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How
the Earth Works December
1994 In his 1864 classic, Journey to the Center of the Earth, Jules Verne described with compelling effect the journey of Professor Otto Lidenbrock, his nephew Axel, and their Icelandic guide Hans into the crater of an extinct volcano. The fictional characters were attempting to follow in the footsteps of an earlier Norse explorer who claimed to have made it all the way to the heart of the planet. Verne's tale, which drew heavily on the geological thinking of the mid-19th century, offered readers a remarkable vision of the Earth's interior landscape, complete with a storm-tossed ocean and air fit to breathe. But in this book, the often-prescient Verne -- whose writings anticipated such things as the submarine, moon landings, and the fax machine -- got it wrong. Exactly 130 years after the book's appearance, Yale geologists are getting it right. Using information from a wide variety of sources, including earthquakes, chemical analyses of volcanoes, CAT-scanning techniques, even a Slinky, scientists at the University are charting their own provocative and controversial journey from the planet's crust, through the mantle, to the core. According to researchers at the Kline Geology Laboratory, not only does the subterranean world turn out to be vastly different from what Verne envisioned, but ideas about the underground landscape have changed dramatically. Take notions about water, for example. Most researchers have long agreed that the high temperatures and pressures which prevailed some 12 to 18 miles below the Earth's surface could lead to the release of small amounts of water. But few scientists were prepared to accept a controversial theory put forth in the 1970s by Danny Rye, professor of geology, and his colleagues, who suggested that while there was certainly no ocean underfoot, the steady creation of water could result in a mighty underground flow through the rocks of an area geologists call the "deep crust." Looking back, Rye is not surprised at the scientific recalcitrance. "I can give you a rock, and you'll say, 'where's the water?' It's not wet." But as Rye and company showed by examining the molecular composition of a variety of rocks and minerals, there are chemical "fingerprints" in the material that could only have resulted through the action of truly oceanic amounts of water working its way through the crust for vast amounts of time. The presence of ore deposits -- concentrations of commercially important minerals -- provides strong support for Rye's fluid flow and transport notion, and any skeptics should be won over by a set of discoveries made during the last few years by Jay Ague, an assistant professor of geology and geophysics. If one travels along the highways outside of New Haven and looks at places where the road has been cut through solid rock, an obvious feature is the pattern of veins that snake through such common kinds of local stone as slate, schist, and gneiss. "Many geologists have looked at these veins, but few people ever sampled them," Ague notes, adding that this lack of interest was a mistake. "What you've got here is a circulatory system, a set of pathways for fluids to move along." In his laboratory, Ague pulls out a tray of what he describes as "extraordinary rocks," many of which were collected with help from the New Haven Mineral Club, a group of amateur geologists. The samples, he explains, started out nearly half a billion years ago as deposits of clay-rich mud. Over time, they were buried deeper and deeper until, about 18 miles below the surface, they began a process of chemical and physical change known as metamorphism. "Basically, they dehydrate," Ague says, adding that the lost water moves outward and upward either through minute pores in the rock or through cracks. Along the way, the water interacts with every surface it meets. The veins represent a record of its travels, a record Ague has learned to read by analyzing the structure of metamorphic rocks that have finally been brought to the surface by the relentless process responsible for building mountains. "This is magnesite," says the geologist, pointing to a thicket of crystals made of magnesium, oxygen, and carbon that jut out of a vein. Ague goes on to say that, because the surrounding rock didn't contain the proper mix of molecules to create this mineral, the raw materials "had to be transported in from someplace else." The necessary building blocks arrived in fluid, but according to Ague, they don't simply zip by on their upward journey. "The cracks seal rapidly," says Ague, "They may be open for a period that lasts for seconds, or months." Whatever the interval, the requisite raw materials produce a bit of crystal. Then the chemistry slows down until the next round of cracking occurs, and more fluid arrives. "What we're seeing," says Ague, "is the net accumulation of small catastrophes," as the scientist calls the cracking, healing, and recracking cycle that he believes is a major feature of crustal life. The power source for each catastrophe is an earthquake, an event that Ague suspects may often be caused by underground fluid generation. That process, he argues provocatively, could be generating even higher pressures than those that caused it in the first place. Seen this way, each metamorphic rock, says Ague, could be "a little time bomb." When enough of them go off at once, the subterranean explosion might be sufficiently strong to cause rapid movement in any nearby faults, slippages of which result in earthquakes. And while Ague's contention is decidedly avant-garde, last year's devastating Northridge quake in southern California may have been initiated, say some geologists, by just such a crustal "time bomb." Tremors, of course, can wreak havoc, but they have also proved invaluable to scientists interested in understanding the planet's internal structure. The shaking of an earthquake -- like a rock thrown in still waters -- sets up a pattern of waves that resonate through the globe. An instrument called a seismometer (essentially a spring, a weight, and a device that records the weight's movements) can read these waves, which researchers are now turning into pictures of what's going on underfoot. On a computer screen in his Kline office, Jonathan Lees, an assistant professor of geology and geophysics, pulls up a colorful portrait of the inside of Mt. St. Helens, the Washington State volcano that erupted with such an enormous bang in 1980. This "seismic image," as Lees calls it, is a study in red, blue, and black. The abstraction owes its patterning, says Lees, to changes in the speed of the waves as they pass through various kinds of materials and conditions. "In this case, the red color indicates that the wave has slowed down -- this occurs when it encounters something that's hot. The blue indicates a faster wave velocity, which happens when it goes through colder material." Black indicates a speed somewhere between the two extremes. A volcano, notes Lees, is a mixture of fiery magma and colder rocks. Using an array of more than three dozen seismographic detectors, the geologist was able to take "snapshots" of Mt. St. Helens at a variety of depths. By stacking these tomographs, or "slice pictures," he could, in the same way a doctor builds a three-dimensional view of the body through a series of CAT-scan X-ray slices, create a picture of a mountain's internal activity. The portrait shows a volcano in uneasy repose. Magma has solidified near the surface, but at a depth of roughly three miles, the slowdown in seismic waves points to the existence of a hot pool of molten rock. Below that, however, there's a plug of cold stone that's nearly two miles thick. Deeper still, at the roots of the volcano, there's more hot magma. When the next eruption is likely to occur is uncertain, says Lees. "I'm not really in the prediction business," he says. Still, having a seismographic window into changes occurring inside volcanoes, as well as within the fault zones from which earthquakes emerge, is critical for scientists bent on geological mind-reading. Mark Brandon, an associate professor of geology and geophysics and a colleague of Lees, uses sound waves -- "earth music" -- as well as his own eyes to glimpse what's going on at places called subduction zones, areas where one of the Earth's tectonic plates is sliding underneath another plate. Brandon has concentrated his efforts off the west coast of North America, particularly in Washington and British Columbia. There, the Juan de Fuca plate, on which the floor of the Pacific Ocean rests, is diving below the vast tectonic "raft" that bears this continent. "Imagine a bulldozer," says Brandon, as he describes how the top part of the subducting plate is literally sheared off by the act of plunging underneath North America. This jumble of material can be "seen" by a ship-borne sonar device that bounces sound waves off the ocean bottom, but when Brandon climbed the Olympic Mountains of northwestern Washington in the early 1980s, he realized that the pattern of the rocks he observed was similar to what he had noticed offshore. And, to make things more intriguing, he found metamorphic rocks on the heights that had clearly come from the subducted plate rather than the continental one. "We're looking at a conveyor belt," says Brandon. "Not only is material being scraped off and piled in front -- we call this an accretionary margin -- but part of the subducting plate is being added to the bottom of the continental plate. This rock is eventually shoved upward as a mountain, then eroded away and exposed. The material is always in motion." Scientists don't have to wait for such uplifting events to see evidence of moving rock, says Associate Professor of Geology and Geophysics Jeffrey Park, who uses earthquake waves to examine the behavior of the upper reaches of the Earth's mantle, an area, more than 100 miles thick, of warm rock that flows like Silly Putty (but much more slowly). The dominant rock of this region, says Park, is called peridotite, which, fortuitously, is composed primarily of a mineral known as olivine. What makes this stuff so useful to geologists interested in the structure of the mantle is that the speed of sound waves passing through peridotite depends on the direction in which the olivine crystals are pointing. For a sound source, Park also depends on earthquakes, particularly the big ones that set the globe "ringing like a bell" for days on end. One such quake occurred on June 28, 1992 in Landers, California, and this event, which was measured at magnitude 7.5 on the Richter scale and caused widespread damage, created waves that were monitored at detection stations in Australia, the Philippines, Japan, and other locations in the western Pacific. Since Park and his Yale colleague Yang Yu knew both precisely when the sound waves should arrive at the detectors and what the waves should look like upon arrival, the researchers could translate any deviations from what they expected to see into a map of how the olivine crystals in the upper mantle were oriented. "This tells you which way the mantle is flowing," says Park. Apparently, it is not moving the way scientists have long supposed. Park explains that once geologists finally accepted the fact that the Earth's tectonic plates were never still, the challenge became how to account for their travels throughout much of the planet's 4.5-billion-year history. Researchers were fond of envisioning the globe's hot innards as behaving like a pan of boiling water, with a circulation pattern of warm, less dense, rising fluid and cooler, more dense, descending fluid swirling around in restless convection cells. These engines would drive the plates, pushing them in the proper direction and dragging them down in the subduction zones. The problem with this simple notion, says Park, is that it doesn't conform with reality. "There's growing evidence that the mantle is going sideways," he notes. In fact, there's brand new research suggesting that the mantle under the Pacific is moving in exactly the opposite direction of the plate it supposedly drives. The author of this "counterintuitive" proposal is Phillip Ihinger, an assistant professor of geology and geophysics, whose work accounts for the baffling pattern of a chain of Pacific volcanoes, including those that created the Hawaiian Islands, which was first noticed by pioneering Yale geologist James Dwight Dana in the 1840s. The volcanoes, many of them long drowned by the ocean, begin on the "Big Island" of Hawaii and then run northwest for more than 1,000 miles before taking a sharp turn to the north. For the past two decades, scientists have agreed that the source of the chain was a peculiar geological feature of the planet called a hot spot: a place on the Earth that serves as a long-term exit point for molten rock that may be coming all the way from the planet's core through the mantle to the crust. Geologists have located more than 40 hot spots around the globe: Yellowstone National Park sits atop one of them; Hawaii is the result of the current activities of another. "The track of these Hawaiian volcanoes not only tells us that the entire gigantic plate is moving, but the kink northwest of Midway Island shows that around 43 million years ago, the plate changed direction," says Ihinger. So the hot spot leaves its "fingerprint" on a moving target, and by studying that imprint in Hawaii, Ihinger has been able to chart what he calls "the winds in the mantle." The process involves analyzing both the age -- younger as one nears the hot spot -- and the distinctive chemical composition of the lavas. "These get seasoned with all sorts of stuff as the lava makes its way up through the mantle," he says. Part of the material is common to hot spots throughout the world, but another chemical fraction is unique to a particular area. When Ihinger looked at the age and chemistry of the Hawaiian volcanoes, he was able to link the observation of James Dwight Dana -- that the chain of mountains was actually a group of chains, something on the order of a train track -- with subterranean events. Think of the material coming up to the hot spot as a balloon, says the scientist. "Once the head pierces the crust, it then travels with the plate. But the tail end doesn't stand still either." Ihinger has shown that the "tail" of the hot-spot balloon, and all the material that comes to the surface with it, is being blown in the opposite direction of the plate it will eventually penetrate. "The only way you can have the pattern we observe is to have two areas of movement, and independent movement at that," he notes. For even though the Pacific plate changed its direction, Ihinger's analysis shows that the mantle wind held steady, blowing southeast for at least the last 73 million years. Alas, even though Verne's underground voyagers eventually wound up in Italy, which is indeed southeast of their starting point, they wouldn't have been helped by the subterranean breeze, says Ihinger. For Iceland sits astride the mid-Atlantic ridge, a crack in the Earth towards which the mantle underneath the trio's Italian destination is flowing. In other words, were Verne true to geology as Yale scientists now understand it, the travelers would have had a very short trip -- and readers would have missed a great adventure story. |
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Yale
Alumni Magazine, PO Box 1905, New Haven, CT 06509-1905. USA.
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