Past plate motions - The Solid Earth - page 67

is always there: Anomalies from a ridge striking − α could also have come from

a ridge striking .

The thickness of the magnetized blocks that give rise to the oceanic magnetic
anomalies is not well determined. In general, the anomalies depend primarily
on the product of the magnetic susceptibility and layer thickness (i.e., 400 m of
material with a susceptibility of 0.05 gives rise to almost the same magnetic
anomaly as 2 km of material with a susceptibility of 0.01). At one time it was
thought that the magnetized layer was only the very top of the oceanic crust (then
termed layer 2A, discussed in Section 9.2.1)but this is not now thought necessarily
to be the case. The axial magnetization is generally greater than magnetization of
old oceanic crust. This high-amplitude axial anomaly/magnetization is probably
caused by highly magnetized extrusive basalts. The high magnetization of young
basalts of 20-30 A m − 1 decays rapidly within the first few million years. This
initial decay is then followed by a slow long-term decay in amplitude with age
until magnetization of 3-6 A m − 1 is reached. This later decay may be due to low
temperature and hydrothermal oxidation of the magnetitic mineral magnetite.
Over old oceanic crust the main source of magnetic anomalies may be deeper
gabbros, rather than the uppermost crust.

+ α

3.3 Reconstruction of past plate motions
3.3.1 Introduction
A magnetic-anomaly profile can be used to construct a magnetized block model
of the ocean crust and to estimate the latitude and orientation of the mid-ocean
ridge which produced it. When several profiles are available and the magnetic
anomalies are plotted on a map, as in Fig. 3.7, such a block model is easily
visualized. To determine the past movements of plates, a substantial amount of
palaeomagnetic data is required. This data collection began with the development
of magnetometers, by Bell Telephone Labs and Gulf Oil, to detect submarines.
The instruments were required to measure magnetic fields to about 1 nT (about
one part in 50 000), which was ideal for measuring marine magnetic anomalies.
Since the oldest oceanic lithosphere is Jurassic (

160 Ma old), magnetic-
anomaly data can only be used to trace the past motions of the plates back to that
time. Continental magnetic data and other geological data provide evidence for
motions of the plates prior to the Jurassic, but the data are necessarily sparser
and more difficult to interpret. The remainder of this chapter provides sections on
the geological histories of the Pacific, Indian and Atlantic Oceans as established
by deciphering magnetic anomalies. The full reconstruction of the geological
history of an ocean is possible only if the ocean contains only ridges. For oceans
such as the Indian and Pacific, in which subduction has taken place, there has
been a loss of information, which prevents a full reconstruction. Deciphering
the geological histories of the oceans, especially the early work by McKenzie
and Sclater on the Indian Ocean, was one of the triumphs of plate tectonics.

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