Contracting Earth v. Continental Drift v. Plate Tectonics
Even though the point of this lesson is for you to create your own knowledge, I think it is worth me giving you the one-page summary as I see it of the history of plate tectonics. Your job is to focus on a scientist who contributed to our current knowledge of how the solid Earth works, but here I will give you a synopsis of how the prevailing wisdom changed gradually from about the time Wegener published "The Origin of the Continents and Oceans" in 1915 up until 1968 which is approximately when Plate Tectonics became the standard.
Contracting Earth theory (early 20th century)
In the early 20th century the prevailing wisdom regarding how mountain belts were formed and why the sea is deep was that the Earth started out as a molten blob and gradually cooled. When it cooled, heavier metals such as iron sank down and formed the core, while lighter metals such as aluminum stayed up in the crust. The cooling also caused contraction and the pressure produced by contraction caused some parts of the crust to buckle upwards, forming mountains. Other parts of the crust buckled downwards, creating ocean basins. Picture in your mind a grape turning into a raisin as it dries out.
The contracting Earth hypothesis was further refined by introducing isostasy. Isostasy is the concept that all elements in a system are in hydrodynamic equilibrium or trying to get there. For example, if you have a bathtub of water, a chunk of balsa wood floats higher than an ice cube because balsa wood is less dense than ice. If you were to push the balsa wood down, it would pop back up when you took your hand away. The popping back up is the balsa wood bringing itself back into equilibrium. It happens very fast because water has low viscosity. Now what if you had a bathtub full of molasses instead of water? When you push the balsa wood down, it will indeed rise back up again after you take your hand away but it will happen more slowly because molasses is more viscous than water.
What does this have to do with the Earth? Well, the pre-continental drift idea went like this. Heavy parts of the crust sank down and lighter parts raised up not only due to the pressure of contraction but also due to isostatic adjustments. The interior of the Earth was thought to be a viscous fluid that could accommodate this sinking and rising. This was the proposed mechanism favored by paleontologists who thought the reason identical fossil species were found on continents separated by oceans was because there had been connecting land bridges that sank.
Now, in fact it is true that isostasy does govern mountain elevations. In fact, most mountain belts have a "root" like the keel of a boat and over long timescales the mantle in fact does flow viscously, but the mantle is solid rock, not a fluid. Land bridges did not sink down into the mantle. That part is wrong.
Enter Alfred Wegener
So, this is where Wegener comes in. He had a PhD in astronomy but most of his scientific contributions were in meteorology. He became very well known in his own lifetime as an explorer of Greenland and as a meteorologist. In fact, he died in 1950 while leading an expedition across Greenland.
His interest in geology was basically a sidelight to his regular academic career and he had no training in geology. He assembled circumstantial evidence for his idea that the continents had once been joined. Let's examine that circumstantial evidence.
Jigsaw puzzle fit
Alfred Wegener was not the first person to notice that the continents fit together across the Atlantic Ocean. In fact, in the 1500's and 1600's when reliable maps of the east coasts of North and South America were produced, this feature was obvious. I've always thought that this piece of evidence has a little bit of Western ego attached to it. For example, if you look at a map centered on the Pacific Ocean, do you notice anything? No, not really, because there is a complete absence of anything that looks like a jigsaw puzzle fit there.
Rock types and geologic structures
Evidence of ancient climates
Statistical analysis of topography
Lack of mechanism for continental drift
Conceptual sketch of plate tectonics
Why so slow?
I think it is interesting to consider how long the plate tectonics revolution took to unfold. Consider that Wegener published "Origin of the Continents and Oceans" in 1915 in which he laid out the circumstantial evidence that indicated the continents had once been joined, but plate tectonics was not accepted as a theory until about 1968. Let's put that length of time into human perspective: