Geology Reference
In-Depth Information
1920s. Holmes' theory was not complete by present standards, to be sure,
but neither is existing knowledge complete.
Holmes' theory may not have been complete in the 1920s, but
by 1944 in the chapter on continental drift he had so hesitated
to include in his
Principles of Physical Geology
, his ideas come very
close to being
'the all-embracing theory which would satis-
factorily correlate all the varied phenomena for which the
earth's internal behaviour is responsible'.
But while recognis-
ing continental drift as the all-encompassing theory it would
become, he nevertheless found it necessary to caution his stu-
dents about such speculation:
'It must be clearly realised, how-
ever, that purely speculative ideas of this kind, specially
invented to match the requirements, can have no scientific
value until they acquire support from independent evidence.'
Little did he realise that he himself had already pioneered the
means by which that 'independent evidence' would be found.
During the Second World War, sensitive instruments were
developed to detect submarines by their magnetic fields, but as
they searched backwards and forwards across the oceans an
extraordinary picture of the sea floor began to emerge. Linear
bands of positive and negative magnetic anomalies, some
stretching hundreds of miles along the ocean floor, were seen
to show an almost perfect symmetry either side of mid-ocean
ridges - large 'cracks' or fissures that extended down the mid-
dle of all the great oceans. This peculiar magnetic pattern puz-
zled scientists for years, until in 1963 the startling proposal was
made that it represented bands of rock on the sea floor that were
magnetized during normal and
reversed
periods of the Earth's
magnetic field.
For some time it had been recognised that when rocks such
as basalt cooled from their molten state, the magnetic particles
within them became 'fossilised', pointing in the direction of the
Earth's magnetic field. Thus it was proposed that the stripes on
the ocean floors represented magnetised basaltic flows that had
erupted from the fissures along the mid-ocean ridges, acquiring
