Evidence for Continental Drift

1. Evidence for
Continental Drift
and The Theory of
Plate Tectonics

2. Did you know that the coal that is
mined in Pennsylvania was actually
formed from tropical plant life near
the Equator? How did it travel
northward to Pennsylvania? Scientists
believe that 200 million years ago,
when the dinosaurs dined upon
tropical ferns and tall tropical
vegetation, what is now Pennsylvania
was at a different location, namely the
equatorial region.

3. 1. Alfred Wegener was the first scientist
to suggest that the fit of the continents of
South America and Africa may not be a
coincidence. He thought that the
continents may have been together at
some point in the past. He called this
“supercontinent” Pangaea
(this means “all land”).

4. 2. In 1912, he proposed the
theory of continental drift, which
means that the continents have
moved to their current locations.

5. 3. Besides the “puzzle-like” fit of
some of the continents, there is
substantial rock & fossil evidence
to support Wegener’s theory:

6. a. Like animal fossils have been
found oceans apart.

8. b. Fossils of the same green
plants have been found on almost
all continents, suggesting that
they all once had a similar
climates.

9. c. Additionally, fossils of tropical
plants have been found on islands
in the Arctic Ocean, suggesting
that area may have been much
warmer in the past.

10. d. Glacial deposits and
weathered rock (scientists think
this from moving ice) have been
found on South America, Africa,
India, and Australia. This could
mean that these lands were once
cold enough for glaciers to form
and eventually move around on
the land.

11. e. Similar rock structures are
found on different continents. For
example, mountains in the
United States are similar to ones
found on Greenland and in
Europe. Also, rocks in South
America and Africa are very
similar.

12. 4. The discovery of sea-floor
spreading on the ocean’s floors
has given Wegener’s theory of
continental drift more support.

14. a. In the 1950’s, it was found
that the ocean floors have
mountains, valleys, and ridges,
just like the land on earth.

15. b. It was suggested (and later
agreed upon) that these
formations have been caused
by magma from the mantle
being pushed up through the
crust. Then, it flows to each
side as the process repeats.
http://www.wwnorton.com/earth/egeo/animations/ch2.htm
(Sea Floor Spreading)

16. c. This idea was supported by the
dating of rock found along the
ocean’s ridges. The youngest
rocks are found right along the
ocean’s ridges. The ages of the
rocks becomes increasingly
older as you move away from
the ridges.
http://www.wwnorton.com/college/geo/earth2/home.asp#animations
(Click on animation to the right)

17. This map shows
the ages of the rocks that
make up the floor of the
Atlantic Ocean. Red
represents the youngest
rocks; the deepest red
marks the Mid-Oceanic
Ridge, where continental
plates are pulling apart
and new crust is being
formed. Older rocks are
yellow, green, and blue.

18. 5. Also supporting the idea of
seafloor spreading are clues
from the earth’s magnetic
poles.

19. a. Think of the earth having a giant
magnet in its core.

20. b. This creates a north and a south
poles on earth. We evidence of
these poles when we use a
compass.

21. c. Anyhow, scientists theorize that
these poles have reversed several
times over the lifespan of the earth.
This is supported by the discovery
of patterns in iron containing
minerals and rocks (remember, iron
is magnetic). Magnetic rocks are
“attracted” to the north pole, so
they are pulled towards it, and away
from the south pole.

23. Interesting…Our planet's magnetic
field reverses about once every
200,000 years on average.
However, the time between
reversals is highly variable. The
last time Earth's magnetic field
flipped was 780,000 years ago,
according to the geologic record
of Earth's polarity.

24. d. When the iron-containing
material on earth is studied, it
shows that this pattern has
changed over time. This is
especially apparent on the
oceans’ floor.

25. e. Did you know that the magnetic
north on earth is still
wandering? The magnetic pole
is currently 966 km (600 miles)
from the geographic one.

27. 6. All of this scientific work has led
scientists to the theory of plate
tectonics, which states that the
earth’s crust is broken in to
sections which move around (or
float) on the mantle below. It is
this theory that helps to explain
how the earth’s surface has
changed and will continue to
change over time.

28. So what will the earth look like in the
future? We can only guess…

29. So what will the earth look like in the
future? We can only guess…