Cascadia's Fault - Jerry Thompson [32]
In the early 1960s, with hot convection cells to power the system, the next question to answer was what happens when two moving portions of crust collide. Like two cars crashing head on, the obvious result of two continents slamming into each other would seem to be crumpled fenders—mountain ranges like the Himalayas and the Alps. When a segment of ocean floor crashes against a continent, the sea floor, being made of heavier, denser, volcanic rock, apparently buckles under the lighter continental crust, creating a deep ocean trench like the Marianas.
As the heavier ocean floor continues to move, it gets forced downward, grinding against the underside of the continent as it goes. Coastal mountain ranges get shoved upward in the process. As the seafloor slab goes deeper, it gets so hot it begins to melt, spewing a volcano up through the overlying continental rock. If two masses get stuck together by friction instead of sliding past each other smoothly, enormous pressure builds up and is finally released in megathrust earthquakes. So the old soup cauldron story had endured, and the conversion of many skeptics into tentative believers was underway. Finally there was a logic to continental drift and a way to put the planet’s jigsaw puzzle together.
By the mid-1960s J. Tuzo Wilson at the University of Toronto would weave the bits and pieces of new discovery together in a comprehensive theory that filled in most of the blanks in Wegener’s original concept. Wilson also changed the terminology. He described the earth’s surface as being divided into “several large rigid plates” rather than continents. By Wilson’s definition, a plate was considerably larger than a continent and could include segments of ocean floor that had been jammed against and welded to the edge of a continent. The North America plate, for example, included everything from the Mid-Atlantic Ridge to the California coast—meaning all that “new ocean floor” being generated underneath the Atlantic had become part of the older continental mass. It was all of a piece, a westward-moving tectonic plate.
His choice of the word plates would allow a new generation of researchers to put the old bogey man of continental drift behind them and move forward into the emerging world of plate tectonics—the new geology. It was a great time to be an earth scientist because there was still so much to figure out about how the system worked, especially along the western coast of North America and around the Pacific Rim.
New voyages of discovery revealed that the mid-ocean ridge system—that twisty baseball seam of volcanic mountains circling the globe—cut through the wide, seafloor prairie of the Pacific Ocean, fracturing the main plate and pushing smaller pieces off to either side as it spread the sea floor wider. The Cocos plate, for example, had apparently been split off from the larger Pacific plate by a convection cell pushing new magma up through the East Pacific Rise: the segment of the baseball seam running parallel to the coast of South America. Upwelling magma had pushed the smaller Cocos plate eastward underneath Central America.
Farther south, researchers learned that another broken slab of sea floor was being thrust under the coast of Chile. To the north another was punching its way down beneath the beaches of Alaska. The same was happening under the coast of Japan and in many other places around the Pacific Rim—all because of seafloor spreading.
Anywhere you looked, broken plates were pushing against one another. At each one of these collision points were large mountain ranges, violent earthquakes, and active volcanoes. The Pacific was circled by a “ring of fire” caused by lumps of the earth’s crust crashing together, melting and erupting.
While scientists around the world were busy piecing it all together in their minds and on paper, the earth itself was providing physical proof of what was really at stake. In 1960, the broken chunk of ocean crust jammed beneath Chile’s continental shelf finally