1. 1

2. Hess was in World War II
He was initially assigned to duty in New York City, where
he was responsible for estimating the positions of enemy
submarines in the North Atlantic. Hess was then assigned
to active sea duty and eventually became commander of
an attack transport ship. This vessel carried equipment
for sounding the ocean floor, and Hess took full
advantage of it. He mapped a large part of the Pacific
Ocean, discovering in the process the underwater flattopped seamounts that he named guyots, in honor of A.H.
Guyot, the first professor of geology at Princeton.
2

3. Sonar
• A device that bounces
sound waves off
underwater objects
and then records the
echoes of these sound
waves.
• Sonar mapped midocean ridges.

4. Hypothesis of sea-floor spreading
Molten rock (magma) oozes up from the
Earth's interior along the mid-oceanic
ridges, creating new seafloor that
spreads away from the active ridge
crest and, eventually, sinks into the
deep oceanic trenches.
4

5. Sea-floor
spreading

6. • Continental drift reexamined in 1960’s with new
information
• New theory developed – Seafloor spreading
– Earthquakes
– Volcanism
– Age of seafloor
– Paleomagnetism
– Heat flow
• Theory combining continental drift and seafloor
spreading termed “Plate Tectonics”

7. • The submersible,
Alvin, found strange
rocks shaped like
toothpaste squeezed
from a tube. Such
rocks can form only
when molten material
hardens quickly after
erupting under water.

8. Scientists discovered that the rock
that makes up the ocean floor lies in a
pattern of magnetized “stripes”.
They hold a record of reversals in
Earth’s magnetic field

9. The earth’s magnetic field does not point exactly north

10. The magnetic field is produced by the motion
of electric charges, i.e., electric current
10

11. The Earth's magnetic field
appears to come from a giant
bar magnet, but with its
south pole located up near
the Earth's north pole (near
Canada).
The magnetic field lines come out of the Earth near Antarctica and enter
near Canada.
11

12. The liquid iron
appears to
produce electric
currents in the
opposite
direction of the
earth rotation.
The currents
create the earth
magnetism.
12

13. The earth’s magnetic field direction
The study of paleomagnetism has demonstrated that
the Earth's magnetic field has changed over time.

15. 15

16. The portion of the ocean crust nearest the
mid-ocean ridge must have what type
of magnetic polarity?
1. normal
2. reverse
3. none
63%
28%
9%

17. We don’t know!!!
Can you find a solution to this
tough scientific problem?
17

18. Scientists did a drilling
sample and found rocks that
the farther away from the
ridge the older the rocks
were. The younger ones
were in the center of the
ridge.

19. 19

20. ocean floors - 200 million years
land - billions of years
Why is seafloor so young relative
to continents?
SUBDUCTION

21. New sea floor created at the mid-ocean
ridge and destroyed in deep ocean trenches

22. Subduction
Ocean floor plunges into deep-ocean trenches.
Subduction is the process by which the ocean floor sinks
beneath a deep-ocean trench and back into the mantle
22

23. Theory of
Plate
Tectonics

24. The theory of plate tectonics
Earth’s lithosphere is broken into giant
plates that move laterally on top of
asthenosphere.
Most earthquakes and volcanic
eruptions happen at plate boundaries.
Three types of relative motions between
plates:
24

25. The plate refers to the pieces of the lithosphere.
The plates of the lithosphere moves atop the
asthenosphere

26. Current Earth’s surface layers are divided into
nine very large plates and several smaller ones

27. Earthquake distribution matches plate boundaries

28. Volcanoes match some plate boundaries;
some are hot spots

29. Convergent
Divergent
Transform
29

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

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

32. Himalayas

33. Continent-Oceanic Crust Collision
• Called SUBDUCTION

34. Earthquake depth indicates subduction zones

35. Ocean-Ocean Plate Collision
• 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!

36. Ocean-Ocean plate collision
Modern example: Japan

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

39. San Andreas Fault near Gorman, California. The grey, metamorphic quartz
monzonite on the left side of the fault are rocks of the Pacific Plate and the brown
sandstone and siltstone on the right of the fault are rocks of the North American
Plate. Photograph copyright by David Lynch.
Plate
39

40. • Plates move away from each other
• New crust is being formed

41. An example  Atlantic Ocean

42. Divergent
boundaries
also can rip
apart (“rift”)
continents

43. Continental Drift
1) Begins with view of Earth with continents in their present
positions, 2) continents move back in time to reunite as Pangaea, 3)
Pangaea label appears, 4) Locations of stratigraphic and fossil
evidence that Wegener used to argue in favor of continental drift is
added.
http://www2.nature.nps.gov/geology/usgsnps/animate/A14.gif

44. What are the
driving forces of
plate motion?

45. Rayleigh-Benard Convection
45

46. Atmospheric Convection
Fig. 5-11, p. 118

47. Street Clouds

48. Street Clouds over City College of New York taken
by Professor Gedzelman

49. How does Earth work?

50. Theory of Plate Tectonics

51. How does convection work? No one knows
but they aren’t afraid to propose models!
Two mantle convection cells
Complex convection

52. Divergent
Convergent
Transform

53. Transform
Convergent
Divergent