An Unexplored Realm in the Heartland of the Southern Gulf ... - Famsi

Magnetic Properties of Basalt
The magnetic properties of basalt are essential to the success of this survey. The
sources of the basalt used in monumental sculpture in the Southern Gulf Lowlands are
located in the Tuxtla Volcanic Ridge (Coe and Diehl 1980a; Heizer et al. 1965; Williams
and Heizer 1965). These basalts are extruded, mafic igneous rock emitted as magmatic
material at the earth's surface during volcanic eruptions in the form of lava rich in
ferromagnesian minerals. As the molten material solidified, its iron-containing minerals
aligned with the Earth’s magnetic field. More precisely, as the volcanic rock cooled past
the Curie Point, a transition temperature that marks a change in the magnetic properties
of the rock, it begins to produce an induced magnetic field (Dunlop and Özdemir 1997).
Below the Curie Point, the blocking temperature is reached, a stage at which the
ferromagnetic grains within the material are no longer free to move. It is at this point that
the magnetic properties of the rock become stable and produce a permanent directional
orientation, a property known as thermoremanent magnetization (TRM) (Butler 1992;
Dunlop and Özdemir 1997). It is the alignment of the stone’s directional polarity that
provides its magnetic coherence, and the level of amplitude demonstrated on the
magnetic anomaly maps are produced by TRM.
This formation process results in two primary magnetic attributes of basalt,
magnetic susceptibility and remanant magnetization (Dobrin and Savit 1988; Dunlop and
Özdemir 1997; Tarling and Hrouda 1993). The measure of magnetic strength of a mineral
or rock is called magnetic susceptibility, which is a function of two factors: 1) the amount
of iron, and to a lesser extent nickel or cobalt, present in the rock and particularly carried
in magnetite, hematite, and related minerals; and 2) the measure of alignment occurring
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