Geology Reference
In-Depth Information
constant, k, in the denominator. The denominator represents the
misaligning influence of thermal energy.
The third type of material magnetism is the kind most important to
rock magnetic cyclostratigraphy, ferromagnetism. Ferromagnetic sub-
stances are permanently magnetized; they remain magnetized even in the
absence of an applied magnetic field.
Remanence
is the permanent mag-
netization of ferromagnetic materials. The apocryphal story is that the
first description of the remnant of magnetization left over when the
applied field was turned off was misspelled as remanent. Ferromagnetism
results from a cooperative interaction between the unpaired spin moments
of electrons in adjacent atoms called exchange interactions. Only three
elements can form ferromagnetic substances: iron, nickel, and cobalt. Iron
is the fourth most common element in the Earth's crust; so ferromagnetic
materials, such as iron oxides, iron sulfides, and iron oxyhydroxides, are
almost ubiquitous. In iron, the unpaired 3D electrons have their spin
moments aligned by exchange interactions giving the crystal lattice a
magnetic order and hence a permanent magnetization. Strictly speaking,
when the adjacent atom's spin moments are aligned parallel, it is called
ferromagnetism, and when the coupling is antiparallel, it is called anti-
ferromagnetism. A third subset of ferromagnetism is called ferrimagne-
tism and is the manner by which magnetite, a very important magnetic
mineral, acquires its magnetization. In ferrimagnetism, sublattices of the
crystal have spin moments that are aligned opposite to each other, but
there is a different number of spin moments in each sublattice, so a net
magnetization results.
Rock magnetism, and its application to environmental magnetism, is the
study of the remanent magnetization of fine particles (micron to submicron
in size). These small particles carry the paleomagnetism recording the
ancient geomagnetic field and retain records of environmental processes
during deposition.
2.3
Ferromagnetic Minerals
Iron is the fourth most common element in the Earth's crust, after oxygen,
silicon, and aluminum, and makes up about 5.6% of crustal rocks; therefore,
tracing the movement of iron, by magnetic methods, can be a powerful way
of tracing the movement of Earth materials between the reservoirs of the
Earth system, as envisioned in Earth system science (Thompson & Oldfield
1986; Liu et al. 2012). Natural magnetic minerals, that always contain iron,
move between the lithosphere, the hydrosphere, the atmosphere, and the bio-
sphere. Iron is not usually found in its native state in the Earth because of
the high oxygen content of the atmosphere, and the most important and
most common magnetic (i.e., ferromagnetic) minerals are the iron oxides,
magnetite (Fe
3
O
4
), and hematite (Fe
2
O
3
). Ferromagnetic materials have orders
