Plate Tectonics
Evidence of the Process
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Evidence for Continental Drift Theory
By looking at the shape of the plates (especially the
continental plates) you will notice that they can fit together
like a puzzle.
At one point in time the continents were joined as one large
mass called Pangaea. (Not to be confused with May 10th,
Pangaea day where the humans of the world celebrate
being together and sharing the earth.)
Look at the diagram (following slide) of Pangaea and see if
you can identify the following 7 plates:
North America, South America, Africa, Eurasia, Australia,
India, and Antarctica.
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Pangaea
A – Eurasia
B – North America
C – South America
D – Africa
E – Antarctica
F – India
G – Australia
Some of the smaller
pieces make up some of
the smaller plates
(Ex Pilipino plate).
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Pellegrini’s illustration
The observation that the continents fit together began in
the 1500’s with map makers. By the 1800’s many others
made the same observations. Look at Antonio SniderPellegrini's illustration from 1858:
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Fossil Evidence
It wasn’t until the early 1900’s that evidence started to be
found that could help prove this theory. Alfred Wegener
(1880 -1930)
pursued the theory of
“continental drift” and
noticed that matching
fossils could be
found in distinct
locations that
corresponded to
where the continents
joined.
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• This was not enough evidence to convince many
scientists and the theory did not become
accepted (in fact it was opposed) until the
1950’s when more evidence on plate tectonics
agreed with the theory of continental drift.
• These other evidences included mountain belts
and paleoglaciation. We have already looked
at why mountain belts form along the edge of
some continents when we looked at plate
boundaries.
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Evidence of Paleoglaciation
Fossils weren’t the only thing that left remains when the
continents were joined as Pangaea. When glaciers cover the
land and then retreat they leave scars, large random rocks
and a definite boundary.
A boundary can be observed along the following locations:
 Antarctica, the east coast of South America, southern
portion of Africa, west coast of India, west coast of Australia
Before the theory of continental drift these glaciations
appeared to be random and did not make sense as many of
these regions are in the tropics.
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However, if you take into account that these regions formed a
polar area during Pangaea it is understood.
The “random”
paleoglaciation
marks form one
boundary when the
continents are joined
as Pangaea.
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Spreading ridges and Magnetic Reversals
The discovery of spreading sea floors convinced many scientists that the
theory of continental drift was true. This revealed that the ocean plates
weren’t as rigid as they were once suspected.
Another observation was made within the spreading seafloor. Think back
once again to the Mid-Atlantic ridge. As new material comes out of the
ridge it is pushed both to the east and to the west.
As the molten rock cools, the compounds in the rock solidify as crystals
with magnetic fields. These magnetic fields align with the North-South
poles on earth.
For some reason the Earth’s magnetic field reverses (the north pole
becomes a south pole). This happens sometimes within 50 000 years and
sometimes it takes millions of years. These were first observed in the
1920’s from cooled lava around Japanese volcanoes.
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In the 1950’s oceanographers observed long bands
running parallel to the Mid-Atlantic ridge and the pattern
was symmetrical on each side of the ridge. This gave
strong evidence that the ocean floor was moving!
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Ring of Fire
As we have noticed earlier, earthquake and volcano activity form
continuous boundaries around the plates.
The combination of
all the above
evidences have
convinced scientists
that the earth’s
surfaces is moving.
But there is still
much work to be
done to better
understand what is
causing the
movements.
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