Past plate motions - The Solid Earth - page 57

that the idea of seafloor spreading, which had then newly been proposed by
H. H. Hess, coupled with the then recently discovered evidence that intermittent
reversals of the Earth's magnetic field have taken place, provide the answer. The
oceanic crust, which is formed along the axes of the mid-ocean ridges as mafic
material wells up, acts as a double-headed magnetic tape recorder that preserves
the past reversals of the magnetic field on each plate (Fig. 3.8(b)). The width
of magnetic stripe is determined by the speed at which 'the tape' is moving
(the half-spreading rate) and the length of time between the magnetic reversals.
Thus, while the Earth's magnetic field is in its normal polarity, a block of oceanic
crust is formed with a strong component of permanent magnetization aligned
with the normal field. When the Earth's field is reversed, new oceanic crust will
have a strong component of permanent magnetization aligned with the reversed
field. In this way a magnetically normal- and reversed-striped oceanic crust is
formed, with the stripes parallel to the ridge axis. To 'decode' a magnetic-anomaly
pattern, however, it is necessary to know either when the Earth's field reversed,
or the half-spreading rate of the ridge.
Example: variability of marine magnetic anomalies
Decoding magnetic anomalies is not as simple in practice as it sounds, partly
because the magnetization of the oceanic crust does not conform to a perfect block
model. The lava flows that make up the magnetized layer are not produced
continuously along the ridge axis. Rather, they are extruded randomly both in time
and in space within an emplacement zone that is centred on the ridge axis. The
effect of this is shown in Fig. 3.9. The simple block model (Fig. 3.8)giveswaytoa
much more complex structure as the width of the emplacement zone is increased.
The variability in magnetic anomalies increases accordingly. An emplacement zone
10 km across, which seems to be appropriate for the Mid-Atlantic Ridge, explains
the variability of Atlantic magnetic anomalies. Pacific magnetic anomalies are much
less variable, partly because the emplacement zone seems to be narrower but mainly
because the much faster spreading rate means that the polarity reversals are much
more widely spaced.
Nevertheless, the block model for the magnetization of oceanic crust is widely
used. All the anomalies shown in the rest of this chapter have been calculated for
block models.
By 1966, researchers had established a reversal timescale extending back
some 4 Ma by using potassium-argon isotopic dating (see Section 6.7)tofita
timescale to the magnetic-reversal sequence that had been measured in conti-
nental lava piles and on oceanic islands. This timescale of reversal was then
used to interpret oceanic anomalies by calculating theoretical magnetic anoma-
lies for assumed spreading rates and latitudes. Figure 3.10 shows the theoretical