Page 91 - Global Tectonics

78 CHAPTER 4

away from the ridge, driving the lithosphere laterally in osphere forms, the crust on either side of the ridge
the same direction by viscous drag on its base, and would consist of a series of blocks running parallel to

finally descended back into the deep mantle at the the crest, which possess remanent magnetizations that
ocean trenches, assisting the subduction of the litho- are either normal or reversed with respect to the geo-

sphere. This possible mechanism will be discussed more magnetic field. A ridge crest can thus be viewed as a
fully in Section 12.7. twin-headed tape recorder in which the reversal history
of the Earth’s magnetic field is registered within oceanic

crust (Vine, 1966).
4.1.5 The Vine–Matthews The intensity of remanent magnetization in oceanic

basalts is significantly larger than the induced magneti-
hypothesis zation. Since the shape of a magnetic anomaly is gov-
erned by the orientation of its total magnetization
It is perhaps surprising to note that magnetic maps of vector, that is, the resultant of the remanent and induced
the oceans showing magnetic lineations (Section 4.2) components, the shapes of magnetic lineations are
were available for several years before the true signifi - effectively controlled by the primary remanent direc-
cance of the lineations was realized. The hypothesis of tion. Consequently, blocks of normally magnetized
Vine & Matthews (1963) was of elegant simplicity and crust formed at high northern latitudes possess a mag-
combined the notion of sea floor spreading (Section netization vector that dips steeply to the north, and the

4.1.4) with the phenomenon of geomagnetic fi eld rever- vector of reversely magnetized material is inclined
sals (Section 4.1.3). steeply upwards towards the south. The magnetic
The Vine–Matthews hypothesis explains the forma- profi le observed over this portion of crust will be char-
tion of magnetic lineations in the following way. New acterized by positive anomalies over normally magne-

oceanic crust is created by the solidification of magma tized blocks and negative anomalies over reversely
injected and extruded at the crest of an ocean ridge magnetized blocks. A similar situation pertains in high
(Fig. 4.5). On further cooling, the temperature passes southern latitudes. Crust magnetized at low latitudes
through the Curie point below which ferromagnetic also generates positive and negative anomalies in this
behavior becomes possible (Section 3.6.2). The solidi- way, but because of the relatively shallow inclination of
fied magma then acquires a magnetization with the the magnetization vector the anomaly over any particu-

same orientation as the ambient geomagnetic fi eld. The lar block is markedly dipolar, with both positive and
process of lithosphere formation is continuous, and negative components. This obscures the symmetry of
proceeds symmetrically as previously formed litho- the anomaly about the ridge crest, as individual blocks
sphere on either side of the ridge moves aside. But, if are no longer associated with a single positive or nega-

the geomagnetic field reverses polarity as the new lith- tive 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).