OC103 Lesson #6: Plate Tectonics The Birth of a Scientific Revolution Every once and a while, a scientific discovery comes along that helps scientists achieve an entirely new understanding of how things work. Famous examples of this include: • Earth is not the center of the universe (Copernicus and Galileo in the 1500s) • Biological evolution by natural selection (Darwin in the 1850s) • Time and space are relative (Einstein in the early 1900s) • The Earth System - Earth is a single, complex system, and human activities are part of that system (going on today) In the 1960s, a revolutionary scientific theory called Plate Tectonics swept the Earth Sciences, and for the first time was able to provide a unifying explanation for many of our observations about Earth. This lesson covers how that scientific revolution came about, and how oceanography played a crucial role in developing the scientific theory of Plate Tectonics. This scientific theory in turn provided a cohesive explanation for many of the features we observe on the ocean floor. But first, a few comments about the important distinction between how scientists use the word “theory” and the everyday use of the word.What is a Scientific “Theory”? Theories are a dime a dozen in everyday life. You often hear someone say they have a “theory” about something: what their cat dreams about, whether or not Elvis is really dead, whatever. A scientific theory is a very different beast though. For something to qualify as a scientific theory, it must be tested, refined, retested, bent, prodded, further refined, and still be able hold up under the scrutiny of numerous skeptical scientists. Long before something becomes a scientific theory, it starts out as an idea that seems to logically explain something pretty well. If that idea stands up to some initial testing and experimenting, it matures into a hypothesis, which is a well-reasoned and rigorously tested explanation for the observations. A hypothesis is still considered a tentative explanation though, until numerous scientists have a chance to test it and try to disprove it in various ways. It is very rare that a hypothesis can be definitively proven, so scientists usually take the opposite tack, and look for ways to disprove it instead. The figure below (from your textbook) shows how a hypothesis must survive repeated cycles of testing and refinement. Only after a hypothesis has survived numerous attacks from all angles is it considered tested enough to become a scientific theory. The hallmark of a good scientific theory is that it can be used to predict something that has not yet been observed, so that you can create an experiment to test if that prediction is correct. The scientific theory of Plate Tectonics underwent exactly this type of repeated testing and refinement (though it had a rough childhood, as seen on the next slide).Continental Drift: An explanation, but flawed and without a cause Not long after early explorers charted the coastlines of South America and Africa, they noticed that the shape of the east coast of South America was an amazing match to the shape of the west coast of Africa, like complementary pieces of a jigsaw puzzle (see figure below left). On its own, this observation lacked evidence that it was anything more than a coincidence. That changed in 1912, when a German meteorologist named Alfred Wegener proposed the idea of “Continental Drift”, which stated that the continents we see today were once parts of a large “supercontinent” (called Pangea, sometimes spelled Pangaea) that has since split into pieces. Wegener based his idea on the combination of 4 observations: • The old observation that the continents appear to fit together like pieces of a jigsaw puzzle. • The crust appears to “float” on the mantle due to isostasy (as we covered in the last lesson), and thus the mantle could be flexible enough to allow the crust to slide around. • If the continents are fit back together like puzzle pieces, seemingly random regions of certain rock types, glacial deposits, and mountain chains become continuous belts that match up between the continents (see figure below right). • The contrast in diversity of plant and animal species before and after Pangea split up, for example: o Animals that existed before the breakup of Pangea, such as dinosaurs, were very similar across all of the continents. o Animals that evolved when continents were isolated from each other after the breakup of Pangea, such as mammals, have unique characteristics that are found on one continent, but not on others (e.g., giraffes, kangaroos, etc.). Although Wegener’s hypothesis was an elegant explanation for these observations, it fell short in the minds of most scientists because it lacked a reasonable driving force or mechanism. Scientists knew that the ocean floor was solid rock, and no one could conceive of forces powerful enough to cause the continents to “plow” through the oceanic crust as they moved around. It turns out that Wegener’s hypothesis was slightly off target, and answers to how the continents were able to move around, and what forces were driving that motion, were later found in the oceans.Modern Oceanography and a Mechanism for “Continental Drift” In Wegener’s time very little was known about what was beneath the oceans, but rapid growth in knowledge motivated by World War II and refinement of SONAR technology brought some new ideas to the fore. Oceanographers sailed on numerous military vessels during and after WWII collecting bathymetric data for the Navy. While compiling and analyzing these data, some facts started to fit together: some of the oceans had long ridges down the middle of them (“mid-ocean ridges”; yellow bands in map below right), and many of those ridges had a valley at their summit that looked very much like the rift valleys we see on land where Earth’s crust is being torn apart (Iceland, Rio Grande Valley, Red Sea; see diagram below left of a mid-ocean ridge with a rift valley). In the 1960s, one of the oceanographers who had been compiling the bathymetric data (Harry Hess) suggested that the ocean floor splits apart at these mid-ocean ridges and volcanism forms new ocean crust along the split in a process called seafloor spreading. He suggested that this process results in the surface of Earth being composed of “plates” that move on “conveyor belts” of oceanic crust