Geoscience Reference
In-Depth Information
how the declination has varied over the last four centuries. In the case of a
dipole field aligned along the Earth's rotation axis, the declination would always
be zero. Obtaining a very detailed image of the historical magnetic field will not
be possible since the measurements were recorded in ship's logs and are therefore
necessarily confined to the main shipping and exploration routes.
3.1.3 Magnetization of rocks
Rocks can become permanently magnetized by the Earth's magnetic field. This
fact has enabled geophysicists to track past movements of the plates.
As any volcanic rock cools, it passes through a series of critical temperatures
at which the various grains of iron minerals acquire spontaneous magnetization.
These critical temperatures, called the
Curie points
or Curie temperatures, are
different for each mineral (e.g., approximately 580
◦
C for magnetite (Fe
3
O
4
) and
680
◦
C for haematite (Fe
2
O
3
)). Once the temperature of the rock is lower than
the
blocking temperature
,(which for most minerals is tens of degrees less than
the Curie point) the magnetized grains cannot be reoriented. This means that the
grains have their magnetic moments aligned with the direction the Earth's mag-
netic field had at the time of cooling. Both of these temperatures are much lower
than the temperatures at which lavas crystallize (typically 800-1100
◦
C), which
means that magnetization becomes permanent some time
after
lavas solidify.
How long afterwards depends on the physical size and other properties of the
intrusion or flow and the rate of cooling, which depends in turn on its environ-
ment (see Section 7.8). This type of permanent residual magnetization is called
thermoremanent magnetization
(TRM) and is considerably larger in magnitude
than the magnetism induced in the basalt by the Earth's present field.
Sedimentary rocks can also acquire remanent magnetization even though they
have never been as hot as 500
◦
C, but this remanent magnetization of sediments
is generally very much less than that of igneous rocks. Sedimentary rocks can
acquire magnetization in two ways:
depositional
or
detrital remanent magneti-
zation
(DRM) and
chemical remanent magnetization
(CRM). Detrital remanent
magnetization can be acquired, as indicated by its name, during the deposition of
sedimentary rocks. If the sediments are deposited in still water, any previously
magnetized small grains will align themselves with their magnetic moments
parallel to the Earth's magnetic field as they fall. Large grains are unaffected.
Chemical remanent magnetization is acquired
in situ after
deposition during the
chemical growth of iron oxide grains, as in a sandstone. When the grains reach
some critical size, they become magnetized in the direction of the Earth's field at
that time. Chemical remanent magnetization is thus a secondary remanent mag-
netization whereas TRM and DRM are both primary remanent magnetizations
dating from the time of formation of the rocks.
The degree to which a rock body can be magnetized by an external mag-
netic field is determined by the
magnetic susceptibility
of the rock.
Induced