Map showing volcanoes that have been active in the last 10,000 years. Similar to earthquakes, volcanoes are located preferentially on or near plate boundaries (see Figure 2). These data helped geologists draw more detailed cross-sections showing that plates are thin at spreading ridges, and that subduction extends long distances, taking plates deep beneath the continents. On the other hand, only shallow earthquakes (depths of 0 to 33 km, shown in red in Figure 1) are recorded at the spreading ridges. Where Hess had postulated that the rocks of the ocean floor were diving down into subduction zones, earthquakes occur at shallow depths of 0 to 33 km below the surface near the trenches and at depths of almost 700 km below the surface further inland (illustrated in Figure 1 by different colored circles). ![]() Not all earthquakes occur at the same depth, however. In fact, earthquakes are concentrated along the plate boundaries drawn by Harry Hess along mid-ocean ridges and subduction zones. Data are from the Advanced National Seismic System. Colors indicate depth of hypocenter, or origin of the earthquake: Red is 0-33 km, yellow is 33-100 km, green is 100-400 km, and blue is >400 km depth. Map showing earthquakes from 2003-2011 with magnitude greater than 3. For example, geologists had long recognized that earthquakes are not randomly distributed on the Earth (see Figure 1). The concepts of continental drift and seafloor spreading had revolutionized geology (see our module The Origins of Plate Tectonic Theory), and scientists excitedly began to revise their interpretations of existing data into a comprehensive theory of plate tectonics.
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