1. 12.1 Evidence for Continental Drift
Wegener came up with the continental drift theory
the idea that the continents used to be connected as one big supercontinent but slowly moved apart over time
He named the original supercontinent Pangaea.

2. Pan - gaea “all” “Earth”

4. Putting the Pieces Together...
The continents do not fit “perfectly” like a jigsaw puzzle.
A better fit is found by matching the continental shelves (original shorelines that are now underwater)

5. Geological/ Rock Evidence
Mountain ranges of one continent, end at coastline  begin again at another continent
(similar folds, ages)

6. Geological/ Rock Evidence
Similarities between rocks of adjacent continents are evidence for continental drift.
The rocks found in Newfoundland are the same type and age as rocks found in Greenland, Ireland, Scotland, Norway.

7. Fossil Evidence
The fossils of some ancient species suggests evidence for ‘pangaea’.
Wegener found fossils of a fresh-water reptile ‘Mesosaurus’ in two places:
- SE South America and SW Africa

8. This small freshwater reptile could not have swam this distance.
6000 km open ocean !
This small freshwater reptile could not have swam this distance.

9. Fossil Evidence Glossopteris (a fern) Found In: South America Africa
Australia
India
Antarctica

10. Fossil Evidence
Problem: Ferns do not grow in cold climates (Antarctica)
No evidence that polar climates were milder 200 million years ago. Antarctica must have been closer to the equator. Since then, Antarctica has moved to present location in South Pole.

12. Glacial Evidence
Evidence of glaciers formed in ice ages provides support to CD theory
During ice ages, glaciers covered large areas of land
Glaciers are vast masses of ice, found today at the poles and high in the mountains.

13. When the glaciers retreat/ advance, they leave behind proof
Deeply scratched rocks (striations), U-shaped valleys, patterns of rock formations
Paleoglaciation refers to the extent of ancient glaciers and to the rock markings they have left behind. Evidence of glaciers in parts of South America, India and Africa (now tropical) puzzling to scientists. If continents were once joined, with much of land mass sitting towards South Pole, then evidence makes sense.
“Paleoglaciation”

14. More Evidence Coal Deposits in Antarctica
Coal forms from dead matter (usually tropical swamp material)
Since the South Pole has never had a tropical climate....it must have been in a warmer location than it is now
If there had been massive global climate change, there would be evidence for warmer climates on all the continents, which there is not.

15. How the Continents Moved
The current theory is that the continents moved due to the movement of tectonic plates.
Tectonic plates = movable slabs of rock that make up the Earth
In order for Wegener’s hypothesis to become accepted as a theory, it would take over 30 years, major technological advancements, and the research of a Canadian geophysicist.

16. Tectonic Plates
These large rigid plates slide over the surface of Earth (over a layer of partially molten rock)
magma
partially molten surface
Tectonic plates explain the location of volcanoes and earthquakes....

17. Volcanoes/ Earthquakes
Volcanoes are openings in the Earth’s surface that spew gases, chunks of molten rock Earthquakes are sudden, ground-shaking releases of built up energy under Earth’s surface

18. Earthquake zones and volcanoes follow a pattern
Earthquake zones and volcanoes follow a pattern.... They occur along the boundaries (edges) of tectonic plates (form an outline)

19. Mid-Atlantic Ridge
Mapping of the ocean floor revealed a long mountain range running down the middle of the Atlantic Ocean.
It is called the “Mid-Atlantic Ridge”
In 1872, British vessel discovered mountain range running north to south down length of Atlantic Ocean.

20. Occurs at rate of ~ 1 inch /yr
How it was formed...
The tectonic plates of the Earth slide apart (sliding is called “conduction”)
Another way to think of this is “seafloor spreading”
Something must
replenish this lost
crust!
Occurs at rate of ~ 1 inch /yr

21. Magma to the Rescue!!!!
Areas of oceanic crust are replenished by volcanic activity....
Magma (molten rock) rises up
Cools and hardens when it reaches surface
Forms the new sea floor
Magma rises because it is less dense than the material that surrounds it.

22. Magma and More Magma This process keeps going.....
Convection currents cause more magma to rise
Magma forces apart hardened material
Older rock is pushed aside as new rock forms!
“Sea Floor Spreading”
Sea floor spreading proposed in 1960 by American geology professor: Harry Hess.

23. An animation....
Spreading Ridge

24. Proof of Sea Floor Spreading!
In the 1940’s...oceanographers took samples of rock from the Mid-Atlantic Ridge
- Younger rocks were found closer to ridge
- Older rocks were found farther from ridge (and in thicker layers...built up!)

25. More evidence in the rocks!!
Scientists have also found that the rocks themselves have a “striped pattern”
To understand this, we must understand the Earth’s magnetism....

26. Earth’s Magnetic Field
N geographic Pole
When Earth’s magnetic poles match its geographic poles (like now), the direction of Earth’s magnetic field is called normal polarity. When they are opposite, the orientation is called reverse polarity. During a period of reverse polarity, a compass needle would point to the geographic South Pole. Magnetic reversal appears random and is not well understood. Scientists think that the magnetic field is produced by the motion of liquid iron in planet’s interior, and if motion of this material changes in some way, magnetic reversal occurs.
S geographic Pole
Every few thousand years....the direction of magnetic field REVERSES!
“MAGNETIC REVERSAL”

27. Consider a rock..... Rock contains metals like iron (Fe)
Little pieces of iron “line up” with magnetic field and then they STAY that WAY!
When MAGNETIC REVERSAL happens...they line up in opposite direction!
Rocks “preserve” the direction of the Earth’s magnetic field that existed when the rock formed! Amazing!
Magma is molten basalt, a dark rock that is rich in iron. As the basalt cools, it becomes magnetic.

28. Studying the magnetic properties of ancient rocks.... “Paleomagnetism”

29. Back to the Sea Floor.... “magnetic striping”
So the sea floor is spreading and new rocks (from magma) are formed...
Looking closely at the sea floor, scientists found a striped pattern!
“magnetic striping”

30. A HOT SPOT....
A geological area where molten rock rises to Earth’s surface!
In mid-1960’s, Canadian geologist J. Tuzo Wilson suggested that chains of volcanic islands were formed when a tectonic plate passed over a stationary hot spot. Developed “plate tectonic theory”...unifying theory of geology.

31. HOT SPOTS.... Hawaiian Islands were formed by a tectonic plate passing
over a hot spot.
The plate slides in the
direction of the arrow....
Forms a CHAIN of islands!
Take the Section 12.1 Quiz