CONTENT CENTER
(PANGAEA TO THE PRESENT )
Lesson # 2
Plate Tectonics
(The Continental Drift Theory)
Earthquakes, volcanoes, and mountains are all produced by the same
natural processes. We know this to be true today, but even as little as
one hundred years ago scientists were unsure as to how these geologic
processes occurred.
The ancient Japanese legend of Namazu
explained why earthquakes occur this way. Namazu was a giant catfish
that lived under the surface of the Earth. It would shake violently
and cause great destruction from time to time. Kashima,
who is a Japanese god, was the only god that was strong enough to control
Namazu. Kashima would hold Namazu down and pin the catfish under a
rock. When Kashima's mind would wonder, Namazu would escape and cause
another earthquake.
Many cultures have tried to explain why earthquakes and volcanoes occur
through stories about their gods and goddesses. The Hawaiian Islanders
thought that volcanoes were the home of the fire goddess Pele. The Romans believed that the
blacksmith god, Vulcan, used volcanoes as
his forge to produce weapons.
For hundreds of years people throughout the world explained earthquakes
and volcanoes through myth and legend. In 1620 however Sir
Francis Bacon of England declared that it was not gods
and goddesses that caused natural disasters. He noticed how the coasts of
Africa and South America were very much alike. In fact they could almost
fit together like pieces of a jigsaw puzzle. The map on the next page
shows how the two continents could fit together.
As humans traveled the world they noticed seashells high in mountains
many miles from the nearest ocean. Why is there a similarity between
the coasts of Africa and South America? How did those seashells end up
high in the mountains? These questions along with new discoveries lead
scientists to believe that the Earth is a dynamic, or constantly changing
planet. It was not until the 1960's though, that scientists started to
agree to the concept that the continents could move across the surface
of the Earth.
A German meteorologist by the name of Alfred
Wegener showed that rock bands in South America and rock
bands in Africa matched mineral content and by age exactly. He also
showed that the magnetic bands in these same rocks did not point to the
magnetic north pole as they should. If the continents could be moved
back into the position that they were created, then they do point to the
magnetic north pole. Wegener concluded that the continents must have
drifted apart hundreds of miles. He did not however have an explanation
as to how these massive continents could move such a great distance.
It was not until the 1960's that geologists gained the technology to
fully understand the processes that could move the Earth's plates.
They concluded that the Earth's surface was composed of not one large
sheet but was composed of more than twelve major pieces of crust.
Geologists call these pieces plates
. These plates float across the surface of the Earth like an iceberg
floats on the ocean. The driving force behind these plate movements are
the convection currents in the mantle. The convection currents turn very
slowly dragging the plates along with these movements. The convection
currents move the plates very slowly. These plates move at only 1 to 4
inches per year!!
The lines on the map of the world on page 4 indicate the position of the
plate boundaries. Boundaries are places where the plates meet.
Now geologists can finally explain the reasons that mountains are built,
volcanoes erupt, and earthquakes occur. The Plate Tectonics Theory of
continental movement can explain scientifically why all of these geologic
processes can occur. Humans no longer have to try to explain these
natural occurrences through myth and legend.
Review Questions
- In your own words explain what the Continental Drift Theory states.
- How did Alfred Wegener try to prove that the continents of Africa and
South America were once connected.
- How did the ancient people of Japan explain earthquakes?
- How did the ancient Romans explain the presence volcanoes.
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