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Appendix A: the Plate Tectonics Paradigm

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Appendix A: the Plate Tectonics Paradigm Appendix A: The Plate Tectonics Paradigm The first five chapters give us CSD theory, now let us have a little practice! Here we introduce the Plate Tectonics Paradigm. We will use it in for on-going Computational Scientific Discovery labs. Studying the Plate Tectonics paradigm has several advantages, including: 1. It was developed in the 1960s, around the same time as Kuhn©s concept of paradigms. Giere and other philosophers of science who came after and were influenced by Kuhn intensively studied the development of the Plate Tectonics paradigm nearly in ªreal timeº, or shortly after it happened. 2. Earthquakes (and geophysics more broadly) are technical without being too technical. There©s math, but nothing as funky as quantum mechanics. 3. The United States Geological Survey keeps a free, Internet-accessible database of earthquakes. It is at http://neic.usgs.gov/neis/epic/. 4. The USGS site has the map-drawing ability. Beyond being just plain cool it also lets us see the data; it lets us really get a feel for it. A.1 The Importance of the Plate Tectonics Paradigm Plate tectonics is currently the dominant way to understand the Earth as a whole over much of its history. The wholestic time-and-space understanding unifies many previously separate terrestrial phenomena. Plate tectonics gives us an understanding of: 1. Most of the world©s earthquakes, 2. Most of the world©s volcanoes, 3. Most of the world©s mountains, including an understanding of which are still growing and why, 4. The continents, and the Earth©s crust more generally, 5. The abundance, location and age of fossils, 6. The abundance, location, age, magnetism, density and composition of rock. As successful as the Plate Tectonics paradigm is, it does not explain everything. As of 2008, when this paragraph was written, Plate Tectonics (as we understand it) has little to say about earthquakes that happen in the middle of continents, or about conditions on the early Earth, or about the natural history of Earth©s sister planets: Mars and Venus. In what follows we will touch on some of the ideas among Western geologists prior to Plate Tectonics, then on Plate Tectonics and some of its issues, and finally on earthquakes and the USGS catalog of them. A.2 Pre-Plate Tectonics During the Medieval era in the Western world the Christian church played a large part in retaining knowledge and training scholars. Naturally early Western geologists schooled in this Copyright (c) 2008 Joseph Phillips All rights reserved. tradition used the Christian Bible as knowledge source. Therefore one of the first theories of the Earth in the West after the fall of Rome was that ªThe Earth was created by Godº. The evidence for this was the Bible. Mountains, perhaps were indirectly created by God during the great flood. During the European Enlightenment geologists were more inclined to look for mechanisms that did not call for continual deistic supervision. Scientists theorized that the Earth was very hot when it was created and that it has been cooling off ever since. As it cools it shrinks, and as it shrinks the excess surface crumples upward. Mountains are a result of this crumpling. While details of both of these whole-Earth creation theories were sparse, details of phenomena more restricted in time and more restricted in space were understood in increasing specifics. For example, in the 1830s Swiss geologist Louis Agassiz looked at living glaciers in the Alps and saw U-shaped valleys and terminal moraines (deposits of gravel and dirt where glaciers stopped advancing and start retreating). He then saw looked around Europe and saw such things where there was no ice. He inferred that much of Europe, and much of the world, must have been covered with ice. This was not a story about the all of the Earth©s history, but one that described a worldwide phenomenon at some particular time. Spatially restricted theories were also proposed and refined in the 1800s. When American geologists explored the breadth of the Appalachian mountains by its eastern coast they found carbonate rock (rock made in the sea from marine shell creatures) at high elevations. From this and other evidence they inferred that there were two previous mountain chains. The long-gone Taconic range was lifted, and then had been eroded down to form the raw material for the Acadian range. The Acadian range was in turn lifted, but it too had eroded into the starting rock of the current Appalachian chain. The Geomorphic Cycle explanation of cycles of mountain building and erosion Copyright (c) 2008 Joseph Phillips All rights reserved. A theory was worked out to explain this, somewhat. A geomorphic cycle begins when somehow (they had no idea how), some rock was lifted to form the original mountain chain. Eventually this mountain chain would gradually erode to the sea, where it would become thick deposits of mud and sand. As time went on the mountains eroded down and the sand and mud deposits, still at sea level, would thicken as their lowest levels sank further down within the Earth. Eventually the mountains would be practically gone. Overtime the Earth would heat and compact the sand into sandstone and the mud into shale. Somehow (perhaps by some sort of slow motion rebound) the Earth would raise the compacted rock to form a new mountain range. This range would itself erode to the sea to form new sedimentary rock and the cycle would begin anew. This cycle of erosion and deposition played itself out over geological time like some immense but slow moving see-saw. This process, of course, was still going on. The Appalachians are a mature mountain range that are well eroded. The ªpiedmontº region east of the Appalachians that stretches from New Jersey to Alabama is such an erosion-created landform. Above all, geologists gradually appreciated that the Earth was much older than the approximately 6000 years old that can be inferred from the Bible. James Hutton developed and Charles Lyell popularized ªuniformitarianismº, that changes happen gradually over time. A lot of time. Time enough for Agassiz©s glaciers to come and go. Time enough even for the Appalachians parent and grandparent ranges to rise from childhood and sink to burial. A.3 Continental Drift or ªHey! Africa matches South America!º Many have noticed the western coast of Africa seems to match the eastern coast of South America, and many have asked if they were at one time joined. As far back as 1596 Dutch cartographer and geographer Abraham Ortelius examined some of the first coastal maps of both continents and was among to hypothesize it. He was followed by British philosopher and statesman Francis Bacon in 1620, British/American publisher, scientist and statesman Benjamin Franklin in the 1700s, and French geographer Antonio Snider-Pellegrini 1858. By the early 1900s more geological evidence became available and ªContinental Driftº as it became known was championed by a generation of scientists including Italian Geologist Roberto Mantovani between 1889 and 1909, American astronomer William Henry Pickering in 1907 and amateur American geologist Frank Bursley Taylor in 1908. In the 1920s German meteorologist Alfred Wegener made the strongest case. He compared not just coastlines by rock types, fossil types, and evidence of prior climates. Copyright (c) 2008 Joseph Phillips All rights reserved. He went beyond linking South America and Africa to join all continents together into a ªsupercontinentº he called Pangaea or ªentire Earthº. This continent split into a northern supercontinent Laurasia and a southern supercontinent Gondwanaland. Laurasia further split into North America and Eurasia. Some time later Gondwanaland split into South America, Africa, India, Antarctica and Australia. With this idea he was able to explain why fossil beds throughout the southern hemisphere line up across seas, how coal fields in Europe matched up with North America, and why dinosaurs in the northern hemisphere were more diverse than those in the global south. Alfred Wegener had a lot of evidence, but his weakness was the same as Copernicus© and Kepler©s: the lack of a believable mechanism. ªHowº skeptical fuddy-duddy Anglo-American geologists asked ªdo continents plow through ocean rock?º Wegener©s mechanism was centrifugal force: Pangaea broke up as the landmasses were forced towards the equator. The skeptics thought that this force was far to weak. Wegener©s ideas died when he did. He died in the 1930s in Greenland while doing experiments looking into the newly hypothesized jet-stream with balloons. A.4 Plate Tectonics Plate tectonics vindicates Alfred Wegener by providing a plausible mechanism: in brief rock is created in some places, destroyed in others, and conveyor-belting continents move themselves and buffet others around. They explain most of earthquakes, volcanoes, Copyright (c) 2008 Joseph Phillips All rights reserved. magnetism, intercontinental fossil distribution and intercontinental rock ages. Further, this it does this for most of the Earth for much of its life. Before discussing Plate Tectonics we need to reformat our minds into accepting two concepts that are unintuitive to most non-geologists: geological time and the flow of rock. A.4.1 Geological Time The concept of ªGeological Timeº is basic geology and pre-dates Plate Tectonics. Indeed, Hutton, Lyell and others set the stage for Plate Tectonics by positing enough time for South America and Africa to separate themselves by thousands of kilometers at a speed of only couple centimeters per year. Geological time is not like the time with which you are generally familiar. Perhaps you may live for 100 years. Perhaps, if you are Chinese, your civilization has lasted 2200 years. Perhaps if you can read ancient Sumerian cuneiform, the oldest thing you might read is about 5200 years old.
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