Geoscience Reference
In-Depth Information
The irradiation being presumed to be constant, the number of charges
accumulated is proportional to time. If such a crystal is picked up and
heated, the electrons in unstable state are mobilized and ejected. They
actually combine with positive ions, giving rise to photons and thus to
light. The amount of light emitted can be measured. It is proportional
to the number of electrons liberated, therefore proportional to the
charges captured, thus proportional to the time that has elapsed since
the epoch of the preceding heating. This last heating could be related
to the deposition of the crystal in a basaltic flow, to rise in temperature
of the soil under the influence of fire, or even at the firing of a pottery
if this object is concerned. We can thus date such events. If the crystal
has never been emptied of free electrons, its electron traps could be
saturated so fully that the time cannot be measured. The method is in
demand in archaeology between 50,000 and 200,000 years BP, that is, in
a time interval too long to be studied with
14
C and too short to apply
the potassium/argon method. Its precision in calculated age is as high
as 5 to 7 per cent.
The Earth's magnetic field changes with time, according to transformations
that affect the Earth's core and probably to certain cosmographic data
parameters. The changes are also correlated with variation in
14
C
indicated above.
Various iron-bearing minerals susceptible to magnetization are
formed or are deposited in this magnetic field that changes with time
in intensity as well as in direction (azimuth, declination). These minerals
mark the field by aligning their dipoles with it. Some environments
(marine and lacustrine sediments) permit us to follow magnetization
over long periods and, thus, changes in this field over time by identifying
the characteristic reversals or alterations. This makes it possible for us
to reconstruct reference chronological sequences that can then serve in
dating specific phenomena, for example, the deposition of a volcanic
flow.
Therefore it is necessary to measure the magnetic susceptibility of
the soil at a given depth. For doing this, a feeble alternating current is
passed through a coil wound around a sample of soil (de Jong
et al.
2000).
The magnetic susceptibility of soils is chiefly related to the presence of
maghemite (g-Fe
2
O
3
) and magnetite (Fe
3
O
4
). The susceptibility depends
on that of the parent material and on the type of pedogenesis that will
concentrate or destroy in the profile the iron-bearing minerals originally
present (Shenggao 2000).
Palaeomagnetism
