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[Professor] (mate)
   OK. Let’s get to work, class. Q12 Today we’re--we’re going to be talking about an extremely important idea in geology: continental drift. The slow movement and reconfiguration of the continents on Earth’s surface.
   It all began pretty early...well, I mean, back in the sixteenth century. Q14(C) Scholars had observed that, um, that the contours of the continents on opposite sides of the Atlantic Ocean, um, bore some resemblance to one another.They wondered if maybe there was some way the continents had "drifted" apart over time...but since there wasn’t any kind of feasible theory that could explain how huge landmasses like continents could move at all--much less move across the entire region now occupied by the Atlantic Ocean--um, since there weren’t any good theories to account for that, um, it just remained an unsolved mystery.
   That is, until 1912, when a German scientist called Alfred Wegener proposed his theory of "continental drift." Q13 Wegener discovered that not only were the coastlines on either side of the Atlantic Ocean similar, the fossils contained in their crusts were too. Theories at the time tried to account for the similarity between fossils by suggesting that a land bridge once existed between the continents, but, to Wegener, this just didn’t seem like enough evidence. He couldn’t get it out of his head that the continents might have once been joined. He gathered a great deal of evidence to support his belief and eventually went public with it. In 1912, he announced his theory of continental drift to...well, we’ll call it an incredibly unreceptive scientific community. The thing is, even with all his evidence, um, Wegener failed to convince his peers of the validity of his theory, because he couldn’t come up with a plausible mechanism for continental drift, an explanation of how continents might move. So...all throughout the 1920s and 1930s, scientists ridiculed the idea of continental drift, calling it unscientific. It simply didn’t fit into their set of beliefs about how the world worked, and so they rejected it, ignoring the evidence that supported it instead of looking for new reasons to explain how it might occur. Q16 Little did they know, within the next 40 years...the theory of continental drift would become universally accepted.
   What changed? What caused this thoroughly despised theory to gain such popularity? Well, as you might have guessed, the change was the result of a new explanation for the mechanism behind continental drift. I’ll tell you how it happened. Um, in the 1960s, the notion of seafloor spreading arose...suggesting that the seafloor actually, um, grew--or spread out--from places on the ocean floor. And as the ocean floor expanded, the continents were carried along. Soon, experts were constructing a picture of a dynamic planet, one gradually transformed by large-scale movements of pieces of its rocky crust. Q14(A) Their new theory, known as plate tectonics, explained what the theory of continental drift never did--how continents could move around on Earth’s surface. And here’s how. The theory of plate tectonics sees the surface of our planet as an outer shell, which covers a softer layer beneath. This--this lower layer, which is called the asthenosphere, uh, it’s pretty solid...but on a geologic timescale, it basically, um, flows. Q17 I know this notion may be a little strange to you, but I don’t want to get into it right now because it’s really a topic for another lecture...so just try to imagine that because of the properties of the asthenosphere, pieces of the Earth’s outer shell--called "tectonic plates"--can move around over really, really long periods of time. Q15(A) In some places, two plates are slowly moving away from each other, and these spots are known as divergent boundaries. Q15(B) in ether places, two plates are colliding, usually causing one of the plates to submerge. These zones are referred to as convergent boundaries.
   So...with all this new information, scientists were forced to reexamine the idea of continental drift. Considered in the new light of plate tectonics, it seemed quite plausible. Plate tectonics basically just updated the theory of continental drift...and many concepts that were rejected when Wegener put them forward are now universally accepted as true.