New Study Explains How Continents Leave Their Roots Behind

In some areas of the seafloor, a tectonic mystery lies buried deep underground.

The ocean floor contains some of the newest rock on Earth, but underneath these young oceanic plates are large swatches of much older continents that have been dislocated from their continental plates and overtaken by the younger, denser oceanic plate.

Researchers have been puzzled by this phenomenon for some time: how does a continental plate leave some of itself behind?

In a new study published in Geophysical Research Letters, a journal of the American Geophysical Union, researchers have linked the displaced pieces of continental plates to a weak link in the plate’s layers called a mid-lithospheric discontinuity.

The crust and the upper mantle make up the lithosphere, the rigid, outer part of the Earth. A mid-lithospheric discontinuity can occur in this layer, running horizontally through the middle of the lithosphere. It is at this place where the lower layer of a continent’s lithosphere can break away from itself and dislocate, leaving behind large pieces of the lower lithosphere, called a root, which can become embedded in the oceanic plate on the trailing side of the continental plate.

The new study finds thicker and weaker mid-lithospheric discontinuity layers are more likely to leave behind roots farther from their continental origins, while thinner layers have more strength to hold onto their roots as the continental plates move, according to the new study.

“This is the first mechanism to explain the large-scale displacement of continental lithosphere being left behind under oceanic lithosphere,” said Timothy Kusky, director for the Center for Global Tectonics at the China University of Geosciences in Wuhan, China, and co-author of the new study.

Kusky likens the process to a peanut butter and jelly sandwich on a table: the sandwich is the Earth’s lithosphere, and the table is the asthenosphere, the weak layer in the upper mantle that accommodates most plate displacements. The peanut butter and jelly is the mid-lithospheric discontinuity that is binding the two halves of the lithosphere together.

If someone pushed the sandwich across the table, the force from the top would move the top layer of bread, but the friction from the table pulls on the bottom slice of bread. As the sandwich moves, the two pieces of bread may get offset, and the sandwich becomes uneven, Kusky said.

Just like the sandwich, as the continental plate slowly moves, the velocity of the upper lithosphere may be faster than the lower lithosphere. If the “peanut butter and jelly” is weak, the top part of the lithosphere begins to surpass its lower half, leaving the lower lithosphere behind to be overtaken by the denser oceanic plate.

The question Kusky and his colleagues attempt to answer in the new study is: Can we model the peanut butter and jelly sandwich?

The study’s authors created a numerical model of the largest documented continental lithosphere offset, a continental root under the southern Atlantic Ocean that was left 1,300 kilometers (more than 800 miles) behind by the African continent from which it originated.

The study’s authors modeled how the minerals in the lithosphere flow and how fast the continental plate would have been moving at the time, approximately 130 million years ago. The researchers ran 225 models of the continental plate, using different thicknesses for the mid-lithospheric discontinuity between 10 and 50 kilometers wide (5 to 31 miles wide) to investigate the layer’s strength holding the two lateral halves together. The models also incorporated a range of plate velocities and viscosity, or stickiness due to friction, of the mid-lithospheric discontinuity.

The model revealed that the thicker the mid-lithospheric discontinuity zone, the larger the plate offset would be. A thinner “jelly” with a high viscosity was less likely to experience shearing from the upper lithosphere, or at least only trail slightly behind. But a thick mid-lithospheric discontinuity layer, more than 25 kilometers thick (about 15 miles thick), can lead to large offsets. Over 100 million years, some roots can wind up 10,000 kilometers (6,200 miles) away from the continent they originated from, according to the models.

In the case of the 1,300-kilometer (800-mile) African offset, the scientists estimate the mid-lithospheric discontinuity was about 40 kilometers thick and traveled apart at a rate of 1 to 3.25 centimeters a year (about 0.39 to 1.28 inches).

Understanding these plate offsets can help researchers understand how the continental pieces of lithosphere can affect oceanic plates and their composition, said Zhensheng Wang, a geoscientist at the China University of Geosciences in Wuhan, China, and co-author of the new study.

One example for further study under this new model would be the Ontong Java Plateau in the Pacific Ocean, the single largest oceanic plateau on Earth.

“Really it represents a new step in plate tectonics,” Kusky said of the new study. “If we can explain the mid-lithospheric discontinuity then we can explain a lot of the enigmatic things in oceanography and plate tectonics in general.”

Study Suggests Hydrogen, Oxygen, Water And Carbon Dioxide Generated In Earth’s Mantle

Research from the University of Texas at Arlington and the Wadia Institute of Himalayan Geology suggests that hydrogen, oxygen, water and carbon dioxide are being generated in the earth’s mantle hundreds of kilometers below the earth’s surface.

“This discovery is important as it shows how earth’s planetary evolution may have happened,” said Asish Basu, UTA professor of earth and environmental sciences and co-author of the cover paper published in Geology in August.

The researchers focused their attention on a seven-kilometer thick portion of the earth’s upper mantle now found in the High Himalayas, at altitudes between 12,000 and 16,000 feet. This section of the mantle was pushed upwards to the top of the mountains as a result of the Indian Plate pushing north into Asia, displacing the ancient Tethys ocean floor and underlying mantle to create the Himalayan Mountain Belt around 55 million years ago.

“This is important as it means that we can analyze the nature of the mantle under the earth’s crust, at depths where drilling cannot reach,” Basu explained. “One key initial discovery was finding microdiamonds whose host rocks originated in the mantle transition zone, at depths between 410 and 660 kilometers below the earth’s surface.”

By studying the host rocks and associated minerals, the scientists had a unique opportunity to probe the nature of the deep mantle. They found primary hydrocarbon and hydrogen fluid inclusions along with microdiamonds by using Laser Raman Spectroscopic study. The discovery also showed that the environment in the deep mantle transition zone depths where the diamond is formed is devoid of oxygen.

The researchers suggest that during the advective transport or mantle up-welling into shallower mantle zones, the hydrocarbon fluids become oxidized and precipitate diamond, a mechanism that may also be responsible for forming larger diamonds like the world’s most valuable, Koh-i-Noor or Mountain of Light diamond, now in the Queen of England’s crown.

“We also found that the deep mantle upwelling can oxidize oxygen-impoverished fluids to produce water and carbon dioxide that are well-known to produce deep mantle melting,” said Souvik Das, UTA post-doctoral research scholar.

“This means that many of the key compounds affecting evolution like carbon dioxide and water are generated within the mantle,” he added.

Earth’s Tectonic Plates Are Weaker Than OnceThought

No one can travel inside Earth to study what happens there. So scientists must do their best to replicate real-world conditions inside the lab.

“We are interested in large-scale geophysical processes, like how plate tectonics initiates and how plates move underneath one another in subduction zones,” said David Goldsby, an associate professor at the University of Pennsylvania. “To do that, we need to understand the mechanical behavior of olivine, which is the most common mineral in the upper mantle of Earth.”

Goldsby, teaming with Christopher A. Thom, a doctoral student at Penn, as well as researchers from Stanford University, the University of Oxford and the University of Delaware, has now resolved a long-standing question in this area of research. While previous laboratory experiments resulted in widely disparate estimates of the strength of olivine in Earth’s lithospheric mantle, the relatively cold and therefore strong part of Earth’s uppermost mantle, the new work, published in the journal Science Advances, resolves the previous disparities by finding that, the smaller the grain size of the olivine being tested, the stronger it is.

Because olivine in Earth’s mantle has a larger grain size than most olivine samples tested in labs, the results suggest that the mantle, which comprises up to 95 percent of the planet’s tectonic plates, is in fact weaker than once believed. This more realistic picture of the interior may help researchers understand how tectonic plates form, how they deform when loaded with the weight of, for example, a volcanic island such as Hawaii, or even how earthquakes begin and propagate.

For more than 40 years, researchers have attempted to predict the strength of olivine in Earth’s lithospheric mantle from the results of laboratory experiments. But tests in a lab are many layers removed from the conditions inside Earth, where pressures are higher and deformation rates are much slower than in the lab. A further complication is that, at the relatively low temperatures of earth’s lithosphere, the strength of olivine is so high that it is difficult to measure its plastic strength without fracturing the sample. The results of existing experiments have varied widely, and they don’t align with predictions of olivine strength from geophysical models and observations.

In an attempt to resolve these discrepancies, the researchers employed a technique known as nanoindentation, which is used to measure the hardness of materials. Put simply, the researchers measure the hardness of a material, which is related to its strength, by applying a known load to a diamond indenter tip in contact with a mineral and then measuring how much the mineral deforms. While previous studies have employed various high-pressure deformation apparatuses to hold samples together and prevent them from fracturing, a complicated set-up that makes measurements of strength challenging, nanoindentation does not require such a complex apparatus.

“With nanoindentation,” Goldsby said, “the sample in effect becomes its own pressure vessel. The hydrostatic pressure beneath the indenter tip keeps the sample confined when you press the tip into the sample’s surface, allowing the sample to deform plastically without fracture, even at room temperature.”

Performing 800 nanoindentation experiments in which they varied the size of the indentation by varying the load applied to the diamond tip pressed into the sample, the research team found that the smaller the size of the indent, the harder, and thus stronger, olivine became.

“This indentation size effect had been seen in many other materials, but we think this is the first time it’s been shown in a geological material,” Goldsby said.

Looking back at previously collected strength data for olivine, the researchers determined that the discrepancies in those data could be explained by invoking a related size effect, whereby the strength of olivine increases with decreasing grain size of the tested samples. When these previous strength data were plotted against the grain size in each study, all the data fit on a smooth trend which predicts lower-than-thought strengths in Earth’s lithospheric mantle.

In a related paper by Thom, Goldsby and colleagues, published recently in the journal Geophysical Research Letters, the researchers examined patterns of roughness in faults that have become exposed at Earth’s surface due to uplifted plates and erosion.

“Different faults have a similar roughness, and there’s an idea published recently that says you might get those patterns because the strength of the materials on the fault surface increases with the decreasing scale of roughness,” Thom said. “Those patterns and the frictional behavior they cause might be able to tell us something about how earthquakes nucleate and how they propagate.”

In future work, the Penn researchers and their team would like to study size-strength effects in other minerals and also to focus on the effect of increasing temperature on size effects in olivine.

Did Life On Earth Start Due To Meteorites Splashing Into Warm Little Ponds?

Life on Earth began somewhere between 3.7 and 4.5 billion years ago, after meteorites splashed down and leached essential elements into warm little ponds, say scientists at McMaster University and the Max Planck Institute in Germany. Their calculations suggest that wet and dry cycles bonded basic molecular building blocks in the ponds’ nutrient-rich broth into self-replicating RNA molecules that constituted the first genetic code for life on the planet.

The researchers base their conclusion on exhaustive research and calculations drawing in aspects of astrophysics, geology, chemistry, biology and other disciplines. Though the “warm little ponds” concept has been around since Darwin, the researchers have now proven its plausibility through numerous evidence-based calculations.

Lead authors Ben K.D. Pearce and Ralph Pudritz, both of the McMaster’s Origins Institute and its Department of Physics and Astronomy, say available evidence suggests that life began when the Earth was still taking shape, with continents emerging from the oceans, meteorites pelting the planet — including those bearing the building blocks of life — and no protective ozone to filter the Sun’s ultraviolet rays.

“No one’s actually run the calculation before,” says Pearce. “This is a pretty big beginning. It’s pretty exciting.”

“Because there are so many inputs from so many different fields, it’s kind of amazing that it all hangs together,” Pudritz says. “Each step led very naturally to the next. To have them all lead to a clear picture in the end is saying there’s something right about this.”

Their work, with collaborators Dmitry Semenov and Thomas Henning of the Max Planck Institute for Astronomy, has been published today in the Proceedings of the National Academy of Science.

“In order to understand the origin of life, we need to understand Earth as it was billions of years ago. As our study shows, astronomy provide a vital part of the answer. The details of how our solar system formed have direct consequences for the origin of life on Earth,” says Thomas Henning, from the Max Planck Institute for Astronomy and another co-author.

The spark of life, the authors say, was the creation of RNA polymers: the essential components of nucleotides, delivered by meteorites, reaching sufficient concentrations in pond water and bonding together as water levels fell and rose through cycles of precipitation, evaporation and drainage. The combination of wet and dry conditions was necessary for bonding, the paper says.

In some cases, the researchers believe, favorable conditions saw some of those chains fold over and spontaneously replicate themselves by drawing other nucleotides from their environment, fulfilling one condition for the definition of life. Those polymers were imperfect, capable of improving through Darwinian evolution, fulfilling the other condition.

“That’s the Holy Grail of experimental origins-of-life chemistry,” says Pearce.

That rudimentary form of life would give rise to the eventual development of DNA, the genetic blueprint of higher forms of life, which would evolve much later. The world would have been inhabited only by RNA-based life until DNA evolved.

“DNA is too complex to have been the first aspect of life to emerge,” Pudritz says. “It had to start with something else, and that is RNA.”

The researchers’ calculations show that the necessary conditions were present in thousands of ponds, and that the key combinations for the formation of life were far more likely to have come together in such ponds than in hydrothermal vents, where the leading rival theory holds that life began in roiling fissures in ocean floors, where the elements of life came together in blasts of heated water. The authors of the new paper say such conditions were unlikely to generate life, since the bonding required to form RNA needs both wet and dry cycles.

The calculations also appear to eliminate space dust as the source of life-generating nucleotides. Though such dust did indeed carry the right materials, it did not deposit them in sufficient concentration to generate life, the researchers have determined. At the time, early in the life of the solar system, meteorites were far more common, and could have landed in thousands of ponds, carrying the building blocks of life. Pearce and Pudritz plan to put the theory to the test next year, when McMaster opens its Origins of Life laboratory that will re-create the pre-life conditions in a sealed environment.

“We’re thrilled that we can put together a theoretical paper that combines all these threads, makes clear predictions and offers clear ideas that we can take to the laboratory,” Pudritz says.

Lost Continent Of Zealandia: Scientists Return From Expedition To Sunken Land

After a nine-week voyage to study the lost, submerged continent of Zealandia in the South Pacific, a team of 32 scientists from 12 countries has arrived in Hobart, Tasmania, aboard the research vessel JOIDES Resolution.

Researchers affiliated with the International Ocean Discovery Program (IODP) mounted the expedition to explore Zealandia. IODP is a collaboration of scientists from 23 countries; the organization coordinates voyages to study the history of the Earth recorded in sediments and rocks beneath the seafloor.

“Zealandia, a sunken continent long lost beneath the oceans, is giving up its 60 million-year-old secrets through scientific ocean drilling,” said Jamie Allan, program director in the U.S. National Science Foundation’s Division of Ocean Sciences, which supports IODP.

“This expedition offered insights into Earth’s history, ranging from mountain-building in New Zealand to the shifting movements of Earth’s tectonic plates to changes in ocean circulation and global climate,” Allan said.

Earlier this year, Zealandia was confirmed as Earth’s seventh continent, but little is known about it because it’s submerged more than a kilometer (two-thirds of a mile) under the sea. Until now, the region has been sparsely surveyed and sampled.

Expedition scientists drilled deep into the seabed at six sites in water depths of more than 1,250 meters (4,101 feet). They collected 2,500 meters (8,202 feet) of sediment cores from layers that record how the geography, volcanism and climate of Zealandia have changed over the last 70 million years.

According to expedition co-chief scientist Gerald Dickens of Rice University in the U.S., significant new fossil discoveries were made. They prove that Zealandia was not always as deep beneath the waves as it is today.

“More than 8,000 specimens were studied, and several hundred fossil species were identified,” said Dickens.

“The discovery of microscopic shells of organisms that lived in warm shallow seas, and of spores and pollen from land plants, reveal that the geography and climate of Zealandia were dramatically different in the past.”

The new discoveries show that the formation 40 to 50 million years ago of the “Pacific Ring of Fire,” an active seafloor zone along the perimeter of the Pacific Ocean, caused dramatic changes in ocean depth and volcanic activity and buckled the seabed of Zealandia, according to Dickens.

Expedition co-chief scientist Rupert Sutherland of Victoria University of Wellington in New Zealand said researchers had believed that Zealandia was submerged when it separated from Australia and Antarctica about 80 million years ago.

“That is still probably accurate, but it is now clear that dramatic later events shaped the continent we explored on this voyage,” Sutherland said.

“Big geographic changes across northern Zealandia, which is about the same size as India, have implications for understanding questions such as how plants and animals dispersed and evolved in the South Pacific.

“The discovery of past land and shallow seas now provides an explanation. There were pathways for animals and plants to move along.”

Studies of the sediment cores obtained during the expedition will focus on understanding how Earth’s tectonic plates move and how the global climate system works. Records of Zealandia’s history, expedition scientists said, will provide a sensitive test for computer models used to predict future changes in climate.

Geologists Study The Drying Up Of The Mediterranean Sea 5.96 Million Years Ago

We already know that climate change influences such Earth processes as erosion and fluctuations in sea levels. But do surface processes in turn have an influence on volcanic activity? This was the question raised by geologists from the University of Geneva (UNIGE, Switzerland) and international collaborators. The researchers analysed volcanic data from the Messinian salinity crisis in the Mediterranean Sea, when the Strait of Gibraltar was blocked and the Mediterranean temporarily isolated from the Atlantic. After observing a sharp rise in volcanic activity during this period, and testing various scenarios, the geologists concluded that the increase in magmatic activity could only be explained by the almost total drying out of the Mediterranean. These results, published in Nature Geoscience, reveal the influence of surface processes, largely controlled by climate, on volcanic activity.

It is known that the Strait of Gibraltar was temporarily shut during the Messinian Era (more precisely, from 5.96 to 5.33 million years ago) and that the Mediterranean Sea was isolated from the Atlantic. In fact, as far back as the 1970s, scientists have found layers of salt several hundred metres thick on the seabed. The only explanation is that there was very limited connection between the Mediterranean and the Atlantic. The scientists also discovered huge underwater canyons dating back to the same period, hollowed out by rivers running over land that is now submerged, suggesting that the sea level was much lower at the time. This also points to the massive drying up of the Mediterranean with enormous geographical and climatic disruption across the entire basin. This hypothesis, however, continues to be a source of debate.

Nevertheless, a team of UNIGE-led geologists has provided new evidence of the Mediterranean’s drying up and the forcing of surface processes on magmatic activity. “We understand that what happens at the Earth’s surface, such as a sudden sea level lowering, causes the pressure to change at depth and has an effect on magma production,” says Pietro Sternai, researcher in the Department of Earth Sciences in UNIGE’s Science Faculty. Given that the salinity crisis was capable of generating these changes in pressure, the geologists, working on the hypothesis that the Mediterranean dried out, studied the changes in volcanic activity during this period.

When a volcano erupts, the magma cools on the Earth’s surface and the minerals crystallise. Based on these silent witnesses of volcanic activity, the scientists were able to establish that there were 13 eruptions around the Mediterranean between 5.9 and 5.3 million years ago. This is over twice the average activity, which is around 4.5 eruptions over a longer time length encompassing the salinity crisis. Why is the figure so high? “The single logical explanation,” suggests Sternai, “is the hypothesis that the sea dried out, since this is the only event powerful enough to alter the Earth’s pressure and magmatic production over the entire Mediterranean.”

The geologists used numerical models to test the hypothesis that the Mediterranean dried up. They reproduced the history of the charging and discharging of the weight of water and sediment in the Mediterranean as it was drying out. Then they calculated the changes in pressure at depth and the impact on magma production.

Two scenarios were examined: The first factored in the salinity crisis with drastic lowering of the sea level, and the second excluded the drawdown. “The simulations showed that the only way to account for the proven increase in volcanic activity was that the level (and thus the weight) of the Mediterranean Sea dropped by about two kilometres,” explains Sternai. “I leave it to you to imagine what the landscape looked like.”

In addition to providing further evidence of the drying out of the Mediterranean, the research also demonstrates the impact of climate change on the deep Earth. Climate change influences magmatic production, in particular via the effects on erosion and hydrology, which modify the pressure exerted at the Earth’s surface on the deep layers. Although we have been aware of the impact of volcanism on the climate for quite some time, the results presented in the study have disclosed that the opposite is also possible. “This pioneering work opens up new perspectives for interdisciplinary studies about the coupling between the solid Earth and the fluid Earth, and—for example—involving volcanologists, geomorphologists and climatologists,” concludes Sternai.

Large Meteorite Impacts Drove Plate-Tectonic Processes On The Early Earth

An international study led by researchers at Macquarie University has uncovered the ways in which giant meteorite impacts may have helped to kick-start our planet’s global tectonic processes and magnetic field. The study, being published in the premier journal Nature Geoscience, explores the effect of meteorite bombardment, in geodynamic simulations of the early Earth.

“Our results indicate that giant meteorite impacts in the past could have triggered events where the solid outer section of the Earth sinks into the deeper mantle at ocean trenches – a process known as subduction. This would have effectively recycled large portions of the Earth’s surface, drastically changing the geography of the planet,” explained lead author Associate Professor Craig O’Neill from Macquarie University.

“Large impact events may have also kick-started the Earth’s magnetic field by triggering the planet’s cold outer crust to suddenly move downward and interact with the Earth’s outer core. This affects convection in the core, and thus the geodynamo – the process that creates the Earth’s magnetic field,” he added.

To date, there is still not clear evidence to show whether plate tectonics operated in Earth’s early history, with the first 500 million years of our planet’s life, called the Hadean, often being dubbed as Earth’s geological dark ages. The little crust that has been preserved from this elusive period – mostly single grains of a mineral called zircon – has been used to argue for early tectonic activity. However, this is at odds with geochemical data and geodynamic simulations, which suggest that the Earth may instead have had a motionless ‘lid’ on its surface – in contrast to the actively moving combination of plates we see today.

“We know that meteorite impacts had a huge effect on the inner solar system at this time,” says Associate Professor O’Neill, “you only need to look at the Moon to see that. What isn’t clear was how our own impact history might have affected the planet’s evolution.”

“We’ve seen evidence of some geological activity that suggests something like subduction acted on the early Earth – but this is hard to reconcile with other geodynamic simulations. But if we consider Earth as part of an evolving early solar system, as opposed to only looking at the planet in isolation, then this evolution starts to make more sense,” he added.

O’Neill also notes that while the magnetic field for much of Earth’s ancient history has been quite low, but recent work has suggested field strengths up to present-day values existed between around 4.0-4.1 billion years ago.
“This is a really important age in the inner solar system. Impacting studies have suggested a big disturbance in the asteroid populations at this time, with perhaps a big upswing in impacts on the Earth. Our simulations show that larger amounts of meteorite collisions with the planet around this time could have driven the subduction process, explaining the formation of many zircons around this period, as well as the increase in magnetic field strength.”

Overall, the study adds evidence towards the fact that meteorite impacts likely had a role in the formation of the Earth that we know today.

“This work shows there is a strong connection between impacts and geophysical evolution capable of drastically altering a planet’s evolution,” said coauthor Dr Simone Marchi from the Southwest Research Institute in the USA.

“One has to wonder, how much of the current Earth, and other terrestrial planets, is the result of collisions that took place eons ago?” Dr Marchi concluded.