Past plate motions - The Solid Earth - page 64

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300

Ma

Figure 3.15. The age of the ocean floor determined using magnetic-anomaly data,
basement ages from deep-sea drilling, anomaly timescales and rotation poles and
angles. For a colour version see Plate 2. (R. D. M uller, personal communication 2004,
after Muller et al .(2002).)

Now imagine another ridge, still spreading east-west, but not at the equator.
Because, in this example (as in the previous one), the ridge is striking north-south,
the horizontal remanent magnetization is in the y direction and has no component
in the x direction (because B φ =

0). As before, magnetization in the y direction
cannot affect the magnetic field outside the block (the block being infinitely long)
and so it is only the vertical component of the remanent magnetization that affects
the magnetic field outside the block. The vertical component of the magnetic field
as measured today by a ship above the block will be either reduced or increased,
depending on whether the magnetic field at the time the block was formed was
reversed or normal. The magnetic anomalies produced by a symmetrical pattern
of such blocks from an east-west spreading ridge are symmetrical.
For all spreading directions of the ridge that are not east-west, there is a
component of the remanent magnetization in the x direction. The effect of this