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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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References

1. file://localhost/vwdocs/vwlessons/plate_tectonics/part8.html
2. file://localhost/vwdocs/vwlessons/atg.html
3. file://localhost/vw.html
4. file://localhost/vwdocs/vwlessons/plate_tectonics/part10.html