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===Paleomagnetic core orientation===
===Paleomagnetic core orientation===
The paleomagnetic technique is based on the fact that nearly all rocks, including quartzite, chert, and chalk, contain at least trace amounts of magnetic minerals such as magnetite and hematite. These minerals act as miniature compasses and lock-in the earth's ambient magnetic field. A primary signal is imprinted near the time of deposition; over time, one or more secondary signals may be overlain on the primary signal. In specially equipped laboratories, the signals in rock samples can be separated, interpreted, and used to orient core to present-day geographic north.
The paleomagnetic technique is based on the fact that nearly all rocks, including quartzite, [[chert]], and chalk, contain at least trace amounts of magnetic minerals such as magnetite and hematite. These minerals act as miniature compasses and lock-in the earth's ambient magnetic field. A primary signal is imprinted near the time of deposition; over time, one or more secondary signals may be overlain on the primary signal. In specially equipped laboratories, the signals in rock samples can be separated, interpreted, and used to orient core to present-day geographic north.
Paleomagnetic core orientation requires that a number of 1-in.-diameter plug samples be collected from the core at the surface ([[:file:core-orientation_fig3.png|Figure 3]]). The number of samples required to achieve a good statistical result is dependent on the rock type and other factors. Plugs are then sent to a laboratory for analysis, and core orientation results are typically returned in one to four weeks.
Paleomagnetic core orientation requires that a number of 1-in.-diameter plug samples be collected from the core at the surface ([[:file:core-orientation_fig3.png|Figure 3]]). The number of samples required to achieve a good statistical result is dependent on the rock type and other factors. Plugs are then sent to a laboratory for analysis, and core orientation results are typically returned in one to four weeks.