Geoscience Reference
In-Depth Information
Figure 3.8.
(a) A magnetic-anomaly profile across the Pacific-Antarctic Ridge plotted
above its end-to-end reverse (mirror image) demonstrates the commonly observed
symmetry of magnetic anomalies about the ridge axis. The half-spreading rate as
determined from this profile is 4.5 cm yr
−1
. Magnetic anomalies are generally about
±500 nT in amplitude, about 1% of the Earth's magnetic field. (After Pitman and
Heirtzler (1966).)
(b)
C
C
B
B
A
A
D
D
C
C
B
B
A
A
Figure 3.8.
(b) A cross section through an idealized ridge illustrates the block model
of normally (black) and reversely (white) magnetized material. In the upper drawing,
the Earth's magnetic field is positive (normal) and has been for some time; thus, the
positively magnetized blocks C have formed. Positively magnetized blocks B and A
were formed during earlier times when the Earth's magnetic field also had normal
polarity. The lower drawing shows the same ridge at a later time; another positive
block D has formed and the magnetic field is in a period of reverse polarity.
generally symmetrical about the axes of the mid-ocean ridges. They are offset by
fracture zones, and are a few tens of kilometres in width and typically
500 nT in
magnitude (Fig. 3.8(a)). This means that magnetic-anomaly maps are very useful
in delineation of ridge axes and fracture zones.
The origin of these magnetic stripes was first correctly understood in 1963 by
F. J. Vine and D. H. Matthews and independently by L. W. Morley. They realized
±
