Earth Probably Began With A Solid Shell

Today’s Earth is a dynamic planet with an outer layer composed of giant plates that grind together, sliding past or dipping beneath one another, giving rise to earthquakes and volcanoes. Others separate at undersea mountain ridges, where molten rock spreads out from the centers of major ocean basins.

But new research suggests that this was not always the case. Instead, shortly after Earth formed and began to cool, the planet’s first outer layer was a single, solid but deformable shell. Later, this shell began to fold and crack more widely, giving rise to modern plate tectonics.

The research, described in a paper published February 27, 2017 in the journal Nature, is the latest salvo in a long-standing debate in the geological research community: did plate tectonics start right away—a theory known as uniformitarianism—or did Earth first go through a long phase with a solid shell covering the entire planet? The new results suggest the solid shell model is closest to what really happened.

“Models for how the first continental crust formed generally fall into two groups: those that invoke modern-style plate tectonics and those that do not,” said Michael Brown, a professor of geology at the University of Maryland and a co-author of the study. “Our research supports the latter—a ‘stagnant lid’ forming the planet’s outer shell early in Earth’s history.”

To reach these conclusions, Brown and his colleagues from Curtin University and the Geological Survey of Western Australia studied rocks collected from the East Pilbara Terrane, a large area of ancient granitic crust located in the state of Western Australia. Rocks here are among the oldest known, ranging from 3.5 to about 2.5 billion years of age. (Earth is roughly 4.5 billion years old.) The researchers specifically selected granites with a chemical composition usually associated with volcanic arcs—a telltale sign of plate tectonic activity.

Brown and his colleagues also looked at basalt rocks from the associated Coucal formation. Basalt is the rock produced when volcanoes erupt, but it also forms the ocean floor, as molten basalt erupts at spreading ridges in the center of ocean basins. In modern-day plate tectonics, when ocean floor basalt reaches the continents, it dips—or subducts—beneath the Earth’s surface, where it generates fluids that allow the overlying mantle to melt and eventually create large masses of granite beneath the surface.

Previous research suggested that the Coucal basalts could be the source rocks for the granites in the Pilbara Terrane, because of the similarities in their chemical composition. Brown and his collaborators set out to verify this, but also to test another long-held assumption: could the Coucal basalts have melted to form granite in some way other than subduction of the basalt beneath Earth’s surface? If so, perhaps plate tectonics was not yet happening when the Pilbara granites formed.

To address this question, the researchers performed thermodynamic calculations to determine the phase equilibria of average Coucal basalt. Phase equilibria are precise descriptions of how a substance behaves under various temperature and pressure conditions, including the temperature at which melting begins, the amount of melt produced and its chemical composition.

For example, one of the simplest phase equilibria diagrams describes the behavior of water: at low temperatures and/or high pressures, water forms solid ice, while at high temperatures and/or low pressures, water forms gaseous steam. Phase equilibria gets a bit more involved with rocks, which have complex chemical compositions that can take on very different mineral combinations and physical characteristics based on temperature and pressure.

“If you take a rock off the shelf and melt it, you can get a phase diagram. But you’re stuck with a fixed chemical composition,” Brown said. “With thermodynamic modeling, you can change the composition, pressure and temperature independently. It’s much more flexible and helps us to answer some questions we can’t address with experiments on rocks.”

Using the Coucal basalts and Pilbara granites as a starting point, Brown and his colleagues constructed a series of modeling experiments to reflect what might have transpired in an ancient Earth without plate tectonics. Their results suggest that, indeed, the Pilbara granites could have formed from the Coucal basalts.
More to the point, this transformation could have occurred in a pressure and temperature scenario consistent with a “stagnant lid,” or a single shell covering the entire planet.

Plate tectonics substantially affects the temperature and pressure of rocks within Earth’s interior. When a slab of rock subducts under the Earth’s surface, the rock starts off relatively cool and takes time to gain heat. By the time it reaches a higher temperature, the rock has also reached a significant depth, which corresponds to high pressure—in the same way a diver experiences higher pressure at greater water depth.

In contrast, a “stagnant lid” regime would be very hot at relatively shallow depths and low pressures. Geologists refer to this as a “high thermal gradient.”

“Our results suggest the Pilbara granites were produced by melting of the Coucal basalts or similar materials in a high thermal gradient environment,” Brown said. “Additionally, the composition of the Coucal basalts indicates that they, too, came from an earlier generation of source rocks. We conclude that a multi-stage process produced Earth’s first continents in a ‘stagnant lid’ scenario before plate tectonics began.”

“Earth’s first stable continents did not form by subduction,” Tim Johnson, Michael Brown, Nicholas Gardiner, Christopher Kirkland and Hugh Smithies, was published February 27, 2017 in the journal Nature.

‘Quartz’ Crystals At Earth’s Core Power Its Magnetic Field

The Earth’s core consists mostly of a huge ball of liquid metal lying at 3000 km beneath its surface, surrounded by a mantle of hot rock. Notably, at such great depths, both the core and mantle are subject to extremely high pressures and temperatures. Furthermore, research indicates that the slow creeping flow of hot buoyant rocks — moving several centimeters per year — carries heat away from the core to the surface, resulting in a very gradual cooling of the core over geological time. However, the degree to which the Earth’s core has cooled since its formation is an area of intense debate amongst Earth scientists.

In 2013 Kei Hirose, now Director of the Earth-Life Science Institute (ELSI) at the Tokyo Institute of Technology (Tokyo Tech), reported that the Earth’s core may have cooled by as much as 1000 degrees Celsius since its formation 4.5 billion years ago. This large amount of cooling would be necessary to sustain the geomagnetic field, unless there was another as yet undiscovered source of energy. These results were a major surprise to the deep Earth community, and created what Peter Olson of Johns Hopkins University referred to as, “the New Core Heat Paradox,” in an article published in Science.

Core cooling and energy sources for the geomagnetic field were not the only difficult issues faced by the team. Another unresolved matter was uncertainty about the chemical composition of the core. “The core is mostly iron and some nickel, but also contains about 10% of light alloys such as silicon, oxygen, sulfur, carbon, hydrogen, and other compounds,” Hirose, lead author of the new study to be published in the journal Nature. “We think that many alloys are simultaneously present, but we don’t know the proportion of each candidate element.”

Now, in this latest research carried out in Hirose’s lab at ELSI, the scientists used precision cut diamonds to squeeze tiny dust-sized samples to the same pressures that exist at the Earth’s core. The high temperatures at the interior of the Earth were created by heating samples with a laser beam. By performing experiments with a range of probable alloy compositions under a variety of conditions, Hirose’s and colleagues are trying to identify the unique behavior of different alloy combinations that match the distinct environment that exists at the Earth’s core.

The search of alloys began to yield useful results when Hirose and his collaborators began mixing more than one alloy. “In the past, most research on iron alloys in the core has focused only on the iron and a single alloy,” says Hirose. “But in these experiments we decided to combine two different alloys containing silicon and oxygen, which we strongly believe exist in the core.”

The researchers were surprised to find that when they examined the samples in an electron microscope, the small amounts of silicon and oxygen in the starting sample had combined together to form silicon dioxide crystals — the same composition as the mineral quartz found at the surface of the Earth.

“This result proved important for understanding the energetics and evolution of the core,” says John Hernlund of ELSI, a co-author of the study. “We were excited because our calculations showed that crystallization of silicon dioxide crystals from the core could provide an immense new energy source for powering the Earth’s magnetic field.” The additional boost it provides is plenty enough to solve Olson’s paradox.

The team has also explored the implications of these results for the formation of the Earth and conditions in the early Solar System. Crystallization changes the composition of the core by removing dissolved silicon and oxygen gradually over time. Eventually the process of crystallization will stop when then core runs out of its ancient inventory of either silicon or oxygen.

“Even if you have silicon present, you can’t make silicon dioxide crystals without also having some oxygen available,” says ELSI scientist George Helffrich, who modeled the crystallization process for this study. “But this gives us clues about the original concentration of oxygen and silicon in the core, because only some silicon:oxygen ratios are compatible with this model.”

Experiments Call Origin Of Earth’s Iron Into Question

New research from The University of Texas at Austin reveals that the Earth’s unique iron composition isn’t linked to the formation of the planet’s core, calling into question a prevailing theory about the events that shaped our planet during its earliest years.

The research, published in Nature Communications on Feb. 20, opens the door for other competing theories about why the Earth, relative to other planets, has higher levels of heavy iron isotopes. Among them: light iron isotopes may have been vaporized into space by a large impact with another planet that formed the moon; the slow churning of the mantle as it makes and recycles the Earth’s crust may preferentially incorporate heavy iron into rock; or, the composition of the raw material that formed the planet in its earliest days may have been enriched with heavy iron.

An isotope is a variety of atom that has a different weight from other atoms of the same element because it has a different numbers of neutrons.

“The Earth’s core formation was probably the biggest event affecting Earth’s history. Materials that make up the whole Earth were melted and differentiated,” said Jung-Fu Lin, a professor at the UT Jackson School of Geosciences and one of the study’s authors. “But in this study, we say that there must be other origins for Earth’s iron isotope anomaly.”

Jin Liu, now a postdoctoral researcher at Stanford University, led the research while earning his Ph.D. at the Jackson School. Collaborators include scientists from The University of Chicago, Sorbonne Universities in France, Argonne National Laboratory, the Center for High Pressure Science and Advanced Technology Research in China, and the University of Illinois at Urbana-Champaign.

Rock samples from other planetary bodies and objects — ranging from the moon, to Mars, to ancient meteorites called chondrites — all share about the same ratio of heavy to light iron isotopes. In comparison to these samples from space, rocks from Earth have about 0.01 percent more heavy iron isotopes than light isotopes.

That might not sound like much, but Lin said it’s significant enough to make the Earth’s iron composition unique among known worlds.

“This 0.01 percent anomaly is very significant compared with, say, chondrites,” Lin said. “This significant difference thus represents a different source or origin of our planet.”

Lin said that one of the most popular theories to explain the Earth’s iron signature is that the relatively large size of the planet (compared with other rocky bodies in the solar system) created high pressure and high temperature conditions during core formation that made different proportions of heavy and light iron isotopes accumulate in the core and mantle. This resulted in a larger share of heavy iron isotopes bonding with elements that make up the rocky mantle, while lighter iron isotopes bonded together and with other trace metals to form the Earth’s core.

But when the research team used a diamond anvil to subject small samples of metal alloys and silicate rocks to core formation pressures, they not only found that the iron isotopes stayed put, but that the bonds between iron and other elements got stronger. Instead of breaking and rebonding with common mantle or core elements, the initial bond configuration got sturdier.

“Our high pressure studies find that iron isotopic fractionation between silicate mantle and metal core is minimal,” said Liu, the lead author.

Co-author Nicolas Dauphas, a professor at the University of Chicago, emphasized that analyzing the atomic scale measurements was a feat unto itself.

“One has to use sophisticated mathematical techniques to make sense of the measurements,” he said. “It took a dream team to pull this off.”

Helen Williams, a geology lecturer at the University of Cambridge, said it’s difficult to know the physical conditions of Earth’s core formation, but that the high pressures in the experiment make for a more realistic simulation.

“This is a really elegant study using a highly novel approach that confirms older experimental results and extends them to much higher pressures appropriate to the likely conditions of core-mantle equilibrium on Earth,” Williams said.

Lin said it will take more research to uncover the reason for the Earth’s unique iron signature, and that experiments that approximate early conditions on Earth will play a key role because rocks from the core are impossible to attain.

New Study Reaffirms Fluctuation of Earth’s Magnetic Field Prior to Full Reversal

A team of researchers from Tel Aviv University, The Hebrew University and the University of California has used ancient jar handles to chart the strength of the Earth’s magnetic field over a 600-year period. In their paper published in Proceedings of the National Academy of Sciences, the team describes how they were able to accurately date the jar handles, which allowed for measuring the geomagnetic field over time.

The geomagnetic field shields life on Earth from a constant stream of cosmic radiation. In this new effort, the researchers sought to learn more about the intensity of the field over time using ancient evidence and to apply this information to understanding how it might behave in the future.

As the team explains, iron oxide particles embedded in clay used to make jars can be used as a measuring device because they become fixed in alignment while the clay is still soft due to the geomagnetic field – once the jar undergoes firing, the particles remain frozen in place. In addition, ancient jar makers stamped and inscribed the handles for tax purposes, leaving clear clues about when they were made.

Thus, to create a single measurement, the researchers would date a given jar handle using historical texts, then examine the iron oxide particles to get a reading regarding magnetic strength. By repeating this process for jar handles created between the 6th and 2nd centuries BCE, the team was able to create a magnetic field strength timeline.

The researchers report that the jar handles revealed a gradual reduction in field strength over the course of the six centuries under study, and that there were also spikes and drops in field strength during some time periods. They found, for example, that field strength spiked at the end of the 8th century BEC, and then sagged again afterwards, losing approximately 27 percent of its strength.

These fluctuations, the team suggests, indicate that we do not need to be worried about the weakening field that has been observed over the past 180 years-they believe it represents normal fluctuations. The new data may also help planet scientists better understand the nature of the geomagnetic field and to answer some questions, such as why fluctuations and changes in direction occur.

Earth’s Hottest, Most Buoyant Mantle Plumes Draw From A Primordial Reservoir Older Than The Moon

Earth’s mantle — the layer between the crust and the outer core — is home to a primordial soup even older than the moon. Among the main ingredients is helium-3 (He-3), a vestige of the Big Bang and nuclear fusion reactions in stars. And the mantle is its only terrestrial source.

Scientists studying volcanic hotspots have strong evidence of this, finding high helium-3 relative to helium-4 in some plumes, the upwellings from Earth’s deep mantle. Primordial reservoirs in the deep Earth, sampled by a small number of volcanic hotspots globally, have this ancient He-3/4 signature.

Inspired by a 2012 paper that proposed a correlation between such hotspots and the velocity of seismic waves moving through Earth’s interior, UC Santa Barbara geochemist Matthew Jackson teamed with the authors of the original paper — Thorsten Becker of the University of Texas at Austin and Jasper Konter of the University of Hawaii — to show that only the hottest hotspots with the slowest wave velocity draw from the primitive reservoir formed early in the planet’s history. Their findings appear in the journal Nature.

“We used the seismology of the shallow mantle — the rate at which seismic waves travel through Earth below its crust — to make inferences about the deeper mantle,” said Jackson, an assistant professor in UCSB’s Department of Earth Science. “At 200 km, the shallow mantle has the largest variability of seismic velocities — more than 6 percent, which is a lot. What’s more, that variability, which we hypothesize relates to temperature, correlates with He-3.”

For their study, the researchers used the latest seismic models of Earth’s velocity structure and 35 years of helium data. When they compared oceanic hotspots with high levels of He-3/4 to seismic wave velocities, they found that these represent the hottest hotspots, with seismic waves that move more slowly than they do in cooler areas. They also analyzed hotspot buoyancy flux, which can be used to measure how much melt a particular hotspot produces. In Hawaii, the Galapagos Islands, Samoa and Easter Island as well as in Iceland, hotspots had high buoyancy levels, confirming a basic rule of physics: the hotter, the more buoyant.

“We found that the higher the hotspot buoyancy flux, the more melt a hotspot was producing and the more likely it was to have high He-3/4,” Jackson said. “Hotter plumes not only have slower seismic velocity and a higher hotspot buoyancy flux, they also are the ones with the highest He-3/4. This all ties together nicely and is the first time that He-3/4 has been correlated with shallow mantle velocities and hotspot buoyancy globally.”

Becker noted that correlation does not imply causality, “but it is pretty nifty that we found two strong correlations, which both point to the same physically plausible mechanism: the primordial stuff gets picked up preferentially by the most buoyant thermochemical upwellings.”

The authors also wanted to know why only the hottest, most buoyant plumes sample high He-3/4.

“The explanation that we came up with — which people who do numerical simulations have been suggesting for a long time — is that whatever this reservoir is with primitive helium, it must be really dense so that only the hottest, most buoyant plumes can entrain some of it to the surface,” Jackson said. “That makes sense and it also explains how something so ancient could survive in the chaotically convecting mantle for 4.5 billion years. The density contrast makes it more likely that the ancient helium reservoir is preserved rather than mixed away.”

“Since this correlation of geochemistry and seismology now holds from helium isotopes in this work to the compositions we examined in 2012, it appears that overall hotspot geochemical variations will need to be re-examined from the perspective of buoyancy,” Konter concluded.

New Ocean Observations Improve Understanding of Motion

Oceanographers commonly calculate large scale surface ocean circulation from satellite sea level information using a concept called “geostrophy“, which describes the relationship between oceanic surface flows and sea level gradient. Conversely, researchers rely on data from in-water current meters to measure smaller scale motion.

New research led by University of Hawai’i at Mānoa (UHM) oceanographer Bo Qiu has determined from observational data the length scale at which using sea level height no longer offers a reliable calculation of circulation.

Upper-ocean processes dissipate heat, transport nutrients and impact the uptake of carbon dioxide—making circulation a critical driver of biological activity in the ocean. The movement of water in the ocean is determined by many factors including tides; winds; surface waves; internal waves, those that propagate within the layers of the ocean; and differences in temperature, salinity or sea level height. Additionally, like high and low pressure systems seen on TV weather maps, the ocean is full of eddies, slowly swirling masses of water.

“As length scales become smaller from several hundred miles to a few tens of miles, we discovered the point at which geostrophic balance becomes no longer valid—meaning that sea level is no longer useful for calculating ocean circulation,” said Qiu, professor at the UHM School of Ocean and Earth Science and Technology (SOEST). “That is due to the presence of oceanic internal wave motions which essentially disrupts the motion that would be caused by geostrophy.”

Scientists use sea level as a means to calculate ocean circulation because satellites circle Earth daily, acquiring sea level data frequently and accurately. Prior to this study, published in Nature Communications, oceanographers knew that sea level can be used to provide a picture of circulation in a general way but not in very fine detail. However, the specific level of detail that can be provided using this approach was not known, until this study.

Further, in areas of the ocean with persistent or frequent eddies, Qiu and co-authors from the Japan Meteorological Agency, Caltech and NASA Jet Propulsion Laboratory determined that sea level can reliably be used to calculate circulation at a fairly high resolution, that is, at fairly small length scales (resolution of 10 miles). However, in areas where motion is dominated by internal waves, satellite sea level can only be used to infer motion on a very large scale (resolution of 125 miles).

“This aspect of the study was a bit of a surprise,” said Qiu. “I didn’t anticipate that the transition point would vary by an order of magnitude within the western North Pacific.”

In the future, Qiu and colleagues hope to develop a mathematical approach to creating more detailed pictures of circulation based on sea level in more locations throughout the Pacific.

The Ancient Indus Civilization in India Adaptation to Climate Change

New research methods and technologies are able to shed light on climate patterns that took place thousands of years ago, giving us a new perspective on how cultures of the time coped with variable and changing environments.

A new article in the February issue of Current Anthropology explores the dynamics of adaptation and resilience in the face of a diverse and varied environmental context, using the case study of South Asia’s Indus Civilization (c.3000-1300 BC). Integrating research carried out as part of the Land, Water and Settlement project—part of an ongoing collaboration between the University of Cambridge and Banaras Hindu University—that worked in northwest India between 2007 and 2014, the article looks at how Indus populations in north-west India interacted with their environment, and considers how that environment changed during periods of climate change.

Lead author, Dr. Cameron Petrie of the Division of Archaeology, University of Cambridge notes that “for most ancient complex societies, water was a critical factor, and the availability of water and the way that it was managed and used provide critical insight into human adaptation and the resilience of subsistence practices”.

Most early complex societies developed in regions where the climatic parameters faced by ancient subsistence farmers were varied, but not especially diverse. The Indus Civilization developed in a specific environmental context, where the winter and summer rainfall systems overlapped. There is now evidence to show that this region was subject to climate change during the period when the Indus Civilization was at its height (c.2500-1900 BC). The Indus Civilization therefore provides a unique opportunity to understand how an ancient society coped with diverse and varied ecologies and change in the fundamental and underlying environmental parameters.

In the early Holocene, the Indus Civilization was situated in proximity to Kotla Dahar, a deep lake, implying regular and consistent rainfall input to offset evaporation, which given its location, would have been primarily monsoonal. The lake showed evidence for two dramatic decreases in monsoon rainfall and a progressive lowering of the lake level. The second of these shows Kotla Dahar becoming completely ephemeral ca. 2200-2000 BC as a result of an abrupt weakening of the monsoon, and the weakening of the monsoon is visible in speleothem records in Oman and northeast India. The proximity of the Kotla Dahar record to the area occupied by Indus populations shows that climate must be formally considered as a contributing parameter in the process of Indus deurbanization, at least in the context of the plains of northwest India.

It has long been hypothesized that there was variation in the subsistence practices used by Indus populations and this fits with the theme of coping with diverse environments. Petrie comments that “we argue that rather than being forced to intensify or diversify subsistence practices in response to climatic change, we have evidence for the use of millet, rice, and tropical pulses in the pre-urban and urban phases of the Indus Civilization. This evidence suggests that local Indus populations were already well adapted to living in varied and variable environmental conditions before the development of urban centers. It is also possible that these adaptations were beneficial when these populations were faced with changes to the local environment that were probably beyond the range of variation that they typically encountered”.