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Figure 3.10. (a) The magnetic-anomaly profile over the Juan de Fuca Ridge at 46 N,
southwest of Vancouver Island, Canada. (b) The profile in (a) is reversed; note the
symmetry. (c) A model magnetic-anomaly profile calculated for this ridge assuming
a half-spreading rate of 2.9 cm yr −1 and the magnetic-reversal sequence shown
below. On the magnetic-reversal timescale, black blocks represent periods of normal
polarity and white blocks periods of reverse polarity. (d) The magnetic-anomaly
profile over the East Pacific Rise at 51 S. (e) The profile in (d) is reversed; note the
symmetry. (f) A model magnetic-anomaly profile calculated for this ridge assuming
a half-spreading rate of 4.4 cm yr −1 and the magnetic-reversal sequence shown
below. The magnetic-reversal timescale is the same as that for the Juan de Fuca
Ridge (see (c)), but with a different horizontal scale because of the faster spreading
rate. (After Vine (1966).)
from this model and the whole seafloor-spreading hypothesis were spectacularly
confirmed by drilling the ocean bottom as part of the Deep Sea Drilling Project
(DSDP), an international enterprise that began in 1968 (see Section 9.2.1).
During Leg 3 of the DSDP a series of holes was drilled into the basalt at the
top of the oceanic crust right across the Atlantic at 30 S(Fig. 3.13). It was not
possible to use radiometric methods to date the lavas sampled from the top of
the crust because they were too altered; instead, the basal sediments were dated
using fossils. The ages are therefore slightly younger than the lava ages would
have been. Figure 3.13(b) shows these sediment ages plotted against the distances
of their sites from the ridge axis. The straight line confirmed that spreading in the
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