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A Tale Of Two Shields: The Canadian And Baltic Shields—Their Archean Rocks, Tectonic History, Beautiful Landscapes, And Melting Ice SheetsMonte Marshall Wednesday, April 9th, 2014 |
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| ABSTRACT
With increasingly good geochemical and paleomagnetic data and zircon age dating, our understanding of the evolution of continents and supercontinents has greatly expanded in recent decades. About 80 % of continental crust was created in the Precambrian, and every continent has patches of Archean crust that are sutured together with crust that was created in Proterozoic orogenies. Because they are cooler and have thicker lithospheric mantles than the younger crust, these areas are stronger and less deformable and are called cratons. Any Phanerozoic sedimentary rocks deposited on them are usually fairly flat lying. Regions of the cratons where the Precambrian igneous and metamorphic rocks are exposed at the surface are called shields. One of the largest and best exposed shields is the Canadian Shield. It extends from the famed Northwest Passage down to the Great Lakes and from west of Hudson Bay to almost all of the coastal/exposed areas of Greenland. Across the Atlantic lies another shield, the Baltic Shield. It is exposed in northern Norway, Sweden, Finland, and in very northwestern Russia between St. Petersburg and the Arctic Ocean. The Baltic Shield is no stranger to North America and the Canadian Shield. It lay next to Greenland in the paleogeographic reconstruction of all three past supercontinents—Pangea, Rodinia, and Nuna/Columbia. Greenland itself is really part of North America. Based on the linear marine magnetic anomalies, the step-wise opening of the North Atlantic during the breakup of Pangea was somewhat complicated. Greenland only rifted from Baffin Island and Labrador during the interval 90 Ma–40 Ma. The Labrador Sea and Baffin Bay were created in those 50 My. At 55 Ma some spreading between Greenland and Northern Europe began and by 40 Ma the current North Atlantic spreading center was firmly established. The oldest rocks found so far in the Baltic Shield are 3.5-3.7 Ga granitic orthogneisses. The largest and best preserved region of early Archean continental crust in the world is the Itsaq Gneiss Complex in Greenland and contains rocks dated at 3.8-3.9 Ga. However, the oldest rocks on earth, found to date, are the Acasta Gneiss found near Great Slave Lake in the Northwest Territory of Canada and dated at 4.0 Ga. As you might expect, we have the Canadian Shield to thank for the oldest rocks in the United States. Just before it dives under the Paleozoic sedimentary rocks in SW Minnesota, the shield contains gneisses dated at 3.5 Ga! There are only 25 to 30 places in all the world’s shields that are known to contain Paleoarchean (3200-3600 Ma) or older rocks! Finally, a few words about the beautiful blue-green ice that remains in the far north of the Canadian Shield area. The ice cap that covers 98% of Greenland and reaches a thickness of two miles in its center contains about 10 percent of the earth’s fresh water. Antarctica has most of the rest. The velocity of the ice flowing out of some of the fjords has increased greatly in the last century. Jacobshaven, on the southwest coast, is its most prolific iceberg producer—it probably produced the iceberg that sank the Titanic. When we approached, so much ice was entering the sea that our icebreaker couldn’t enter the harbor! But Baffin Island, which was probably covered with ice a few thousand years ago, now has only a few isolated ice caps that feed small glaciers that flow down to the sea. The Arctic is warming more quickly than the Antarctic and ironically the number of tourists (who, in part, want “to see the ice before it melts”) is also increasing rapidly! Ships burn a lot of fuel. I have visited parts of these areas in recent years and will show you the landscape, largely fashioned by the recent Ice Age, and vegetative cover—tundra in the far north and stunted forests (taiga) to the south. You will see polar bears and killer whales frolicking at the bow of our ice breaker. And you will see what some of the oldest rocks in the world look like. For the most part they are gneisses and their foliation is often flat-lying, but at times is contorted into folds whose wavelengths range from centimeters to entire hill sides. The dark rocks are often laced with bands of beautiful pink granite that has either been intruded as veins or formed in place as migmatites. One of the most moving moments for me was when I stood on the western edge of Greenland’s ice cap, now a hundred miles inland from the Labrador Sea, and visualized how much ice had melted in just a few thousand years, and how much more the ice cap may continue to shrink in years to come! BIOGRAPHY Those of you who have read my recent articles or attended my recent talks, know that I have become interested in deep space and deep time in my declining years. So, I’ve decided to write this biography in that vein! I was born on a sunny Sunday morning in April, in Mercy Hospital, San Diego. But I had birth complications and the physicians had to use tongs to pull my head out first. My mother told me that she cried when she saw my somewhat misshaped head laced with tong bruise marks! I doubt that she realized that my birth trauma was partially due to the most important anatomical change in hominid evolution—the five-fold increase in the size of our brains/skulls, since we split from chimpanzees about 7 million years ago! Since that day, advances in biology, especially molecular biology and genetics, new fossils finds in Africa, and much more accurate geochronology have opened amazing chapters in our evolution. By 4 Ma a group of African apes, the Australopithecines (Remember Lucy?) began to use their hind limbs less for tree climbing and more for walking. Around 3 Ma the first hominids of our genus appeared, Homo habilis. They were full-time bipedals, made primitive stone tools, and began losing much of their body hair. There have been some 20 species of Homo and those extant at 1.5 Ma had learned the controlled use of fire, made more sophisticated stone tools, and developed black skin since they no longer had much fur to protect them from the sun. At about 200 ka, a species, Homo sapiens, appeared in the forests and grasslands of Africa that had an anatomy very similar to ours. In fact, the study of mitochondrial DNA has shown that everyone on earth today is the descendant of one Homo sapiens woman, called the “mitochondrial Eve”! Successive innovations in culture and lifestyle and a change in the climate of northern Africa led to waves of migration out of northeast Africa and up into Europe and Asia at 60 ka. But the wonder doesn’t end here! Anthropologists had been finding the bones of other hominids mixed with those of our ancestors in caves in Europe and Asia. In Europe it was Neanderthal man and in the Middle East it was the Denisovans. Since they were considered a different species, and therefore mating should only produce at most a single generation of sterile hybrids, their disappearance shortly after ‘we’ arrived was assumed to be their elimination/genocide by our ‘superior’ technology and intelligence. More modern research and genetics suggests that our merging was more peaceful—AND that everyone on earth, except the sub-Saharan Africans, have 1-5 % Neanderthal genes and as-yet-not-precisely quantified percentage of Denisovan genes! The National Geographic/IBM have collected and analyzed the DNA of 500,000 people all over the world (including MM). Wouldn’t my mother be surprised to learn that her poor, bruised baby was two percent Neanderthal, about three percent Denisovan, and that our early African ancestors spent some time in the Middle East, then pulled up stakes and moved to NW Europe, Ireland, and the British Isles about 10 thousand years ago! And……, as they say, the rest is history :>) |
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