1. The Earth is layered, with a dense inner core surrounded by less dense layers. New evidence in the late 1800s showed the Earth has an interior that is hot due to volcanoes. 2. The crust varies in thickness and density around the world, with thick continental crust and thin oceanic crust. Below the crust is the mantle, which is solid but flows like a liquid over geologic timescales. 3. Seismic waves provided evidence that helped reveal the layered structure of the Earth. P and S waves travel at different speeds and through different materials, allowing scientists to map the interior of the Earth.

1. 1

2. The Earth is
layered
knowledge of layering is recent (late
1800s); prior to that, only knew
interior must be hot (volcanoes)

3. mean density ~ 5500 kg/m3, but surface rocks ~2300 kg/m3
therefore, density gradient exists

4. Crust
1000 o C

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

6. Crust
Mantle is
essentially solid,
but behaves like
liquid.
1000 o C
Lithosphere
Asthenosphere

7. Crust
1000 o C
Lithosphere
Asthenosphere
Mantle
is essentially solid,
but behaves like
liquid.
2885 km
4000 o C
Outer core
is liquid of iron and nickel.
2230 km
Inner core
is solid of iron and nickel.
7000 o C
1230 km

8. The lithosphere is
a. Continental crust
b. Oceanic crust
c. A combination of the crust and the uppermost
mantle
d. Athenosphere

9. How do we
know the Earth
is layered?

10. Earthquakes:
One way to learn about the Earth’s interior
Kump et al (2000)
10

11. Seismic body
waves: P and
S waves
P-waves:
-Primary waves
-Series of compression
and expansions in the
direction of wave
movement
- much like a slinky
S-waves:
- Secondary waves
- Perpendicular
replacement
P-Waves
S-Waves
Kump et al (2000) 11

12. How to remember them
• P waves
– Primary waves or pressure waves
• S waves
– Secondary waves or shear waves
12

13. Where do P and S waves
propagate?
• P waves travel though both
solids and liquids
• S waves travel only through
solids
13

14. Fig. 5.04

15. Outer core
(liquid)
P-wave
Inner core
(solid)
S-wave
Mantle
(solid)
15

16. Moho
Andrija Mohorovicic (1857-1936)
Found seismic discontinuity at
30 km depth in the Kupa Valley
(Croatia).
Mohorovicic discontinuity or ‘Moho’
Boundary between crust and mantle

17. Moho discontinuity
Boundary 
Crust/Mantle
Boundary
 Mantle/outer core
Boundary
 Outer core/Inner core
17

19. the originator of
the theory of
continental drift
Photograph courtesy of the Alfred Wegener Institute
for Polar and Marine Research, Bremerhaven,
Germany
Kump et al (2000)
19

21. 1. Supercontinent Pangaea started to break up
about 200 million years ago.
2. Continents "drifted" to their present positions.
3. Continents "plowed" through the ocean crust.
21

22. Earth ~200 million years ago

23. Wegener used land features, fossils, and
climate as evidence to support his
hypothesis of continental drift.
Examples of land features such as mountain ranges in
Africa and South America lined up; also coal fields on Europe
matched up with coal fields in North America. Wegener
noticed that fossils from reptiles that were found in places
that are now separated by oceans. Since neither reptile
could have swum great distances, Wegener inferred that
these reptiles had once lived on a single landmass that split
apart.
23

24. Continental Drift: Evidence
Fossil critters and plants

25. Continental Drift:
Evidence
Correlation of
mountains with nearly
identical rocks and
structures

26. Continental Drift:
Evidence
Glacial features of the
same age restore to a
tight polar distribution.

27. Alfred Wegener published his book
with title “The Origin of Continents
and Oceans” in 1915.
In his book, Dr. Wegener drew
together evidence from various
fields. Expanded editions during
the 1920s presented the
accumulating evidence.
27

28. Alfred Wegener also came up with
a theory to explain continental
drift, although it was in error.
His theory of continental drift proposed that centrifugal
force moved the heavy continents toward the equator as
the Earth spun. He thought that inertia, from centrifugal
movement combined with tidal drag on the continents
(caused by the gravitational pull of the sun and moon)
would account for continental drift.
28

29. -- Newton’s first law of motion: an object will not
Newton’s first law of motion: an object will not
accelerate unless a force is applied.
accelerate unless a force is applied.
-- Of cause, our reference frame is accelerating
Of cause, our reference frame is accelerating
(rotating)!
(rotating)!
-- Centrifugal force.
Centrifugal force.
-- What is difference between Centrifugal force and
What is difference between Centrifugal force and
centripetal force?
centripetal force?
-- Action and reaction
Action and reaction
-- So the magnitude as what centripetal force is:
So the magnitude as what centripetal force is:

30. What are Centrifugal force and centripetal force?
What are Centrifugal force and centripetal force?
-- Action and reaction
Action and reaction

31. Alfred Wegener’s explanation of
continental drift was like “ice drift”
Centrifugal and tidal forces were far too weak to move continents!
31

32. Fig. 5.02a

33. Alfred Wegener’s explanation of
continental drift was like “ice drift”
Centrifugal and tidal forces were far too weak to move continents!
Another problem was that flaws in Wegener's original data caused
him to make some incorrect: he suggested that North America and
Europe were moving apart at over 250 cm per year (too fast, about
a hundred times faster than the measured rate).
33

34. Reaction of Alfred Wegener’s
continental drift theory
While his ideas attracted a few early supporters from South Africa
and Europe, the hypothesis was generally met with skepticism.
The one American edition of Wegener's work, published in 1924, was
received so poorly that the American Association of Petroleum
Geologists organized a symposium specifically in opposition to the
continental drift hypothesis. Also its opponents could argue that the
oceanic crust was too firm for the continents to "simply plow through".
through
By the 1930s, Wegener's geological work was almost universally
dismissed by the scientific community and remained obscure for some
thirty years.
34

35. 35

36. Resurrection of Alfred Wegener’s
continental drift theory
•In the 1950s and 1960s, several developments in geology,
notably the discoveries of seafloor spreading, led to the
rapid resurrection of the continental drift hypothesis and
its direct descendant, the theory of plate tectonics.
tectonics
•Alfred Wegener was quickly recognized as a founding
father of one of the major scientific revolutions of the
20th century.
36

37. Who resurrected the theory of
continental drift?
Rear Admiral Dr. Harry
Hammond Hess was born
in New York City,
received B.S. from Yale
University,
He received Ph.D. from
Princeton University, was
a Professor of geology
from 1934 to 1969 at
Princeton University
Harry Hammond Hess
(May 24, 1906 – August 25, 1969)
Considered one of the "founding fathers" of the unifying theory of plate tectonics,
The founding father of seafloor spreading theory.
37

38. Who resurrected the theory of
continental drift?
Dr. Harry Hammond Hess
He believe in many of the observations Wegener used in defending his theory of
continental drift, but he had very different views about large-scale movements of
the Earth, suggesting that the convection of the Earth's mantle was the driving
force behind this process. This work provided a conceptual base for the
development of the theory of plate tectonics.,
38

39. 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.
39

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