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 as mafic 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 by glacial 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. 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 of paleomagnetism involves the identification of older magnetic fields that surrounded Earth in the geologic past.
The best source rocks for detailed paleomagnetic studies are thick accumulations of flood basalts in the interiors of continental plates. There have been over twenty paleomagnetic reversals in about the last 5 million years; our current magnetic field orientation has been stable for the last 700,000 years. An average for the past 20 million years is about one field reversal every 500,000 years.
It has been theorized that magnetic reversals are a result of changes in direction of convection flow in the liquid outer core or of periods of no convection. It is likely, before the onset of a paleomagnetic reversal, that there is a brief period of zero magnetic field, which may allow ultraviolet radiation to bombard the earth's surface and damage or kill various species; in fact, some species extinctions and mutations correlate with some paleomagnetic reversals.