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“To see a world in a grain of sand”, the opening sentence of the poem by William Blake, is an oft-used phrase that also captures some of what geologists do. We observe the composition of mineral grains, smaller than the width of a human hair. Then, we extrapolate the chemical processes they suggest to ponder the construction of our planet itself. Now, we’ve taken that minute attention to new heights, connecting tiny grains to Earth’s place in the galactic environment. Looking out to the universe At an even larger scale, astrophysicists seek to understand the universe and our place in it. They use laws of physics to develop models that describe the orbits of astronomical objects. Although we may think of the planet’s surface as something shaped by processes entirely within Earth itself, our planet has undoubtedly felt the effects of its cosmic environment. This includes periodic changes in Earth’s orbit, variations in the Sun’s output, gamma ray bursts, and of course meteorite impacts. Just looking at the Moon and its pockmarked surface should remind us of that, given Earth is more than 80 times more massive than its gray satellite. In fact, recent work has pointed to the importance of meteorite impacts in the production of continental crust on Earth, helping to form buoyant “seeds” that floated on the outermost layer of our planet in its youth. We and our international team of colleagues have now identified a rhythm in the production of this early continental crust, and the tempo points to a truly grand driving mechanism. This work has just been published in the journal Geology. The rhythm of crust production on Earth Many rocks on Earth form from molten or semi-molten magma. This magma is derived either directly from the mantle – the predominantly solid but slowly flowing layer below the planet’s crust – or from recooking even older bits of pre-existing crust. As liquid magma cools, it eventually freezes into solid rock. Through this cooling process of magma crystallization, mineral grains grow and can trap elements such as uranium that decay over time and produce a sort of stopwatch, recording their age. Not only that, but crystals can also trap other elements that track the composition of their parental magma, like how a surname might track a person’s family. With these two pieces of information – age and composition – we can then reconstruct a timeline of crust production. Then, we can decode its main frequencies, using the mathematical wizardry of the Fourier transform. This tool basically decodes the frequency of events, much like unscrambling ingredients that have gone into the blender for a cake. Our results from this approach suggest an approximate 200-million-year rhythm to crust production on the early Earth. Our place in the cosmos But there is another process with a similar rhythm. Our Solar System and the four spiral arms of the Milky Way are both spinning around the supermassive black hole at the galaxy’s center, yet they are moving at different speeds. The spiral arms orbit at 210 kilometers per second, while the Sun is speeding along at 240km per second, meaning our Solar System is surfing into and out of the galaxy’s arms. You can think of the spiral arms as dense regions that slow the passage of stars much like a traffic jam, which only clears further down the road (or through the arm). This model results in approximately 200 million years between each entry our Solar System makes into a spiral arm of the galaxy. So, there seems to be a possible connection between the timing of crust production on Earth and the length of time it takes to orbit the galactic spiral arms – but why? Strikes from the cloud In the distant reaches of our Solar System, a cloud of icy rocky debris named the Oort cloud is thought to orbit our Sun. As the Solar System periodically moves into a spiral arm, interaction between it and the Oort cloud is proposed to dislodge material from the cloud, sending it closer to the inner Solar System. Some of this material may even strike Earth. Earth experiences relatively frequent impacts from the rocky bodies of the asteroid belt, which on average arrive at speeds of 15km per second. But comets ejected from the Oort cloud arrive much faster, on average 52km per second. We argue it is these periodic high-energy impacts that are tracked by the record of crust production preserved in tiny mineral grains. Comet impacts excavate huge volumes of Earth’s surface, leading to decompression melting of the mantle, not too dissimilar from popping a cork on a bottle of fizz. This molten rock, enriched in light elements such as silicon, aluminum, sodium, and potassium, effectively floats on the denser mantle. While there are many other ways to generate continental crust, it’s likely that impacting on our early planet formed buoyant seeds of crust. Magma produced from later geological processes would adhere to those early seeds. Harbingers of doom, or gardeners for terrestrial life? Continental crust is vital in most of Earth’s natural cycles – it interacts with water and oxygen, forming new weathered products, hosting most metals and biological carbon. Large meteorite impacts are cataclysmic events that can obliterate life. Yet, impacts may very well have been key to the development of the continental crust we live on. With the recent passage of interstellar asteroids through the Solar System, some have even gone so far as to suggest they ferried life across the cosmos. However we came to be here, it is awe-inspiring on a clear night to look up at the sky and see the stars and the structure they trace, and then look down at your feet and feel the mineral grains, rock, and continental crust below – all linked through a very grand rhythm indeed. By Chris Kirkland and Phil Sutton, THE CONVERSATION – from Science Alert

By measuring radioactive elements in rocks from Earth and other parts of the solar system, scientists can develop a timeline of our planet’s early years. Earth is roughly 4.54 billion years old. In that time, it has seen continents form and disappear, ice caps expand and retreat, and life evolve from single-celled organisms into blue whales. But how do we know Earth’s age? We start by looking inside it. “When you’re an Earth scientist who looks at a rock, it’s not just a rock; it’s like that rock has a story that you can try to decipher,” said Becky Flowers, a geologist at the University of Colorado Boulder. When minerals form out of magma or lava, they often contain traces of radioactive material, such as uranium. Over time, those radioactive elements decay, meaning they spew radiation, eventually transforming them into new, more stable elements that remain trapped inside the mineral. Take radioactive uranium-238, a common form of uranium. Its atoms will release energy until they eventually turn into lead. That process occurs at a fixed rate known as a half-life, which corresponds to the amount of time it takes for half of the atoms to decay. The half-life of uranium-238 is more than 4 billion years, meaning it takes more than 4 billion years for half of the uranium-238 in a sample to become lead. This makes it perfect for dating objects that are very, very old. By knowing these half-lives, we can calculate how old a rock is based on the ratio of the “parent” radioactive element and the “daughter” stable element — a method called radiometric dating. The mineral zircon is commonly used for radiometric dating because it contains a relatively large amount of uranium, Flowers said. Uranium-lead dating is just one type of radiometric dating. Other types use different elements; for example, radiocarbon dating, one of the most common methods, uses a radioactive isotope of carbon that has a half-life of thousands of years and is useful for dating organic matter. Using these methods, geologists have found minerals on Earth that date as far back as 4.4 billion years, meaning the planet has been around at least that long. But if scientists say Earth is more than 4.5 billion years old, where did those extra 100 million years or so come from? Earth, as mentioned, has changed a lot over billions of years, especially through processes such as plate tectonics, which shift the crust, birthing new land out of magma and subducting old land back underground. As a result, rocks from the very beginning of the planet’s history are hard to find; they’ve long since eroded or melted back into raw material. But scientists can use radiometric dating to determine the age of rocks from other parts of the solar system, too. Some meteorites contain materials that are more than 4.56 billion years old, and rocks from the moon and Mars have also been dated to around 4.5 billion years ago. Those dates are pretty close to the time scientists think the solar system started to take shape out of the cloud of gas and dust surrounding the newborn sun. And by knowing all of these relative ages, we can start to piece together a timeline of how Earth, the moon, Mars and all of the other little rocks floating around in nearby space started to form. Yet the transition from primordial dust cloud to planet Earth didn’t happen all at once but rather over millions of years, Rebecca Fischer, an Earth and planetary scientist at Harvard University, told Live Science. That means our understanding of Earth’s age will always be less about a specific year when the planet formed and more about a general sense of the era when our home planet started to take shape. By Ethan Freedman (lifes-little-mysteries ) reprinted from LiveScience.com

To date, Earth is the only planet we know of that has continents. Exactly how they formed and evolved is unclear, but we do know – because the edges of continents thousands of miles apart match up – that, at one time long ago, Earth’s landmass was concentrated in one big supercontinent. Since that’s not what the planet looks like today, something must have triggered that supercontinent to break apart. Now, we have new evidence to suggest that giant meteorite impacts played a significant role. The smoking gun consists of crystals of the mineral zircon, excavated from a craton in Western Australia, a piece of Earth’s crust that has remained stable for over a billion years. Known as the Pilbara Craton, it is the best-preserved chunk of crust on the planet… and the zircon crystals within it contain evidence of ancient meteorite impacts before the continents broke apart. “Studying the composition of oxygen isotopes in these zircon crystals revealed a ‘top-down’ process starting with the melting of rocks near the surface and progressing deeper, consistent with the geological effect of giant meteorite impacts,” explained geologist Tim Johnson of Curtin University in Australia. Our research provides the first solid evidence that the processes that ultimately formed the continents began with giant meteorite impacts, similar to those responsible for the extinction of the dinosaurs, but which occurred billions of years earlier.” The work was conducted on 26 rock samples containing fragments of zircon, dating between 3.6 and 2.9 billion years old. The research team carefully analyzed isotopes of oxygen; specifically, the ratios of oxygen-18 and oxygen-16, which have 10 and 8 neutrons, respectively. These ratios are used in paleogeology to determine the formation temperature of the rock in which the isotopes are found. Based on these ratios, the team was able to distinguish three distinct and fundamental stages in the formation and evolution of the Pilbara Craton. The first stage is the formation of a large proportion of zircons consistent with partial melting of the crust. This partial melting, the researchers show, was likely the result of bombardment by meteorites, which heated the planetary crust on impact. The oldest cluster of these zircons, according to the team’s interpretation, was the result of a single giant impact that led to the formation of the craton. The second stage was a period of reworking and stabilization of the crustal nucleus, followed by the third stage – a period of melting and granite formation. This stabilized nucleus would then, much later, evolve to become today’s continents, as did the cratons found on other continents around the world. Many meteorites have pelted Earth in eons past, in numbers much higher than the number of continents. It’s only the largest impacts that could generate enough heat to create the cratons, which appear to be twice as thick as their surrounding lithosphere. These findings are consistent with previously proposed models for the formation of cratons around the world – but constitute, the researchers said, the strongest evidence yet for the theory. However, it’s just one craton, out of around 35 known. To make the evidence even stronger still, the team will need to compare their results with more samples from other cratons, to see if their model is consistent globally. “Data related to other areas of ancient continental crust on Earth appears to show patterns similar to those recognized in Western Australia,” Johnson said. “We would like to test our findings on these ancient rocks to see if, as we suspect, our model is more widely applicable.” Article by MICHELLE STARR, Science Alert,10 August 2022 – The research has been published in Nature.

Among the planets in the Solar System, Earth is unique for having plate tectonics. Mapping our planet through its long history creates a beautiful continental dance — mesmerizing in itself and a work of natural art. This is the first time Earth’s geological record has been used to look so far back in time in an attempt to map the planet over the last 40% of its history. The work, led by Xianzhi Cao from the Ocean University in China, is now published in the open-access journal Geoscience Frontiers.  Our planets rocky surface is split into fragments (plates) that grind into each other and create mountains, or split away and form chasms that are then filled with oceans. There are 4.6 billion years of plate motion to investigate, and the rocks we walk over contain the evidence for how Earth has changed over this time. This is a first attempt at mapping the last 1.8 billion years of Earth’s history – a leap forward in the scientific grand challenge to map our world. Modelling our planet’s past is essential if we’re to understand how nutrients became available to power evolution. Apart from causing earthquakes and volcanoes, plate tectonics also pushes up rocks from the deep earth into the heights of mountain ranges. This way, elements which were far underground can erode from the rocks and end up washing into rivers and oceans. From there, living things can make use of these elements. A number of critical metals – like copper and cobalt – are more soluble in oxygen-rich water. In certain conditions, these metals are then precipitated out of the solution: in short, they form ore deposits. Many metals form in the roots of volcanoes that occur along plate margins. By reconstructing where ancient plate boundaries lay through time, we can better understand the tectonic geography of the world and assist mineral explorers in finding ancient metal-rich rocks now buried under much younger mountains. Such a model will allow us to test hypotheses about Earth’s past. For example, why Earth’s climate has gone through extreme “Snowball Earth” fluctuations, or why oxygen built up in the atmosphere when it did. Indeed, it will allow us to much better understand the feedback between the deep planet and the surface systems of Earth that support life as we know it. Excerpted from a Science Alert article by Alan Collins, Professor of Geology, University of Adelaide

During the last ice age, humans ventured into two vast and completely unknown continents: North and South America. For nearly a century, researchers thought they knew how this wild journey occurred: The first people to cross the Bering Land Bridge, a massive swath of land that connected Asia with North America when sea levels were lower, were the Clovis, who made the journey shortly before 13,000 years ago. According to the Clovis First theory, every Indigenous person in the Americas could be traced to this single, inland migration, said Loren Davis, a professor of anthropology at Oregon State University. But in recent decades, several discoveries have revealed that humans first reached the so-called New World thousands of years before we initially thought and probably didn’t get there by an inland route. So who were the first Americans, and how and when did they arrive? Genetic studies suggest that the first people to arrive in the Americas descend from an ancestral group of Ancient North Siberians and East Asians that mingled around 20,000 to 23,000 years ago and crossed the Bering Land Bridge sometime between then and 15,500 years ago. Geneticists studying the first Americans tend to paint a more consistent picture than archaeologists do, mainly because they’re using the same human remains and genetic datasets. Genetic analyses have found that Ancient North Siberians and a group of East Asians paired up around 20,000 to 23,000 years ago. Soon after, the population split into two genetically distinct groups: one that stayed in Siberia, and another, the basal American branch, which emerged around 20,000 to 21,000 years ago. Genetic data suggest the descendants of this basal American branch crossed the Bering Land Bridge and became the first Americans. But some archaeological sites hint that people may have reached the Americas far earlier than that. There are fossilized human footprints in White Sands National Park in New Mexico that may date to 21,000 to 23,000 years ago. That would mean humans arrived in North America during the Last Glacial Maximum (LGM), which occurred between about 26,500 to 19,000 years ago, when ice sheets covered much of what is now Alaska, Canada and the northern U.S. Other, more equivocal data suggest the first people arrived in the Western Hemisphere by 25,000 or even 31,500, years ago. If these dates can be confirmed, they would paint a much more complex picture of how and when humans reached the Americas. Almost all scientists agree, however, that this incredible journey was made possible by the emergence of Beringia — a now-submerged, 1,100-mile-wide (1,800 kilometers) landmass that connected what is now Alaska and the Russian Far East. During the last ice age, much of Earth’s water was frozen in ice sheets, causing ocean levels to fall. Beringia surfaced once waters in the North Pacific dropped roughly 164 feet (50 meters) below today’s levels; it was passable by foot between 30,000 and 12,000 years ago. From there, the archaeological picture gets muddier. The older version of the story originated in the 1920s and 1930s, when Western archaeologists discovered sharp-edged, leaf-shaped stone spear points near Clovis, New Mexico. The people who made them, now dubbed the Clovis people, lived in North America between 13,000 and 12,700 years ago, based on a 2020 analysis of bone, charcoal and plant remains found at Clovis sites. At the time, it was thought that the Clovis traveled across Beringia and then moved through an ice-free corridor, or “a gap between the continental ice sheets,” in what is now part of Alaska and Canada. However, new discoveries have turned back the clock on the first Americans’ arrival. In 1976, researchers learned about the site of Monte Verde II in southern Chile, which radiocarbon dating showed was about 14,550 years old. It took decades for archaeologists to accept the dating of Monte Verde, but soon, other sites also pushed back the date of humans’ arrival in the Americas. The Paisley Caves in Oregon contain human coprolites, or fossilized poop, dating to about 14,500 years ago. Page-Ladson, a pre-Clovis site in Florida with stone tools and mastodon bones, dates to about 14,550 years ago. And Cooper’s Ferry — a site that includes stone tools, animal bones and charcoal — dates to around 16,000 years ago. Then, in 2021, scientists announced much more ancient traces of human occupation: fossilized footprints in White Sands, New Mexico dating to between 21,000 and 23,000 years ago. Sites such as White Sands and Cooper’s Ferry have big implications for how the first people arrived in the Americas. It’s thought that the ice-free corridor through North America didn’t fully open until about 13,800 years ago. So if humans were in the Americas long before then, they likely traveled there along the Pacific coast. That coastal journey could have been made by foot, by watercraft, or both. But no fossil or archaeological evidence of this journey has been unearthed. Ideally, archaeologists would like to find more sites from all of these branches, especially any remains that could explain the genetics behind the people at White Sands between 23,000 and 21,000 years ago. Evidence of these long-lost people can be found in the remains of the animals they butchered, the charcoal they burned, the tools they crafted and the loved ones they buried. Local Indigenous Peoples’ stories strongly suggest humans were in the area during the Ice Age Floods, but tangible scientific evidence is sparse and yet to be found in the area. Eedited from a Live Science article by Laura Geggel