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In-Depth Information
a narrow colatitude band about ™, in the GAD
hypothesis also the
inclination anomaly
,
I
(™),
will be approximately zero:
field direction could not be sufficient to prove
that the rock formed during a time interval of
reversed polarity. In fact, it is known that the non-
dipole field components can produce large local
deviations of the observed declination values.
Therefore, the evidence for magnetic polarity
reversals must be based upon many independent
observations distributed over the Earth's surface.
The recognition of the existence of recurrent
events of field inversion led to the development of
geomagnetic polarity time scales
since the 1960s.
In these time scales, a time interval with constant
magnetic polarity (normal or inverted) is called
chron
and the time boundaries are established on
the basis of radiometric dating. The first time
scales had a rather restricted temporal range,
which was based on potassium
argon (K-Ar)
dating of Pliocene and Pleistocene igneous rocks.
Therefore, they spanned the interval 0-5 Ma. An
example of this class of polarity time scales is il-
lustrated in Fig.
4.9
. This time scale was built us-
ing 354 igneous rock samples, from which K-Ar
ages and magnetic polarity had been determined.
Figure
4.9
shows that the average duration of the
chrons was
0.25 Myrs since 5 Ma. The number
of observations that were classified as “interme-
diate polarity” was
1.5 % of the sample. These
field directions were probably acquired during
the short phases of switching of the GAD polarity
at the transition between two adjacent chrons.
It is estimated that these phases have duration
between 1,000 and 8,000 years (see McElhinny
and McFadden
2000
, and references therein).
Therefore, in terms of geological time they are
global synchronous events. An important feature
of the alternate sequence of polarity chrons in
Fig.
4.9
is represented by their variable length, so
that the specific
pattern
of normal and reversed
chrons for a given time interval can be used as a
distinctive “fingerprint” that identifies the inter-
val. We shall make extensive usage of this feature
in the procedure of identification of marine mag-
tool for dating the oceanic sea floor. In the case
of the time scale 0-5 Ma, historically four major
magnetic polarity epochs
were identified, along
with shorter intervals that were called
events
.
These epochs, to which was given the name of
I .™/
D
I
tan
1
.2 cot™/
0
(4.50)
where
I
is
the
inclination of the time-averaged
field vector F . To date, many studies have shown
that the time-averaged geomagnetic field is essen-
tially a GAD field, possibly with small additional
non-dipole axial components that will be consid-
of the geomagnetic poles over a time interval of
the order of hundreds of thousands of years is
indistinguishable from the geographic poles. In
general, on a time scale of thousands of years,
the magnetic moment wobbles (so that the field
inclination changes), precesses, and changes its
intensity (e.g., Merrill and McFadden
2003
). The
characteristic periodicity of these processes is
several times longer than that of the non-dipolar
component. For an in-depth discussion about the
secular variation of the geomagnetic field the
reader is referred to the topics of Butler (
1992
)
and McElhinny and McFadden (
2000
),andtothe
paper of McElhinny et al. (
1996
).
4.4
Polarity Inversions, Chrons,
and Geomagnetic
Timescales
On a time scale larger than that associated
with secular variation, we observe another
phenomenon that has had a dramatic impact for
the construction of the plate tectonics paradigm.
It consists into the recurrent inversion of the
GAD field polarity, with a periodicity between
10
4
and 10
8
years. A time interval such that the
GAD magnetic moment has the same direction
as the present day field, pointing southward,
is said to be of
normal polarity
, while the
opposite configuration defines a
reversed polarity
time interval. Although magnetic reversals
determine a change of declination of 180
ı
at
any point on the Earth's surface, the fact that
an observed paleomagnetic field direction at a
site is approximately opposite to the present day
