Plate tectonics theory proposes that Earth's outermost layer is broken into rigid tectonic plates that constantly move atop the asthenosphere in response to convection currents in the mantle, interacting along plate boundaries through divergent, convergent, and transform motion that creates geologic features like mid-ocean ridges, trenches, volcanoes, and earthquakes. Mapping of the seafloor revealed evidence like magnetic stripes and the ages of rocks that supported seafloor spreading and subduction, leading scientists in the 1960s to combine these ideas with continental drift into the unified theory of plate tectonics.

1. Plate Tectonics
Drifting continents:
Theory of Continental Drift

2.  Alfred Wegener suggested that all
continents were joined together at
some time in the past. This is called
the hypothesis of “continental drift”.
 According to continental drift,
continents have moved slowly to their
current locations.
 He called this large landmass as
Pangaea (“all land”)

4. Evidence
 1. Landforms
 2. Fossils
 3. Evidence from climate
changing

5. Landforms
• Mountains in
South Africa
line up with
mountains in
Argentina
• Coal mines
line up on
different
continents

6. Mountain Structures
 Similar rock structures are
found on different continents.
 For example, parts of
Appalachian mountains are
similar to those found in
Greenland and western Europe.

7. Appalachian Mountains
strike through the
eastern United States and
Canada then end at the
sea off of Newfoundland.
The Caledonides
Mountains are found in
eastern Greenland,
Ireland, Great Britain
and Norway.
These two mountain
ranges are the same age
with the same rock types
and structures.

8. Fossils
 Besides the puzzlelike fit of continents, fossils provided
support for continental drift.
 For example: fossils of the reptile Mesosaurus have
been found in South America and Africa.

9.  Another fossil that supports the continental
drift is Glossopteris which has been found in
Africa, Australia, India, South America,
and Antarctica.

10. Climate Clues
 Fossils of tropical plants were found in Spitsbergen,
an ice covered island in the Arctic Ocean (north of
Norway).

11. Glacier Activity
 Glacial deposits and rock
surfaces scoured and polished by
glaciers are found in South
America, Africa, India and
Australia.

12. Why was it not accepted?
 Wegener could not provide an adequate
explanation for the force that pushes or pulls
the continents.

13. Although Wegener provided evidence to support his
hypothesis, he couldn’t answer two questions:
1. What was causing the continents to move?
Wegener said that this force might be the rotation of Earth,
however, physicists were able to show that this force was not
great enough to move continents.

14. 2. How were the continents moving?
Wegener proposed that the continents were
plowing through a stationary ocean floor. But, his
peers argued that continents could not push through
the ocean floor without fracturing, because crustal
rock is too brittle. And no evidence of fracturing had
been found.

15. And the rest of the story….
Undaunted by rejection, Wegener devoted the rest
of his life to doggedly pursuing additional
evidence to defend his theory. He froze to death in
1930 during an expedition crossing the Greenland
ice cap, but the controversy he spawned raged on.
However, after his death, new evidence from
ocean floor exploration and other studies rekindled
interest in Wegener's theory, ultimately leading to
the development of the theory of plate tectonics.
http://bumileluhur.blogspot.com/2011/01/this-dyniamic-earth-story-of-plate.html

16. The controversy continues!
Plate tectonics has proven to be as important to the earth
sciences as the discovery of the structure of the atom was to
physics and chemistry. Even though the theory of plate
tectonics is now widely accepted by the scientific
community, aspects of the theory are still being debated
today. Ironically, one of the chief outstanding questions is the
one Wegener failed to resolve: What is the nature of the
forces propelling the plates? Scientists also debate how plate
tectonics may have operated (if at all) earlier in the Earth's
history and whether similar processes operate, or have ever
operated, on other planets in our solar system.
Will you be the one to solve the
mystery?

17. Plate Tectonics
Seafloor Spreading
Harry
Hess

18. Mapping The
Ocean Floor
 Scientists began using sound waves on moving
ships to map large areas of ocean floor in detail.
 Sound waves echo off the ocean bottom – the
longer the sound waves take to return to the
ship, the deeper the water is.

19. Mapping the Mid-Ocean Ridge
 Also known as the Mid-
Atlantic Ridge
 Longest mountain
chain in the world
 Divided by a trench

20.  Most is hundreds of meters under water, but part
reaches above the ocean surface.
 It is almost twice as deep as the Grand Canyon.
 Iceland is part of the mid-ocean ridge

21.  In the early 1960s, Harry Hess proposed that hot,
less dense material below Earth’s crust rises
toward the surface at the mid-ocean ridges.
Then, it flows sideways, carrying seafloor away
from the ridge in both directions. This theory is
known “seafloor spreading”.

22.  In 1968, a research ship, Glomar Challenger, began
gathering rock samples from mid-ocean ridges.
 They made a remarkable discovery as they studied the
ages of rock samples.
 The younger rocks are closer to the mid-ocean ridges
and older rocks are farther from the ridges.

23. Evidence of Sea-Floor Spreading
 Molten Material
 Magnetic Stripes
 Drilling Samples

24. Magnetic Clues
 Iron-bearing minerals,
such as magnetite, that
are found in the rocks of
the seafloor can record
Earth’s magnetic field
direction when they
form.
 The magnetic alignment
in the rocks reverses
back and forth over time
in strips parallel to the
mid-ocean ridges.

25. Seafloor
Spreading

26. Subduction of Deep-Ocean
Trenches
 Subduction is the
process by which
the ocean floor
sinks beneath a
deep-ocean trench
and back into the
mantle.

27. Subduction and Earth’s Ocean
 Subduction and sea-floor spreading can
change the size and shape of the oceans.
 The ocean floor is renewed about every 200
million years.

28.  The Pacific Ocean is shrinking; a trench swallows
more oceanic crust than the mid-ocean ridge can
produce.
 The Atlantic Ocean is expanding.
 There are only a few trenches in the Atlantic.
 The continental crust is attached to the ocean floor, so
as the Atlantic expands and moves, the continents
move with it.

29. Section 3:
theory of Plate
tectonicS
 In the 1960s, scientists developed a new theory that
combined continental drift and seafloor spreading .
 According to the theory of plate tectonics, Earth’s
crust and part of upper mantle are broken into
sections. These sections are called plates, move on a
plastic-like layer of the mantle.

30. Where did the theory come
from?
 J. Tuzo Wilson combined Earth’s plates,
continental drift, and sea-floor spreading into a
single theory---PLATE TECTONICS

31. A Theory of Plate Tectonics
 Plate tectonics states that pieces of Earth’s
lithosphere are in constant, slow motion, driven by
convection currents in the mantle, and it explains
the formation, movement, and subduction of
Earth’s plates.

32. Basically….
 The plates on the lithosphere float on the
asthenosphere, convection currents rise in the
asthensophere and spread out beneath the
lithosphere.
 http://youtu.be/ryrXAGY1dmE

33.  Earth’s crust and a part of upper mantle combined
are the lithosphere. (100km=62 mile thick)
 The plastic-like layer below the lithosphere is
called asthenosphere. The rigid plates of the
lithosphere float and move around on the
asthenosphere.

34. Causes of Plate Tectonics
 Convection currents cause the movements of plates
 Hot, less dense liquid or gas is forced upward, as it
reaches the surface, it cools down and sinks back
down. This entire cycle of heating, rising, cooling
and sinking is called a convection current.

35.  No plate can budge without affecting
the other plates surrounding it!

38. Plate Boundaries
 When plates move, they can interact in several
ways. Three different moving types of plates are:
 Plates moving apart (Divergent)
 Plates moving together (Convergent)
 Plates slide past each other (Transform)

39.  Faults break in Earth’s crust
where rocks have slipped past each
other.
 Plate boundaries are where the
edges of different pieces of the
lithosphere meet.

40. Gummy Time
 Boundary Activity

41. Transform Boundary
 Two plates slip past each other, moving in
opposite directions. (Do not push or pull)
 Crust is neither created nor destroyed near a
transform boundary.
 Earthquakes occur frequently along these
boundaries.

42. Divergent Boundary
 Two plates move apart or diverge.
 Most occur at the mid-ocean ridge; they can also
occur on land (poor Africa).
 A rift valley is a deep valley that forms along the
divergent valley.
 The Great Rift Valley in Africa marks a deep
crack in the African continent that runs for about
3,000 km.
 When divergent boundaries develop on land, two
of Earth’s plates pull apart.

43. Convergent Boundary
 Two plates come together or collide (converge)
 Results: earthquakes, trenches, volcanoes,
mountains
 When two plates collide the denser of the two
plates will sink or slide beneath the other.

44. 3 types of convergent boundary
outcomes
1) Oceanic crust meets continental crust:

45.  Oceanic crust meets oceanic crust:

46.  Continental crust meets continental crust:
 Example: Folded Mountains

47. The continents’ slow dance
 The plates move at slow rates about 1 to 10 cm
per year.
http://youtu.be/hSdlQ8x7cuk

48. To prevent confusion
 Not all volcanoes
occur at plate
boundaries. For
example, the
Hawaiian islands
formed from a hot
spot volcano. As the
Pacific plate moves
across a fixed hot spot
(mantle plume), new
volcanoes (and
islands) form.

49. Summary
Theories: Continental Drift Theory of Plate
Theory Tectonics
Forces of Plate Cycle of Ridge Push/Gravity Slab Pull
Movement: Convection ******
Currents
Types of Plate Divergent Convergent Transform
Movement/
Interactions:
(Boundary)
Processes or Sea-floor Subduction Earthquakes
Types of Spreading (Volcanoes) and
Movement Mountains
(Results):
Stress: Tension Compression Shearing
Faults: Normal Reverse Strike Slip

50. Cookie Time
 Boundary Activity