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at the time, which will be discussed in the next chapter, that the mantle must be
deformable to accommodate slow vertical motions of the surface. Wegener died on
the Greenland ice sheet in 1930, and his theory fell into disrepute in the anglophone
world, with the notable exceptions of Arthur Holmes [16] in the UK, Reginald Daly
[17] in the USA, Alex du Toit [18] in South Africa and Sam Carey [19] in Australia.
The idea of continental drift was revived in the late 1950s on the basis of
geomagnetic observations of apparent polar wander [20], though it did not gain
wide currency. Exploration of the sea floor after World War II led to discoveries of
features that had no equivalents on land, such as the mid-ocean ridge system and
fracture zones thousands of kilometres long. These, in turn, generated a ferment
of ideas that might explain them, among which were an expanding Earth and
continental drift [7]. Eventually the marine evidence led Hess and Dietz [6, 21]
independently to the idea of seafloor spreading. In their conception there also had
to be a complementary subduction of sea floor at ocean trenches, but the evidence
was more circumstantial at trenches.
Mantle convection was very much a part of the exploratory thinking during this
period, but it is interesting that in some ways this was a hindrance. For example,
Heezen [22] traced the Mid-Atlantic Ridge south of Africa and into the Indian
Ocean, and then perceived a problem. The ridges were inferred to be the sites of
horizontal extension of the crust, because wherever they came on land they were
extensional, such as in Iceland and around the Red Sea, and there is commonly a
trough at their crests, which is consistent with the presence of an extensional graben.
If both the Atlantic and Indian ridges are extending, then, to Heezen, Africa should
be undergoing shortening, but there is no evidence for such shortening. Heezen's
suggested resolution of this problem was that the Earth must be expanding.
There is an important lesson to be learned from looking at Heezen's logic.
Heezen, like most geologists of the time, thought of convection in terms of 'cells',
with an active upwelling on one side and an active downwelling on the other,
as depicted in Figure 3.1(a). Figure 3.1(a) also includes schematic locations of
continents and ridges. There is subduction on the western side of the Americas
that can accommodate the extension of both the East Pacific Rise (EPR) and the
Mid-Atlantic Ridge (MAR), so that part seems reasonable. However, there is no
subduction between the Mid-Atlantic Ridge and the Mid-Indian Ridge (MIR).
Therefore the extension at the ridges must be increasing the area between them.
Therefore, according to Heezen, the Earth must be expanding.
The other possibility is that the ridges are moving relative to each other, that is,
the MIR is moving away from the MAR. The reason this did not seem possible to
Heezen is that he presumed that there would be hot, active upwellings under the
ridges, and if the ridge moved it would move away from the upwelling and cease
to spread. The resolution of this puzzle is that convection does not have to take
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