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Paleo-Magnetizm
Paleo Magnetizm is, effectively, the study of magnetic materials present in rocks. Depending upon the type of rocks being studied, this could be due to one of the following:-
Thermal remanent magnetization occurs when rocks are laid down or transformed as a result of intense heat normally as lava or other forms of magma. In such situations, the magnetic material in the rock (normally magnetite) will align with the earth's magnetic field.
Detrital remanent magnetization occurs when rocks are formed by conglomoration and subsequent pressure (e.g. sandstone or shale). Here, magnetic particles in the rock will try to align with the earth's magnetic field. This alignment normally occurs on deposition but can also occur post deposition.
Chemical remanent magnetization occurs when minerals are formed from a chemical solution. Here, magnetic particles in the mineral (normally hematite) will try to align with the earth's magnetic field as the mineral is formed.
These basic elements allowed the development of the theories of the "Dancing Continents" or continental drift through the analysis of igneous rocks particularly those on ocean floors. Reverse use of such techniques allow the dating (geochronology) of igneous, sandstone and limestone deposits with relation to strata level and continental drift.
An excellent introduction to Paleo-Magnetizm by R. M. Butler can be found by clicking here.