From Carnegie Institution for Science (US) : “Deep diamonds contain evidence of deep-Earth recycling processes”

From Carnegie Institution for Science (US)

March 31, 2021

Diamonds that formed deep in the Earth’s mantle contain evidence of chemical reactions that occurred on the seafloor. Probing these gems can help geoscientists understand how material is exchanged between the planet’s surface and its depths.

Given that the bottom of the ocean is just 11 kilometers (6.8 miles) down at its deepest point, this may seem rather odd – but those diamonds are a really valuable clue for understanding the exchange of material between Earth’s surface and its crushing depths, researchers say.

An illustration showing how diamonds can offer researchers a glimpse into the processes occurring inside our planet, including deep-Earth recycling of surface material. Credit: Katherine Cain/Carnegie Institution for Science.

Examples of rough CLIPPIR diamonds from the Letseng mine, Lesotho. These are the same kinds of diamonds as the ones analyzed in this study. Largest stone is 91.07 carats. Credit: Robert Weldon; copyright GIA; courtesy of Gem Diamonds Ltd.

New work published in Science Advances confirms that serpentinite—a rock that forms from peridotite, the main rock type in Earth’s mantle, when water penetrates cracks in the ocean floor—can carry surface water as far as 700 kilometers deep by plate tectonic processes.

“Nearly all tectonic plates that make up the seafloor eventually bend and slide down into the mantle—a process called subduction, which has the potential to recycle surface materials, such as water, into the Earth,” explained Carnegie’s Peng Ni, who co-led the research effort with Evan Smith of the Gemological Institute of America.

Diamonds are prized for their beauty when shaped into faceted gems, but the little clear lumps of carbon can tell us a lot about the conditions in which they formed. Not all diamonds are perfectly clear; some contain what we call inclusions, fragments of other minerals that got caught up in the diamond formation process (and, once, a whole other diamond).

Sometimes, we can tell these inclusions are from the deep environment where the diamond formed; calcium silicate perovskite, for instance, is unstable at depths above about 650 kilometers except when it’s been trapped in a diamond, so it’s unlikely to have formed at the surface.

Where commercial diamond companies see impurities, scientists see inclusions, and the team took this opportunity to study the planet’s interior. Instead of deep minerals, though, the researchers found heavy isotopes of iron, outside the known values for iron from those mantle depths, or the products of reactions we’d expect at those depths.

This, they say, is the first evidence confirming a geochemical pathway to capture and transport surface materials deep into the mantle.

Serpentinite residing inside subducting plates may be one of the most significant, yet poorly known, geochemical pathways by which surface materials are captured and conveyed into the Earth’s depths. The presence of deeply-subducted serpentinites was previously suspected—due to Carnegie and GIA research about the origin of blue diamonds and to the chemical composition of erupted mantle material that makes up mid-ocean ridges, seamounts, and ocean islands. But evidence demonstrating this pathway had not been fully confirmed until now.

The research team—which also included Carnegie’s Steven Shirey, and Anat Shahar, as well as GIA’s Wuyi Wang and Stephen Richardson of the University of Cape Town (SA)—found physical evidence to confirm this suspicion by studying a type of large diamonds that originate deep inside the planet.

“Some of the most famous diamonds in the world fall into this special category of relatively large and pure gem diamonds, such as the world-famous Cullinan,” Smith said. “They form between 360 and 750 kilometers down, at least as deep as the transition zone between the upper and lower mantle.”

Sometimes they contain inclusions of tiny minerals trapped during diamond crystallization that provide a glimpse into what is happening at these extreme depths.

This cartoon shows a subducting oceanic plate traveling like a conveyor belt from the surface down to the lower mantle. The white arrows show the comparatively well-established shallow recycling pathway in the top layer of the plate (crust and sediments), that feeds into arc volcanoes. Our new findings from studying diamonds reveal a deeper recycling pathway, shown in light blue. Water infiltrating fractures in the seafloor hydrates the rocks in the interior of the plate (forming “serpentinite”), and these hydrated rocks can sometimes be carried down to the top of the lower mantle. This is a major pathway that transfers water, carbon, and other surficial elements deep down into the mantle. Credit: Wenjia Fan/ W. Design Studio.

“Studying small samples of minerals formed during deep diamond crystallization can teach us so much about the composition and dynamics of the mantle, because diamond protects the minerals from additional changes on their path to the surface,” Shirey explained.

In this instance, the researchers were able to analyze the isotopic composition of iron in the metallic inclusions. Like other elements, iron can have different numbers of neutrons in its nucleus, which gives rise to iron atoms of slightly different mass, or different “isotopes” of iron. Measuring the ratios of “heavy” and “light” iron isotopes gives scientists a sort of fingerprint of the iron.

The diamond inclusions studied by the team had a higher ratio of heavy to light iron isotopes than typically found in most mantle minerals. This indicates that they probably didn’t originate from deep-Earth geochemical processes. Instead, it points to magnetite and other iron-rich minerals formed when oceanic plate peridotite transformed to serpentinite on the seafloor. This hydrated rock was eventually subducted hundreds of kilometers down into the mantle transition zone, where these particular diamonds crystallized.

“Our findings confirm a long-suspected pathway for deep-Earth recycling, allowing us to trace how minerals from the surface are drawn down into the mantle and create variability in its composition,” Shahar concluded.

__________________

This work was supported by the Diamonds and Mantle Geodynamics Group of the Deep Carbon Observatory, a U.S. National Science Foundation grant, and the research program of the Gemological Institute of America.

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Carnegie Institution for Science (US)

Andrew Carnegie established a unique organization dedicated to scientific discovery “to encourage in the broadest and most liberal manner investigation; research; and discovery and the application of knowledge to the improvement of mankind…” The philosophy was and is to devote the institution’s resources to “exceptional” individuals so that they can explore the most intriguing scientific questions in an atmosphere of complete freedom. Carnegie and his trustees realized that flexibility and freedom were essential to the institution’s success and that tradition is the foundation of the institution today as it supports research in the Earth, space, and life sciences.

The Carnegie Institution of Washington (US) (the organization’s legal name), known also for public purposes as the Carnegie Institution for Science (US) (CIS), is an organization in the United States established to fund and perform scientific research. The institution is headquartered in Washington, D.C. As of June 30, 2020, the Institution’s endowment was valued at $926.9 million. In 2018 the expenses for scientific programs and administration were $96.6 million.

History

When the United States joined World War II Vannevar Bush was president of the Carnegie Institution. Several months before on June 12, 1940 Bush had been instrumental in persuading President Franklin Roosevelt to create the National Defense Research Committee (later superseded by the Office of Scientific Research and Development) to mobilize and coordinate the nation’s scientific war effort. Bush housed the new agency in the Carnegie Institution’s administrative headquarters at 16th and P Streets, NW, in Washington, DC, converting its rotunda and auditorium into office cubicles. From this location Bush supervised, among many other projects the Manhattan Project. Carnegie scientists cooperated with the development of the proximity fuze and mass production of penicillin.

Research

Carnegie scientists continue to be involved with scientific discovery. Composed of six scientific departments on the East and West Coasts the Carnegie Institution for Science is involved presently with six main topics: Astronomy at the Department of Terrestrial Magnetism (Washington, D.C.) and the Observatories of the Carnegie Institution of Washington (Pasadena, CA and Las Campanas, Chile); Earth and planetary science also at the Department of Terrestrial Magnetism and the Geophysical Laboratory (Washington, D.C.); Global Ecology at the Department of Global Ecology (Stanford, CA); Genetics and developmental biology at the Department of Embryology (Baltimore, MD); Matter at extreme states also at the Geophysical Laboratory; and Plant science at the Department of Plant Biology (Stanford, CA).

Carnegie 6.5 meter Magellan Baade and Clay Telescopes located at Carnegie’s Las Campanas Observatory, Chile. over 2,500 m (8,200 ft) high.

Carnegie Las Campanas 2.5 meter Irénée Dupont telescope, Atacama Desert, over 2,500 m (8,200 ft) high approximately 100 kilometres (62 mi) northeast of the city of La Serena,Chile.

Carnegie Institution 1-meter Swope telescope at Las Campanas, Chile, 100 kilometres (62 mi) northeast of the city of La Serena, near the north end of a 7 km (4.3 mi) long mountain ridge, Cerro Las Campanas, near the southern end and over 2,500 m (8,200 ft) high, at Las Campanas, Chile.

From University of Bern [Universität Bern](CH): “Volcanoes might light up the night sky of this planet”

From University of Bern [Universität Bern](CH)

3.6.21

Tobias G. Meier
Center for Space and Habitability (CSH),
University of Bern [Universität Bern](CH) and NCCR PlanetS
tobias.meier@csh.unibe.ch

Dr. Dan J. Bower
Center for Space and Habitability (CSH),
University of Bern [Universität Bern](CH) and NCCR PlanetS
daniel.bower@csh.unibe.ch

This artist’s illustration represents the possible interior dynamics of the super-Earth exoplanet LHS 3844b. The planet’s interior properties and the strong stellar irradiation might lead to a hemispheric tectonic regime. © University of Bern [Universität Bern](CH) Credit: Thibaut Roger.

Until now, researchers have found no evidence of global tectonic activity on planets outside our solar system. Under the leadership of the University of Bern [Universität Bern](CH) and the National Center of Competence in Research NCCR PlanetS(CH), scientists have now found that the material inside planet LHS 3844b flows from one hemisphere to the other and could be responsible for numerous volcanic eruptions on one side of the planet.

On Earth, plate tectonics is not only responsible for the rise of mountains and earthquakes.

The tectonic plates of the world were mapped in 1996, USGS.

It is also an essential part of the cycle that brings material from the planet’s interior to the surface and the atmosphere, and then transports it back beneath the Earth’s crust. Tectonics thus has a vital influence on the conditions that ultimately make Earth habitable.

Until now, researchers have found no evidence of global tectonic activity on planets outside our solar system. A team of researchers led by Tobias Meier from the Center for Space and Habitability (CSH) at the University of Bern [Universität Bern](CH) and with the participation of ETH Zürich [Eidgenössische Technische Hochschule Zürich)](CH), the University of Oxford(UK) and the National Center of Competence in Research NCCR PlanetS(CH) has now found evidence of the flow patterns inside a planet, located 45 light-years from Earth: LHS 3844b. Their results were published in The Astrophysical Journal Letters.

An extreme contrast and no atmosphere

“Observing signs of tectonic activity is very difficult, because they are usually hidden beneath an atmosphere”, Meier explains. However, recent results suggested that LHS 3844b probably does not have an atmosphere. Slightly larger than Earth and likely similarly rocky, it orbits around its star so closely that one side of the planet is in constant daylight and the other in permanent night – just like the same side of the Moon always faces the Earth. With no atmosphere shielding it from the intense radiation, the surface gets blisteringly hot: it can reach up to 800°C on the dayside. The night side, on the other hand, is freezing. Temperatures there might fall below minus 250°C. “We thought that this severe temperature contrast might affect material flow in the planet’s interior”, Meier recalls.

To test their theory, the team ran computer simulations with different strengths of material and internal heating sources, such as heat from the planet’s core and the decay of radioactive elements. The simulations included the large temperature contrast on the surface imposed by the host star.

Flow inside the planet from one hemisphere to the other

“Most simulations showed that there was only upwards flow on one side of the planet and downwards flow on the other. Material therefore flowed from one hemisphere to the other”, Meier reports. Surprisingly, the direction was not always the same. “Based on what we are used to from Earth, you would expect the material on the hot dayside to be lighter and therefore flow upwards and vice versa”, co-author Dan Bower at the University of Bern and the NCCR PlanetS explains. Yet, some of the teams’ simulations also showed the opposite flow direction. “This initially counter-intuitive result is due to the change in viscosity with temperature: cold material is stiffer and therefore doesn’t want to bend, break or subduct into the interior. Warm material, however, is less viscous – so even solid rock becomes more mobile when heated – and can readily flow towards the planet’s interior”, Bower elaborates. Either way, these results show how a planetary surface and interior can exchange material under conditions very different from those on Earth.

A volcanic hemisphere

Such material flow could have bizarre consequences. “On whichever side of the planet the material flows upwards, one would expect a large amount of volcanism on that particular side”, Bower points out. He continues “similar deep upwelling flows on Earth drive volcanic activity at Hawaii and Iceland”. One could therefore imagine a hemisphere with countless volcanoes – a volcanic hemisphere so to speak – and one with almost none.

“Our simulations show how such patterns could manifest, but it would require more detailed observations to verify. For example, with a higher-resolution map of surface temperature that could point to enhanced outgassing from volcanism, or detection of volcanic gases. This is something we hope future research will help us to understand”, Meier concludes.

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University of Bern [Universität Bern](CH) is a university in the Swiss capital of Bern and was founded in 1834. It is regulated and financed by the Canton of Bern. It is a comprehensive university offering a broad choice of courses and programs in eight faculties and some 150 institutes. With around 17,512 students, Universität Bern is the third biggest University in Switzerland.

Universität Bern operates at three levels: university, faculties and institutes. Other organizational units include interfaculty and general university units. The university’s highest governing body is the Senate, which is responsible for issuing statutes, rules and regulations. Directly answerable to the Senate is the University Board of Directors, the governing body for university management and coordination. The Board comprises the Rector, the Vice-Rectors and the Administrative Director. The structures and functions of the University Board of Directors and the other organizational units are regulated by the Universities Act. Universität Bern offers about 39 bachelor and 72 master programs, with enrollments of 7,747 and 4,523, respectively. The university also has 2,776 doctoral students. Around 1,561 bachelor, 1,489 master’s degree students and 570 PhD students graduate each year. For some time now, the university has had more female than male students; at the end of 2016, women accounted for 56% of students.

From Yale University: “Going with the grains to explain a fundamental tectonic force”

From Yale University

January 18, 2021
Science contact
Fred Mamoun
fred.mamoun@yale.edu
203-436-2643

Writer
Jim Shelton

Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Credit: Wikipedia.

A new study suggests that tiny, mineral grains — squeezed and mixed over millions of years — set in motion the chain of events that plunge massive tectonic plates deep into the Earth’s interior.

The theory, proposed by Yale scientists David Bercovici and Elvira Mulyukova, may provide an origin story for subduction, one of the most fundamental forces responsible for the dynamic nature of the planet.

The study appears in the PNAS.

Subduction occurs when one tectonic plate slides underneath another plate and then sinks into the Earth’s mantle. Its role in major geological processes is immense: It is the main engine for tectonic motion. It builds mountains, triggers earthquakes, forms volcanoes, and drives the geologic carbon cycle.

The tectonic plates of the world were mapped in 1996, USGS.

Yet researchers have been uncertain about what initiates subduction.

“Why Earth even has subduction, unlike other terrestrial planets as far as we know, is a mystery,” said Bercovici, Yale’s Frederick William Beinecke Professor and chair of Earth and Planetary Sciences.

“Mantle rock near the surface that has cooled for hundreds of millions of years has two competing effects,” he said. “While it’s gotten colder and heavier and wants to sink, it’s also gotten stiffer and doesn’t want to sink. The stiffening effect should win out, as it does on most planets, but on Earth, for some reason, it doesn’t.”

A conceptual sketch of the ocean basin setting for the new model. Inset images from a computer model show mineral fraction, grain size, and weakness. Credit: Elvira Mulyukova and David Bercovici.

According to the theoretical model developed by Bercovici and Mulyukova, a research scientist at Yale, subduction may initiate at the margins between Earth’s sea floor and continents.

The model shows that tectonic stresses in an oceanic plate cause its mineral grains to mix with each other, become damaged, and eventually shrink. Over a period of approximately 100 million years, this process weakens the oceanic plate and makes it susceptible to vertical shear and bending — which are associated with the start of subduction.

“The real bottleneck for tectonic plate activity on a terrestrial planet is how fast its massive, rocky layers can deform,” said Mulyukova. “The rocks can deform only as fast as their tiny mineral grains allow. Our model explains how these changes in mineral grains can dramatically weaken the rock and make subduction possible on a planet like Earth.”

This research was supported by a grant from the National Science Foundation.

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Yale University comprises three major academic components: Yale College (the undergraduate program), the Graduate School of Arts and Sciences, and the professional schools. In addition, Yale encompasses a wide array of centers and programs, libraries, museums, and administrative support offices. Approximately 11,250 students attend Yale.

From Science News: “How the Earth-shaking theory of plate tectonics was born”

From Science News

1.13.21
Carolyn Gramling

Scientists have peppered the planet with seismometers that can detect the rumble of moving magma within a volcano, and GPS stations can spot changes in land elevation as magma swells below. But anticipating eruptions remains tricky. Credit: BRUCE DAVIDSON/NATURE PICTURE LIBRARY/ALAMY.

Pure insights plus a boom in data transformed our understanding of Earth.

Some great ideas shake up the world. For centuries, the outermost layer of Earth was thought to be static, rigid, locked in place. But the theory of plate tectonics has rocked this picture of the planet to its core. Plate tectonics reveals how Earth’s surface is constantly in motion, and how its features — volcanoes, earthquakes, ocean basins and mountains — are intrinsically linked to its hot interior.

The tectonic plates of the world were mapped in 1996, USGS.

The planet’s familiar landscapes, we now know, are products of an eons-long cycle in which the planet constantly remakes itself.

When plate tectonics emerged in the 1960s it became a unifying theory, “the first global theory ever to be generally accepted in the entire history of earth science,” writes Harvard University science historian Naomi Oreskes, in the introduction to Plate Tectonics: An Insider’s History of the Modern Theory of the Earth. In 1969, geophysicist J. Tuzo Wilson compared the impact of this intellectual revolution in earth science to Einstein’s general theory of relativity, which had produced a similar upending of thought about the nature of the universe.

Plate tectonics describes how Earth’s entire, 100-kilometer-thick outermost layer, called the lithosphere, is broken into a jigsaw puzzle of plates — slabs of rock bearing both continents and seafloor — that slide atop a hot, slowly swirling inner layer. Moving at rates between 2 and 10 centimeters each year, some plates collide, some diverge and some grind past one another. New seafloor is created at the center of the oceans and lost as plates sink back into the planet’s interior. This cycle gives rise to many of Earth’s geologic wonders, as well as its natural hazards.

“It’s amazing how it tied the pieces together: seafloor spreading, magnetic stripes on the seafloor … where earthquakes form, where mountain ranges form,” says Bradford Foley, a geodynamicist at Penn State. “Pretty much everything falls into place.”

With so many lines of evidence now known, the theory feels obvious, almost inevitable. But the conceptual journey from fixed landmasses to a churning, restless Earth was long and circuitous, punctuated by moments of pure insight and guided by decades of dogged data collection.

Continents adrift

In 1912, German meteorologist Alfred Wegener proposed at a meeting of Frankfurt’s Geological Association that Earth’s landmasses might be on the move. At the time, the prevailing idea held that mountains formed like wrinkles on the planet as it slowly lost the heat of formation and its surface contracted. Instead, Wegener suggested, mountains form when continents collide as they drift across the planet’s surface. Although now far-flung, the continents were once joined together as a supercontinent Wegener dubbed Pangaea, or “all-Earth.” This would explain why rocks of the same type and age, as well as identical fossils, are found on either side of the Atlantic Ocean, for example.

The San Andreas Fault (shown) is the boundary between the Pacific and North American plates. Credit: KEVIN SCHAFER/ALAMY.

This idea of drifting continents intrigued some scientists. Many others, particularly geologists, were unimpressed, hostile, even horrified. Wegener’s idea, detractors thought, was too speculative, not grounded enough in prevailing geologic principles such as uniformitarianism, which holds that the same slow-moving geologic forces at work on Earth today must also have been at work in the past. The principle was thought to demand that the continents be fixed in place.

In 1989, a slip of the San Andreas Fault triggered a magnitude 6.9 earthquake that rocked the San Francisco Bay area, causing 63 deaths and billions of dollars in damag. Credit: David Weintraub/Science Source.

German geologist Max Semper disdainfully wrote in 1917 [JSTOR] that Wegener’s idea “was established with a superficial use of scientific methods, ignoring the various fields of geology,” adding that he hoped Wegener would turn his attention to other fields of science and leave geology alone.“O holy Saint Florian, protect this house but burn down the others!” he wrote sardonically.

The debate between “mobilists” and “fixists” raged on through the 1920s, picking up steam as it percolated into English-speaking circles. In 1926, at a meeting in New York City of the American Association of Petroleum Geologists, geologist Rollin T. Chamberlin dismissed Wegener’s hypothesis as a mishmash of unrelated observations. The idea, Chamberlin said, “is of the foot-loose type, in that it takes considerable liberty with our globe, and is less bound by restrictions or tied down by awkward, ugly facts than most of its rival theories.”

One of the most persistent sticking points for Wegener’s idea, now called continental drift, was that it couldn’t explain how the continents moved. In 1928, English geologist Arthur Holmes came up with a potential explanation for that movement. He proposed that the continents might be floating like rafts atop a layer of viscous, partially molten rocks deep inside Earth. Heat from the decay of radioactive materials, he suggested, sets this layer to a slow boil, creating large circulating currents within the molten rock that in turn slowly shift the continents about.

Holmes admitted he had no data to back up the idea, and the geology community remained largely unconvinced of continental drift. Geologists turned to other matters, such as developing a magnitude scale for earthquake strength and devising a method to precisely date organic materials using the radioactive form of carbon, carbon-14.

Data flood in

Rekindled interest in continental drift came in the 1950s from evidence from an unexpected source — the bottom of the oceans. World War II had brought the rapid development of submarines and sonar, and scientists soon put the new technologies to work studying the seafloor. Using sonar, which pings the seafloor with sound waves and listens for a return pulse, researchers mapped out the extent of a continuous and branching underwater mountain chain with a long crack running right down its center. This worldwide rift system snakes for over 72,000 kilometers around the globe, cutting through the centers of the world’s oceans.

Armed with magnetometers for measuring magnetic fields, researchers also mapped out the magnetic orientation of seafloor rocks — how their iron-bearing minerals are oriented relative to Earth’s field. Teams discovered that the seafloor rocks have a peculiar “zebra stripe” pattern: Bands of normal polarity, whose magnetic orientation corresponds to Earth’s current magnetic field, alternate with bands of reversed polarity. This finding suggests that each of the bands formed at different times.

The Pacific Ocean’s Mariana Trench is the deepest known subduction zone, where a tectonic plate sinks back into Earth’s interior. Here, the Deep Discoverer explores the trench at a depth of 6,000 meters in 2016. Credit: NOAA OFFICE OF OCEAN EXPLORATION AND RESEARCH.

Meanwhile, growing support for the detection and banning of underground nuclear testing also created an opportunity for seismologists: the chance to create a global, standardized network of seismograph stations [USGS]. By the end of the 1960s, about 120 different stations were installed in 60 different countries, from the mountains of Ethiopia’s Addis Ababa to the halls of Georgetown University in Washington, D.C., to the frozen South Pole. Thanks to the resulting flood of high-quality seismic data, scientists discovered and mapped rumbles along the mid-ocean rift system, now called mid-ocean ridges, and beneath the trenches. The quakes near very deep ocean trenches were particularly curious: They originated much deeper underground than scientists had thought possible. And the ridges were very hot compared with the surrounding seafloor, scientists learned by using thin steel probes inserted into cores drilled from shipboard into the seafloor.

In the early 1960s, two researchers working independently, geologist Harry Hess and geophysicist Robert S. Dietz, put the disparate clues together — and added in Holmes’ old idea of an underlying layer of circulating currents within the hot rock. The mid-ocean ridges, each asserted, might be where circulation pushes hot rock toward the surface. The powerful forces drive pieces of Earth’s lithosphere apart. Into the gap, lava burbles up — and new seafloor is born. As the pieces of lithosphere move apart, new seafloor continues to form between them, called “seafloor spreading.”

Research suggests that volcanic island chains form as plates move over upwellings of magma. But the origin of the Hawaiian Islands (Kilauea volcano shown) and other similar chains remains something of a geologic puzzle. Credit:ART WOLFE/GETTY IMAGES.

The momentum culminated in a two-day gathering of perhaps just 100 earth scientists in 1966, held at the Goddard Institute for Space Studies in New York. “It was quite clear, at this conference in New York, that everything was going to change,” University of Cambridge geophysicist Dan McKenzie told the Geological Society of London in 2017 in a reflection on the meeting.

But going in, “no one had any idea” that this meeting would become a pivotal moment for the earth sciences, says seismologist Lynn Sykes of Columbia University. Sykes, then a newly minted Ph.D., was one of the invitees; he had just discovered a distinct pattern in the earthquakes at mid-ocean ridges. This pattern showed that the seafloor on either side of the ridges was pulling apart, a pivotal piece of evidence for plate tectonics.

At the meeting, talk after talk piled data on top of data to support seafloor spreading, including Sykes’ earthquake data and those symmetrical patterns of zebra stripes. It soon became clear that these findings were building toward one unified narrative: Mid-ocean ridges were the birthplaces of new seafloor, and deep ocean trenches were graves where old lithosphere was reabsorbed into the interior. This cycle of birth and death had opened and closed the oceans over and over again, bringing the continents together and then splitting them apart.

The evidence was overwhelming, and it was during this conference “that the victory of mobilism was clearly established,” geophysicist Xavier Le Pichon, previously a skeptic of seafloor spreading, wrote in 2001 in his retrospective essay My conversion to plate tectonics, included in Oreskes’ book.

Plate tectonics emerges

The whole earth science community became aware of these findings the following spring, at the American Geophysical Union’s annual meeting. Wilson laid out the various lines of evidence for this new view of the world to a much larger audience in Washington, D.C. By then, there was remarkably little pushback from the community, Sykes says: “Right away, they accepted it, which was surprising.”

Scientists now knew that Earth’s seafloor and continents were in motion, and that ridges and trenches marked the edges of large blocks of lithosphere. But how were these blocks moving, all in concert, around the planet? To plot out the choreography of this complex dance, two separate groups seized upon a theorem devised by mathematician Leonhard Euler way back in the 18th century. The theorem showed that a rigid body moves around a sphere as though it is rotating around an axis. McKenzie and geophysicist Robert Parker used [Nature] this theorem to calculate the dance of the lithospheric blocks — the plates. Unbeknownst to them, geophysicist W. Jason Morgan independently came up with a similar solution [JGR].

Shifting landmasses — such as the opening of the Drake Passage between South America and Antarctica (icebergs around Elephant Island shown) — can alter currents, and climates. Credits: NASA IMAGE BY JEFF SCHMALTZ, LANCE/EOSDIS RAPID RESPONSE.

With this last piece, the unifying theory of plate tectonics was born. The hoary wrangling over continental drift now seemed not only antiquated, but also “a sobering antidote to human self-confidence,” physicist Egon Orowan told Science News in 1970.

People have benefited greatly from this clearer vision of Earth’s workings, including being able to better prepare for earthquakes, tsunamis and volcanoes. Plate tectonics has also shaped new research across the sciences, offering crucial information about how the climate changes and about the evolution of life on Earth.

And yet there’s still so much we don’t understand, such as when and how the restless shifting of Earth’s surface began — and when it might end. Equally puzzling is why plate tectonics doesn’t appear to happen elsewhere in the solar system, says Lindy Elkins-Tanton, a planetary scientist at Arizona State University in Tempe. “How can something be a complete intellectual revolution and also inexplicable at the same time?”

Jupiter’s frozen moon Europa (shown) has its own form of icy plate tectonics.Credit: NASA/JPL-Caltech, SETI Institute.

Signs of plate tectonics are clearly visible on Earth’s surface (Piqiang Fault in China’s Xinjiang Province shown). Scientists wonder whether similar features on other planets could be clues to habitability.Credit: NASA.

Mars shows signs of volcanic activity (Olympus Mons shown) but no known plate tectonics.Credit: NASA/JPL.

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From MIT: “Rock magnetism uncrumples the Himalayas’ complex collision zone”

From MIT News

January 7, 2021
Lauren Hinkel Department of Earth, Atmospheric and Planetary Sciences (EAPS)

MIT EAPS researchers find the impressive mountain range formed over a series of impacts, not a single event, as previously thought.

Craig Martin holds a pickaxe in the Himalayan mountains. Credit: Craig Martin.

With some of the world’s tallest peaks, Asia’s “the abode of snow” region is a magnet for thrill seekers, worshipers, and scientists alike. The imposing 1,400-mile Himalayan mountain range that separates the plains of the Indian subcontinent from the Tibetan Plateau is the scene of an epic continent-continent collision that took place millions of years ago and changed the Earth, affecting its climate and weather patterns. The question of how the Indian and Eurasian tectonic plates collided, and the mountains came into existence, is one that scientists are still unfolding. Now, new research published in PNAS and led by MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS) confirms that it’s more complicated than previously thought.

“The Himalayas are the textbook example of a continent-continent collision and an excellent laboratory for studying mountain-building events and tectonics,” says EAPS graduate student Craig Martin, the paper’s lead author.

The story begins around 135 million years ago, when the Neotethys Ocean separated the tectonic plates of India and Eurasia by 4,000 miles. The common view of geologists is that the Neotethys Ocean plate began subducting into Earth’s mantle under Eurasia, on its southern border, pulling India north and the tectonic plates above it together to ultimately form the Himalayas in a single collision event around 55-50 million years ago. However, geologic evidence suggested that the high rate of subduction observed didn’t seem to quite fit this hypothesis, and model reconstructions place the continental plates thousands of kilometers apart at the time of this inferred collision. To account for the time delay and subduction strength required, MIT’s Oliver Jagoutz, associate professor of geology, and Leigh “Wiki” Royden, the Cecil and Ida Green Professor of Geology and Geophysics, proposed that because of the high speed, orientation, and location of the final continental collision, there needed to be another oceanic plate and subduction zone in the middle of the ocean, called the Kshiroda plate and the Trans-Tethyan subduction zone (TTSZ), which ran east to west. Additionally, EAPS geologists and others postulated that an arc of volcanic islands, like the Marianas, existed in between the two, called the Kohistan-Ladakh arc. Located near the equator, they took the brunt of the force from India before being squished between the two continental crusts.

Tiny magnets point the way

This chain of events, its timing and geological configuration, was speculation based on models and some geological evidence until EAPS researchers tested it — but first, they needed rocks. Along with professor of planetary sciences Ben Weiss of the MIT Paleomagnetism Laboratory, Martin, Jagoutz, Royden, and their colleagues visited northwest India’s Ladakh region, bordering the Eurasian plate. Over multiple excursions, the team, which included EAPS undergraduate Jade Fischer for one trip, scrambled over outcrops and drilled rock cores, slightly larger than the size of a cork. As they pulled them out, the geologists and paleomagnetism experts marked the samples’ orientation in the rock layer and its location in order to determine when and where on Earth the rock was formed. The team was looking for evidence showing whether a volcano, which was active around 66-61 million years ago, was part of a volcanic island chain in the ocean south of Eurasia, or part of the Eurasian continent. This would also help determine the plausibility of a double subduction zone scenario.

Back in the lab, the MIT researchers used rock dating and paleomagnetism to understand this ancient geologic car crash. They leveraged the fact that, as lava cools and rock forms, it captures a signature of the Earth’s magnetic field, which runs north-south toward Earth’s magnetic poles. If rock forms near the equator, the magnetization (electron) spins in its iron-bearing minerals, like magnetite and hematite, will be oriented parallel to the ground. As you move further away from the equator, the rock’s magnetization will tip into the Earth; however subsequent heating and remagnetization can print over the original signature.

After checking for this, and correcting for the tilt of the bedrock at the site, Martin and his colleagues were able to pinpoint the latitude at which the rocks were created. Uranium-lead dating of the samples’ zircon minerals provided the other piece of the puzzle to constrain the timing of formation. If there was a single collision, these rocks would have formed at a latitude somewhere around 20 degrees north, above the equator, near Eurasia; if the islands existed, they would have originated near the equator.

“It’s cool that we can reconstruct the deep-time atlas of the world using the tiny magnets preserved in rocks,” says Martin.

A two-part system

With their time and latitudinal measurements and models, the MIT researchers found the evidence they were looking for — the presence of an island chain and double subduction system. From 80 to likely 55-50 million years ago, the Neotethys Ocean was subducted in two locations: along the Eurasian plate’s southern edge (the Kshiroda plate sank) and the mid-ocean TTSZ, just south of the Kshiroda plate and near the equator. Together, these events closed the ocean, and the tectonic activity worked with erosion and weathering to sequester and draw down carbon, until the Paleocene Epoch (66-23.03 million years ago). “The presence of two subduction zones and the timing of their destruction at low latitudes explains the cooling global climate in the Cenozoic (66 million years ago to present day),” says Martin.

Most importantly: “Our results mean that instead of India colliding directly with Eurasia to form the Himalayas, India first collided with a volcanic island chain (similar to the Mariana Islands today), and then with Eurasia up to 10 million years later than is generally accepted,” says Martin. This is because Kohistan-Ladakh arc and India collision slowed the India-Eurasia convergence rate, which kept decreasing until 45-40 million years ago when the final collision occurred. “This finding is contrary to the long-held view that the India-Eurasia collision was a single-stage event that started at 55-60 million years ago,” says Martin. “Our results strongly support Oli and Wiki’s double subduction hypothesis explaining why India moved north so anomalously fast in the Cretaceous period.”

Further, Martin, Jagoutz, Royden, and Weiss were able to determine the maximum extent of the Indian plate before it was forced under Eurasia. The convergence between India and Eurasia since 50-55 million years ago was around 2,800-3,600 kilometers. Much of this is explained by the subduction of the Kshiroda plate, which the MIT researchers estimated to be roughly 1,450 kilometers wide, at the time of the first collision with the island arc, 55-50 million years ago. After the first stage of collision between the island chain and India, the Kshiroda plate continued to disappear underneath Eurasia. Then, 15-10 million years later, as the two continents came together, the continental crust began shortening, folding, and thrusting rocks upward, the force of which caused observable changes to the composition and structure of the rocks. “Our results also directly constrain the size of the part of India ‘lost’ in the collision to less than 900 kilometers in the north-south direction, which is much less than the 2,000 kilometers previously required to explain the timing of collision.”

The newly-gained insights into the mechanisms and geometry of such an archetypal mountain system have important implications for using the Himalayas to study continental collision, says Martin. Revising the number of subduction zones, the age of final collision, and the amount of continental crust involved in the formation of the Himalayas changes some key parameters required to accurately model the growth of mountain belts, the deformation of continental crust, and the relationships between plate tectonics and global climate.

Martin hopes to take this further throughout the rest of his graduate studies by focusing in on the intensely deformed collision zone between the volcanic island chain and Eurasia. He hopes to understand the closure of the Kshiroda ocean and the geological structures produced during the continental collision.

Not only is the finding impressive, but as Martin remarks, “I think it is cool to imagine idyllic tropical volcanic islands, with dinosaurs roaming around on them, having been sandwiched between two colliding tectonic plates and uplifted to form the roof of the world.”

This study was funded, in part, by NSF Tectonics Program and MISTI-India.

See the full article here .

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From Science Magazine: “Slowdown in plate tectonics may have led to Earth’s ice sheets”

From Science Magazine

Dec. 22, 2020
Paul Voosen

The extrusion of fresh ocean crust at midocean ridges began to slacken 15 million years ago, perhaps cooling the planet.
Credit: geogphotos/Alamy Stock Photo.

In seafloor trenches around the world, slabs of old ocean crust fall in slow motion into the mantle, while fresh slabs are built at midocean ridges, where magma emerges at the seams between separating tectonic plates. The engine is relentless—but maybe not so steady: Beginning about 15 million years ago, in the late Miocene epoch, ocean crust production declined by one-third over 10 million years to a slow pace that pretty much continues to today, says Colleen Dalton, a geophysicist at Brown University who presented the work this month at a virtual meeting of the American Geophysical Union. “It’s a global phenomenon.”

Although previous ocean-spreading records showed hints of a slowdown, nothing suggested such a steep decline, says Clint Conrad, a mantle dynamicist at the University of Oslo who is unaffiliated with the work. The lag was also widespread: Dalton found crust production slowed down or stayed steady at 15 of Earth’s 16 ocean ridges. And its effect on the climate may have been stark, Conrad says. “If you dramatically slow down plate tectonics in such a short time, you can put out a lot less carbon dioxide (CO2) gas from volcanism.” The slowdown corresponds to a 10°C drop in temperatures in the late Miocene, when ice sheets began to grow across Antarctica after a long hiatus.

Seafloor spreading is captured in magnetic zones on the ocean floor. Every million years or so, Earth’s magnetic field flips, and this reversal is frozen in the rocks forged at the midocean ridges. Ship-based observations of the alternating magnetic “stripes” that result as slabs of ocean crust unfurl from the seafloor spreading centers helped give credence to the theory of plate tectonics in the 1960s.

The ridges in the Atlantic and Indian oceans spread slowly, however, which means ships have been able to map these stripes with a temporal resolution of only about 10 million years. But geophysicists Charles DeMets of the University of Wisconsin, Madison, and Sergey Merkuryev of Saint Petersburg State University drew on previously unused data from Russian naval ships, which—like those of other nations—tow magnetometers to aid the hunt for enemy submarines. The new data sharpened the resolution in these ocean basins to 1 million years. “And it turns out there are surprising signals hiding in a lot of places that we didn’t know about,” says DeMets, who identified part of the slowdown in his records.

Dalton and her colleagues added to the picture by assembling a complementary high-resolution record for the Pacific Ocean, where seafloor spreading is faster and more complex. With that global view, the slackening immediately became apparent. It appears the deceleration came in two waves, DeMets says: first between 12 million and 13 million years ago in the Pacific and then 7 million years ago in the Atlantic and Indian oceans.

Maybe the subducting slabs stopped tugging as hard on the moving sea floor during this time, Dalton speculates, because they grew thinner or less dense. Or maybe the subduction zones, typically as long as the midocean ridges, shrunk in length, reducing their pull. Another possibility is that the zones changed their orientation, causing the subducting slabs to meet more resistance as they dove into the mantle, which has a kind of natural, grainlike wood. Or a slab could have broken off entirely, changing the flow of heat inside the mantle and altering the glide of the tectonic plates overhead, Conrad says. “Even if you change one plate, it affects all the plates.”

By taking volcanic CO2 emissions tied to today’s ocean crust production and adjusting them for late Miocene speeds, the team found a drop in atmospheric CO2 that could plausibly explain the global cooling at the time. But Dalton says other explanations are possible—for example, ancient volcanic rocks, uplifted out of the ocean to form fresh mountain peaks in places like Indonesia, might have started to soak up more CO2. Both mechanisms likely explain some of the drop, says Nicholas Swanson-Hysell, a paleogeographer at the University of California, Berkeley. “But which is more important?”

Beyond lowering CO2, the crustal slowdown would have reshaped Earth’s surface. With less seafloor volcanism, the midocean ridges would have been smaller, increasing the capacity of the oceans. Sea levels would have fallen by 22 meters, Dalton calculates, exposing vast new stretches of land. And as the volcanoes went quiet, the planet itself would have grown 5% less efficient at shedding its internal heat, losing some 1.5 terawatts of output—roughly equal to the production of 1500 nuclear power plants. That decline in heat flow wouldn’t have made much difference to atmospheric temperatures, but Dalton says it calls into question reconstructions of Earth’s cooling history that assume constant heat loss across the ages.

Although there’s much to be teased out, it’s clear that, when viewed over relatively short geological time spans, there’s nothing constant about plate tectonics, says Karin Sigloch, a geophysicist at the University of Oxford.

The tectonic plates of the world were mapped in 1996, USGS.

“Variation should always be expected.” Slabs break, monster plumes of seafloor magma suddenly erupt—all with huge climatic repercussions for the thin biosphere clinging to life at the surface. Yet they are just burps in a planetary engine that churns away in a deep and hidden underworld.

See the full article here .

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From Florida State University: “FSU MagLab geochemists solve mystery of Earth’s vanishing crust”

From Florida State University

June 26, 2020 [Just found this.]
Kristen Coyne
kcoyne@magnet.fsu.edu

Scientists examined hundreds of samples taken along the global ridges that contain recycled ancient oceanic crust in variable amounts. “Depleted” segments of the ridge received lower than “normal” amounts of recycled crust, while “enriched” segments contain a larger proportion of recycled crust. Credit: Caroline McNiel/National MagLab.

Thank goodness for the Earth’s crust: It is, after all, that solid, outermost layer of our planet that supports everything above it.

The tectonic plates of the world were mapped in 1996, USGS.

But much of what happens below that layer remains a mystery, including the fate of sections of crust that vanish back into the Earth. Now, a team of geochemists based at the Florida State University-headquartered National High Magnetic Field Laboratory has uncovered key clues about where those rocks have been hiding.

The researchers provided fresh evidence that, while most of the Earth’s crust is relatively new, a small percentage is actually made up of ancient chunks that had sunk long ago back into the mantle then later resurfaced. They also found, based on the amount of that “recycled” crust, that the planet has been churning out crust consistently since its formation 4.5 billion years ago — a picture that contradicts prevailing theories.

Their research is published in the journal Science Advances .

“Like salmon returning to their spawning grounds, some oceanic crust returns to its breeding ground, the volcanic ridges where fresh crust is born,” said co-author Munir Humayun, a MagLab geochemist and professor at Florida State’s Department of Earth, Ocean and Atmospheric Science (EOAS). “We used a new technique to show that this process is essentially a closed loop, and that recycled crust is distributed unevenly along ridges.”

In addition to Humayun, the research team included MagLab postdoctoral researcher Shuying Yang, lead author on the paper, and MagLab Geochemistry Group Director and EOAS Chair Vincent Salters.

The Earth’s oceanic crust is formed when mantle rock melts near fissures between tectonic plates along undersea volcanic ridges, yielding basalt. As new crust is made, it pushes the older crust away from the ridge toward continents, like a super slow conveyer belt. Eventually, it reaches areas called subduction zones, where it is forced under another plate and swallowed back into the Earth.

Scientists have long theorized about what happens to subducted crust after being reabsorbed into the hot, high-pressure environment of the planet’s mantle. It might sink deeper into the mantle and settle there, or rise back to the surface in plumes, or swirl through the mantle, like strands of chocolate through a yellow marble cake. Some of that “chocolate” might eventually rise up, re-melt at mid-ocean ridges, and form new rock for yet another millions-year-long tour of duty on the sea floor.

This new evidence supports the “marble cake” theory.

Scientists had already seen clues supporting the theory. Some basalts collected from mid-ocean ridges, called enriched basalts, have a higher percentage of certain elements that tend to seep from the mantle into the melt from which basalt is formed; others, called depleted basalts, had much lower levels.

New tool cracks an ancient case

To shed more light on the mystery of the disappearing crust, the team chemically analyzed 500 samples of basalt collected from 30 regions of ocean ridges. Some were enriched, some were depleted and some were in between.

Early on, the team discovered that the relative proportions of germanium and silicon were lower in melts of recycled crust than in the “virgin” basalt emerging from melted mantle rock. So they developed a new technique that used that ratio to identify a distinct chemical fingerprint for subducted crust.

They devised a precise method of measuring that ratio using a mass spectrometer at the MagLab. Then they crunched the numbers to see how these ratios differed among the 30 regions sampled, expecting to see variations that would shed light on their origins.

At first the analysis revealed nothing of note. Concerned, Yang, a doctoral candidate at the time, consulted with her adviser. Humayun suggested looking at the problem from a wider angle: Rather than compare basalts of different regions, they could compare enriched and depleted basalts.

After quickly re-crunching the data, Yang was thrilled to see clear differences among those groups of basalts.

“I was very happy,” recalled Yang, lead author on the paper. “I thought, ‘I will be able to graduate!’”

The team had detected lower germanium-to-silicon ratios in enriched basalts — the chemical fingerprint for recycled crust — across all the regions they sampled, pointing to its marble cake-like spread throughout the mantle. Essentially, they solved the mystery of the vanishing crust.

It was a lesson in missing the forest for the trees, Humayun said.

“Sometimes you’re looking too closely, with your nose in the data, and you can’t see the patterns,” he said. “Then you step back and you go, ‘Whoa!’”

Digging deeper into the patterns they found, the scientists unearthed more secrets. Based on the amounts of enriched basalts detected on global mid-ocean ridges, the team was able to calculate that about 5 to 6 percent of the Earth’s mantle is made of recycled crust, a figure that sheds new light on the planet’s history as a crust factory. Scientists had known the Earth cranks out crust at the rate of a few inches a year. But has it done so consistently throughout its entire history?

Their analysis, Humayun said, indicates that, “The rates of crust formation can’t have been radically different from what they are today, which is not what anybody expected.”

The MagLab is funded by the National Science Foundation and the State of Florida. It is headquartered at Florida State University with additional locations at University of Florida and Los Alamos National Laboratory.

Recycled ancient crust returns to the oceanic ridges. Credit: Caroline McNiel/National MagLab.

See the full article here .

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From University of Houston: “Lost and Found- UH Geologists ‘Resurrect’ Missing Tectonic Plate”

From University of Houston

October 20, 2020
Sara Tubbs
sstubbs2@uh.edu
713-743-4248

(l-r) Jonny Wu, assistant professor of geology in the UH Department of Earth and Atmospheric Sciences and Spencer Fuston, a third-year geology doctoral student, applied a technique developed by the UH Center for Tectonics and Tomography called slab unfolding to reconstruct what tectonic plates in the Pacific Ocean looked like during the early Cenozoic Era.

A 3D block diagram across North America showing a mantle tomography image reveals the Slab Unfolding method used to flatten the Farallon tectonic plate. By doing this, Fuston and Wu were able to locate the lost Resurrection plate.

This image shows plate tectonic reconstruction of western North America 60 million years ago showing subduction of three key tectonic plates, Kula, Farallon and Resurrection.

Fuston Wu 2020 Preferred Model.

Long-Debated Plate Located in Northern Canada Using 3D Mapping Technology

The existence of a tectonic plate called Resurrection has long been a topic of debate among geologists, with some arguing it was never real. Others say it subducted – moved sideways and downward – into the earth’s mantle somewhere in the Pacific Margin between 40 and 60 million years ago.

A team of geologists at the University of Houston College of Natural Sciences and Mathematics believes they have found the lost plate in northern Canada by using existing mantle tomography images – similar to a CT scan of the earth’s interior. The findings, published in Geological Society of America Bulletin, could help geologists better predict volcanic hazards as well as mineral and hydrocarbon deposits.

“Volcanoes form at plate boundaries, and the more plates you have, the more volcanoes you have,” said Jonny Wu, assistant professor of geology in the Department of Earth and Atmospheric Sciences. “Volcanoes also affect climate change. So, when you are trying to model the earth and understand how climate has changed since time, you really want to know how many volcanoes there have been on earth.”

Wu and Spencer Fuston, a third-year geology doctoral student, applied a technique developed by the UH Center for Tectonics and Tomography called slab unfolding to reconstruct what tectonic plates in the Pacific Ocean looked like during the early Cenozoic Era. The rigid outermost shell of Earth, or lithosphere, is broken into tectonic plates and geologists have always known there were two plates in the Pacific Ocean at that time called Kula and Farallon. But there has been discussion about a potential third plate, Resurrection, having formed a special type of volcanic belt along Alaska and Washington State.

“We believe we have direct evidence that the Resurrection plate existed. We are also trying to solve a debate and advocate for which side our data supports,” Fuston said.

Using 3D mapping technology, Fuston applied the slab unfolding technique to the mantle tomography images to pull out the subducted plates before unfolding and stretching them to their original shapes.

“When ‘raised’ back to the earth’s surface and reconstructed, the boundaries of this ancient Resurrection tectonic plate match well with the ancient volcanic belts in Washington State and Alaska, providing a much sought after link between the ancient Pacific Ocean and the North American geologic record,” explained Wu.

This study is funded by a five-year, $568,309 National Science Foundation CAREER Award led by Wu.

See the full article here.

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From Macquarie University via COSMOS: “Tracking the birth of a supercontinent”

From Macquarie University

via

COSMOS

7 August 2020
Huaiyu Yuan, Macquarie University

Scientists find some old and intriguing clues.

The present landscape near Dongshen, China. Credit: Wan et al., Author provided.

Far beneath the city of Dongshen in northern China, we have discovered what may be the 2 billion-year-old birthmarks of Earth’s first supercontinent.

An ancient dipping structure in the planet’s crust appears to be a trace of an early collision between two continental masses like the one that created the Himalaya – and may record the origin of the global system of plate tectonics that persists today.

The tectonic plates of the world were mapped in 1996, USGS.

When did plate tectonics begin?

The theory of plate tectonics is one of the key scientific advances of the past century. It explains how Earth’s crust is made of enormous rocky “plates” floating on the planet’s molten interior, which slowly move around. These movements are responsible for earthquakes and mountain ranges.

Earth is the only planet we know of with plate tectonics. The motion of the plates gradually cycles elements between the interior of the planet, the surface and the atmosphere, generating the resources and environment that make human life possible.

At some point in the deep past, plate tectonics began as Earth cooled. When this happened, however, has remained controversial. Dates spanning three-quarters of Earth’s history have been proposed, from the Hadean eon (between 4.5 billion and 4 billion years ago) to the late Proterozoic eon (less than a billion years ago).

Many of these dates come from isolated samples showing the existence of single plates. However, plate tectonics is a global phenomenon in which plates interact with each other. We studied one of these early interactions: a collision in what is now northern China, in which the edge of one plate was thrust upwards while the other was pushed down.

The dipping Moho

Our new study suggests plate tectonics began globally somewhere between 2 billion and 1.8 billion years ago. The research, published in Science Advances, was carried out by an international team from China, Germany and Australia, led by Wan, Bo from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS).

The global network of ancient collisions that show the creation of supercontinent Nuna. Credit: Wan et al. / Science Advances, Author provided.

We studied an area geologists call the Ordos block, which is part of the North China craton, a very stable chunk of the Asian continent that takes in parts of northeastern China, Mongolia and North Korea.

In April 2019, we deployed 609 seismic recording stations spaced every 500 metres along a 300-kilometre line. By combining the earthquake data from these stations, we were able to form a detailed picture of Earth’s crust in this area.

Beneath the city of Dongsheng, we found a feature called a dipping Moho in which the bottom of Earth’s crust dips from around 35km deep to more than 50km deep over a horizontal distance of only 40km.

This dipping structure looks nearly identical to what is found beneath the Himalayan mountains, except it is around 2 billion years old.

A global pattern

Next, we collected seismic evidence from other studies around the world for similar dipping Moho structures that are about the same age. Putting observations from six continents together, we can form a picture of the creation of the ancient supercontinent Nuna.

Nuna (sometimes also called Columbia) is believed to have been made up of parts of most of the continents that exist today. If Nuna was the first supercontinent, we can interpret these tectonic collisions that occurred around 2 billion years ago as the oldest evidence of plate tectonics in the global sense. Even though such collisions may have occurred here and there early on, it is likely that plate tectonics did not become a global network until this time.

See the full article here .

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From Caltech: “Slow Earthquakes in Cascadia are Predictable”

From Caltech

July 01, 2020

Robert Perkins
(626) 395‑1862
rperkins@caltech.edu

Cascadia subduction zone

Evidence mounts that slow-slip seismic events follow a deterministic pattern.

If there is one word you are not supposed to use when discussing serious earthquake science, it is “predict.” Seismologists cannot predict earthquakes; instead they calculate how likely major earthquakes are to occur along a certain fault over a given period of time.

It is a matter of debate among seismologists whether the process that drives earthquakes—the loading of strain along a fault followed by the sudden, sharp release of energy as two tectonic plates grind against one another—is a stochastic (random) process, for which only an estimate of the probability of occurrence can be made, or whether it is a deterministic, and potentially predictable, process.

Seismologists at Caltech studied a decade’s worth of so-called “slow-slip events,” which result from episodic fault slip like regular earthquakes but only generate barely perceptible tremors, in the Cascadia region of the Pacific Northwest. Their analysis shows that this particular type of seismic event is deterministic and potentially could be predictable days or even weeks in advance.

A paper about the work was published in the journal Science Advances on July 1.

“Deterministic chaotic systems, despite the name, do have some predictability. This study is a proof of concept to show that friction at the natural scale behaves like a chaotic system, and consequently has some degree of predictability,” says Adriano Gualandi, the lead and corresponding author of the paper. Gualandi was a postdoctoral scholar in the lab of Jean-Philippe Avouac, the Earle C. Anthony Professor of Geology and Mechanical and Civil Engineering, while working on this research. Gualandi and Avouac collaborated with Sylvain Michel, who worked on this project as a graduate student at Caltech, and Davide Faranda of Institut Pierre Simon Laplace in France on the study.

Slow-slip events were first noted about two decades ago by geoscientists tracking otherwise imperceptible shifts in the earth using global positioning system (GPS) technology. The events occur when tectonic plates grind incredibly slowly against each other, like an earthquake in slow motion. A slow-slip event that occurs over the course of weeks might release the same amount of energy as a one-minute-long magnitude 7.0 earthquake. However, because these quakes release energy so slowly, the deformation that they cause at the surface is on the scale of millimeters, despite affecting areas that may span thousands of square kilometers.

As such, slow-slip events were only discovered when GPS technology was refined to the point that it could track those very minute shifts. Slow-slip events also do not occur along every fault; so far, they have been spotted in just a handful of locations including the Pacific Northwest, Japan, Mexico, and New Zealand.

Slow-slip events are useful to researchers because they build up and reoccur frequently, making it possible to study how strain loads and releases along a fault. Over a 10-year period, 10 magnitude 7.0 or greater slow-slip earthquakes might occur along a given fault. By contrast, most regular earthquakes of that magnitude only reoccur on the order of hundreds of years. Because of this time lag between regular large earthquakes and the lack of instrumental records from hundreds of years ago, it is impossible to precisely compare past events with recent ones.

GPS stations reveal activity beneath Cascadia where the oceanic floor slides beneath North America. The plate interface is locked at shallow depths (the shaded area), but we see recurring slow-slip events (in blue) that unzip the plate interface, generating tremors (the black dots).

Despite their name, slow-slip events offer seismologists a way to press “fast-forward” on the loading/slipping process that drives earthquakes. In a short time frame of around 10 years, seismologists using state-of-the-art GPS equipment can observe the cycle repeat itself several times.

Slow-slip events represent what is known as a “forced non-linear dynamical system.” The motion of the tectonic plates is the force driving the system, while the friction between the plates, which causes pressure to build up and then eventually be released in a slip event, makes the system non-linear; in a non-linear system, the change in output is not proportional to the change in input. Despite the fact that both the motion and the friction can be modeled using fully deterministic differential equations, the starting conditions of the system—how much strain the fault is already under, for example—have a significant impact on long-term outcomes. Not knowing those exact starting conditions is one of the possible reasons that the overall system is unpredictable in the long run. However, an examination of the fault slip history can reveal how often and for how long similar patterns repeated over time. In this way, the team was able to assess the predictability horizon time of slow-slip events.

“This result is very encouraging,” Gualandi says. “It shows that we are on the right track and, if we manage to get more precise data, we could attempt some real-time prediction experiments for slow earthquakes.”

Gualandi likens the potential prediction of a slow-slip event to the current science of forecasting the weather, which also involves predictions about a complex, chaotic process (and similarly falls off in accuracy after a week or so). “We already know that approximately every 12 to 14 months there will be a new slow earthquake, but we do not know exactly when it will happen. What we have shown is that it seems to be possible to determine when the fault will slip some days before it happens, similar to the way weather can be forecast fairly accurately a couple days in advance.”

One key question is whether the findings for slow-slip quakes can translate to the regular earthquakes that shake cities and endanger lives and property. Last year Michel, Avouac, and Gualandi reported evidence that slow-slip earthquakes are a good analogue for their more destructive cousins.

“If the analogy that we’re drawing between slow earthquakes and regular earthquakes is correct, then regular earthquakes are predictable,” Avouac says. “But even if regular earthquakes are deterministic, the predictability horizon may be very short, possibly on the order of a few seconds, which may be of limited utility. We don’t know yet.”

See the full article here .

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