Читать книгу A Wilder Time - William E. Glassley - Страница 19
ОглавлениеWE WERE THERE TO OBSERVE, and to collect samples of anything that would provide evidence of the terrain’s history—stretched crystals, folded and distorted rock layers, and any other indicators of tectonic movements. Noting on a map the places where each observation was made and where the samples were collected would allow us to piece together a tentative story while in the field. The samples we collected would be shipped back to our laboratories, where we could later assemble other facets of the history—how hot the rocks had been and how deeply buried they were when the deformation had occurred. The field observations combined with the results from the laboratory would provide the factual framework for the history we would write of what had happened thousands of millions of years ago.
The vanished mountains we envisioned were simple possibilities, tentative interpretations of passages written subtly in the obtuse patterns and features of Greenland’s rocks. The patterns match those seen in the Alps and the Himalayas—zones that seemed to be huge thrust faults, folds of immense proportion, metamorphism at extreme conditions. Through the inspired power of analogy, Kai, John, their coworkers, and those who had come before them had surmised that the Greenland landscape was an old ancestor, a forerunner of the young mountain systems that today so dramatically exalt Earth’s skin. But the Greenland ancestors are long gone, erased by the incessant hunger of flowing water, blowing wind, and grinding ice to achieve a form of topographic equality between sea and land. Erosion always wins.
The first clear hint of those lost mountains had come years earlier. Just after World War II, the Geological Survey of Greenland (GGU) was founded in Denmark. Through its offices, a small group of geologists, including Arne Noe-Nygaard and Hans Ramberg, began the first systematic study of the west coast of Greenland, sailing along the complex coastline in motorized sailing vessels strengthened to resist collisions with ice. They found a two-hundred-mile-wide belt of rock that seemed to preserve evidence of multiple complex episodes of protracted and intense deformation. The belt was called the Nagssugtoqidian mobile belt, named for the region it cut through—Nagssugtoq—and the fact that the rocks seemed to have been twisted into structures that implied extreme plasticity and flow. The mobile belt ran east-west all the way across Greenland. Although the mobile belt seemed to represent a major orogenic, or “mountain-building,” event, how or why it formed remained enigmatic. Cutting through this region were several distinct zones, each zone a few miles to tens of miles wide, in which the rocks were steeply inclined and consistently aligned in the same direction. For some years, the significance of the zones of aligned rocks remained obscure, their tectonic significance unknown. But by the late 1960s and early 1970s it had been suggested by Arthur Escher and Juan Watterson, among others, that these zones contained rocks that had been severely sheared into steeply inclined parallel sheets and layers. The individual zones were eventually called shear zones and were named after the regions they ran through—Isortoq, Ikertoq, Itivdleq, and Nordre Strømfjord. The latter, the Nordre Strømfjord shear zone (NSSZ), became the center of attention because it marked the northern edge of the entire Nagssugtoqidian mobile belt. It was the only one for which observations were made near the ice—the others were only mapped while sailing along the coast, and their inland extent was unknown.
Geology is not generally considered an enterprise rich with drama. Rocks stolidly await inspection, slowly providing, through insightful consideration, a glacially paced story of incremental change. But there are occasions when perspectives are radically altered, new story lines emerge, and the field is caught by surprise.
In 1987, such a change shook the world of Greenland geology. Although it played out subtly, the consequences for all involved were profound. Feiko Kalsbeek, Bob Pidgeon, and Paul Taylor reported finding along the northern limits of the mobile belt, near the inland ice, remnants of the same type of rocks as those found today in the Andes and the Sierra Nevada range in California.* Although nearly 2,000 million years older, those rocks were evidence that what is happening in the Andes today had happened in Greenland. In the case of the Andes, the continent of South America moves west, riding over the floor of the Pacific Ocean and pushing it hundreds of miles below the surface. Plunging into the incandescent heat of Earth’s interior, generating massively destructive earthquakes, the ocean floor partially melts, giving rise to bodies of molten rock that slowly make their way back to the surface. The volcanoes of the Andes and the mountainous spine they decorate are the result of that process. If the analogy was accurate, somewhere hidden within the Nagssugtoqidian mobile belt there should be evidence of a vanished Pacific, but no evidence of such a thing had yet been found.
