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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