Page 90 - Global Tectonics

SEA FLOOR SPREADING AND TRANSFORM FAULTS 77

rapid overturn in the outer core will ensure that the 4.1.4 Sea floor spreading

temperature at the core–mantle boundary is essentially
uniform. The inferred temperature differences in the
In the early 1960s, Dietz (1961) and Hess (1962) had
lower mantle, however, will give rise to a nonuniform
proposed that continental drift might be accomplished
distribution of heat flux at the core–mantle boundary.

by a process that Dietz termed sea fl oor spreading
Anomalously cold material near the boundary will
(Section 1.2). It was suggested that new oceanic litho-
steepen the temperature gradient and increase the heat
sphere is created by the upwelling and partial melting
flow, whereas hotter material will decrease the gradient

of material from the asthenosphere at the ocean ridges.

and heat flow. The new advances in computer simula-
As the ocean gradually grows wider with the progres-
tions of the geodynamo make it possible to explore this
sive creation of lithosphere, the continents marginal to
possibility. The initial results of such computations
the ocean are moved apart. The drift between North
(Glatzmaier et al., 1999) are very interesting and encour-
America and Europe, for example, would have been
aging in that different heat flow distributions do produce

accomplished by the gradual growth of the Atlantic
significant changes in the reversal frequency and might

Ocean over the past 180 Ma. Since the Earth is not
well explain the variations observed in Fig. 4.4.
increasing in surface area by any signifi cant amount
The results obtained from numerical simulations of
(Section 12.3), the increase in size of those oceans
the geodynamo since the mid 1990s represent remark-

growing by sea floor spreading would be balanced by
able breakthroughs in our modeling and understanding
the destruction of lithosphere at the same rate in
of the possible origin of the Earth’s magnetic fi eld.
another, shrinking, ocean by subduction at deep sea
However one has to bear in mind that, although the
trenches situated around its margins.
physical formulation of these models is thought to be
The driving mechanism of these movements was
complete, the parameters assumed are not in the range
believed to be convection currents in the sub-litho-
appropriate for the Earth. This is because the comput-
spheric mantle (Fig. 1.5). These were thought to form
ing power available is still not adequate to cope with the
cells in which mantle ascended beneath ocean ridges,
spatial and temporal resolution that would be required
bringing hot material to the surface and giving rise to
in the integrations.

new lithosphere. The flow then moved horizontally
6 Number of intervals per analysis = 50
5
Estimate of reversal rate λ 4 2 Cretaceous
3
superchron
1
0
0 20 40 60 80 100 120 140 160
Mid-age of sliding window (Ma)
Figure 4.4 Estimated frequency of geomagnetic reversals over the past 160 Ma (redrawn from Merrill et al., 1996, with
permission from Academic Press. Copyright Elsevier 1996).