Plate tectonics - Introducing Tectonics, Rock Structures and Mountain Belts - page 11

When this continental reconstruc-
tion was examined, many geologi-
cal features shared by the separated
continents could be explained. For
example, similar assemblages of fossil
land animals and plants that existed
over 200 million years ago, prior to the
splitting up of the supercontinents, are
found in the various separate conti-
nents that resulted from the split. The
existence of the same land animals and
plants in widely separated continents
is almost impossible to explain (how
did they cross the oceans?) and con-
trasts with the obvious differences that
exist between the assemblages that
characterise these continents now.
Another telling piece of evidence is
the presence of glacier-derived clays
and glacial striations in rocks of late
Palaeozoic age (~300 Ma old) in all the
Gondwana continents, which in their
present positions cover about half the
globe, but when restored to their pre-
sumed Gondwana fit, make a reasona-
bly-sized polar ice cap (Figure 3.2). The
distribution of other climatic indicators
in these 300-200 Ma-old rocks also
makes sense when in the Gondwana
fit; these include dune-bedded sand-
stones and evaporite deposits, which
mark out two desert belts on either side
of a central equatorial belt indicated
by the presence of coal deposits and
coral reefs, indicators of tropical condi-
tions. The distribution of these climatic
indicator rocks makes no sense in their
present locations; for example, coals
representing the product of equatorial
forests now lie near the North Pole, and
glacial deposits lie near the equator!
Wegener's ideas caused consider-
able debate among the geological
community, failing to obtain universal

acceptance mainly because of the lack
of a convincing mechanism for the
movements. Physicists, in particular,
opposed continental drift because
their calculations of the strength of the
Earth's crust 'proved' that it was inca-
pable of the type of behaviour required.
However, work on radioactivity led
Arthur Holmes in 1931 to demonstrate
that the Earth must be much hotter, and
therefore much weaker, than previously
thought, and suggested that the mantle
could be capable of transferring heat
by slow flow in the solid state by means
of convection currents. Such mantle
currents, it was thought, could carry
continents laterally across the Earth's
surface. Debate carried on, however,
until the 1960s when work on palaeo-
magnetism (the magnetic directions of
old rocks) showed that the positions of
magnetic north for 200 million-year-old
rocks in different continents plotted
in different places. However, when
the continents were fitted together

in their presumed original positions,
the locations of magnetic north poles
coincided (Figure 3.3). This was con-
vincing proof that the continents had
drifted to their present positions from
their previous locations in the super-
continent 200 million years ago.

3

10 11

The ocean floor - static or
mobile?
The next stage in the evolution of ideas
came from studies of the ocean floor,
where mapping by various remote-sens-
ing techniques had revealed a topog-
raphy that was as varied as that of the
continents. As described in the previous
chapter, the generally even ocean floor
is interrupted by a system of great ridges
and deep, narrow trenches (Figure 2.2).
Much of the objection to Wegener's
ideas on continental drift had centred
on the failure to visualise how a conti-
nent could plough across static ocean
crust. However, this objection was coun-
tered by the proposal that the ocean

A B

North
American
pole

European
pole

combined
pole

North
America

NP

Asia

Europe

North
America

Europe

America

Africa

Figure 3.3 Palaeomagnetic evidence for continental drift. A. Polar projection of the northern hemisphere
showing the different positions of the north magnetic pole for 200-year-old rocks from Europe and
North America. B. When the continents of North America and Europe are fitted together in the positions
occupied in the Pangaea reconstruction, the magnetic north poles coincide. NP, north pole. Based on
McElhinny (1973).