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that the Earth's crust, including the ocean bottoms, is broken up into great
plates that fl oat on the molten magma that lies below them. Convective cells
In the 1960s the discovery of traces of residual magnetism in old rocks
showed Holmes' theory to be partly correct. The discovery relied on the fact
that every few hundred thousand years the Earth's magnetic fi eld switches
poles, so that the north magnetic pole becomes the south and vice versa.
When lava is vented and solidif ies, it tends to preserve the way in
which the magnetic f ield was oriented at the time of the eruption. And
it was the pattern in which this residual magnetism is preserved in the
solidif ied lava of the mid-Atlantic ridge and other great underwater
mountain ridges in the ocean basins that provided evidence for Holmes'
tectonic plate theory.
The midline of each of these oceanic ridges turns out to be fl anked by
alternating bands of residual magnetism with switched orientations. The
striped pattern of these alternating bands forms a perfect mirror image on
each side of the ridge. And the widths of the various bands themselves refl ect
perfectly the various times at which the Earth's magnetic fi eld has switched
in the past. This alternating pattern could only be explained if the ocean bot-
tom on each side of the mid-ocean ridge is being continually spread apart as
fresh fl ows of magma at the ridge accumulate.
Continuing pressure from the spreading ridges helps to push tectonic
plates apart as they fl oat on the liquid magma. The spreading plates bump up
against the margins of other plates and begin to descend beneath them. Much
of the motive force that moves the plates comes from these descending plate
margins, which pull the plates across the magma, rather than from the con-
vective cells that Holmes originally envisioned. These collisions can trigger
earthquakes and eruptions far from the original source of pressure itself, such
as the collision between the Nazca and the South American plate that threw
Darwin of his feet. Nowhere are the consequences of these collisions more
dramatic than in the world's most tectonically active region, Indonesia.
Just north of the Indonesian island of Sumatra, the Indo-Australian plate
and the immense Eurasian plate grind up against each other along a ridge of
