Geology Reference
In-Depth Information
and which correlated with the field reversal chronology established on land. These
magnetic stripes provided strong and startling evidence in favour of seafloor spread-
ing. They also opened the prospect of assigning ages to vast areas of the sea floor
on the basis of the reversal sequence, which was rapidly correlated from ocean to
ocean.
We should reflect on the magnitude of that last paragraph. Assigning ages to
rocks always has been and still is a central occupation of geologists. It is painstaking
work, whether the method is correlation of fossils or measurement of radioactive
decay. It took much of the twentieth century to develop the ability to get reliable
ages accurate to within a few per cent or less for many kinds of rocks. As Menard
remarked ([7], p. 212):
To general astonishment, magnetic reversals provide the long-sought global stratigraphic
markers that are revolutionising most of geology. At sea, as though by a miracle, magnetic
anomalies give the age of the sea floor without even collecting a sample of rock.
3.4 Evidence for motion - seismology
Seismology had already provided a key piece of evidence even before Wilson
conceived of transform faults, as will be reiterated shortly. The Lamont (now
Lamont-Doherty) Geological Observatory of Columbia University in New York
state, directed by Maurice Ewing, had pioneered the exploration of the Atlantic
sea floor, and then of other oceans. After Dietz' paper on seafloor spreading,
Ewing turned much of the effort to testing the hypothesis. Part of this programme
was to study the earthquakes in oceanic regions, and it was already known that
these occur mainly on mid-ocean ridges. By 1963 there was a better distribution of
modern seismographs around the world, including the newly deployed World-Wide
Standardised Seismograph Network. This permitted earthquakes in remote regions
to be located with an accuracy about ten times better than previously.
Lynn Sykes, working at Lamont, found that the earthquakes are located within
a very narrow zone along the crests of mid-ocean ridges, and along the joining
segments of fracture zones, where these were known or could be inferred [31, 32].
He made the explicit point that earthquakes on fracture zones occur predominantly
on the segments that connect segments of ridge crest, and hardly at all on segments
beyond ridge crests (Figure 3.6). This had been very puzzling when it was thought
that fracture zones had offset ridges by motion along the length of the fracture
zone. However, it was explicitly predicted by the transform fault concept, and was
noted (barely) by Wilson as evidence in its favour (Figure 3.3).
When Sykes saw the evidence of his colleagues for symmetric magnetic anoma-
lies, he was convinced of seafloor spreading, but realised that he could make another
decisive test through seismology. The elastic waves emitted by an earthquake have
