anomalies
of large positive and negative magnetic anomalies? The are simulated by a model in which the intensities of the
shape of a magnetic anomaly is determined by both the magnetization vary, and that relatively high values of
−1
geometric form of the source and the orientation of some 10 A m are required to produce the necessary
its magnetization vector. Oceanic layer 2 maintains a contrasts.
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 4.1.3 Geomagnetic reversals
amplitude of the anomalies observed on the surface
3–7 km above the seabed. Consequently, the lineations The possibility that the geomagnetic fi eld reverses
must arise because adjacent blocks of layer 2 are mag- polarity was fi rst suggested during the early part of the
netized in different directions. Figure 4.3 shows an 20th century, when it was noted that reversed magneti-
interpretation of magnetic anomalies observed over the zations were present in some rock samples, and that the
Juan de Fuca Ridge in the northeastern Pacifi c. Layer 2 low amplitudes of magnetic anomalies observed over