Geology Reference
In-Depth Information
(a)
6.0E-03
Linear acquisition plot-LAP
MC3
5.0E-03
4.0E-03
3.0E-03
2.0E-03
1.0E-03
0.0E+00
0
0.5
1
1.5 2
Log 10 applied field (mT)
2.5
3
3.5
4
(b)
6.0E-03
Gradient acquisition plot-GAP
MC3
5.0E-03
4.0E-03
3.0E-03
2.0E-03
1.0E-03
0.0E+03
0
0.5
1
1.5 2
Log 10 applied field (mT)
2.5
3
3.5
4
Figure 2.6
IRM acquisition modeling of samples from the Carboniferous Mauch Chunk Formation. (a) The
linear acquisition plot shows the raw IRM acquisition data. (b) The gradient acquisition plot shows the fitting
of log normal distributions for magnetic particle coercivity distributions to the first derivative of the IRM linear
acquisition plot. Source: Bilardello & Kodama 2010.
2.4.2
Magnetic Particle Anisotropy
The reason that paleomagnetism is such an important field of study is that
the small ferromagnetic grains in most rocks retain their magnetization in
the same direction for geological significant time periods, i.e., billions of
years. The magnetic stability of ferromagnetic grains is the result of the
magnetization of a grain being fixed in place by the magnetic anisotropy of
the grains. Anisotropy means simply that individual magnetic grains
are more easily magnetized in one direction, in the grain, than another
direction. Magnetic anisotropy can arise from three different causes; the
shape of the magnetic grain, the crystallography of the magnetic grain, and
the effects of stress deforming the grain. For magnetite, shape is the impor-
tant type of anisotropy for stabilizing its magnetization. For hematite,
crystallography is important in controlling the magnetic grain's stability.
