According to the theory of plate tectonics: - The Earth's crust is broken into plates that slowly move over time; - Plates interact at boundaries where they can collide, pull apart, or slide past one another; - These interactions are responsible for geologic events like earthquakes, volcanoes, and mountain building.

1. Continental Drift
Theory &
Plate Tectonic Theory

2. •Alfred Wegener in the early
1900’s proposed the hypothesis
that continents were once joined
together in a single large land
mass he called Pangea (meaning
“all land” in Greek).
• He proposed that Pangea had
split apart and the continents had
moved gradually to their present
positions - a process that became
known as continental drift.
CONTINENTAL DRIFT

3. CONTINENTAL DRIFT
According to the hypothesis of
continental drift, continents
have moved slowly to their
current locations.

4. Pangaea about 200 million years ago, before it began breaking up.
Wegener named the southern portion of Pangaea Gondwana, and
the northern portion Laurasia.

5. The continents about 70 million years ago. Notice that the
breakup of Pangea formed the Atlantic Ocean. India’s eventual
collision with Eurasia would form the Himalayan Mountains.

6. The position of the continents today. The continents are still
slowly moving, at about the speed your fingernails grow. Satellite
measurements have confirmed that every year the Atlantic Ocean
gets a few inches wider!

8. Continents fit together
like a puzzle….e.g. the
Atlantic coastlines of
Africa and South
America.
The Best fit includes the
continental shelves (the
continental edges under
water.)
Wegener’s Evidence for
Continental Drift
Picture from
http://www.sci.csuhayward.edu/~lstrayer/geol2101/2101_Ch19_03.pdf

9. Wegener’s Evidence for
Continental Drift
Fossils of plants and animals of the
same species found on different
continents.
Picture from
http://volcano.und.edu
/vwdocs/vwlessons/pl
ate_tectonics/part3.h
tml

10. Wegener’s Evidence for
Continental Drift
• Rock sequences (meaning
he looked at the order of
rock layers) in South
America, Africa, India,
Antarctica, and Australia
show remarkable
similarities.
• Wegener showed that the
same three layers occur
at each of these places.
• Picture from
http://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/p
art4.html

11. Wegener’s Evidence for
Continental Drift
• The same three layers are
in the same order in areas
now separated by oceans.
• Wegener proposed that
the rock layers were made
when all the continents
were part of Pangaea.
• He proposed that they
formed in a smaller small
joined land mass that was
later broken and drifted
apart. Picture from
http://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/p
art4.html

12. Seafloor Spreading
• Everyone agreed that Wegener’s evidence
was compelling. But wouldn’t we feel the
movement?
• Also, wouldn’t there be evidence to show
that the continents were still moving
today?
• Wegener was a meteorologist and his
theory was not well accepted.

13. Seafloor Spreading
• One reason scientists
had a hard time with
Wegener’s theory is
that there was no
mechanism for the
continents motion.

14. Seafloor Spreading
• In the 1960’s, a scientist
named Henry Hess made a
discovery that would
vindicate Wegner.
• Using new technology, radar,
he discovered that the
seafloor has both trenches
and mid-ocean ridges.
• Henry Hess proposed the sea-
floor spreading theory.
Picture from USGS
http://pubs.usgs.gov/gip/dynamic/HHH.html

15. Seafloor Spreading
• 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 the seafloor
away from the ridge in
both directions.
Picture from http://library.thinkquest.org/17457/platetectonics/4.php

16. Seafloor Spreading
• As the seafloor spreads apart at a mid-
ocean ridge, new seafloor is created.
• The older seafloor moves away from the
ridge in opposite directions.
• This helped explain how the crust could
move—something that the continental
drift hypothesis could not do.
Picture from
http://www.pbs.org/wgbh/aso/tryit/tectonics/divergent.html

17. • In 1968, scientists aboard the
research ship Glomar Challenger
began gathering information about
the rocks on the seafloor.
• Scientists found that the youngest
rocks are located at the mid-ocean
ridges.
Evidence for Spreading

18. Mechanism for
Plate Tectonics
• Seafloor Spreading provided
insight to the mechanism for
how the continents moved.
• The magma which pushes up
at the mid-ocean ridge
provides the new land
pushing the plates, and the
subduction zones gobble up
the land on the the other
side of the plates. The mechanism was
convection currents!
Picture from
http://library.thinkquest.org/17457/platetectonics/2.php

19. Plate Tectonic Theory
• Both Hess’s discovery and
Wegner’s continental drift
theory combined into what
scientists now call the Plate
Tectonic Theory.
• Theory of plate tectonics:
• The Earth’s crust and part of the
upper mantle are broken into
sections, called plates which
move on a plastic-like layer of
the mantle

20. Plate Tectonic Theory
• Plate Tectonics explains
– Earthquakes
– Mountains
– Volcanoes

22. Structure of the Earth
• The Earth is
made up of 3
main layers:
– Core
– Mantle
– Crust
Inner core
Outer core
Mantle
Crust

23. The Crust
• This is where we live!
• The Earth’s crust is made
of:
Continental Crust
- thick (10-70km)
- buoyant (less
dense than oceanic
crust)
- mostly old
Oceanic Crust
- thin (~7 km)
- dense (sinks under
continental crust)
- young

24. Plate Tectonics
• The Earth’s crust is divided into 12 major
plates which are moved in various directions.
• This plate motion causes them to collide,
pull apart, or scrape against each other.
• Each type of interaction causes a
characteristic set of Earth structures or
“tectonic” features.
• The word, tectonic, refers to the
deformation of the crust as a consequence
of plate interaction.

25. What are tectonic plates made
of?
• Plates are
made of rigid
lithosphere.
The lithosphere is made
up of the crust and the
upper part of the
mantle.

26. What lies beneath the tectonic
plates?
• Below the
lithosphere
(which makes
up the tectonic
plates) is the
asthenosphere.

27. Plate Movement
• “Plates” of lithosphere are moved around
by the underlying hot mantle convection
cells

28. • Divergent
• Convergent
• Transform
Three types of plate boundary

29. • Spreading ridges
– As plates move apart new material is erupted
to fill the gap
Divergent Boundaries

30. • Iceland has a divergent
plate boundary running
through its middle
Iceland: An example of continental rifting

31. • There are three styles of convergent
plate boundaries
– Continent-continent collision
– Continent-oceanic crust collision
– Ocean-ocean collision
Convergent Boundaries

32. • Forms mountains, e.g. European Alps, Himalayas
Continent-Continent Collision

33. Himalayas

34. • Called SUBDUCTION
Continent-Oceanic Crust Collision

35. • Oceanic lithosphere
subducts underneath the
continental lithosphere
• Oceanic lithosphere heats
and dehydrates as it
subsides
• The melt rises forming
volcanism
• E.g. The Andes
Subduction

36. • When two oceanic plates collide, one runs over the
other which causes it to sink into the mantle
forming a subduction zone.
• The subducting plate is bent downward to form a
very deep depression in the ocean floor called a
trench.
• The worlds deepest parts of the ocean are found
along trenches.
– E.g. The Mariana Trench is 11 km deep!
Ocean-Ocean Plate Collision

38. • Where plates slide past each other
Transform Boundaries
Above: View of the San Andreas
transform fault

40. Volcanism is
mostly focused
at plate margins
Pacific Ring of Fire

41. - Subduction - Rifting - Hotspots
Volcanoes are formed by:

42. Pacific Ring of Fire
Hotspot
volcanoes

43. • Hot mantle plumes breaching the
surface in the middle of a tectonic
plate
What are Hotspot Volcanoes?
Photo: Tom Pfeiffer /
www.volcanodiscovery.com
The Hawaiian island chain are
examples of hotspot volcanoes.

44. The tectonic plate moves over a fixed hotspot forming a
chain of volcanoes.
The volcanoes get younger from one end to the other.

45. • As with volcanoes, earthquakes are not
randomly distributed over the globe
• At the boundaries between plates,
friction causes them to stick together.
When built up energy causes them to
break, earthquakes occur.
Figure showing
the distribution
of earthquakes
around the globe

46. Where do earthquakes
form?
Figure showing the tectonic setting of earthquakes

47. Plate Tectonics Summary
• The Earth is made up of 3 main layers
(core, mantle, crust)
• On the surface of the Earth are tectonic
plates that slowly move around the globe
• Plates are made of crust and upper mantle
(lithosphere)
• There are 2 types of plate
• There are 3 types of plate boundaries
• Volcanoes and Earthquakes are closely
linked to the margins of the tectonic plates