The rocks of oceanic crust become older and older with increased distance from ridges. This suggests that new crust is constantly being created at the mid-ocean ridges.
The ages of the rocks of the oceanic crust are determined by an analysis of their remnant magnetism.
Many rocks, particularly volcanic rocks, contain Fe-bearing minerals such as magnetite that act like natural magnets.
When such minerals crystallize from lava, they orient relative to the existing magnetic field of the Earth.
The orientation or remnant magnetism of such minerals can be measured in the laboratory or field with a magnetometer.
Paleomagnetic studies of sequences of volcanic rocks have
revealed that the Earth's magnetic field changes polarity periodically
throughout geologic time.
denotes normal polarity anomalies, when Earth's polarity was the
same as it is presently.
denote reverse polarity anomalies, when Earth's polarity was the
opposite the present polarity.Paleomagnetic studies of this type have led to the development of a magnetic-polarity time scale.
Magnetometers reveal that there are parallel and symmetrical bands of alternating normal and reverse anomalies in the volcanic rocks on both lava plateaus and mid-ocean ridges.
The times of the magnetic reversals between the anomalies are indicated by magnetic-polarity time scale.
This provides the age of the oceanic crust itself
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