An Introduction to Plate Tectonics
Testing the Sea-Floor Spreading Hypothesis
Before being widely accepted, a new hypothesis must be tested. One test for the
sea-floor-spreading hypothesis involved magnetic patterns on the sea floor.
In the late 1950's, scientists mapped the present-day magnetic field generated by
rocks on the floor of the Pacific Ocean. The volcanic rocks which make up the sea
floor have magnetization because, as they cool, magnetic minerals within the rock
align to the Earth's magnetic field. The intensity of the magnetic field they measured
was very different from the intensity they had calculated. Thus, the scientists
detected magnetic anomalies, or differences in the magnetic field from place to
place. They found positive and negative magnetic anomalies. Positive magnetic
anomalies are places where the magnetic field is stronger than expected. Positive
magnetic anomalies are induced when the rock cools and solidifies with the Earth's
north magnetic pole in the northern geographic hemisphere. The Earth's magnetic field
is enhanced by the magnetic field of the rock. Negative magnetic anomalies are
magnetic anomalies that are weaker than expected. Negative magnetic anomalies are
induced when the rock cools and solidifies with the Earth's north magnetic pole in
the southern geographic hemisphere. The resultant magnetic field is less than expected
because the Earth's magnetic field is reduced by the magnetic field of the rock.
When mapped, the anomalies produce a zebra-striped pattern of parallel positive and
negative bands. The pattern was centered along, and symmetrical to, the mid-ocean
ridge.
A hypothesis was presented in 1963 by Fred Vine and Drummond Matthews to
explain this pattern. They proposed that lava erupted at different times along the
rift at the crest of the mid-ocean ridges preserved different magnetic anomalies.
For example, lava erupted in the geologic past, when the north magnetic pole was in
the northern hemisphere, preserved a positive magnetic anomaly. In contrast, lava
erupted in the geologic past, when the north magnetic pole was in the southern
hemisphere, preserved a negative magnetic anomaly. Lava erupting at the present time
would preserve a positive magnetic anomaly because the Earth's north magnetic pole is
in the northern hemisphere. Vine and Matthews proposed that lava erupted on the sea
floor on both sides of the rift, solidified, and moved away before more lava was
erupted. If the Earth's magnetic field had reversed (changed from one geographic pole
to the other) between the two eruptions, the lava flows would preserve a set of
parallel bands with different magnetic properties. The ability of Vine and Matthews'
hypothesis to explain the observed pattern of ocean floor magnetic anomalies provided
strong support for sea floor spreading.
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