Geoscience Reference
In-Depth Information
Example: calculation of latitude and longitude of the
palaeomagnetic pole
If the angle of declination for the basalt flow of the previous example is 80
◦
,
calculate the latitude and longitude of the palaeomagnetic pole.
The latitude of the palaeomagnetic pole is calculated by using Eq. (3.20) with
λ
=
16
◦
and
D
=
80
◦
:
sin
λ
p
=
sin 47 sin 16
+
cos 47 cos 16 cos 80
Thus,
λ
p
=
18
◦
.
The longitude of the palaeomagnetic pole can now be calculated from Eq. (3.21)
after checking whether sin
λ
≥
sin
λ
p
sin
λ
x
or not:
cos 16 sin 80
cos 18
Thus,
φ
p
−
φ
x
=
84
◦
, and so
φ
p
=
104
◦
. The position of the palaeomagnetic pole is
therefore 18
◦
N, 104
◦
E.
sin(
φ
p
−
φ
x
)
=
(a)
(b)
Figure 3.6.
(a) Polar-
wander curves for North
America (circles) and
Europe (squares).
(b) Polar-wander curves
for North America and
Europe when allowance
has been made for the
opening of the Atlantic
Ocean. The two curves are
now almost coincident.
(After McElhinny (1973).)
5
5
0
100
100
3
0
0
250
180
230
400
220
240
270
300
400
550
3.2 Dating the oceanic plates
3.2.1 Magnetic stripes
To use measurements of the magnetic field to gain information about the mag-
netization of the crust, it is first necessary to subtract the regional value of the
geomagnetic field (e.g., IGRF 1995). What remains is the magnetic anomaly.
Over the oceans, magnetic-field measurements are made by towing a magne-
tometer behind a ship. A magnetometer can be routinely towed while a research
ship is on passage or doing other survey work that does not involve slow, tight
manoeuvring. These marine instruments measure the magnitude
B
,butnot the
