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.
