Continental Drift 2

#2: Closer to an Answer

With new technology developed during the world wars, scientists were able
to make detailed maps of the ocean floor.
These maps held the secret to how the continents moved.

The Ridge

After the Second World War, submarines were available,
and so was their technology. Once nuclear submarines were developed
it became important that submariners could navigate safely underwater.
Although there were plenty of maps showing what the world looked like above
the oceans, no one knew much about the ocean floor.
Technology had been developed that allowed the scientists to map the ocean
floor by bouncing sound waves off it. This is called sonar.
After the sonar survey was done, maps were made showing the ocean floor
in great detail. It was soon discovered that an underwater mountain range
stretched all the way around the earth on the ocean floor.
It was also noticed that on either side of this mountain range there was a
very wide trench.

Magnetic Stripes

Scientists had known for some time that the earth has not always been
polarized to the North. Now when you hold a compass the needle points to
the North. In the past though it has pointed to the South. We know that
over billions of years the earth has changed its polarity many times.
Soon after the discovery of the underwater ridge came another discovery.
This was that the ocean floor is made up of strips of rock with different
polarities. A map showing the polarity of the ocean floor in black and white
looks very much like the pattern on a zebra’s coat.
The black stripes indicate normal polarity, and the white strips indicate
reversed polarity.

An important point to remember is that volcanic rock can only be
polarized while it is still molten. Once it cools its polarity is set,
and will not change, even if the polarity of the earth changes.

The Age and Thickness of the Crust

The final important discovery was that the earth’s crust is the same
thickness over the entire ocean floor, and that it has very little build
up of sediment on it. If it had been there for billions of years, it should
have a thick layer of sediment sitting on top of the rock floor.
This led scientists to decide that the ocean floor was in fact quite young,
in comparison to the continents.

Beginning to Believe

It had taken 50 years to get this far, but all of a sudden Wegener’s
theory was making much more sense. Instead of the continents drifting
through the earth’s crust, scientists began to believe that the earth’s
crust had pushed the continents apart.

At first the scientists proposed that new rock was being formed
along the underwater mountain ridge. They decided that the ridge was a
weakness in the crust and that magma, from below the earth’s crust was
being forced upwards through these weak areas and flowing down the sides
of the mountain ridges as lava. Underwater, lava cools very rapidly,
so it was becoming solid, and gradually pushing the crust, on either side
of the ridge away from the mountains. If this could be proved, then it would
explain how the continents had been pushed apart.

The Final Proof

Still there was a problem; nobody could come up with conclusive
proof that this theory was correct. Then finally the scientists managed
to put it all together. They put together two more pieces of information
that had somehow slipped past them, although they had had the evidence for
some time.

  • The rocks at the ridges were the youngest, gradually becoming older as they moved apart
  • The stripes of polarity on the ocean floor were symmetrical on either side of the ridge.
  • Now it became clear that the new rock was being formed, and pushing the older rock to the sides,
    causing the entire earth’s crust to be pushed apart. Since rock was formed and polarized on both
    sides of the ridge at the same time, the stripes were symmetrical.
    Sometimes there was a wide strip of south polarized rock followed by a narrow strip of
    north polarized rock, and sometimes the strips were the same width.
    The most important thing was that the strips were the same on both sides of the ridge;
    they were like a mirror image of each other.

    So the theory of drifting continents was at least partly correct.
    They had at one time been a single landmass, which had split apart.
    However, they hadn’t drifted across the ocean, through the earths crust,
    as Wegener had thought. Instead they had been pushed apart, with the movement
    of the earth’s crust.

    A Problem

    From Wegener’s theory of drifting continents, we now had a theory of a
    spreading ocean floor. But still there was a problem.
    If all this new rock was being formed, and pushing aside the old rock,
    why wasn’t the earth getting any larger? Where was all this rock going?


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