1. Plate Tectonics What is it and what makes it work? Oceanography

2. The Earth’s interior
1000 C
4000 C
7000 C

3. Pea-soup analogy

4. mantle convection Heat transfer from the core to the mantle
Produces slow convection currents
of the mantle material
(in the order of centimetres per year)

5. Mantle convection currents produce lateral forces at the boundary between the mantle and the crust

6. Lateral forces at the crust-mantle boundary push the crust around – in different directions leading to the motion of crustal plates.

7. The plates are moving at rates of a few centimeters per year San Diego is moving at 5 cm/year vs Imperial valley is moving 1 cm/year

8. Plate motions

9. Divergent boundary

10. Convergent boundary
Subduction

11. Transform boundary

12. Plates of the Earth’s crust

13. Plates around North America

14. The Theory of Plate Tectonics
Unifying principle of geology.
Evolved in the late 1960s, early 1970s.
Realization that certain parts of the Earth were more
affected by earthquakes and volcanoes.
Also noted that large mountainous regions occur on
margins of continents.
Changes in sea level were related to these events.
They were not a random occurrences.

15. Distribution of major volcanoes and earthquakes “Ring of Fire”

16. The Theory of Plate Tectonics
Many hypotheses were presented to answer these
questions.
None answered all of the questions at all localities.
This led to the hypothesis of PLATE TECTONICS.
It became widely accepted in a few short decades.

17. Steps in Development The Theory of Plate Tectonics
1. Benjamin Franklin (late 1700s)
Recognized that crust of Earth was a shell.
Surface could be broken and parts moved about.
2. Alfred Wegener (1912)
German meteorologist-geophysicist
Proposed theory of Continental Drift.
Proposed continents float on a denser
underlying interior of the Earth.
CONTINENTS periodically break up and
DRIFT apart.

18. The Theory of Plate Tectonics
Wegener believed all continents were joined together.
Supercontinent of Pangaea existed about 200 million
years ago.
Pangaea covered 40% of the Earth’s surface.

19. The Theory of Plate Tectonics
Majority of Pangaea was in Southern Hemisphere.
Pangaea was surrounded by a single ocean, the
Panthalassic Sea.
Pangaea broke up ~ 180 million years ago.

20. The Theory of Plate Tectonics
Wegener first published his theory 1912.

21. The Theory of Plate Tectonics
Evidence in support of Continental Drift
1. Continental Fit
Sir Francis Bacon (1620)
noted that the continents
might fit together.
Made observation after
seeing some newly made
maps.

22. The Theory of Plate Tectonics
2. Habitats of Modern
Organisms
Hippopotamus found in
Africa and Madagascar.
Marsupials in Australia.
Indicate some migration
and evolution took place
before and after drift
began.

23. Marsupial Mammals Marsupials are the group of mammals commonly thought of as pouched mammals. They give live birth, but they do not have long gestation times like placental mammals. Instead, they give birth very early and the young animal, essentially a helpless embryo, climbs from the mother's birth canal to the nipples. There it grabs on with its mouth and continues to develop, often for weeks or months depending on the species. Like other mammals, the marsupials are covered with hair. Mothers nurse their young — a young kangaroo may nurse even when it has grown almost to the mother's size. The only naturally occurring marsupial in the United States is the opossum, Didelphis marsupialis. In the past, however, marsupials were quite common. During the Mesozoic marsupials were very common in North America; more common, in fact, than placental mammals. They persisted here until the mid- to late-Tertiary. In South America and Australia, however, marsupials continued to be an important group of land mammals.

24. Many South American forms are similar to the North American opossum
Many South American forms are similar to the North American opossum. The marsupials of South America began to go extinct in the late Miocene and Early Pliocene when a land connection with North America formed, allowing placental mammals to cross into South America. In Australia, though, marsupials continue to be very diverse, and are the dominant native mammals. They include kangaroos, koalas, tasmanian devils, wombats, and other typical Australian mammals. Until recently, they also included the marsupial wolf, Thylacinus. Like the quagga, the marsupial wolf is now extinct. The last individual was seen in Tasmania in the 1950s.

25. The Theory of Plate Tectonics
3. Fossil Record
Wegener used the
fossil record.
Found fossils of
plants and
animals that were
found on several
continents.
Included animals, Cynognathus, Lystrosaurus, Mesosaurus,
and plants Glossopteris.

26. The Theory of Plate Tectonics
4. Similar Rock Types Across Ocean Basins
Mountains of Northern Hemisphere similar in Greenland, NA, and
Europe.
Also similar rocks between South America and Africa.

27. The Theory of Plate Tectonics
5. Ancient Climates
Glacial striations found in India, Australia, South America
and Africa.
Radiate from a point in southern Africa.
Also coal deposits found in presently cold regions,
such as Norway.

28. The Theory of Plate Tectonics
Wegener seemed to have a great idea, and lots of supporting evidence.
However, Plate Tectonics was not widely accepted until the early 1970’s.
Why?
Wegener could not explain how the continents drift.
He could provide no mechanism for the theory.

29. Captain Harry Hammond Hess
The Theory of Plate Tectonics
Captain Harry Hammond Hess
(later Professor of Geology at Princeton)
Believed in Wegener’s hypothesis
Conducted echo-soundings of the oceans during WWII
Mapped the mid-ocean ridges (3000 m high and 2000 m wide)
Located deep-sea trenches (10,000 m deep) associated with large continental mountain belts (the Andes) and island arcs (Aleutians, Japan)
Hess presented sea-floor spreading as a mechanism in 1960’s