theory not only on the match of the South American and Africa continents at the edge
of their continental shelves, but also on matching rock types, geological structures
and fossils such as the seed plant Glossopteris, which is found across all the southern
continents. The directions of the striations on the glaciated early Permian pavement
rocks found in South America, South Africa, Australia and India showed the ice had
been moving away from a seemingly impossible source, from the oceans to the south.
Wegener then concluded that 270 million years ago these continents were joined
to form a supercontinent centred on Antarctica. He called his revolutionary theory
‘Continental Drift’ and supposed that the mechanism for this spreading might be the
centrifugal force of the earth’s rotation.
Wegener’s theory of continental drift was not accepted by many geologists because
it required the continents to move across the oceans without an obvious mechanism for
this to happen. It was not until the 1960s when seafloor magnetometer measurements
showed residual magnetic ‘stripes’ in the ocean floor extending out parallel to the
Mid-Atlantic Ridge and
forming a perfect mirror
image in both directions,
that his theory was fully
accepted.
Convection currents
within the earth’s mantle
cause upwelling along the
Mid-Atlantic Ridge and
the intrusion of magma,
forcing the older oceanic
material to move away in
both directions. As the lava
solidifies iron particles
take up the direction of
the earth’s magnetic field
at that time, which can
reverse every few hundred
thousand years. This
movement away from the
mid-ocean ridges allows
the continents to ‘drift’
apart by being rafted on
top of this very slowly
moving oceanic material.
Distribution of the fossil Glossopteris across the southern continents Unfortunately, Alfred
The Voyage of Continent Australia 13