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Michael Wysession

Washington University in St. Louis
A Modern Journey to the Center of the Earth
The study of earthquakes is fascinating, with an unusual history. Early attention to earthquakes was largely given because of a desire to locate, and later predict these potentially destructive earth movements. With the reinvention of the theory of Continental Drift in the form of Plate Tectonics, seismology took on an important geological role. Most earthquakes occur along the active boundaries between mobile ""plates"" at earth's surface. It took the cold war for seismology to become a fully funded field of research, as governments realized that the monitoring of nuclear tests could best be done with seismology. Bombs are similar to, but distinctly different from, earthquakes in their seismic characteristics. However, with the wealth of seismic information available due to the politically-driven establishment of world-wide seismic networks, new possibilities emerged for investigating the structure of the deep earth. Since light doesn't pass through rock, and the deepest we have ever been able to drill is about 8 miles, we knew relatively little about the interior of our own planet until quite recently. Using seismic waves from earthquakes in a manner similar to medical tomography, we are now able to create amazing images of the complex structure of earth's interior. What we have found is a fascinating and complex half-billion year long cycle that rock passes through during a journey from the surface to the core and back again. Ocean seafloors subduct beneath the edges of continents and sink into the mantle. Some times these giant sheets of rock punch straight down into the deep mantle. Other times they drape across a mid-mantle boundary, and pile up until plunging all the way down to the boundary between the rocky mantle and liquid iron outer core. Here the rock or the ancient seafloors heats up, and eventually rises back up to surface, sometimes in the form of hotspot volcanoes like Hawaii and Iceland. This complex core-mantle boundary, Earth's ""other surface,"" turns out to be a geological sister to the Earth's crust, and is the site of many unusual newly discovered geological processes. It is not only the graveyard of ancient seafloors and birthplace of hotspot volcanoes, but also controls the fluctuations in Earth's magnetic field by channeling the flow of the liquid outer core.
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