By Leo Hindery, Jr.The coronavirus pandemichas already wreaked havoc on the U.S. economy, bringing business to a grinding halt and leaving millions of Americans worrying about where their next paycheck will come from. This is a nearly unprecedented public health crisis that demands meticulous coordination by top scientists and doctors. It’s also an economic emergency that calls for drastic intervention.In fact, experts say New Jersey’s economy is among those that could be hardest hit, as many of the state’s growth centers depend on a robust hospitality industry.But the lack of leadership from ...
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However, we still don’t have firm answers to some basic questions about continents: how did they come to be, and why did they form where they did?
In new research published in Nature, we studied ancient minerals from Western Australia and found tantalizing clues suggesting the giant impact hypothesis might be right.
How do you make a continent?
The continents form part of the lithosphere, the rigid rocky outer shell of Earth made up of ocean floors and the continents, of which the uppermost layer is the crust.
The crust beneath the oceans is thin and made of dark, dense basaltic rock which contains only a little silica. By contrast, the continental crust is thick and mostly consists of granite, a less dense, pale-colored, silica-rich rock that makes the continents “float”.
Beneath the lithosphere sits a thick, slowly flowing mass of almost-molten rock, which sits near the top of the mantle, the layer of Earth between the crust and the core.
If part of the lithosphere is removed, the mantle beneath it will melt as the pressure from above is released. And impacts from giant meteorites – rocks from space tens or hundreds of kilometers across – are an extremely efficient way of doing exactly that!
What are the consequences of a giant impact?
Giant impacts blast out huge volumes of material almost instantaneously. Rocks near the surface will melt for hundreds of kilometers or more around the impact site. The impact also releases pressure on the mantle below, causing it to melt and produce a “blob-like” mass of thick basaltic crust.
This mass is called an oceanic plateau, similar to that beneath present-day Hawaii or Iceland. The process is a bit like what happens if you are hit hard on the head by a golf ball or pebble – the resulting bump or “egg” is like the oceanic plateau.
Our research shows these oceanic plateaus could have evolved to form the continents through a process known as crustal differentiation. The thick oceanic plateau formed from the impact can get hot enough at its base that it also melts, producing the kind of granitic rock that forms buoyant continental crust.
Are there other ways to make oceanic plateaus?
There are other ways oceanic plateaus can form. The thick crusts beneath Hawaii and Iceland formed not through giant impacts but “mantle plumes”, streams of hot material rising up from the edge of Earth’s metallic core, a bit like in a lava lamp. As this ascending plume reaches the lithosphere it triggers massive mantle melting to form an oceanic plateau.
So could plumes have created the continents? Based on our studies, and the balance of different oxygen isotopes in tiny grains of the mineral zircon, which is commonly found in tiny quantities in rocks from the continental crust, we don’t think so.
Zircon is the oldest known crustal material, and it can survive intact for billions of years. We can also determine quite precisely when it was formed, based on the decay of the radioactive uranium it contains.
What’s more, we can find out about the environment in which zircon formed by measuring the relative proportion of isotopes of oxygen it contains.
We looked at zircon grains from one of the oldest surviving pieces of continental crust in the world, the Pilbara Craton in Western Australia, which started forming more than 3 billion years ago. Many of the oldest grains of zircon contained more light oxygen isotopes, which indicate shallow melting, but younger grains contain a more mantle-like balance isotopes, indicating much deeper melting.
Zircon δ18O (‰) vs age (Ma) for individual dated magmatic zircon grains from the Pilbara Craton. The horizontal grey band shows the array of δ18O in mantle zircon (5.3 +/– 0.6‰, 2 s.d.). The vertical grey bands subdivide the data into three stages, as discussed in the paper. The pink boxes represent the age of deposition of high-energy impact deposits (spherule beds) from the Pilbara Craton and more widely.
This “top-down” pattern of oxygen isotopes is what you might expect following a giant meteorite impact. In mantle plumes, by contrast, melting is a “bottom-up” process.
Sounds reasonable, but is there any other evidence?
Yes, there is! The zircons from the Pilbara Craton appear to have been formed in a handful of distinct periods, rather than continuously over time.
Except for the earliest grains, the other grains with isotopically-light zircon have the same age as spherule beds in the Pilbara Craton and elsewhere.
Spherule beds are deposits of droplets of material “splashed out” by meteorite impacts. The fact the zircons have the same age suggests they may have been formed by the same events.
The sun sets in the Pilbara, and the hunt for firewood is on. Chris Kirkland, 2021.
Further, the “top-down” pattern of isotopes can be recognized in other areas of ancient continental crust, such as in Canada and Greenland. However, data from elsewhere have not yet been carefully filtered like the Pilbara data, so it will take more work to confirm this pattern.
The next step of our research is to reanalyze these ancient rocks from elsewhere to confirm what we suspect – that the continents grew at the sites of giant meteorite impacts. Boom.
For the first time, we have shown that a soft heel pad was crucial to how sauropod dinosaurs supported their immense weight, according to a new digital reconstruction of their feet.
Sauropods, which weighed up to 50 tonnes and dominated the world’s ecosystems for around 100 million years, appear to have developed soft heel pads early in their evolution, and it was likely a key step that allowed sauropods to become the largest animals to have ever walked the earth. Our work appears this week in the journal Science Advances.
One of the most notable things about sauropods is the immense size of some species: the feet of sauropod dinosaurs would have shaken the earth as they walked. Indeed, the name of one of the first described sauropods to gain popular appeal, Brontosaurus, means “thunder lizard”.
Sauropods had long necks and tails, and walked on four long, pillar-like legs, but they didn’t start out gigantic. Around 230 million years ago, the ancestors of these dinosaurs were small, two-legged animals that would have looked very much like their saurischian cousins, the theropods; most probably wouldn’t have weighed more than an ostrich.
But starting around 210 million years ago, sauropod ancestors increased in size, with an estimated body mass approaching one tonne. The largest sauropods such as Argentinosaurus, Patagotitan and Australotitan probably reached adult sizes in excess of 50 tonnes more than ten times the size of the largest living terrestrial animal today, the African elephant.
It goes without saying that animals of that size had immense feet. Some sauropod footprints found in the Kimberley region of Western Australia are more than 1.7 meters long – big enough for most people to bathe in!
But what did sauropod feet really look like, and how did they support the titanic adult body weight of their owners?
An accumulation of sauropod tracks in the Lower Cretaceous Broome Sandstone, Walmadany area, Dampier Peninsula, Western Australia. Steven W. Salisbury, Author provided
On the trail of sauropods
Having spent many years tracking sauropods in the Kimberley, I [Steve Salisbury] have long pondered what their feet might have looked like in life. The front feet appear to have been like those of elephants, with the bones arranged in a near-vertical, semi-circular column, with greatly reduced finger bones except for the thumb. The “hand” prints of most sauropods are typically rounded or “bean-shaped”.
Despite their commonly portrayed columnar look, however, sauropod feet were very different to those of elephants. Sauropods had long, flexible toes, as evidenced by the mobility between the bones. Fossilized tracks show they could spread their toes, adjusting the splay of the foot as they walked across different surfaces – this is not what we find in elephants today.
Computer modeling shows sauropod feet had a soft tissue pad. Andreas Jannel, Author provided
It has long been assumed that like other dinosaurs, sauropods walked on their toes, with the ankle joint elevated off the ground. Yet many sauropod tracks include the impression of a large “heel”.
This has led many paleontologists to speculate that sauropods had some kind of “heel pad”. But apart from tracks, definitive evidence of a heel pad in sauropods has remained just that – academic speculation. Our work aims to change that.
Walking in the feet of giants
Armed with knowledge of what the foot skeleton of various sauropods looked like, along with information about their tracks, Andréas Jannel went about trying to figure out how their feet may have worked, as part of his PhD at The University of Queensland. We also teamed up with Olga Panagiotopoulou, an expert in the foot mechanics of modern animals, and elephants in particular.
Andréas generated 3D digital models for the foot skeleton of various sauropods and sauropod precursors. He and Olga then went about testing the strength of these models using a technique known as finite element analysis. They compared how different postures influenced the mechanical behavior of the foot with and without the addition of a soft-tissue pad.
Forces exerted on sauropod foot bones with and without a soft tissue pad. Andreas Jannel, Author provided
Regardless of the posture of the foot – toes on the ground, toes partially on the ground, or only the tips of the toes on the ground — none of the models could sustain the magnitude of mechanical forces that sauropods would have encountered in life, unless they also had a soft tissue pad beneath the “heel”.
Our findings indicate that a soft tissue pad would have cushioned the entire foot skeleton, allowing it to absorb mechanical forces during weight bearing. Put simply, without that pad beneath the heel, bones in the feet of sauropods would have crumpled under their immense weight.
The sauropods had soft tissue pads to absorb their enormous weight and enable them to walk on land. Andreas Jannel, Author provided
Arrival of the giants
Sauropod precursors such as Plateosaurus have traditionally been reconstructed as having walked with their toes slightly raised off the ground and with no heel pad. Our models now indicate their foot skeleton could not have supported their body weight without some form of additional padding.
Goolarabooloo Law Boss Richard Hunter alongside a 1.75 metre sauropod track in the Lower Cretaceous Broome Sandstone, Western Australia. The sauropod that made these tracks would have been around 5.4 metres high at the hips. From Salisbury et al. (2017). Photo: Steven W. Salisbury; image Anthony Romilio, Author provided
Some fossil tracks thought to belong to animals such as Plateosaurus do show evidence of pads starting to coalesce behind the toes. This “incipient” heel pad – one just starting to develop – would be consistent with our models.
The presence of an incipient heel pad in sauropod precursors laid the foundations for the evolution of a more substantial structure. By 170 million years ago, the first “true” sauropods were exceeding 10 metric tonnes, and tracks attributed to them show a well-developed heel pad.
The stage had been set, and within 10 million to 15 million years, titans weighing more than 30 tonnes were walking the earth, and the diversification of giant sauropods had begun. They would dominate world ecosystems for the next 100 million years.
Steven W. Salisbury, PhD; Associate Professor, School of Biological Sciences, The University of Queensland; Andréas Jannel, Postdoctoral researcher, Museum für Naturkunde, Berlin, and Olga Panagiotopoulou, Senior lecturer PhD, Monash University
A clip of New York Starbucks workers walking off the job to protest the firing of a fellow employee believed to have been targeted for union organizing has now been viewed nearly 20 million times on the popular video platform TikTok, bringing greater public visibility to the coffee giant's aggressive attempts to beat back a movement that has spread nationwide.
Sam Amato was fired from a Buffalo-area store last week after working for Starbucks for more than 13 years. The location, whose employees voted to unionize in March, has been closed since the Friday walkout as workers strike to demand Amato's reinstatement.
Michael Sainato reported for The Guardian on Thursday that months after they voted to unionize, "Amato and his co-workers were transferred to different stores while their own store underwent remodeling, where he said workers were heavily scrutinized by the store manager through disciplinary actions and write-ups."
While Starbucks claims Amato was terminated for closing his store's lobby without permission, Starbucks Workers United says that's a ruse to disguise the deliberate firing of a union leader. Amato says he didn't make the decision to close the lobby.
"This is the week of my thirteenth anniversary at Starbucks and I was fired," Amato told a local media outlet. "I was fired because I was a union leader at my store. My manager couldn't even explain to me why they were firing me. She couldn't look me in the eye."
Amato is one of dozens of union leaders that Starbucks has fired in recent months as the organizing wave continues, expanding to more than 200 locations across the U.S. even as management terminates employees, shutters entire stores, and threatens to deny new wage increases and benefits to organized workers.
In South Carolina, a Starbucks store manager recently went as far as accusing unionized workers of "kidnapping and assault" after they laid out a list of demands during a meeting.
"I cannot believe I have to say this, but workers who are organizing for better working conditions should not have to deal with their boss accusing them of kidnapping and assault. How outrageous," said Sen. Bernie Sanders (I-Vt.). "This campaign of rampant union-busting from Starbucks has got to end."
According to a tally by More Perfect Union, a progressive media outlet, "Starbucks has now illegally fired at least 70 pro-union workers since February."
"Overall," More Perfect Union notes, "members and supporters have been fired in at least 20 states."