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Started by Recusant, May 02, 2020, 08:30:19 PM
QuoteEarth's continents are constantly on the move, it's a key feature of our planet, but that wasn't always the case.While some scientists think Earth's tectonic plates began pushing and pulling only a billion years ago, others think the whole process started nearly four billion years ago, when our planet was but an infant. That's quite the discrepancy, and as usual, general agreement lies somewhere in between. Today, it's commonly thought Earth's tectonic plates began moving around 2.8 billion years ago, when the interior of our planet was just the right temperature to allow for the formation of 15 rigid plates.Even still, disagreement reigns. Direct evidence from this time is hard to come by, and now some of the oldest rocks on Earth suggest we may have been more than 400 million years off the mark.Analysing magnetism in ancient rocks from Australia and South Africa, researchers at Harvard and MIT claim tectonic plates were moving at least 3.2 billion years ago and maybe earlier. "Basically, this is one piece of geological evidence to extend the record of plate tectonics on Earth farther back in Earth history," says Alec Brenner, who researches paleomagnetics at Harvard University."Based on the evidence we found, it looks like plate tectonics is a much more likely process to have occurred on the early Earth and that argues for an Earth that looks a lot more similar to today's than a lot of people think."The Pilbara craton in Western Australia is one of the oldest slices of Earth's ancient crust and contains fossils for some of the earliest organisms on our planet. Stretching nearly 500 kilometres across (300 miles), this chunk of primordial crust was formed as early as 3.5 billion years ago.Drilling into a portion of this craton, known as the Honeyeater Basalt, researchers used state of the art magnetometers and demagnetising equipment to uncover the region's magnetic history. Roughly 3.2 billion years ago, their data reveals a shift from one point to another, a latitudinal drift of 2.5 centimetres a year.Or, as the authors put it, "a velocity comparable with those of modern plates."[Continues . . .]
Quote from: Tank on May 06, 2020, 06:37:52 AMI missed this! I have to say that with my layman's understanding of plate tectonics (Dunning Krugar warning) that I would have thought that when the Earth was younger and hotter plate tectonics would have been more active than now.
QuoteThe geologic record is exactly that: a record. The strata of rock tell scientists about past environments, much like pages in an encyclopedia. Except this reference book has more pages missing than it has remaining. So geologists are tasked not only with understanding what is there, but also with figuring out what's not, and where it went.One omission in particular has puzzled scientists for well over a century. First noticed by John Wesley Powell in 1869 in the layers of the Grand Canyon, the Great Unconformity, as it's known, accounts for more than one billion years of missing rock in certain places.Scientists have developed several hypotheses to explain how, and when, this staggering amount of material may have been eroded. Now, UC Santa Barbara geologist Francis Macdonald and his colleagues at the University of Colorado, Boulder and at Colorado College believe they may have ruled out one of the more popular of these. Their study appears in the Proceedings of the National Academy of Sciences."There are unconformities all through the rock record," explained Macdonald, a professor in the Department of Earth Science. "Unconformities are just gaps in time within the rock record. This one's called the Great Unconformity because it was thought to be a particularly large gap, maybe a global gap."A leading thought is that glaciers scoured away kilometers of rock around 720 to 635 million years ago, during a time known as Snowball Earth, when the planet was completely covered by ice. This hypothesis even has the benefit of helping to explain the rapid emergence of complex organisms shortly thereafter, in the Cambrian explosion, since all this eroded material could have seeded the oceans with tremendous amounts of nutrients.Macdonald was skeptical of this reasoning. Although analogues of the Great Unconformity appear throughout the world -- with similar amounts of rock missing from similar stretches of time -- they don't line up perfectly. This casts doubt as to whether they were truly eroded by a global event like Snowball Earth.[. . .]"The basic hypothesis is that this large-scale erosion was driven by the formation and separation of supercontinents," Macdonald said.The Earth's cycle of supercontinent formation and separation uplifts and erodes incredible extents of rock over long periods of time. And because supercontinent processes, by definition, involve a lot of land, their effects can appear fairly synchronous across the geologic record.However, these processes don't happen simultaneously, as they would in a global event like Snowball Earth. "It's a messy process," Macdonald said. "There are differences, and now we have the ability to perhaps resolve those differences and pull that record out."[Continues . . .]
QuoteScientists have reconstructed the paleoecology the Paleo-Agulhas Plain, a now-drowned landscape on the southern tip of Africa that was high and dry during glacial phases of the last 2 million years and may have been instrumental in shaping the evolution of early modern humans.Early humans lived in South African river valleys with deep, fertile soils filled with grasslands, floodplains, woodlands, and wetlands that abounded with hippos, zebras, antelopes, and many other animals, some extinct for millennia.In contrast to ice age environments elsewhere on Earth, it was a lush environment with a mild climate that disappeared under rising sea levels around 11,500 years ago.An interdisciplinary, international team of scientists has now brought this pleasant cradle of humankind back to life in a special collection of articles that reconstruct the paleoecology of the Paleo-Agulhas Plain, a now-drowned landscape on the southern tip of Africa that was high and dry during glacial phases of the last 2 million years."These Pleistocene glacial periods would have presented a very different resource landscape for early modern human hunter-gatherers than the landscape found in modern Cape coastal lowlands, and may have been instrumental in shaping the evolution of early modern humans," said Janet Franklin, a distinguished professor of biogeography in the department of Botany and Plant Sciences at UC Riverside, an associate member of the African Centre for Coastal Palaeoscience at Nelson Mandela University in South Africa, and co-author of several of the papers.Some of the oldest anatomically modern human bones and artifacts have been found in cliff caves along the coast of South Africa. For many years, the lack of shellfish in some layers at these sites puzzled archaeologists. In spite of apparently living near the ocean, the inhabitants hunted mostly big game -- the sort of animals that typically live farther inland.Scientists knew a submerged landscape existed on the continental shelf just offshore, but it wasn't until recently, perhaps inspired by rising sea levels of our current human-caused global warming, they realized these caves might have made up the westernmost edge of a long-lost plain.During most of the Pleistocene, the geological era before the one we live in now, these caves were not located on the coast. With so much of the Earth's water locked up in continent-sized glaciers, sea level was much lower, and humans could have thrived between the cliffs and a gentler coastline miles and miles to the east.A special issue of Quaternary Science Reviews presents papers using a wide range of techniques to reconstruct the environment and ecology of the Paleo-Agulhas Plain. They reveal a verdant world rich with game, plant, and coastal resources, periodically cut off from the mainland during warm spells between glacial periods when sea level rose to levels similar to those of today, which would have played an important role in human evolution.[Continues . . .]
QuoteOur planet wears its magnetic field like an oversized coat that just won't sit comfortably. All that sliding means the north magnetic pole is destined to move ever closer to Siberia's coastline over the coming decade.There's no conspiracy behind it - but the geological forces responsible have been something of a mystery. Now, we might be a little closer to understanding what's going on.Researchers from the University of Leeds in the UK and the Technical University of Denmark have analysed 20 years of satellite data, finding that a monolithic competition between two lobes of differing magnetic force near the core is likely to be behind the pole's wanderlust. When the precise position of Earth's magnetic north was located for the first time back in 1831, it was squarely in Canada's corner of the Arctic, on the Boothia Peninsula in the territory of Nunavut.Ever since, fresh sets of measurements have recorded this spot drift north by an average of around 15 kilometres (about 9 miles) every year.Advanced technology means we can now keep a careful watch on the pole's location with unprecedented accuracy. Prior to the 1970s, the north magnetic pole's position was like a drunken stagger. Since then, it's had a mission, marching in a straight line, building speed.Since the 1990s, its movement has quadrupled in speed, to a current rate of between 50 and 60 kilometres (about 30 and 37 miles) a year. In late 2017, the pole's sprint brought it within 390 kilometres (240 miles) of the geographical north pole.[Continues . . .]
Quote from: Tank on May 16, 2020, 06:12:39 AMOh for a time machine
Quote from: Icarus on May 24, 2020, 12:57:15 AMHoly cow Hermes! That link is spectacular. Not so much about the chemistry but about the whole bit of beautiful SA information.Most of we dullard Americans envision Africa as a place where Hutus and Tutsi kill each other. I know better but your link has made me think of that continent in a different way.As a matter of fact an Amrerican doctor lives across the street from me. He got some of his Med education in SA. His wife is a very charming South African woman who is entirely civilized and sophisticated. A Brit friend who was here for a while once, when in a philosophic and wine fueled discussion, said this to me: "Americans are so insular". That statement stuck in my brain to this day. He was polite enough not to tell me that a lot of Americans are ill informed clods. He was correct of course.
Quote from: Icarus on May 24, 2020, 10:13:55 PMThat word, insular, was to imply that we see our country as an island of brilliant, productive, inspired, and generally better than the rest of the world. We are on a really big island, separated from a lot of other countries by large oceans. Our AMERICA IS NUMBER ONE mind set misses the mark of course, but it works for a lot of us.