Geoscience Reference
In-Depth Information
80
21
20
60
19
14
40
17
15
18
20
16
0
0
400
800
1200
1600
Figure 3.13.
(a) Geographic locations of the drilling sites on Leg 3 of the Deep Sea
Drilling Project. Site numbers refer to the order in which the holes were drilled.
(b) The age of the sediment immediately above the basalt basement versus the
distance of the drill site, as shown in (a), from the axis of the Mid-Atlantic Ridge.
(From Maxwell
et al
.(1970).)
follows in the same way as that in which the remanent magnetization of continen-
tal lavas gives their magnetic latitude (Eq. (3.17)). The magnitude and direction
of the remanent magnetization of oceanic crust depend on the latitude at which
the crust was formed and are unaffected by later movements and position. This
means that, although the magnetic anomaly resulting from this magnetization of
the crust is dependent on the present location of that piece of crust, it can be used
to determine the original latitude and orientation of the mid-ocean ridge, though
not the longitude.
Imagine a mid-ocean ridge spreading symmetrically, producing infinitely long
blocks of new crust (Fig. 3.16). The magnetic field measured above any block will
include a contribution from the permanent magnetization of the block. Suppose
first that, at the time the block was formed, the ridge was at the equator, where
the magnetic field is north-south and has no vertical component (Eq. (3.9)), and
was spreading east-west. In this case, the permanent magnetization of the block
would be along the block,
M
(0,
M
y
, 0), and so the magnetic field lines cannot
leave the block (the block being infinitely long). It is therefore impossible for the
magnetic field of this block to affect the magnetic field outside the block. Such a
block would not produce an anomaly!
=
