Cascadia's Fault - Jerry Thompson [31]
I can still recall the lecture. The professor, whose name is lost to me now, asked us to imagine the earth as a large cauldron of thick soup that has been brought to a slow boil. Bubbles of heat rise up from the bottom of the cauldron. At the surface the soup cools and forms a crust that floats atop the hotter liquid material below. When new heat bubbles rise to the surface, they push the older crust aside.
Propelled against the outer walls of the pot, the older crust is pulled down into the interior of the cauldron, where it gets reheated and eventually bubbles back to the surface to form crust again. This continuous, circular motion of a heated liquid is now known as a convection cell. And that, concluded the professor, is how we might explain the way continents get dragged or pushed across the surface of the earth.
Just think of continents as great rafts of floating soup crust. In the 1920s, however, this was still just a wild idea with no solid evidence to back it up. Wegener and his supporters might have been cheered by these new developments, as they seemed to provide an explanation—the mysterious force that could move continents around like pieces of a jigsaw puzzle—and make sense of continental drift. Sadly that’s not how the story ended for Wegener himself. A meteorologist by profession, he got lost in a blizzard during a research trip to Greenland in 1930 and did not live to see the discoveries that would rehabilitate his theory more than three decades later.
If continents were indeed moving around like rafts of soup crust, there had to be some way to prove it once and for all. The next several breakthroughs in earth science came as a result of military research begun during World War II. The U.S. Navy needed a new technology to detect German U-boats and better maps of the ocean floor to keep track of where enemy (and their own) submarines might be able to hide. In those days the bottom of the sea was as uncharted as outer space and a generation of young scientists was eager, willing, and able to explore the planet’s last frontier.
As warships sailed from one battle to the next, echo sounders pinged day and night, creating detailed, never-before-seen profiles of the ocean floor. In the Pacific, they charted undersea volcanoes and steep canyons like the Marianas Trench which, according to the new measurements, was seven miles (11 km) deep. What process had created such a steep canyon in a mostly flat ocean floor? Could this be where the soup crust buckled under and got recycled into the earth’s interior cauldron?
The threat of enemy subs seemed just as real during the Cold War, so the Office of Naval Research continued sending exploration teams to sea during the 1950s and early ’60s. Along the way scientists discovered amazing new details about the Mid-Atlantic Ridge, a chain of undersea mountains halfway between Europe and North America. The so-called ridge turned out to be a set of parallel mountain ranges with volcanic vents oozing hot magma onto the ocean floor.
As the magma spewed out and cooled in seawater, it expanded and hardened to a rocky crust, forming a new piece of ocean floor. Over millions and millions of years, the lava had piled up into those volcanic ridges while a seemingly constant spew of new magma kept pushing the ridge flanks farther and farther apart. Here, at last, was direct physical evidence of the convection currents that might be causing continental drift.
At first it was thought this mid-ocean ridge existed only in the North Atlantic. Further mapping confirmed that it wandered down between Africa and South America and then snaked around the entire globe like the seam on a baseball, a fifty-thousand mile (80,000 km) chain of volcanic ridges. When they put all the new charts together, these mid-ocean ridges turned out to be the longest continuous mountain range in the world. In terms of scientific significance, the undersea ridges had morphed into the most prominent geologic structure on the planet. Research done on these