Sea-Floor Spreading:�Evolution of a Theory

After Wegener and his Theory of Continental Drift rocked the scientific world in the early 1900’s, things began to settle down in the 1920’s and 1930’s (at least in Geology), and world events turned attention to political and economic matters, and the earth sciences were placed on a back burner.

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This is where we pick up our story…

While wars are usually thought of for death and destruction, they are often the grounds for many new scientific discoveries and inventions. World War II (1939-1945) was certainly no exception, and inventions like jet engines, nuclear power, and RADAR appeared then, and were tools that not only helped win the war, but are commonly used today.

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Another invention that was invented for a wartime purpose but wound up having a huge impact on geology was a device called SONAR. Originally invented to hunt down German and Japanese submarines, it also discovered a feature on the ocean floor that was to forever revolutionize the way we see our planet.

While chasing Nazi submarines all over the Atlantic Ocean, it was also discovered that running down the middle of the ocean floor was a huge chain of mountains! This came as a major shock, because up until that time, the ocean floors were thought to be as flat and featureless as a table top.

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Because most of the submarine hunting was done in the Atlantic, this was where the range was best studied, so it was named the Mid-Atlantic Ridge. However, after the war, geologists began studying all of the oceans in detail, and here’s what was soon discovered:

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a. The mountains of the Mid-Atlantic Ridge were really one huge mountain range that wrapped all the way around the Earth! It was then renamed the Mid-Ocean Ridge (MOR).

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b. The MOR is some 50,000 miles long, and averages 10,000 feet high above the rest of the ocean floor.

This makes it the largest mountain range on the planet!!!

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c. The center of the MOR is a rift valley.

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d. The rocks making up the MOR were basalts, and had very little sediment and mud covering them.

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e. The basalts of the MOR were in a shape called pillow lavas. These had puzzled geologists for centuries. They were found on continents worldwide, but it made no sense that they could form above sea-level. Now that they had been seen forming deep underwater at the MOR, it was obvious that any pillow lava on land was once a MOR long ago!

f. The rift valleys at the center of the MOR were much warmer than the rest of the rocks making up the ocean floor.

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g. The youngest rocks in the ocean floor were found right at the MOR, and the rocks got older as you moved away towards the edges of the oceans. They did this in a repeating pattern until the continents were reached.

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h. As geologists developed tools to measure the magnetism of rocks, much like the rock ages, they found that there was an identical repeating pattern of magnetism on either side of the MOR.

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All of this evidence led geologists to realize that they had a new theory on their hands, so in the late 1950’s, the Theory of Sea-Floor Spreading (SFS) was proposed. It simply stated that new ocean crust was created at the MOR’s, and this pushed the older crust away to both sides. This process creates new ocean floors, which makes the oceans wider, and moves the continents.

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Sea Floor Spreading Animations

• This was the missing data Wegener needed to show how continents move, and it was now obvious he was right- SFS proved that Continental Drift was correct. Unfortunately for Wegener, he had been dead for some 40 years, but a major breakthrough in geology had been achieved.

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• Is Sea-Floor Spreading a good scientific theory? Why or why not?

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Characteristics of a good theory:� -based on data

-connects several ideas

-can be modified or proven wrong

-allows us to predict the future