Geoscience Reference
In-Depth Information
Figure 3.11.
Details of the recent reversals of the Earth's magnetic
field as determined from detailed radiometric dating of continental
and oceanic-island lavas and palaeomagnetism of marine sediments.
The
epochs
, time intervals during which the Earth's magnetic field
was either predominantly normal or predominantly reversed in
polarity, have been named after prominent scientists in the study of
the Earth's magnetic field. (William Gilbert was a sixteenth-century
English physician, Carl Friedrich Gauss was a nineteenth-century
German mathematician, Bernard Brunhes (1906) was the first person
to propose that the Earth's magnetic field was reversed at the time
lavas were formed and Motonori Matuyama (1929) was the first
person to attempt to date these reversals.) The
polarity events
(subchrons)
, short fluctuations in the magnetic polarity, are named
after the geographic location where they were first recognized (e.g.,
Olduvai Gorge, Tanzania, the site of the early hominid discoveries of
Leakey; Mammoth, California, U.S.A.; and Jaramillo Creek, New
Mexico, U.S.A.). (Based on McDougall
et al.
(1992) and Spell and
McDougall (1992).)
Epoch
Polarity
event
Age
(Ma)
0
0.78
0.92
1.01
Jaramillo
1
1.78
1.96
Olduvai
2
2.11
2.27
Reunion
2.60
3
3.02
3.09
3.21
3.29
3.57
Kaena
Mammoth
South Atlantic had been continuous and had been occurring at a fairly steady rate
for the last 80 Ma, and that the geomagnetic timescale (Fig. 3.12)was reasonably
accurate. Since then, considerable effort has been put into ensuring that geo-
logical and magnetic timescales are as precise as possible. Figure 3.14 shows a
timescale from the middle Jurassic to the present.
To use a geomagnetic timescale to date the oceanic plates, it is necessary to
recognize specific anomalies. Fortunately, the reversal sequence is sufficiently
irregular (Fig. 3.14) for this to be possible for the trained eye. The prominent
anomalies up to age 83 Ma have been numbered from one to thirty-three. For
ages 125-162 Ma they are labelled with the prefix M (M standing for Mesozoic).
Particularly prominent is the long Magnetic Quiet Zone in the Cretaceous
(83-124 Ma numbered C34), during which no reversals occurred.
Figure 3.12 showed profiles from the Pacific and their corresponding theoret-
ical profiles, as well as the South Atlantic profiles. It is clear that, in contrast to
the history of the Atlantic, spreading rates in the Pacific have changed markedly
with time. This is also the case in the Indian Ocean. Figure 3.15 is an isochron
map of the ocean floor. This shows that the ridges have undergone a number of
changes both in rate and in direction of spreading in the past. The oldest parts
of the ocean floor are Jurassic. This is an interesting fact in itself and the subject
of conjecture about the density and stability of older oceanic lithosphere and
causes of initiation of a subduction zone in any particular location.
