Geologist Alfred Wegener first proposed the theory of continental drift in 1912. He noticed that the continents seemed to fit together and referred to their original combined landmass as Pangaea. The theory proposed that the continents were once joined together but have since drifted apart. Supporting evidence included matching fossil and rock formations between continents, as well as evidence that mountain ranges and glacial scars on separate continents aligned when the continents were fitted together. Exploration of the seafloor in the 1960s provided further evidence, revealing mid-ocean ridges and magnetic stripe patterns that supported the new theory of seafloor spreading, where new crust is formed at ocean ridges and pushes existing crust apart over geologic time.

1. CONTINENTAL DRIFT
http://www.tectonics.caltech.edu/movi
es/outreach/sumatra/pangaea.mov

5. Alfred L. Wegener
▪ Geologist Alfred Wegener
noticed similar rocks & fossil
remains were found on
continents which seemed to
fit together
▪ He called this “super”
continent called Pangaea
▪ In 1912 Wegener
published the
first version &
died defending
his theory

6. CONTINENTAL DRIFT THEORY
Theory that
continents were
once part of a
single landmass
that broke apart
and have moved to
their present
locations.

7. PANGAEA
the large landmass that included all of Earth’s
present day continents

8. Support for Continental Drift
1. SHAPE OF THE CONTINENTS
Continents fit together like puzzle pieces

9. Support for Continental Drift
▪ Wegener was not the only
one to notice the fit of the
continents.
▪ 1858: Antonio Pellegrini
depicted Africa and South
America connected.
▪ He was one of the first
scientists to publish maps
depicting the apparent fit of
the continents.
1. SHAPE OF THE CONTINENTS

10. 2. FOSSIL EVIDENCE
SAME FOSSILS: DIFFERENT CONTINENTS
Support for Continental Drift

12. 3. ROCK EVIDENCE – Mountain ranges
Support for Continental Drift
▪ SAME ROCK:
DIFFERENT RANGE
Existing mountain ranges
separated by vast
oceans contain rocks of
identical mineral content.
▪ A prime example are the
Appalachian Mountains
in the eastern U.S and
the Caledonian
Mountains in the British
Isles.

13. different continents:
- Rocks found in Appalachian
Mountains similar to rocks in
Greenland and western Europe
- Rock structures from eastern
South America similar to
western
Africa

15. SAME SCARS:
DIFFERENT CONTINENTS
Support for Continental Drift
4. GLACIAL SCARS

16. 5. LOCATION OF COAL DEPOSITS
Coal deposits have been found in temperate and polar
regions; however, coal is formed in tropical regions.
Support for Continental Drift

17. CONTINENTAL DRIFT IN DOUBT
Why didn’t people believe in continental drift?
▪ People couldn’t image how the earth could
be millions of years old
▪ People couldn’t image a force great enough
to move the continents
BUT by the 1960’s evidence would prove
continental drift is TRUE and…. The story
continues (as does all good science!)

18. Support
▪ Echo sounding was used to map the seafloor
Seafloor features were
discovered using SONAR.

19. ▪ What information do you know
regarding the seafloor that may
be used to help understand the
hypothesis of seafloor spreading?
Exploration of the Ocean
Basins

20. Exploration of the Ocean
Basins
▪ Seafloor features
▪ Composition of the seafloor
▪ Age of the seafloor
▪ Hot spots
▪ Seismicity
▪ Paleomagnetic pattern produced
on the seafloor

21. Exploration of Ocean Basins
Ocean basins contain a variety of landforms not discovered
until WW II.

22. Seafloor
features
▪ Mid-oceanic
ridges
▪ Trenches
▪ Volcanism
adjacent to
trenches
▪ Seamounts
▪ Fracture
zones

23. Continental
shelf
Submarine
canyons
(cut into the c.
slope)
Abyssa
l
plain
Continental
rise
Continent
al
slope
Seafloor Features:
Continental Margins
Abyssa
l
plain

24. Seamoun
ts
Abyssal
Hills
(linear
hills)
Mid-Atlantic
RIDGE)
Deep Ocean

29. ▪ In the 1963, Harry
Hess developed
the idea of
seafloor spreading
to explain the
seafloor’s
formation
▪ Continental
movement is the
result of ocean
floor movement
Seafloor Spreading

31. Seafloor Spreading
▪ Mafic lava is extruded along the mid-oceanic
ridge to produce ocean floor.
▪ New material is added, the cooler material is
split and is “pushed” away from the ridge

33. DIVERGENT BOUNDARIES
Make new crust: Sea Floor Spreading
▪ As magma rises and cools, iron
and magnesium minerals align
themselves to the Earth’s
magnetic field
▪ “Magnetic Stripes” are formed
▪ The Earth’s magnetic field
reverses polarity
▪ Magnetic stripes are mirrored on
each side of the mid-ocean ridge
Every ~20,000 years the polarity of the Earth changes.
This is change is reflected in the rocks, whose
polarity is fixed once the magma is cooled.

34. Findings that support
Seafloor Spreading Theory
1. Rocks are younger at the mid-
ocean ridge and rocks far from
the mid-ocean ridge are older
2. Sediments are thinner at the
ridge
3. Magnetic Stripes

35. WILL THE CONTINENTS REJOIN
SOMETIME IN THE FUTURE?

37. What Drives It: Convection

38. Driving Mechanisms
Convection cells
within the mantle
Pushing at the
mid-oceanic ridges
Pulling at the
subduction zones
Push
Pull
Convection