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Magnetic Fields

The magnetic field and the poles. The earth is surrounded by a magnetic

field. Magnetic lines of force originate from north and south magnetic poles,

which are about 11.5 degrees away from the geographic North and South Poles.The magnetic field is strongest at the magnetic poles. The positions of the

magnetic poles have changed over time and appear to be rotating around the

geographic poles on an axis tilted from the geographic axis by 11.5 degrees. The

magnetic field traps high-energy particles created by the sun's ultraviolet

radiation, thus protecting our environment on Earth.

The magnetic field is thought to be generated by the liquid outer core. If this

liquid material is metallic, as geophysical studies suggest, its flow as a result of

heat convection would create an electric current. Electric currents induce

magnetic fields.

Magnetic anomalies. The intensity of the magnetic field is measured at the

earth's surface with a magnetometer. Large-scale patterns may be related to

convection patterns in the liquid outer core. Local magnetic features or

anomalies are usually related to different rock types. Rocks have different

magnetic characteristics that, when added to the overall regional magnetic

pattern, create anomalies. Magnetic anomalies are areas of magnetism that

are either higher or lower than the average magnetic field for the area. A

positive magnetic anomaly is a reading that exceeds the average magnetic

field strength and is usually related to more strongly magnetic rocks, such asmafic rocks or magnetite-bearing rocks, underneath the magnetometer. A

negative magnetic anomaly is a reading that is lower than the average

magnetic field. Positive anomalies can also be created by irregularities in the

bedrock surface beneath sedimentary cover; a rock that is only 10 meters from

the surface and buried by sediment will have a more positive magnetic reading

than the same rock that is 80 meters from the surface and covered by sediment.

Similarly, negative anomalies can result from troughs or grabens that have

developed on the bedrock surface.

The magnetic characteristics of the bedrock, especially in areas covered byglacial sediments, can be mapped in great detail using magnetic-field values.

The magnetic data can even show the strike and dip of the rock units and outline

the contacts between rock units of different magnetism.

Polarity reversals. The earth's magnetic field has periodically reversed its

polarity in the geologic past: north becomes south, and south becomes north.

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This phenomenon is known from rocks that formed during these periods of

reversal. Magnetic minerals crystallize in cooling lava flows and point themselves

toward the north magnetic pole. This magnetic record is permanently trapped in

the rocks when they harden. The study ofpaleomagnetism involves the

identification of older magnetic fields that surrounded Earth in the geologic past.