116 | Great Geologists
Modern understanding of Late Cenozoic paleomagnetic reversals.
Vine and Wilson collaborated to explain the magnetic anomaly patterns seen around the Juan de Fuca Ridge off Vancouver
Island (Magnetic anomalies over a young ocean ridge off Vancouver Island, published in the journal Science in 1965). This
ridge was, according to Wilson, bounded by transform faults, one being an extension of the San Andreas Fault. Wilson
had introduced the concept of transform faults in 1965. These faults displace mid-ocean ridges and are an aspect of the
mechanism by which lateral movement (spreading) of the ocean crust is accommodated.
Vine and Wilson noted the symmetry of the magnetic anomalies on either side of the ridge and, moreover, that this pattern
could be recognised across the transform faults once the faults were restored to a pre-movement position. They next
addressed the age of the anomalies. If spreading occurred at a uniform rate, then there should be a correlation between the
widths of the normal and reversed magnetised stripes and their ages. If the ocean floor forms at 4 cm a year (2 cm on each
side of the ridge), then a strip of ocean floor represents a record of 10,000 years of the Earth’s magnetic history. These ages
should match time scales based on continental rocks, where the age of reversals had been determined.
Vine and Wilson were able to show that a predictive model of anomaly width matched closely, but not perfectly, with the
observed data. A near-perfect match was achieved after Vine, when attending the 1965 meeting of the Geological Society of
America, learnt of a new short reversal event (the “Jaramillo Event”) c. 0.9 Ma. The paleomagnetic timescale, when combined
with the assumption of a constant rate of spreading outward from an ocean ridge, precisely predicted the symmetrical pattern
of anomalies on either side of that ridge. The evidence for seafloor spreading now seemed undeniable.
Corroboration was provided by the work of Neil Opdyke and Walter Pitman at the Lamont Geological Observatory at Columbia
University in New York. Pitman noted identical symmetrical patterns in paleomagnetic data collected by the research vessel
Eltanin from the East Pacific Rise (the data were said to be “too perfect” by some who opposed seafloor spreading). Opdyke
demonstrated that the continental record of magnetic reversals, including the Jaramillo Event, could be detected in analyses of
sediments from the South Atlantic, with ages constrained by microfossil analysis.
In November 1966, a by-invitation-only symposium on continental drift was held in New York. Vine presented a summary of
the interpretation of magnetic anomalies around mid-ocean ridges, including new data collected by an airborne magnetometer
from the Reykjanes Ridge south-west of Iceland. Although the debate was vigorous (a notable exchange being - Frank Press:
“Have you tested the symmetry statistically?” Fred Vine: “I never touch statistics. I just deal with the facts”), there was now
little doubt that paleomagnetic evidence supported the notion of seafloor spreading and helped determine its rate. Vine ably
summarised the situation in his paper, “Spreading of the ocean floor: New evidence” (Science, 1966).