Geology Reference
In-Depth Information
Fig. 4.11
C
series geomagnetic polarity time scale of
Cande and Kent (
1995
). Only the major anomalies are
labeled. Numbers on the upper scale are distances from
the ridge when the spreading rate is 20 mm year
1
.The
red line
shows the theoretical magnetic signal associated
with this time scale in the case of an N-S profile in
the northern hemisphere. Ages of the labeled anomalies
are: 2A D 2.58 Ma, 3A D 5.89 Ma, 4A D 8.70 Ma, 5 D
10.95 Ma, 5C D 16.01 Ma, 6 D 20.13 Ma, 7 D 24.73 Ma,
9
D
27.03
Ma,
12
D
30.48
Ma,
13
D
33.06
Ma,
18
D
40.13 Ma, 20
D
42.54 Ma, 21
D
47.91 Ma, 25
D
55.90
Ma,
28
D
62.50
Ma,
30
D
65.58
Ma,
31
D
67.74 Ma, 32
D
71.07 Ma, 33
D
73.62 Ma, and
34
D
83.50 Ma
In these time scales, the polarity chrons for latest
Cretaceous and Cenozoic are numbered within
the
C
-
series
, from C34 (the oldest) to C1 (the
youngest). For older times, the middle Jurassic
through Early Cretaceous sequence, which is
used in marine geophysics studies, consists of
the
M
-
series
(polarity chrons from M0 to M41).
This segment of the time scale is based upon the
assumption that the Hawaiian sequence of marine
magnetic anomalies in the western Pacific formed
at constant spreading rate (Gradstein et al.
1994
).
Also in this case the sequence was pinned to tie
points with known radiometric age.
The naming conventions of magnetochronol-
ogy are simple. For each normal polarity chron,
there is a corresponding older chron with reversed
polarity. Then, a suffix “n” or “r” is used to
distinguish the two time intervals. For instance,
C13n identifies the normal polarity chron within
the interval C13. A complication arises from the
presence in the
C
-series of smaller intervals with
inverted polarity within a chron. For example,
C2r starts at 2.581 Ma and ends at 1.950 Ma
in the scale of Cande and Kent (
1995
). This
reversed chron includes a short normal polarity
sub-chron between 2.140 and 2.150 Ma, which
was discovered later. Therefore, it is now divided
into three sub-chrons, named respectively C2r.1r,
C2r.1n, and C2r.2r. Figures
4.11
and
4.12
show
respectively the geomagnetic polarity time scales
for the
C
-and
M
-series.
An important feature of the late Cretaceous -
Cenozoic geomagnetic polarity time scale is the
presence of a long chron with normal polarity
(C34), from
120 to
84 Ma (Fig.
4.11
). This
is called the
Cretaceous normal
-
polarity super-
chron
or, alternatively,
Cretaceous Quiet Zone
(CQZ). During this time interval, which lasted
36 Myrs, the geomagnetic polarity remained
fixed. Another interesting feature can be observed
on the Jurassic - early Cretaceous time scale
(Fig.
4.12
). It is represented by a long sequence
of short polarity chrons before M25, between the
Oxfordian and the Callovian, which determines
blurring of the magnetic signal. This time interval
partly coincides with the so-called
Jurassic Quiet
Zone
(JQZ), which is observed offshore North
America, along the coast of northwest Africa,
and in the western Pacific. This zone includes
anomalies older than M29 and is characterized by
the low amplitude of the magnetic signal, which
makes the analysis difficult.
4.5
Ionosphere
and Magnetosphere
The large region around the Earth where the
geomagnetic field dominates is called
magneto-
sphere
. It is occupied by
plasma
, that is, high-
energy charged particles, of solar wind prove-
nance or terrestrial origin. The
solar wind
is a
