The Vine Matthews hypothesis

It is perhaps surprising to note that magnetic maps of the oceans showing magnetic lineations (Section 4.2) were available for several years before the true significance of the lineations was realized. The hypothesis of Vine & Matthews (1963) was of elegant simplicity and combined the notion of sea floor spreading (Section 4.1.4) with the phenomenon of geomagnetic field reversals (Section 4.1.3).

The Vine-Matthews hypothesis explains the formation of magnetic lineations in the following way. New oceanic crust is created by the solidification of magma injected and extruded at the crest of an ocean ridge (Fig. 4.5). On further cooling, the temperature passes through the Curie point below which ferromagnetic behavior becomes possible (Section 3.6.2). The solidified magma then acquires a magnetization with the same orientation as the ambient geomagnetic field. The process of lithosphere formation is continuous, and proceeds symmetrically as previously formed lithosphere on either side of the ridge moves aside. But, if the geomagnetic field reverses polarity as the new lith-

osphere forms, the crust on either side of the ridge would consist of a series of blocks running parallel to the crest, which possess remanent magnetizations that are either normal or reversed with respect to the geomagnetic field. A ridge crest can thus be viewed as a twin-headed tape recorder in which the reversal history of the Earth's magnetic field is registered within oceanic crust (Vine, 1966).

The intensity of remanent magnetization in oceanic basalts is significantly larger than the induced magnetization. Since the shape of a magnetic anomaly is governed by the orientation of its total magnetization vector, that is, the resultant of the remanent and induced components, the shapes of magnetic lineations are effectively controlled by the primary remanent direction. Consequently, blocks of normally magnetized crust formed at high northern latitudes possess a magnetization vector that dips steeply to the north, and the vector of reversely magnetized material is inclined steeply upwards towards the south. The magnetic profile observed over this portion of crust will be characterized by positive anomalies over normally magnetized blocks and negative anomalies over reversely magnetized blocks. A similar situation pertains in high southern latitudes. Crust magnetized at low latitudes also generates positive and negative anomalies in this way, but because of the relatively shallow inclination of the magnetization vector the anomaly over any particular block is markedly dipolar, with both positive and negative components. This obscures the symmetry of the anomaly about the ridge crest, as individual blocks are no longer associated with a single positive or negative anomaly. However, at the magnetic equator, where

Figure 4.5 Sea floor spreading and the generation of magnetic lineations by the Vine-Matthews hypothesis (redrawn from Bott, 1982, by permission of Edward Arnold (Publishers) Ltd).

the field is horizontal, negative anomalies coincide with normally magnetized blocks and positive anomalies with reversely magnetized blocks, precisely the reverse situation to that at high latitudes. In addition, the amplitude of the anomaly decreases from the poles to the equator as the geomagnetic field strength, and hence the magnitude of the remanence, decreases in this direction. Figure 4.6 illustrates how the shape and amplitude of the magnetic anomalies over an ocean ridge striking east-west vary with latitude.

The orientation of the ridge also affects anomaly shape and amplitude, because only that component of the magnetization vector lying in the vertical plane through the magnetic profile affects the magnetic anomaly. This component is at a maximum when the ridge is east-west and the profile north-south, and at a minimum for ridges oriented north-south. The variation in amplitude and shape of the magnetic anomalies with orientation for a ridge of fixed latitude is shown in Fig. 4.7. In general, the amplitude of magnetic anomalies decreases as the latitude decreases and as the strike of the ridge progresses from east-west to north-south. The symmetry of the anomalies is most apparent for ridges at high magnetic latitudes (e.g. greater than 64°,

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Figure 4.6 Variation of the magnetic anomaly pattern with geomagnetic latitude. All profiles are north-south. Angles refer to magnetic inclination. No vertical exaggeration.

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Figure 4.6 Variation of the magnetic anomaly pattern with geomagnetic latitude. All profiles are north-south. Angles refer to magnetic inclination. No vertical exaggeration.

which is equivalent to geographic latitudes greater than 45°), north-south trending ridges at all latitudes and east-west trending ridges at the magnetic equator.

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