From Natural Selection to Plate Tectonics

CHAPTER FOURTEEN From Natural Selection to Plate Tectonics I found, on landing [at Guafo Island], that the formation of the island, like that of [Guamblin] and [Ipun] Islands, is a sof sandstone, which can be cut with a knife as easily as a cake of chocolate. —Robert FitzRoy aro Guafo, faro Guafo, cambio,” Daniel barked into the walkie- talkie, imitating the monotone and cadence of military speech, F a mischievous smile covering his face. Receiving no response, he repeated, “Guafo lighthouse, Guafo lighthouse, over.” Six of us sat around an aluminum camp table in an orange-and-blue dome tent sheltering us from the mist on the beach outside. As I finished a Chilean breakfast of mashed avocado on toast while tottering on a three-legged campstool sinking into the sand, I contemplated the day’s task. Carry a couple of hundred pounds of GPS equipment, tools, and batteries five miles down the shore and up a 500-foot cliff to the lighthouse, home of the only permanent inhabitants on Isla Guafo. The Beagle visited here twice in 1834–5. On the first visit— while Darwin was with Sulivan surveying the eastern shore of 273 ROB WESSON Chilóe—FitzRoy thought the geology of the island interesting and unusual. He brought Darwin back for a look, but Darwin was not too impressed. It would be a century and a half before the real importance of this island emerged. On May 22, 1960, the sailors in the lighthouse on Isla Guafo had the ride of their lives during the greatest earthquake of the twentieth century—and a ringside seat for what was to follow. About ten minutes after the earthquake, the sailors watched in awe as the sea withdrew about a third of a mile to the west. Farther out to sea they could see a great wave forming, then moving rapidly toward them. The first megawave was followed by three more, reaching more than thirty feet above the beach. Two days later the sailors discovered that their dock, a flat shelf carved in the rock, was useless, far above the water. The island had been uplifted about ten feet. The understanding of this great earthquake represents a water- shed in the development of what is now called plate tectonics—the concept that the geology and physiography of Planet Earth are determined by the movements of great plates on the surface of the planet above a viscous mantle. And also marked another success in the energetic and ultimately fruitful, but sometimes disharmonious back and forth between the geologists, like Darwin, who base their observations on what the they see for themselves in the field, and their more mathematically inclined brethren who argue from a basis in physical theory. Our expedition to Isla Guafo aimed to push that understanding even further, to advance understanding of the processes going on in the time period between great earthquakes. B The branch of geology that deals with the large-scale processes that shape the earth’s crust—how seemingly solid layers of rock are folded and faulted to form mountain ranges, how continents are formed, and including how and why an island might bob up and down—has its own name: tectonics. And the ripening of the 274 DARWIN’S FIRST THEORY ideas that make up modern tectonics, plate tectonics—between the time that Darwin abandoned his explanation for Glen Roy and our adventures on Isla Guafo—is a tangled skein of story lines, a heady mix of personality and institutions, of observation and theory, and of curiosity versus pragmatism. The story involves not only purely geology, but advances in physics, chemistry, and . evolution. One key thread through these snarls is the maturation of Dar- win’s idea of a fluid beneath the earth’s crust, an idea that he shared with his contemporaries Charles Babbage and John Herschel. This thread twists through many corners of science, including the study of that most basic physical force, gravity. To back up just a little, the force of gravity is proportional to mass, and for a planet, the gravity on its surface is related to its density, and the variation of density within it. Isaac Newton, writing in Principia in 1687, made an intuitive estimate of the density of the earth. Newton guessed the density was “five or six times greater than if it consisted all of water.” Not too far off from the modern value of 5.51 gm/cm3 (with water at 1.0). Pierre Bouguer, the real brains of the French expedition to South America in the 1700s, tried to test another of Newton’s ideas. Newton suggested that a plumb bob suspended at a point near a large mountain would be deflected slightly toward the mountain by its gravitational attraction. Bouguer tried this experiment near the large volcano Chim- borazo, which was then thought to be the highest mountain on earth. He calculated that the plumb bob should be deflected about 103 seconds of arc from the vertical, but was disappointed that the measured value was only about 7½ seconds. “One can say,” he concluded, “. [that] mountains act at a distance, but that their action is much less considerable than the scale of their volume.” Since Chimborazo was a volcano, he thought that it might be par- tially hollow, and that perhaps this was the problem. Measurement techniques improved, but understanding of how and why the measurements of gravity varied upon the surface of the earth was elusive. When Sir George Everest, the surveyor general of India, discovered that his plumb bobs were deflected toward the 275 ROB WESSON mass of the Himalayas—but again not as much as was predicted by theory—a whole new area of inquiry, later named isostasy, began. Was the attraction reduced because the rocks of the Himalayas were less dense than those of the surrounding region, or because the mountains had the same density, but had a root of low-density rocks that penetrated into the higher density of the earth’s mantle below? Many geologists were a little confused by the debate and why it was important. But once again, those curious raised beaches and that pesky ice played a role. Thomas Jamieson—to whom Darwin had finally yielded on the question of Glen Roy—first pieced together a history sea level during the ice ages in Scotland. Jamieson argued that Scotland had been significantly depressed by the weight of the thousands of feet of ice piled up on it, but then gradually rose after the melting of the ice. This, he concluded, was the best explanation for the raised beaches of England and Scotland and the slow rising of Sweden. Jamieson also noted one peculiar fact that he did not explain. Parts of Scotland experienced a period of submergence just after the ice had melted. Jamieson did not anticipate that the melting of the glaciers led to a rapid rise in sea level throughout the oceans, inundating low-lying land. In contrast, the rebound of the land after the melting of the ice was a much slower process—one that is still going on today. In the late 1880s as Jamieson’s ideas were finding their way to America, Grove Karl Gilbert, a pioneering geologist of the U.S. Geological Survey, found the Glen Roy of the United States—on a massive scale—in the region surrounding the Great Salt Lake. Gilbert found and mapped four ancient shorelines of the lake, the highest about one thousand feet about the current level. The ancient lake—he named it Lake Bonneville—covered about ten times the area of the current Great Salt Lake. But unlike Glen Roy, Lake Bonneville filled a closed depression. There was no mystery about what dammed this lake. From careful measurements of the elevations along the shorelines of the former lake, Gilbert discovered something amazing. 276 DARWIN’S FIRST THEORY The shorelines were not flat, as the shorelines at Glen Roy were perceived to be. Shorelines on mountains that were once islands and peninsulas near the center of the lake were nearly two hundred feet higher than those on the periphery. The level of the shorelines was a mirror image, albeit subdued, of the depth of water in the ancient lake. Gilbert hypothesized that the earth’s crust was like an elastic plate—overlying a highly viscous, yielding substrate—and had been bowed down by the extra weight of two thousand cubic miles of water. Gilbert was an exceptional geologist, combining skill in the field with an appreciation for mathematical approaches and physical understanding. At the time, he was unique. What geologists saw led them to believe that the crust of the earth had had the ability to move around, to be mobile. This belief was anathema for many scientists grounded in physics and mathematics. If the common sense-interpretation of a geologic observation didn’t agree with the physics as understood at that time, it couldn’t be cor- rect. This erroneous way of thinking extended even into the twentieth century. The leader of this group was William Thomson, the British physicist later ennobled as Lord Kelvin, and not only did Thomson oppose mobility, but his views about the age of the earth also came to vex Darwin, endangering the acceptance of natural selection. Fortunately, eventually, physics would catch up. B Geologists who adopted Lyell’s point of view—that the geology of the earth was shaped by ongoing processes—quickly realized that the earth had to be very old. Darwin estimated at least three hundred million years. Lord Kelvin estimated the age of the earth by imagining it as a cooling iron sphere, and settled on an estimate between twenty and one hundred million years, a number that Darwin believed was inadequate to allow the progress of evolution by natural selection.

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