New model refutes leading theory on how Earth's continents formed

The formation of Earth's continents billions of years ago set the stage for life to thrive. But scientists disagree over how those land masses formed and if it was through geological processes we still see today.

A recent paper from the University of Illinois Chicago's David Hernández Uribe in Nature Geoscience adds new information to that debate, poking holes in the leading theory of continent formation.

Hernández Uribe used computer models to study the formation of magmas thought to hold clues to the origin of continents.

Magma is the molten substance that, when it cools, forms rocks and minerals.

Hernández Uribe looked for magmas that match the compositional signature of rare mineral deposits called zircons that date back to the Archaean period of 2.5 to 4 billion years ago, when scientists believed that continents first formed.

Last year, scientists from China and Australia published a paper arguing that Archaean zircons could only be formed by subduction -- when two tectonic plates collide underwater, pushing land mass to the surface.

That process still happens today, causing earthquakes and volcanic eruptions and reshaping the coasts of continents.

But Hernández Uribe, assistant professor of earth and environmental sciences, found that subduction was not necessary to create Archaean zircons.

Instead, he found that the minerals could form through high pressure and temperatures associated with the melting of the Earth's thick primordial crust.

"Using my calculations and models, you can get the same signatures for zircons and even provide a better match through the partial melting of the bottom of the crust," Hernández Uribe said.

"So based on these results, we still do not have enough evidence to say which process formed the continents."

The results also raise uncertainty about when plate tectonics started on Earth.

If Earth's first continents formed by subduction, that meant that continents started moving between 3.6 to 4 billion years ago -- as little as 500 million years into the planet's existence.

But the alternative theory of melting crust forming the first continents means that subduction and tectonics could have started much later.

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What turned Earth into a giant snowball 700m years ago? Scientists now have an answer

Australian geologists have used plate tectonic modelling to determine what most likely caused an extreme ice-age climate in Earth's history, more than 700 million years ago.

The study, published in Geology, helps our understanding of the functioning of the Earth's built-in thermostat that prevents the Earth from getting stuck in overheating mode.

It also shows how sensitive global climate is to atmospheric carbon concentration.

"Imagine the Earth almost completely frozen over," said the study's lead author, ARC Future Fellow Dr Adriana Dutkiewicz.

"We now think we have cracked the mystery: historically low volcanic carbon dioxide emissions, aided by weathering of a large pile of volcanic rocks in what is now Canada; a process that absorbs atmospheric carbon dioxide."

The project was inspired by the glacial debris left by the ancient glaciation from this period that can be spectacularly observed in the Flinders Ranges in South Australia.

A recent geological field trip to the Ranges, led by co-author Professor Alan Collins from the University of Adelaide, prompted the team to use the University of Sydney EarthByte computer models to investigate the cause and the exceptionally long duration of this ice age.

The extended ice age, also called the Sturtian glaciation after the 19th century European colonial explorer of central Australia, Charles Sturt, stretched from 717 to 660 million years ago, a period well before the dinosaurs and complex plant life on land existed.

Dr Dutkiewicz said: "Various causes have been proposed for the trigger and the end of this extreme ice age, but the most mysterious aspect is why it lasted for 57 million years -- a time span hard for us humans to imagine."

The team went back to a plate tectonic model that shows the evolution of continents and ocean basins at a time after the breakup of the ancient supercontinent Rodina.

They connected it to a computer model that calculates CO2 degassing of underwater volcanoes along mid-ocean ridges -- the sites where plates diverge and new ocean crust is born.

They soon realised that the start of the Sturtian ice age precisely correlates with an all-time low in volcanic CO2 emissions.

In addition, the CO2 outflux remained relatively low for the entire duration of the ice age.

Dr Dutkiewicz said: "At this time, there were no multicellular animals or land plants on Earth. The greenhouse gas concentration of the atmosphere was almost entirely dictated by CO2 outgassing from volcanoes and by silicate rock weathering processes, which consume CO2."

Co-author Professor Dietmar Müller from the University of Sydney said: "Geology ruled climate at this time. We think the Sturtian ice age kicked in due to a double whammy: a plate tectonic reorganisation brought volcanic degassing to a minimum, while simultaneously a continental volcanic province in Canada started eroding away, consuming atmospheric CO2.

"The result was that atmospheric CO2 fell to a level where glaciation kicks in -- which we estimate to be below 200 parts per million, less than half today's level."

The team's work raises intriguing questions about Earth's long-term future.

A recent theory proposed that over the next 250 million years, Earth would evolve towards Pangea Ultima, a supercontinent so hot that mammals might become extinct.

However, the Earth is also currently on a trajectory of lower volcanic CO2 emissions, as continental collisions increase and the plates slow down.

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Source rocks of the first real continents

Geoscientists have uncovered a missing link in the enigmatic story of how the continents developed -- a revised origin story that doesn't require the start of plate tectonics or any external factor to explain their formation. Instead, the findings published last week in Nature Communications, rely solely on internal geological forces that occurred within oceanic plateaus that formed during the first few hundred million years of Earth's history.

A major hurdle in understanding how the continents formed during the Archean Eon (four to 2.5 billion years ago) has been identifying the building blocks of Earth's early crust.

Much of the "new" Archean crust formed during this period comprised a very distinct association of three types of granitoid rocks -- tonalite, trondhjemite and granodiorite (TTG).

Understanding what went into making TTGs and the magmas they formed from has been difficult, because so many geological processes occurred between their initial melting and ultimate crystallization.

Earlier researchers focused on the trace element composition of these rocks, hoping to find clues about TTG magmas and their source.

"We tracked a specific set of trace elements that aren't affected by alteration and pristinely preserve signatures from the original magma that made new TTG crust," said Dr. Matthijs Smit, associate professor and Canada Research Chair at the University of British Columbia's (UBC) Department of Earth, Ocean and Atmospheric Sciences.

"These elements allowed us to look back through the chemical changes that TTG magmas go through and trace the melt compositions back to their initial state and source -- most likely a sort of gabbro."

"Funnily enough, many people have varieties of this type of rock as a kitchen countertop," Dr. Smit says.

"In a way, many people are preparing their dinner on the type of rock that was responsible for making our modern continents."

The Archean TTG crust is still part of the continents today.

For instance, in North America they make up much of the Canadian Interior between the Cordillera mountain belt in the west and the Grenville and Appalachian mountain belts in the east.

The majority of Ontario, Quebec, Manitoba, Saskatchewan, Northwest Territories and Nunavut is made up of Archean crustal fragments that are dominated by TTGs and their slightly younger and more evolved granite counterparts.

"All of these rocks -- and especially their combination -- can be explained by the model we present," said Dr. Smit.

"Ours is a simple model in which TTGs, as well as the younger rocks that TTGs are typically associated with, resulted from the slow burial, thickening and melting of precursor crust that likely resembled oceanic plateaus. The continental crust was destined to develop the way it did, because it kept getting buried further and the rocks at its base had no choice but to melt. In doing so, they made the TTGs that proved a winning recipe for continental survival and growth."

The UBC researchers' discovery of a stand-alone "intra-crustal" mechanism to make TTGs dispels the long-standing theory that Archean TTGs are formed in Earth's first subduction zones and mark the start of plate tectonics.

"There's always been a 'chicken-and-egg' question of which came first -- the start of plate tectonics or TTG magmatism to make new continental crust," says Dr. Smit.

"We show that these things may actually not be directly related. The recognition of the type of source rock makes this leap possible and also takes away the need to have other mechanisms, such as meteorite impact, explain the growth of the first real continents."

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Venus had Earth-like plate tectonics billions of years ago, study suggests

Venus, a scorching wasteland of a planet according to scientists, may have once had tectonic plate movements similar to those believed to have occurred on early Earth, a new study found. The finding sets up tantalizing scenarios regarding the possibility of early life on Venus, its evolutionary past and the history of the solar system.

Writing in Nature Astronomy, a team of scientists led by Brown University researchers describes using atmospheric data from Venus and computer modeling to show that the composition of the planet's current atmosphere and surface pressure would only have been possible as a result of an early form of plate tectonics, a process critical to life that involves multiple continental plates pushing, pulling and sliding beneath one another.

On Earth, this process intensified over billions of years, forming new continents and mountains, and leading to chemical reactions that stabilized the planet's surface temperature, resulting in an environment more conducive to the development of life.

Venus, on the other hand, Earth's nearest neighbor and sister planet, went in the opposite direction and today has surface temperatures hot enough to melt lead. One explanation is that the planet has always been thought to have what's known as a "stagnant lid," meaning its surface has only a single plate with minimal amounts of give, movement and gasses being released into the atmosphere.

The new paper posits that this wasn't always the case. To account for the abundance of nitrogen and carbon dioxide present in Venus' atmosphere, the researchers conclude that Venus must have had plate tectonics sometime after the planet formed, about 4.5 billion to 3.5 billion years ago. The paper suggests that this early tectonic movement, like on Earth, would have been limited in terms of the number of plates moving and in how much they shifted. It also would have been happening on Earth and Venus simultaneously.

"One of the big picture takeaways is that we very likely had two planets at the same time in the same solar system operating in a plate tectonic regime -- the same mode of tectonics that allowed for the life that we see on Earth today," said Matt Weller, the study's lead author who completed the work while he was a postdoctoral researcher at Brown and is now at the Lunar and Planetary Institute in Houston.

This bolsters the possibility of microbial life on ancient Venus and shows that at one point the two planets -- which are in the same solar neighborhood, are about the same size, and have the same mass, density and volume -- were more alike than previously thought before diverging.

The work also highlights the possibility that plate tectonics on planets might just come down to timing -- and therefore, so may life itself.

"We've so far thought about tectonic state in terms of a binary: it's either true or it's false, and it's either true or false for the duration of the planet," said study co-author Alexander Evans, an assistant professor of Earth, environmental and planetary sciences at Brown. "This shows that planets may transition in and out of different tectonic states and that this may actually be fairly common. Earth may be the outlier. This also means we might have planets that transition in and out of habitability rather than just being continuously habitable."

That concept will be important to consider as scientists look to understand nearby moons -- like Jupiter's Europa, which has shown proof of having Earth-like plate tectonics -- and distant exoplanets, according to the paper.

The researchers initially started the work as a way to show that the atmospheres of far-off exoplanets can be powerful markers of their early histories, before deciding to investigate that point closer to home.

They used current data on Venus' atmosphere as the endpoint for their models and started by assuming Venus has had a stagnant lid through its entire existence. Quickly, they were able to see that simulations recreating the planet's current atmosphere didn't match up with where the planet is now in terms of the amount nitrogen and carbon dioxide present in the current atmosphere and its resulting surface pressure.

The researchers then simulated what would have had to happen on the planet to get to where it is today. They eventually matched the numbers almost exactly when they accounted for limited tectonic movement early in Venus' history followed by the stagnant lid model that exists today.

Overall, the team believes the work serves as a proof of concept regarding atmospheres and their ability to provide insights into the past.

"We're still in this paradigm where we use the surfaces of planets to understand their history," Evans said. "We really show for the first time that the atmosphere may actually be the best way to understand some of the very ancient history of planets that is often not preserved on the surface."

Upcoming NASA DAVINCI missions, which will measure gasses in the Venusian atmosphere, may help solidify the study's findings. In the meantime, the researchers plan to delve deep into a key question the paper raises: What happened to plate tectonics on Venus? The theory in the paper suggests that the planet ultimately became too hot and its atmosphere too thick, drying up the necessary ingredients for tectonic movement.

"Venus basically ran out of juice to some extent, and that put the brakes on the process," said Daniel Ibarra, a professor in Brown's Department of Earth, Environmental and Planetary Sciences and co-author on the paper.

The researchers say the details of how this happened may hold important implications for Earth.

"That's going to be the next critical step in understanding Venus, its evolution and ultimately the fate of the Earth," Weller said. "What conditions will force us to move in a Venus-like trajectory, and what conditions could allow the Earth to remain habitable?"

Read more at Science Daily

Evidence that Venus is volcanically active

Venus appears to have volcanic activity, according to a new research paper that offers strong evidence to answer the lingering question about whether Earth's sister planet currently has eruptions and lava flows.

Venus, although similar to Earth in size and mass, differs markedly in that it does not have plate tectonics. The boundaries of Earth's moving surface plates are the primary locations of volcanic activity.

New research by University of Alaska Fairbanks Geophysical Institute research professor Robert Herrick revealed a nearly 1-square-mile volcanic vent that changed in shape and grew over eight months in 1991. Changes on such a scale on Earth are associated with volcanic activity, whether through an eruption at the vent or movement of magma beneath the vent that causes the vent walls to collapse and the vent to expand.

The research was published today in the journal Science.

Herrick studied images taken in the early 1990s during the first two imaging cycles of NASA's Magellan space probe. Until recently, comparing digital images to find new lava flows took too much time, the paper notes. As a result, few scientists have searched Magellan data for feature formation.

"It is really only in the last decade or so that the Magellan data has been available at full resolution, mosaicked and easily manipulable by an investigator with a typical personal workstation," Herrick said.

The new research focused on an area containing two of Venus' largest volcanoes, Ozza and Maat Mons.

"Ozza and Maat Mons are comparable in volume to Earth's largest volcanoes but have lower slopes and thus are more spread out," Herrick said.

Maat Mons contains the expanded vent that indicates volcanic activity.

Herrick compared a Magellan image from mid-February 1991 with a mid-October 1991 image and noticed a change to a vent on the north side of a domed shield volcano that is part of the Maat Mons volcano.

The vent had grown from a circular formation of just under 1 square mile to an irregular shape of about 1.5 square miles.

The later image indicates that the vent's walls became shorter, perhaps only a few hundred feet high, and that the vent was nearly filled to its rim. The researchers speculate that a lava lake formed in the vent during the eight months between the images, though whether the contents were liquid or cooled and solidified isn't known.

The researchers offer one caveat: a nonvolcanic, earthquake-triggered collapse of the vent's walls might have caused the expansion. They note, however, that vent collapses of this scale on Earth's volcanoes have always been accompanied by nearby volcanic eruptions; magma withdraws from beneath the vent because it is going somewhere else.

The surface of Venus is geologically young, especially compared to all the other rocky bodies except Earth and Jupiter's moon Io, Herrick said.

"However, the estimates of how often eruptions might occur on Venus have been speculative, ranging from several large eruptions per year to one such eruption every several or even tens of years," he said.

Herrick contrasts the lack of information about Venusian volcanism with what is known about Jupiter's moon Io and about Mars.

"Io is so active that multiple ongoing eruptions have been imaged every time we've observed it," he said.

On a geological time scale, relatively young lava flows indicate Mars remains volcanically active, Herrick said.

"However, nothing has occurred in the 45 years that we have been observing Mars, and most scientists would say that you'd probably need to watch the surface for a few million years to have a reasonable chance of seeing a new lava flow," he said.

Herrick's research adds Venus to the small pool of volcanically active bodies in our solar system.

"We can now say that Venus is presently volcanically active in the sense that there are at least a few eruptions per year," he said. "We can expect that the upcoming Venus missions will observe new volcanic flows that have occurred since the Magellan mission ended three decades ago, and we should see some activity occurring while the two upcoming orbital missions are collecting images."

Read more at Science Daily

Laying geological groundwork for life on Earth

New research analyzing pieces of the most ancient rocks on the planet adds some of the sharpest evidence yet that Earth's crust was pushing and pulling in a manner similar to modern plate tectonics at least 3.25 billion years ago. The study also provides the earliest proof of when the planet's magnetic north and south poles swapped places.

The two results offer clues into how such geological changes may have resulted in an environment more conducive to the development of life on the planet.

The work, described in PNAS and led by Harvard geologists Alec Brenner and Roger Fu, focused on a portion of the Pilbara Craton in western Australia, one of the oldest and most stable pieces of the Earth's crust. Using novel techniques and equipment, the researchers show that some of the Earth's earliest surface was moving at a rate of 6.1 centimeters per year and 0.55 degrees every million years.

That speed more than doubles the rate the ancient crust was shown to be moving in a previous study by the same researchers. Both the speed and direction of this latitudinal drift leaves plate tectonics as the most logical and strongest explanations for it.

"There's a lot of work that seems to suggest that early in Earth's history plate tectonics wasn't actually the dominant way in which the planet's internal heat gets released as it is today through the shifting of plates," said Brenner, a Ph.D. candidate in the Graduate School of Arts and Sciences and member of Harvard's Paleomagnetics Lab. "This evidence lets us much more confidently rule out explanations that don't involve plate tectonics."

For example, the researchers can now argue against phenomena called "true polar wander" and "stagnant lid tectonics," which can both cause the Earth's surface to shift but aren't part of modern-style plate tectonics. The results lean more toward plate tectonic motion because the newly discovered higher rate of speed is inconsistent with aspects of the other two processes.

In the paper, the scientists also describe what's believed to be the oldest evidence of when Earth reversed its geomagnetic fields, meaning the magnetic North and South Pole flipped locations. This type of flip-flop is a common occurrence in Earth's geologic history with the pole's reversing 183 times in the last 83 million years and perhaps several hundred times in the past 160 million years, according to NASA.

The reversal tells a great deal about the planet's magnetic field 3.2 billion years ago. Key among these implications is that the magnetic field was likely stable and strong enough to keep solar winds from eroding the atmosphere. This insight, combined with the results on plate tectonics, offers clues to the conditions under which the earliest forms of life developed.

"It paints this picture of an early earth that was already really geodynamically mature," Brenner said. "It had a lot of the same sorts of dynamic processes that result in an Earth that has essentially more stable environmental and surface conditions, making it more feasible for life to evolve and develop."

Today, the Earth's outer shell consists of about 15 shifting blocks of crust, or plates, which hold the planet's continents and oceans. Over eons the plates drifted into each other and apart, forming new continents and mountains and exposing new rocks to the atmosphere, which led to chemical reactions that stabilized Earth's surface temperature over billions of years.

Evidence of when plate tectonics started is hard to come by because the oldest pieces of crust are thrust into the interior mantle, never to resurface. Only 5 percent of all rocks on Earth are older than 2.5 billion years old, and no rock is older than about 4 billion years.

Overall, the study adds to growing research that tectonic movement occurred relatively early in Earth's 4.5-billion-year history and that early forms of life came about in a more moderate environment. Members of the project revisited the Pilbara Craton in 2018, which stretches about 300 miles across. They drilled into the primordial and thick slab of crust there to collect samples that, back in Cambridge, were analyzed for their magnetic history.

Using magnetometers, demagnetizing equipment, and the Quantum Diamond Microscope -- which images the magnetic fields of a sample and precisely identifies the nature of the magnetized particles -- the researchers created a suite of new techniques for determining the age and way the samples became magnetized. This allows the researchers to determine how, when, and which direction the crust shifted as well as the magnetic influence coming from Earth's geomagnetic poles.

The Quantum Diamond Microscope was developed in a collaboration between Harvard researchers in the Departments of Earth and Planetary Sciences (EPS) and of Physics.

For future studies, Fu and Brenner plan keep their focus on the Pilbara Craton while also looking beyond it to other ancient crusts around the world. They hope to find older evidence of modern-like plate motion and when the Earth's magnetic poles flipped.

Read more at Science Daily

Pioneering study shows climate played crucial role in changing location of ancient coral reefs

The study, published in Nature Communications, demonstrates how changes in temperature and plate tectonics, where the positions of Earth's continents were in very different positions than today, have determined the distribution of corals through the ages.

Although climate has often been regarded as the main driver of the location of coral reefs, this had yet to be proven due to limited fossil records. Now, for the first time, a team of international scientists used habitat modelling and reconstructions of past climates to predict the distribution of suitable environments for coral reefs over the last 250 million years.

The researchers, from the University of Vigo, in Spain, the University of Bristol and University College London in the UK, then checked their predictions using fossil evidence of warm-water coral reefs. They showed that corals in the past, from 250 to about 35 million years ago, existed much further from the equator than today, due to warmer climatic conditions, and a more even distribution of shallow ocean floor.

"Our work demonstrates that warm-water coral reefs track tropical-to-subtropical climatic conditions over geological timescales. In warmer intervals, coral reefs expanded poleward. However, in colder intervals, they became constrained to tropical and subtropical latitudes," said first author Dr Lewis Jones, a computational palaeobiologist research fellow at the University of Vigo.

Suitable coral habitats became restricted to the tropical regions from about 35 million years ago, driven by global cooling and increases in shallow oceans resulting from tectonic changes of the Indo-Australian Archipelago which is recognised as a marine biodiversity hotspot.

Although this suggests warm temperatures permitted long-term poleward expansions of corals in the past, the researchers say coral reef ecosystems are unlikely to match the rapid rate of human-induced climate change.

"Current anthropogenic climate change will result in the poleward expansion of suitable habitat for coral reefs. In fact, we are already witnessing the expansion of some tropical reef corals. However, whether coral reef ecosystems -- and all the biodiversity they support -- can keep pace with the current rapid rate of anthropogenic climate change is another question," Jones said.

"Limiting global warming is fundamental to saving coral reefs, as well as the biodiversity they house. Yet, perhaps even more important is reducing the rate of global warming."

Warm-water coral reefs, also known as 'rainforests of the sea', support the greatest biodiversity of marine organisms on Earth. In today's oceans, these biologically rich ecosystems, including reef fishes, are limited to the tropics and subtropics, where temperatures of the ocean surface typically do not fall below 18ºC. A substantial proportion of this modern biodiversity is found in the Indo-Australian Archipelago. However, in the geological past, coral reef ecosystems also existed outside of the tropics and subtropics, with their fossil remains found much further from the equator.

Co-author Dr Alex Farnsworth, Senior Research Associate in meteorology and climate modelling from the University of Bristol Cabot Institute for the Environment, said: "Climate has changed significantly throughout geological time, however understanding how it has impacted coral reef ecosystems has been difficult due to a lack of quantifiable data which has significant gaps.

"Using this new combined data-model approach we can start to better understand reef ecosystems evolution and behaviour."

Previous work has failed to find a strong relationship between temperature and the distribution of coral reefs because the fossil record is incomplete and biased. For example, not all the remains of organisms or ecosystems that existed in the past are recorded in the fossil record, and it has been shown the single most important factor explaining the sampled distribution of ancient reefs is Gross Domestic Product, with the majority of known fossil reef data stemming from wealthy countries, purely because these are the regions where we have looked hardest.

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Earliest geochemical evidence of plate tectonics found in 3.8-billion-year-old crystal

A handful of ancient zircon crystals found in South Africa hold the oldest evidence of subduction, a key element of plate tectonics, according to a new study published today in AGU Advances, AGU's journal for high-impact, open-access research and commentary across the Earth and space sciences.

These rare time capsules from Earth's youth point to a transition around 3.8 billion years ago from a long-lived, stable rock surface to the active processes that shape our planet today, providing a new clue in a hot debate about when plate tectonics was set in motion.

Earth's crust and the top layer of mantle just under it are broken up into rigid plates that move slowly on top of viscous but mobile lower layers of mantle rock. Heat from Earth's core drives this slow but inexorable motion, responsible for volcanoes, earthquakes, and the uplift of mountain ranges.

Estimates for when this process revved up and modern crust formed range from over 4 billion years ago to just 800 million years ago. Uncertainty arises because the geologic record from Earth's youth is sparse, due to the surface recycling effect of plate tectonics itself. Almost nothing remains from the Hadean Eon, Earth's first 500 million years.

"The Hadean Earth is this big mystery box," said Nadja Drabon, a geologist at Harvard University and the lead author of the new study.

Tiny time capsules

In an exciting step forward in solving this mystery, in 2018 Drabon and her colleagues unearthed a chronological series of 33 microscopic zircon crystals from a rare, ancient block of crust in the Barberton Greenstone Belt in South Africa, that formed at different times over a critical 800-million-year span from 4.15 to 3.3 billion years ago.

Zircon is a relatively common accessory mineral in Earth's crust, but ancient representatives from the Hadean Eon, 4 to 4.56 billion years ago, are exceedingly rare, found in only 12 places on Earth, and usually in numbers fewer than three at each location.

Hafnium isotopes and trace elements preserved in the Greenstone Belt zircons told a story about the conditions on Earth at the time they crystalized. Zircons 3.8-billion-years-old and younger appeared to have formed in rock experiencing pressures and melting similar to modern subduction zones, suggesting the crust may have started moving.

"When I say plate tectonics, I'm specifically referring to an arc setting, when one plate goes under another and you have all that volcanism -- think of the Andes, for example, and the Ring of Fire," Drabon said, describing a classic example of subduction.

"At 3.8 billion years there is a dramatic shift where the crust is destabilized, we have new rocks forming and we see geochemical signatures becoming more and more similar to what we see in modern plate tectonics," Drabon said.

In contrast, the older zircons preserved evidence of a global cap of "protocrust" derived from remelting mantle rock that had remained stable for 600 million years, the study found.

Signs of global change

The new study found a similar transition to conditions resembling modern subduction in zircons from other locations around the world, dating to within about 200 million years of the South African zircons.

"We see evidence for a significant change on the Earth around 3.8 to 3.6 billion years ago and evolution toward plate tectonics is one clear possibility." Drabon said.

While not conclusive, the results suggest a global change may have begun, Drabon said, possibly starting and stopping in scattered locations before settling into the efficient global engine of constantly moving plates we see today.

Plate tectonics shapes Earth's atmosphere as well as its surface. Release of volcanic gasses and production of new silicate rock, which consumes large amounts carbon dioxide from the atmosphere, temper large temperature swings from too much or too little greenhouse gas.

"Without all of the recycling and new crust forming, we might be going back and forth between boiling hot and freezing cold," Drabon said. "It's kind of like a thermostat for the climate."

Plate tectonics has, so far, only been observed on Earth, and may be essential to making a planet livable, Drabon said, which makes the origins of plate motions of interest in research into the early development of life.

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Hawaiian-Emperor undersea mystery revealed with supercomputers

The Hawaiian-Emperor seamount chain spans almost four thousand miles from the Hawaiian Islands to the Detroit Seamount in the north Pacific, an L- shaped chain that goes west then abruptly north. The 60-degree bend in the line of mostly undersea mountains and volcanic islands has puzzled scientists since it was first identified in the 1940s from the data of numerous echo sounding ships.

A team of scientists have now used supercomputers allocated by the Extreme Science and Engineering Discovery Environment (XSEDE) to model and reconstruct the dynamics of Pacific tectonic plate motion that might explain the mysterious mountain chain bend.

Major Findings

"We've shown with computer models for the first time how the Pacific plate can abruptly change direction from the north to the west," said Michael Gurnis, professor of Geophysics at the California Institute of Technology.

"It's been a holy grail to figure out why this change happened," he said. Gurnis co-authored the study on the origins of the seamount chain that was published in Nature Geoscience in January 2022.

Besides Gurnis, the team consisted of geoscientists Jiashun Hu, a post-doctoral scholar at Caltech, and Dietmar Mu?ller of Sydney University in Australia and computational scientists Johann Rudi of Argonne National Laboratory and Georg Stadler of New York University.

Plate Motion Clues

The plate motion provides a key to understanding how the seamount chain reflects plate motions. Gigantic tectonic plates in Earth's crust basically move over the hot, weak rock of the mantle.

The Pacific Plate is one of the largest. The plate spans about 40 million square miles undersea, outlined by the mountains and volcanos of the 'Ring of Fire' that are created by the return of the plates to the mantle.

But the volcanos of Hawaii and the Hawaiian-Emperor seamount chain weren't caused by this process. Instead, scientists theorize that plumes of Earth's hottest rock, from its core, travel upward through the mantle to generate a volcanic hotspot. And it's theorized that the seamount chain was created by the plate moving over the hot plume, something like a trail of burn marks on a paper moved over a candle.

About 80 million years ago, the Pacific plate traveled mostly north for about 30 million years, as evidenced by the line of Emperor seamounts. But about 50 million years ago, something odd happened. The Pacific plate apparently changed direction, and the mantle plume also shifted.

"Maybe there's an underlying physical reason why they would happen simultaneously," Gurnis said.

Prior Gordon Bell Prize

He pointed to previous work using techniques such as adaptive mesh refinement on the dynamics of mantle convection, computational work that scales well to a large number of CPUs and used the Stampede1 system of TACC and earned the team spearheaded by Johann Rudi the Gordon Bell Prize in 2015.

"Moreover, earlier work with Mu?ller, Gurnis and others showed how the physics of plumes could work inside the mantle such that the you could have a plume which rapidly migrated to the south and then stopped at 50 million years ago," Gurnis said.

"These two studies are complementary because going into the present study, we actually had a model which could explain the motion of the plume to the south and then stop abruptly, but we didn't have a model that could explain how the plate could change its direction," he added.

The team's computations of the physics of tectonic plates had to account for the faults at their boundaries but yet allow the movement of plates.

Computational Challenges

The challenge of getting both of those pieces of physics computed simultaneously meant that they needed computational methods that can handle vast changes in the mechanical properties from one plate to another plate as well as their faults.

Yet, the traditional ideas of plate motion failed to add up to enough force in the models to pull the Pacific Plate to the west and explain the bend.

"We discovered that there was another idea that had existed in the literature, but it wasn't getting much attention," Gurnis said.

New Factor

The new factor accounted for in the study was a subduction zone in the Russian Far East, a Kronotsky arc that terminated at about 50 million years ago. They built new plate tectonic reconstructions with these subduction zones.

When they put the zones in the models, they discovered that they could make the Pacific plate go to the north. And when that subduction terminated, the Pacific plate started to move to the west, slowly building up other subduction zones that over time provided more force to pull the Pacific plate.

"It's a new hypothesis that's much firmer in terms of the physics which it's based upon," Gurnis concluded. "It will allow other scientists to see if it will hold up to further scrutiny and if there are other ideas that can be tested on its assumptions."

Computational Resources

For the study, Gurnis was awarded access to the Stampede2 supercomputer at TACC through XSEDE funded by the National Science Foundation (NSF). He was also awarded access to the NSF-funded Frontera system also at TACC, the most powerful supercomputer in academia and the first phase of the NSF "Towards a Leadership Class Computing Facility" program.

"Both XSEDE and Frontera are absolutely vital for our research," Gurnis said.

"This capability computing is essential," he added. "We're spinning up projects with this collaboration that will be substantially larger than this, that are going to require something even beyond Frontera to compute."

This basic research aims to investigate mysteries about the dynamics of the past and present Earth.

"When you deal with some of the most fundamental processes in the earth, it's important to correctly figure out how they work," Gurnis said.

New Directions

He also highlighted the interplay between domain science and the applied work with computational scientists.

"The algorithms we've developed for adaptive mesh refinement can be applied to many pure and applied problems," Gurnis added. "That was a huge breakthrough."

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Tug of sun, moon could be driving plate motions on ‘imbalanced’ Earth

A study led by geophysicist Anne M. Hofmeister in Arts & Sciences at Washington University in St. Louis proposes that imbalanced forces and torques in the Earth-moon-sun system drive circulation of the whole mantle.

The new analysis provides an alternative to the hypothesis that the movement of tectonic plates is related to convection currents in the Earth's mantle. Convection involves buoyant rise of heated fluids, which Hofmeister and her colleagues argue does not apply to solid rocks. They argue that force, not heat, moves large objects. The new research is published in a special paper of the Geological Society of America, as part of a forthcoming collection assembled in honor of geologist Warren B. Hamilton.

Earth's internal workings are popularly modeled as dissipating heat generated by internal radioactivity and from leftover energy created during collisions when our planet formed. But even mantle convection proponents recognize that that amount of internal heat-energy is insufficient to drive large-scale tectonics. And there are other problems with using convection to explain observed plate motions.

Instead, Earth's plates might be shifting because the sun exerts such a strong gravitational pull on the moon that it has caused the moon's orbit around Earth to become elongated.

Over time, the position of the barycenter -- the center of mass between the orbiting bodies of the Earth and the moon -- has moved closer to Earth's surface and now oscillates 600 km per month relative to the geocenter, Hofmeister said. This sets up internal stresses, as the Earth continues to spin.

"Because the oscillating barycenter lies ~4600 km from the geocenter, Earth's tangential orbital acceleration and solar pull are imbalanced except at the barycenter," Hofmeister said. "The planet's warm, thick and strong interior layers can withstand these stresses, but its thin, cold, brittle lithosphere responds by fracturing."

Daily spin flattens the Earth from a perfect spherical shape, which contributes to this brittle failure of the lithosphere. These two independent stresses create the mosaic of plates observed in the outer shell, the authors suggest. The variety of plate motions comes from the changes in size and direction of the imbalanced gravitational forces with time.

But how to test this alternative? Hofmeister suggested: "One test would be a detailed examination of the tectonics of Pluto, which is too small and cold to convect, but has a giant moon and a surprisingly young surface."

The study includes a comparison of rocky planets that shows that the presence and longevity of volcanism and tectonism depend on the particular combination of moon size, moon orbital orientation, proximity to the sun and rates of body spin and cooling.

Earth is the only rocky planet with all the factors needed for plate tectonics, Hofmeister noted.

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Mars habitability limited by its small size, isotope study suggests

Water is essential for life on Earth and other planets, and scientists have found ample evidence of water in Mars' early history. But Mars has no liquid water on its surface today. New research from Washington University in St. Louis suggests a fundamental reason: Mars may be just too small to hold onto large amounts of water.

Remote sensing studies and analyses of Martian meteorites dating back to the 1980s posit that Mars was once water-rich, compared with Earth. NASA's Viking orbiter spacecraft -- and, more recently, the Curiosity and Perseverance rovers on the ground -- returned dramatic images of Martian landscapes marked by river valleys and flood channels.

Despite this evidence, no liquid water remains on the surface. Researchers proposed many possible explanations, including a weakening of Mars' magnetic field that could have resulted in the loss of a thick atmosphere.

But a study published the week of Sept. 20 in the Proceedings of the National Academy of Sciences suggests a more fundamental reason why today's Mars looks so drastically different from the "blue marble" of Earth.

"Mars' fate was decided from the beginning," said Kun Wang, assistant professor of earth and planetary sciences in Arts & Sciences at Washington University, senior author of the study. "There is likely a threshold on the size requirements of rocky planets to retain enough water to enable habitability and plate tectonics, with mass exceeding that of Mars."

For the new study, Wang and his collaborators used stable isotopes of the element potassium (K) to estimate the presence, distribution and abundance of volatile elements on different planetary bodies.

Potassium is a moderately volatile element, but the scientists decided to use it as a kind of tracer for more volatile elements and compounds, such as water. This is a relatively new method that diverges from previous attempts to use potassium-to-thorium (Th) ratios gathered by remote sensing and chemical analysis to determine the amount of volatiles Mars once had. In previous research, members of the research group used a potassium tracer method to study the formation of the moon.

Wang and his team measured the potassium isotope compositions of 20 previously confirmed Martian meteorites, selected to be representative of the bulk silicate composition of the red planet.

Using this approach, the researchers determined that Mars lost more potassium and other volatiles than Earth during its formation, but retained more of these volatiles than the moon and asteroid 4-Vesta, two much smaller and drier bodies than Earth and Mars.

The researchers found a well-defined correlation between body size and potassium isotopic composition.

"The reason for far lower abundances of volatile elements and their compounds in differentiated planets than in primitive undifferentiated meteorites has been a longstanding question," said Katharina Lodders, research professor of earth and planetary sciences at Washington University, a coauthor of the study. "The finding of the correlation of K isotopic compositions with planet gravity is a novel discovery with important quantitative implications for when and how the differentiated planets received and lost their volatiles."

"Martian meteorites are the only samples available to us to study the chemical makeup of the bulk Mars," Wang said. "Those Martian meteorites have ages varying from several hundred millions to 4 billion years and recorded Mars' volatile evolution history. Through measuring the isotopes of moderately volatile elements, such as potassium, we can infer the degree of volatile depletion of bulk planets and make comparisons between different solar system bodies.

"It's indisputable that there used to be liquid water on the surface of Mars, but how much water in total Mars once had is hard to quantify through remote sensing and rover studies alone," Wang said. "There are many models out there for the bulk water content of Mars. In some of them, early Mars was even wetter than the Earth. We don't believe that was the case."

Zhen Tian, a graduate student in Wang's laboratory and a McDonnell International Academy Scholar, is first author of the paper. Postdoctoral research associate Piers Koefoed is a co-author, as is Hannah Bloom, who graduated from Washington University in 2020. Wang and Lodders are faculty fellows of the university's McDonnell Center for the Space Sciences.

The findings have implications for the search for life on other planets besides Mars, the researchers noted.

Being too close to the sun (or, for exoplanets, being too close to their star) can affect the amount of volatiles that a planetary body can retain. This distance-from-star measurement is often factored into indexes of "habitable zones" around stars.

"This study emphasizes that there is a very limited size range for planets to have just enough but not too much water to develop a habitable surface environment," said Klaus Mezger of the Center for Space and Habitability at the University of Bern, Switzerland, a co-author of the study. "These results will guide astronomers in their search for habitable exoplanets in other solar systems."

Wang now thinks that, for planets that are within habitable zones, planetary size probably should be more emphasized and routinely considered when thinking about whether an exoplanet could support life.

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Anatomy of the red planet: Mars-quakes reveal interior

Since early 2019, researchers have been recording and analysing marsquakes as part of the InSight mission. This relies on a seismometer whose data acquisition and control electronics were developed at ETH Zurich. Using this data, the researchers have now measured the red planet's crust, mantle and core -- data that will help determine the formation and evolution of Mars and, by extension, the entire solar system.

Mars once completely molten

We know that Earth is made up of shells: a thin crust of light, solid rock surrounds a thick mantle of heavy, viscous rock, which in turn envelopes a core consisting mainly of iron and nickel. Terrestrial planets, including Mars, have been assumed to have a similar structure. "Now seismic data has confirmed that Mars presumably was once completely molten before dividing into the crust, mantle and core we see today, but that these are different from Earth's," says Amir Khan, a scientist at the Institute of Geophysics at ETH Zurich and at the Physics Institute at the University of Zurich. Together with his ETH colleague Simon Stähler, he analysed data from NASA's InSight mission, in which ETH Zurich is participating under the leadership of Professor Domenico Giardini.

No plate tectonics on Mars

The researchers have discovered that the Martian crust under the probe's landing site near the Martian equator is between 15 and 47 kilometres thick. Such a thin crust must contain a relatively high proportion of radioactive elements, which calls into question previous models of the chemical composition of the entire crust.

Beneath the crust comes the mantle with the lithosphere of more solid rock reaching 400-600 kilometres down -- twice as deep as on Earth. This could be because there is now only one continental plate on Mars, in contrast to Earth with its seven large mobile plates. "The thick lithosphere fits well with the model of Mars as a 'one-plate planet'," Khan concludes.

The measurements also show that the Martian mantle is mineralogically similar to Earth's upper mantle. "In that sense, the Martian mantle is a simpler version of Earth's mantle." But the seismology also reveals differences in chemical composition. The Martian mantle, for example, contains more iron than Earth's. However, theories as to the complexity of the layering of the Martian mantle also depend on the size of the underlying core -- and here, too, the researchers have come to new conclusions.

The core is liquid and larger than expected

The Martian core has a radius of about 1,840 kilometres, making it a good 200 kilometres larger than had been assumed 15 years ago, when the InSight mission was planned. The researchers were now able to recalculate the size of the core using seismic waves. "Having determined the radius of the core, we can now calculate its density," Stähler says.

"If the core radius is large, the density of the core must be relatively low," he explains: "That means the core must contain a large proportion of lighter elements in addition to iron and nickel." These include sulphur, oxygen, carbon and hydrogen, and make up an unexpectedly large proportion. The researchers conclude that the composition of the entire planet is not yet fully understood. Nonetheless, the current investigations confirm that the core is liquid -- as suspected -- even if Mars no longer has a magnetic field.

Reaching the goal with different waveforms

The researchers obtained the new results by analysing various seismic waves generated by marsquakes. "We could already see different waves in the InSight data, so we knew how far away from the lander these quake epicentres were on Mars," Giardini says. To be able to say something about a planet's inner structure calls for quake waves that are reflected at or below the surface or at the core. Now, for the first time, researchers have succeeded in observing and analysing such waves on Mars.

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New research uncovers continental crust emerged 500 million years earlier than thought

 The first emergence and persistence of continental crust on Earth during the Archaean (4 billion to 2.5 billion years ago) has important implications for plate tectonics, ocean chemistry, and biological evolution, and it happened about half a billion years earlier than previously thought, according to new research being presented at the EGU General Assembly 2021.

Once land becomes established through dynamic processes like plate tectonics, it begins to weather and add crucial minerals and nutrients to the ocean. A record of these nutrients is preserved in the ancient rock record. Previous research used strontium isotopes in marine carbonates, but these rocks are usually scarce or altered in rocks older than 3 billion years.

Now, researchers are presenting a new approach to trace the first emergence of old rocks using a different mineral: "barite."

Barite forms from a combination of sulfate coming from ocean water mixing with barium from hydrothermal vents. Barite holds a robust record of ocean chemistry within its structure, useful for reconstructing ancient environments. "The composition of the piece of barite we pick up in the field now that has been on Earth for three and a half billion years, is exactly the same as it was when it when it actually precipitated," says Desiree Roerdink, a geochemist at University of Bergen, Norway, and team leader of the new research. "So in essence, it is really a great recorder to look at processes on the early Earth."

Roerdink and her team tested six different deposits on three different continents, ranging from about 3.2 billion to 3.5 billion years old. They calculated the ratio of strontium isotopes in the barite, and from there, inferred the time where the weathered continental rock made its way to the ocean and incorporated itself into the barite. Based on the data captured in the barite, they found that weathering started about 3.7 billion years ago -- about 500 million years earlier than previously thought.

"That is a huge time period," Roerdink says. "It essentially has implications for the way that we think about how life evolved." She added that scientists usually think about life starting in deep sea, hydrothermal settings, but the biosphere is complex. "We don't really know if it is possible that life could have developed at the same time on land," she noted, adding "but then that land has to be there."

Lastly, the emergence of land says something about plate tectonics and the early emergence of a geodynamic Earth. "To get land, you need processes operating to form that continental crust, and form a crust that is chemically different from the oceanic crust," Roerdink says.

From Science Daily

Extra-terrestrial impacts may have triggered 'bursts' of plate tectonics

When -- and how -- Earth's surface evolved from a hot, primordial mush into a rocky planet continually resurfaced by plate tectonics remain some of the biggest unanswered questions in earth science research. Now a new study, published in Geology, suggests this earthly transition may in fact have been triggered by extra-terrestrial impacts.

"We tend to think of the Earth as an isolated system, where only internal processes matter," says Craig O'Neill, director of Macquarie University's Planetary Research Centre. "Increasingly, though, we're seeing the effect of solar system dynamics on how the Earth behaves."

Modelling simulations and comparisons with lunar impact studies have revealed that following Earth's accretion about 4.6 billion years ago, Earth-shattering impacts continued to shape the planet for hundreds of millions of years. Although these events appear to have tapered off over time, spherule beds -- distinctive layers of round particles condensed from rock vaporized during an extra-terrestrial impact -- found in South Africa and Australia suggest the Earth experienced a period of intense bombardment about 3.2 billion years ago, roughly the same time the first indications of plate tectonics appear in the rock record.

This coincidence caused O'Neill and co-authors Simone Marchi, William Bottke, and Roger Fu to wonder whether these circumstances could be related. "Modelling studies of the earliest Earth suggest that very large impacts -- more than 300 km in diameter -- could generate a significant thermal anomaly in the mantle," says O'Neill. This appears to have altered the mantle's buoyancy enough to create upwellings that, according to O'Neill, "could directly drive tectonics."

But the sparse evidence found to date from the Archaean -- the period of time spanning 4.0 to 2.5 billion years ago -- suggests that mostly smaller impacts less than 100 km in diameter occurred during this interval. To determine whether these more modest collisions were still large and frequent enough to initiate global tectonics, the researchers used existing techniques to expand the Middle Archaean impact record and then developed numerical simulations to model the thermal effects of these impacts on Earth's mantle.

The results indicate that during the Middle Archaean, 100-kilometer-wide impacts (about 30 km wider than the much younger Chixculub crater) were capable of weakening Earth's rigid, outermost layer. This, says O'Neill, could have acted as a trigger for tectonic processes, especially if Earth's exterior was already "primed" for subduction.

"If the lithosphere were the same thickness everywhere, such impacts would have little effect," states O'Neill. But during the Middle Archean, he says, the planet had cooled enough for the mantle to thicken in some spots and thin in others. The modelling showed that if an impact were to happen in an area where these differences existed, it would create a point of weakness in a system that already had a large contrast in buoyancy -- and ultimately trigger modern tectonic processes.

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