Geoscience Reference
In-Depth Information
Example: calculation of palaeomagnetic latitude
Magnetic measurements have been made on a basalt flow at present at 47
◦
N, 20
◦
E.
The angle of inclination of the remanent magnetization of this basalt is 30
◦
.
Calculate the magnetic latitude of this site at the time the basalt was magnetized.
The magnetic latitude
λ
is calculated by using Eq. (3.17):
tan
I
=
2 tan
λ
I
is given as 30
◦
,so
λ
=
tan
−
1
tan 30
◦
2
=
tan
−
1
(0
.
2887)
=
16
.
1
◦
Therefore, at the time the sample was magnetized, it was at a magnetic latitude of
16
◦
N, which indicates that between then and now the site has moved 31
◦
northwards
to its present position at 47
◦
N.
After
λ
p
has been calculated, the sine formula for a spherical triangle (e.g.,
Eq. (2.10)) can be used to give the difference between the longitudes of the
palaeomagnetic pole and the sample location,
φ
p
−
φ
x
:
sin(90
−
λ
) sin D
sin(90
−
λ
p
)
sin(
φ
p
−
φ
x
)
=
cos
λ
sin D
cos
λ
p
=
sin
λ
≥
sin
λ
p
sin
λ
x
cos
λ
sin D
cos
sin(180
+
φ
p
−
φ
x
)
=
sin
λ<
sin
λ
p
sin
λ
x
(3.21)
λ
p
Therefore, by using Eqs. (3.17) and (3.19)-(3.21), we can calculate past magnetic-
pole positions.
If palaeomagnetic pole positions can be obtained from rocks of different ages
on the same continent, these poles can be plotted on a map. Such a plot is called a
polar-wander path
(a name that is a 'fossil' of the older notion that it was the poles,
not the continents, that drifted) and shows how the magnetic pole moved relative
to that continent. If such polar-wander paths from two continents coincide, then
the two continents cannot have moved relative to each other during the times
shown. However, if the paths differ, there must have been relative motion of the
continents. Figure 3.6 shows polar-wander paths for Europe and North America
for the last 550 Ma. Although these two paths have almost the same shape, they
are certainly not coincident. When the opening of the Atlantic Ocean is taken
into account, however, the two paths can be rotated on top of each other, and they
then do approximately coincide.
