I. Continental drift is the predecessor to plate tectonic theory
- A. 19th century observers had noticed that the coastlines of North America and South America generally matched the coastlines of Europe and Africa.
- B. A German climatologist named Alfred Wegener proposed scientific evidence to support the idea that the continents once fit together.
- 1. He noted that much of the rock in the Southern Hemisphere between 200 and 300 million years old showed evidence of glaciation.
- 2. In certain places he found fossil remains of an organism that thrives in much warmer environments than exists today in those places.
- 3. The mountain ranges in Greenland, Scandinavia, the British Isles, the East Coast of North America, and Northwest Africa correlated quite well.
- 4. The stratigraphy between the east coast of South America and the west coast of Africa fit quite nicely.
- C. The argument that the continents fit together like a jig-saw puzzle has been further refined.
- 1. If the continental boundaries of Africa and South America are defined as the edge of the continental shelf, then the continents fit together even better than at the coastlines.
- a. The chances of this fit occurring randomly are 1 in 100,000
- b. The place where the overlap is the greatest between South America and Africa is the place where the Niger River delta is located; this feature was formed relatively recently in geologic history.
- 2. If the continental boundaries of North America, Europe, and South America are defined as the edge of the continental shelf, then those continents also fit together very well. There are only two problems:
- a. The Iberian Peninsula (Spain and Portugal) must be moved slightly.
- b. There is considerable overlap where Iceland is. This is due to the fact that Iceland was formed only very recently in geologic history.
- 3. Evidence for "polar wandering" could be better explained with stationary poles and drifting continents.
- D. Wegener assembled the continents together into a supercontinent he called Pangea.
- 1. When Pangea split up, the north fragment was called Laurasia. This became what are now North America and Europe.
- 2. The southern fragment was called Gondwanaland. This later split into Africa and South America.
- E. The biggest problem Wegener had in getting his ideas accepted was coming up with a mechanism.
- 1. He proposed that continents plowed through the ocean crust by a combination of centrifugal force, and gravitational forces from the moon & sun.
- 2. Many geologists were skeptical, because the forces he proposed were not strong enough to move continents.
II. Important discoveries in paleomagnetism led to the development of plate tectonics.
- A. The behavior of the earth's magnetic field also gives evidence of plate movement by showing how the continents have moved relative to the magnetic pole.
- 1. A magnet can induce other objects to become magnetic.
- a. When a hand magnet is brought into contact with a nail, it causes the nail to become weakly magnetic.
- b. A similar process works with basalts in the presence of the earth's magnetic field.
- i. Basalts contain a lot of iron, which behaves magnetically.
- ii. When molten basaltic magma cools and crystallizes into a rock, the earth, itself a magnet, endows the rock basalt with a weak magnetic field oriented with the earth's magnetic field as it was at the time of formation.
- 2. By measuring the orientation of the magnetic field in a basalt, it is possible to locate the position of the magnetic north pole at the time the basalt was formed.
- 3. Basalts in North America indicate that 200 million years ago the magnetic north pole was located where Siberia is today. The basalts give the "wrong" answer because they have been moved since they froze 200 m.y. ago.
- 4. Similar studies done on basalts of other continents indicate different positions for the magnetic pole, significantly far from where North American basalts indicate.
- a. The younger basalts from other continents show closer agreement on the position of the north pole.
- b. Older basalts from other continents show a gradual divergence in the position of the north pole.
- 5. If the continents had remained fixed throughout earth's history, then each continent would have indicated a different magnetic north pole.
- 6. We know this would have been impossible -- there is only one magnetic north pole -- hence such evidence indicates that the continents moved.
- E. The behavior of terrestrial magnetism also supports plate tectonics through the phenomenon of magnetic reversals.
- 1. Rock basalt indicates the position of the magnetic north pole.
- 2. For the past several million years, basalts have told us that the magnetic north pole was located within the vicinity of the present day arctic circle or Siberia.
- 3. However some older basalts indicate that the magnetic north pole was located somewhere on the Antarctic continent.
- 4. Because it is impossible that the north pole could drift so far so quickly, the concept of magnetic reversal now explains this phenomenon.
- a. The strength of the magnetic field fluctuates over time.
- b. Sometimes this field weakens and suddenly switches pole orientation, then grows again.
- 5. This is a process that has occurred many times throughout geologic history.
- 6. Ocean floor basalts indicate a pattern of magnetic reversals.
- a. Basalts of the same age show the same polarity.
- b. This pattern produces a series of magnetic stripes parallel to the spreading centers (mid-ocean ridges) at the ocean floor.
- 7. Because basaltic rocks of the same age can be found on either side of a spreading center, the pattern of magnetic stripes on one side of the spreading center mirrors the pattern found on the other side of the spreading center.
- 8. The discovery of this phenomenon was pivotal evidence proving plate tectonics.
III. Summary: What evidence exists that favors plate tectonics?
- A. The continental boundaries fit together well when the continents are placed next to each other.
- B. Evidence shows that climates have changed drastically over time (i.e., there is evidence that glaciers once existed in the Sahara Desert).
- C. Rocks and mountain chains on both sides of the Atlantic correlate quite well.
- D. Continents had to have moved to accomodate magnetic evidence.
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