1. Continental Drift & Plate Tectonics

2. Plate Tectonics Explains…
The formation of:
1. Oceans
2. Mid-ocean Ridges
3. Trenches
4. Mountains
5. Earthquakes
6. Volcanoes

3. Drifting Continents
Continental Drift
• (1912) Wegener proposed that continents
began to break apart about 200 million years
ago
• He called this continental drift, where
continents continue to slowly move to their
present positions.
• Pangaea, a Greek word that means “all the
earth,” refers to the combined landmass.
Wegener proposed that the continents were
joined as a single landmass called Pangaea.

4. Drifting Continents
A Rejected Hypothesis
• In the early 1900s, most scientists rejected
Wegener’s hypothesis of continental drift.
• Two unanswered questions—what forces
could move continents and how continents
could move without shattering—were the
main reasons that the hypothesis of
continental drift was rejected.

5. Pangaea

6. The Tethys Sea existed between the continents of Gondwana
and Laurasia before the opening of the Indian Ocean.

7. Historical Overview

8. Drifting Continents
Continental Drift
Evidence from Rock Formations
a) Wegener reasoned mountain ranges would
have fractured as the continents separated.
b) Using this reasoning, Wegener hypothesized
that similar rock types are on opposite sides of
the Atlantic Ocean.
c) That similar groups of rocks were observed in
the South America, Africa, United States,
Greenland, and Europe supported Wegener’s
idea.

10. Evidence of Continental Drift
Rock Record

11. Evidence of Continental Drift
Fossil Record
Similar animal and plant fossils were found on opposite
sides of the Atlantic Ocean.

12. Drifting Continents
Continental Drift
Evidence from Fossils

14. Drifting Continents
Continental Drift
Ancient Climatic Evidence
• Coal deposits
found in
Antarctica.
• Glacial rock
deposits in
Africa.

15. Seafloor Spreading Seafloor Spreading
• Seafloor spreading states that new ocean
crust is formed at ocean ridges and destroyed
at deep-sea trenches.
• An American scientist named Harry Hess proposed
the theory of seafloor spreading.
– Magma is forced
toward the crust
along an ocean
ridge and fills the
gap that is
created.

16. Seafloor Spreading
Seafloor Spreading
– When the magma hardens,
a small amount of new
ocean floor is added to
Earth’s surface.
– Each cycle of spreading
and the intrusion of
magma results in the
formation of another
small section of ocean
floor, which slowly moves
away from the ridge.

17. Theory of Plate Tectonics
Theory of Plate Tectonics
• The theory of plate tectonics states that
Earth’s crust is broken into slabs called
plates.
• There are a dozen major plates and
several smaller ones.
• Tectonic plates move in different
directions and at different rates over
Earth’s surface.

18. Theory of Plate Tectonics
Theory of Plate Tectonics

19. Theory of Plate Tectonics
Plate Boundaries
• Tectonic plates interact at places called
plate boundaries.
• At some plate boundaries:
– Plates come together, or converge
– Plates move away from one another, or diverge
– Plates move horizontally past one another, or
transform.

20. Theory of Plate Tectonics
Plate Boundaries (3)
1. Divergent Boundaries
– Divergent boundaries are places where two
tectonic plates are moving apart.
– Most divergent boundaries are found in rifts, or
fault-bounded valleys, which form along the axis
of an
ocean ridge.
– A rift valley, which is a
narrow depression, is
created when a
divergent boundary
forms on a continent.

21. Divergent Boundaries
cont…
– Some of the magma forms new oceanic crust at
the ridge or is forced back to the surface, forming
an arc of volcanic islands that parallel the trench.
– Oceanic plates are always YOUNGER than
continental plates.

22. Theory of Plate Tectonics
Plate Boundaries
2. Convergent Boundaries
– Convergent boundaries are places where
two tectonic plates are moving toward each
other.
– There are three types of convergent boundaries:
a. Oceanic crust converging with oceanic crust
b. Continental crust converging with continental crust.
c. Oceanic crust converging with continental crust

23. Plate Boundaries
Convergent Boundaries cont…
– Subduction occurs when one of the two converging
plates descends beneath the other.
– A subduction zone forms when one oceanic plate, which has
become denser as a result of cooling, descends below another
plate creating a deep-sea trench.
– The subducted plate descends into the mantle
and melts.

24. Theory of Plate Tectonics
Plate Boundaries
3. Transform Boundaries
– A transform boundary is a
place where two plates slide
horizontally past each other,
deforming or fracturing the
crust.
– Transform boundaries are
characterized by long faults
and usually offset sections
of ocean ridges.
– The San Andreas Fault is an
exception to the fact that transform
boundaries rarely occur on
continents.

25. Plate Boundaries Stress/Force Description/Facts
1. Divergent TENSION -Plates move apart.
-Lava creates new ocean
Example: floor.
Mid Ocean Ridge (MOR)
2. Convergent COMPRESSION -Plates move together.
a.Ocean – Continent -Ocean floor is destroyed by
Example: Andes Mtns. subduction.
-Mountains, volcanoes, and
b. Ocean – Ocean islands can be formed.
Example: Aleutian
Islands, Alaska. Japan.
c. Continent – Continent
Example: Himalayas.
3. Transform SHEARING -Plates move past each
Example: other.
-San Andreas Fault, CA
-MOR.

29. Theory of Plate Tectonics
Plate Boundaries
Convergent Boundaries

30. Activity: When Plates Create
You need:
4 Pages: Instructions, 114, 115, & 116.
5 colored pencils: orange, gray (or black), blue, brown, and red.
1. COLOR and LABEL puzzle pieces:
a. Orange (Magma)
b. Gray or Black(Oceanic Crust)
c. Gray or Black (Oceanic Crust)
d. Gray or Black(Oceanic Crust)
e. Orange (Magma)
f. Blue (water)
Brown (Islands)
g. Gray or Black (Oceanic Crust)
h. Orange (Magma)
i. Blue (water)
Brown (Islands/volcanoes)
Orange (Magma)
2. Answer questions and STAPLE on your “Plates,” and turn in.