Divergent
Boundaries
*
Plates are constructed by successive injections of magma at divergent plate
boundaries.
Divergent boundaries in oceans are defined by ridges and rises,
generally with a narrow rift valley marking the
boundary between plates along a central ridge crest.
The remnant magnetism of the mafic oceanic crust, which records
the Earth’s magnetic field at the time the crust formed as it cooled through
its Curie point (~580˚C for magnetite) (Fig. 17.3A),
shows reversals (Fig. 17.3B) in the magnetic
field as well as the magnetic inclination (i.e., where the crust was on the
surface of the Earth when it cooled).
Therefore, in terms of magnetism, the oceanic crust is striped with normal and reverse magnetism.
When a magnetometer passes over
normally magnetized (i.e., the same direction as the present day magnetic
direction), the magnetic intensity reinforces the present day field and positive anomalies result.
When passing over reversely
magnetized zones, the magnetism is subtracted from the present day magnetic
field and weak negative anomalies are observed.
Symmetry of magnetic anomalies about the mid-ocean
ridge is observed.
