Activity 2: Magnetic Reversals and Sea-Floor Striping
 

A key factor in establishing the theory of plate tectonics was recognition of past reversals in the Earth’s magnetic field and the mapping of normal and reverse magnetic striping on the ocean floor. The motion of the Earth’s molten outer core produces a magnetic field that is characterized by force lines emanating from near the geographic poles (Fig. 1). At present, the magnetic field causes compasses to point toward magnetic north. This polarity is normal polarity.

 

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Fig. 1: Normal magnetic polarity (left) and reverse magnetic polarity (right).

 

 

At various times in the past, the magnetic field was oriented in the opposite direction, and compasses would have pointed toward magnetic south. These time periods were ones of reverse polarity (Fig. 1). New oceanic crust inherits the Earth’s magnetic field at the time it solidifies. Reversals of the Earth’s magnetic field produce magnetic anomalies or variations in the magnetic character of the oceanic crust.  Thus the sea floor has regions where its magnetic minerals point north (normal) and areas where they point south (reverse). These magnetic rocks produce highs and lows in the local magnetic field. Because they were originally produced at the midocean ridge crest, the anomalies are oriented parallel the midocean ridge. We can map these anomalies by towing a magnetometer behind a ship and measuring the strength of the local magnetic field. The resulting map of the sea floor will have a distinctive magnetic signature (Fig. 2).

 
development of magnetic anomalies
 

Fig. 2: Sea-floor spreading and magnetic anomalies.

 
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