Evidence for Sea-Floor Spreading: the Magnetic Stripes
33
What is "proof' in the geologic sciences? Can we prove that a fossil was once part
of a living organism? Can we prove that the Earth has an age of 4.6 billion years?
Can we prove that large continental glaciers once covered vast areas of North
America and northern Europe?
It all depends on what one is willing to accept as proof. Certainly, all the above
questions were once vehemently answered with NO by experts, and would be
answered with disbelief today - diesbelief that anyone would ask such a silly question.
What, then, about proof for sea-floor spreading? Since 1968, the year in which
several major articles appeared on the subject, "sea-floor spreading" has joined the
various other propositions about the Earth which are accepted as fact.
Why can we be so confident that the sea-floor spreading story is correct?
Our confidence derives from the global pattern of magnetic anomalies on the sea
floor (Fig. 1.15). For each of the observations on morphology, heat flow, seismic
activity, etc., which are so nicely explained by sea-floor spreading, it is possible to
conceive of some other way to produce the phenomenon. However, for the magnetic
anomalies no reasonable alternative to sea-floor spreading has ever been proposed -
and not for lack of trying.
The patterns, the "magnetic stripes", were first discovered by geophysicists at
Scripps Institutions of Oceanography (R. G. Mason, A. D. Raff, V. Vacquier). However, their origin remained a complete mystery for several years. One of the problems
was that the area for which they had been mapped is tectonically complicated, and the
symmetry of the patterns about the Ridge, which holds the key to the explanation, is
not obvious at all there.
The first successful attempt to account for the "stripes" was by F. J. Vine (then a
graduate student at Cambridge University) and D. H. Matthews (his advisor), in 1963.
Their suggestion was strinkingly simple: put together the ideas on sea-floor spreading
of H. H. Hess and of R. S. Dietz, and combine them with the evidence for periodic
reversals in the Earth's magnetic field, as presented by A. Cox and co-workers (in
1963). The newly upwelled, hot material at the Ridge Crest (or within the central rift)
is magnetized upon cooling below 525 0 (the Curie Point), in accordance with the
prevailing magnetic field. If this field reverses periodically, and, they said, "if spreading of the ocean floor occurs, blocks of alternately normal and reversely magnetized
material would drift away from the center of the ridge and parallel to the crest of it."
Here, in a nutshell, was the key to proving the reality of sea-floor spreading (Fig.
1.16). The sea floor could be seen to act as a tape recorder of the earth's magnetic
field.
Eventually, the magnetic anomaly sequence on each side of the ridge, in every
major ocean basin, was found to be exactly the same as in the lava flows studied (and
dated) on land.
The tape recorder is perhaps not of the finest quality, but it works remarkably well
(Fig. 1.16a). With a time scale at hand (Fig. 1.16c), we can now read off the spreading rate, simply by matching the sea-floor anomalies to the magnetic reversal scale.
Thus, we can make an age map of the sea floor (Fig. 1.17).
If the age map is correct, we should then find that the oldest sediment lying on the
basaltic substrate shows the same age progression. The deep-sea drilling ship Glomar
33
What is "proof' in the geologic sciences? Can we prove that a fossil was once part
of a living organism? Can we prove that the Earth has an age of 4.6 billion years?
Can we prove that large continental glaciers once covered vast areas of North
America and northern Europe?
It all depends on what one is willing to accept as proof. Certainly, all the above
questions were once vehemently answered with NO by experts, and would be
answered with disbelief today - diesbelief that anyone would ask such a silly question.
What, then, about proof for sea-floor spreading? Since 1968, the year in which
several major articles appeared on the subject, "sea-floor spreading" has joined the
various other propositions about the Earth which are accepted as fact.
Why can we be so confident that the sea-floor spreading story is correct?
Our confidence derives from the global pattern of magnetic anomalies on the sea
floor (Fig. 1.15). For each of the observations on morphology, heat flow, seismic
activity, etc., which are so nicely explained by sea-floor spreading, it is possible to
conceive of some other way to produce the phenomenon. However, for the magnetic
anomalies no reasonable alternative to sea-floor spreading has ever been proposed -
and not for lack of trying.
The patterns, the "magnetic stripes", were first discovered by geophysicists at
Scripps Institutions of Oceanography (R. G. Mason, A. D. Raff, V. Vacquier). However, their origin remained a complete mystery for several years. One of the problems
was that the area for which they had been mapped is tectonically complicated, and the
symmetry of the patterns about the Ridge, which holds the key to the explanation, is
not obvious at all there.
The first successful attempt to account for the "stripes" was by F. J. Vine (then a
graduate student at Cambridge University) and D. H. Matthews (his advisor), in 1963.
Their suggestion was strinkingly simple: put together the ideas on sea-floor spreading
of H. H. Hess and of R. S. Dietz, and combine them with the evidence for periodic
reversals in the Earth's magnetic field, as presented by A. Cox and co-workers (in
1963). The newly upwelled, hot material at the Ridge Crest (or within the central rift)
is magnetized upon cooling below 525 0 (the Curie Point), in accordance with the
prevailing magnetic field. If this field reverses periodically, and, they said, "if spreading of the ocean floor occurs, blocks of alternately normal and reversely magnetized
material would drift away from the center of the ridge and parallel to the crest of it."
Here, in a nutshell, was the key to proving the reality of sea-floor spreading (Fig.
1.16). The sea floor could be seen to act as a tape recorder of the earth's magnetic
field.
Eventually, the magnetic anomaly sequence on each side of the ridge, in every
major ocean basin, was found to be exactly the same as in the lava flows studied (and
dated) on land.
The tape recorder is perhaps not of the finest quality, but it works remarkably well
(Fig. 1.16a). With a time scale at hand (Fig. 1.16c), we can now read off the spreading rate, simply by matching the sea-floor anomalies to the magnetic reversal scale.
Thus, we can make an age map of the sea floor (Fig. 1.17).
If the age map is correct, we should then find that the oldest sediment lying on the
basaltic substrate shows the same age progression. The deep-sea drilling ship Glomar