Cascadia's Fault (10 page)

CHAPTER 5

Cauldron and Crust: The Rehabilitation of Continental Drift

A non-geologist might wonder why it really mattered whether the Alaska fault was vertical or horizontal, but the implications for the West Coast were dire. If Plafker and Benioff were right about that slab of ocean floor poking down underneath the continent, then in all likelihood the same would be true for British Columbia, Washington, Oregon, and northern California. An earthquake like the Good Friday disaster would affect millions of people if it happened to the Pacific Northwest. But the debate was by no means resolved by Plafker's research even though in hindsight the evidence appeared overwhelming and obvious.

The debate about faults and earthquakes and how mountains were built had been raging for five decades. In January 1912, Berlin-born meteorologist Alfred Wegener dared to express for the first time his idea of “continental displacement.” Like others before him, Wegener had noticed that the coastlines of Africa and South America seemed to fit together as if they were pieces of a jigsaw puzzle and that parts of North America and Europe also seemed to match up. He speculated that all these far-flung land masses might once have been part of a
single supercontinent (he named it Pangaea) approximately 200 million years ago.

His idea was that Pangaea had broken apart and that huge masses of land had moved sideways across the surface of the earth, jostling and crashing into each other over millions of years. While others had speculated about the significance of matching coastlines, Wegener had the audacity to put it in writing, making himself an instant lightning rod. Published in 1915, his book
The Origin of Continents and Oceans
triggered a controversy that was still raging in 1964. If Plafker and Benioff turned out to be right about how that fault in Alaska worked, then the essence of Wegener's big idea might be right as well.

Wegener's notion that “tidal friction” and differences in gravity caused by the earth's imperfect, oblate shape had caused the continental breakup and that huge slabs of the earth's crust somehow plowed across the ocean floors like ships through pack ice was considered by physicists to be impossible. What on earth—what mechanism—could possibly generate a force strong enough to fracture and move entire continents horizontally? To many it sounded like utter nonsense. The evidence that big land masses had once been joined together, however, was harder to dismiss.

The dominant view among scientists at the time was that the continents had been locked rigidly in place from the very beginning of time as the earth solidified from a molten state. With the interior of the planet gradually cooling, the outer crust began to shrink and slump, to crack and wrinkle like a drying apple's skin—creating mountains along the way. Others thought parts of the crust rose and fell periodically as if they were floating on a semi-fluid interior.

Most who doubted Wegener were aware of the work of other scientists pointing to fossil match-ups, the remarkable similarities between rock layers on different continents, and the evolution of nearly identical plants and animals on opposite sides of the oceans. They realized that sooner or later there would have to be some way to account for all this.
Wegener offered a theory to explain how the various bits and pieces
might
have fit together, even if he couldn't say for sure why they had come apart.

He argued that if continents could move downward as the planet cooled and contracted, or even upward—rising slowly as ice ages ended and the enormous weight of glaciers melted away—then they could probably move horizontally as well. Figuring out how and why this happened would be the crucial next step. Even before Wegener published his theory there were reasons to question the orthodox view that the earth was cooling and shrinking. In fact, some researchers already thought the opposite might be true.

With the discovery of nuclear radiation at the turn of the century came the understanding that some elements generated energy all by themselves, that rocks containing these elements deep underground might be pumping out an enormous amount of heat that accumulates faster than it can dissipate into space. If true, then perhaps the earth was heating up rather than cooling. The surface of the planet might actually be expanding rather than shrinking. It might also explain how continents could slide or drift sideways.

Scientists began to speculate that heat generated in the earth's interior might get trapped beneath the continents. Radioactive elements could be generating enormous “convection currents” of melted rock that would rise from the planet's white-hot mantle toward the surface, like bubbles in a pot of soup. In fact the soup analogy seemed to make so much sense it was still being taught in Geology 101 courses when I entered university in 1970.

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 volcanic slag heaps would make or break the theory of continental drift.

 

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 reached its breaking point and snapped loose in the largest earthquake ever recorded. Scarcely four years later, George Plafker was collecting evidence that the same kind of horizontal fault had caused the 1964 earthquake in Alaska. Even though the old guard had still not accepted the idea of plate tectonics, Plafker was pretty sure he was right.

 

One could argue that this should have been the dawning of awareness of the megathrust earthquake threat to British Columbia, the Pacific Northwest, and California as well. A 1965 paper by Tuzo Wilson pointed to the existence of what he called the Juan de Fuca Ridge. The
name was chosen because the upper end of the ridge lay due west of the Strait of Juan de Fuca, which runs between Vancouver Island and Washington State. Here was an undersea mountain range that had previously been discovered and then dismissed as an insignificant, amorphous hump of rock running parallel to the coast. But if Wilson and the young turks of plate tectonics were right, the Juan de Fuca Ridge was in fact another part of that fiery seam of volcanic mountains running through the oceans.

If this ridge turned out to be spreading apart sideways, powered by a cauldron of hot magma, it must also be thrusting a slab of sea floor underneath the edge of British Columbia, Washington, Oregon, and California. Presumably some kind of trench would be located where the two plates met, a “convergent plate boundary” just like the ones off Chile, Alaska, and Mexico. If so, giant earthquakes must surely follow.