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New research published in the journal ‘Science Advances’, has focused their study off the west coast of North America giving seismologists a better understanding of what one scientist describes as “the single greatest geophysical hazard to the continental United States”.
Zach Eilon, a geophysicist at the University of California Santa Barbara, has developed a new method that uses an array of scientific instruments spread across the sea floor to measure shock waves that travel through the planet’s crust. “Because we think this particular phenomenon is strongly related to temperature and to molten rock beneath the Earth, this is a technique that can be applied to volcanoes to get a better sense of their plumbing system,” says Eilon.
Eilon’s research targets the Juan de Fuca plate, which runs several hundred kilometers off the coast between southern British Columbia and northern California and is the youngest and smallest of the planet’s 13 major tectonic plates. The collision zone in this region has the potential to generate massive quakes and destructive tsunamis, which occur when the plates overcome friction and slip past one another, quickly displacing huge amounts of water.
His data suggest the interior of the Juan de Fuca plate is cooler than previously believed, meaning the edge that is being pushed westward below the North American plate is able to bring with it more water. The water acts as a lubricant and increases the likelihood of the slipping that leads to a quake.
Geoff Abers, an earth-sciences professor at Cornell University who co-authored the paper with Eilon, said improvements in sea-floor technology and the sheer number of sensors that were deployed make this project the first time researchers have been able to study an entire tectonic plate in the ocean. “We’re not directly looking at the just earthquake cycles, but we’re looking at the broader, theoretical framework for how the Earth works and getting a much better handle on that,” Abers said.
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As outlined in my article Cosmic Ray Penetration More Prevalent Than Realized, a new study published July 27th in the journal ‘Science’, identifies mantle plumes – viscous molten rock coming from the Earth’s outer core – as the source heated surfaces which include volcanoes and ocean bottom fissures.
For more than 2 decades, scientists have pondered the nature of these mysterious regions, sometimes called Ultra Low Velocity Zones (ULVZs). Researchers examining one below Iceland at a depth of nearly 3000 kilometers, now have their answer. This discovery shows molten plumes that shoot out as roots of hot rock that slowly rise through the mantle to feeding a system of volcanoes and fissures.
Earth scientists have long suspected that upwellings in these mantle convection currents would manifest themselves as the plumes responsible for Earth’s volcanic hot spots. Now we have started to see them with sophisticated computer models that use the waves from large earthquakes to create CT scan–like tomographic pictures of Earth’s interior; says Barbara Romanowicz, a seismologist at the University of California, Berkeley, and led author of the study.
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The Intertropical Convergence Zone (ITCZ), also known as the doldrums, is one of the dramatic features of Earth’s climate system. Prominent enough to be seen from space, the ITCZ appears in satellite images as a band of bright clouds around the tropics. Here, moist warm air accumulates in this atmospheric region near the equator, where the ocean and atmosphere heavily interact. Intense solar radiation and calm, warm ocean waters produce an area of high humidity, ascending air, and rainfall, which is fed by converging trade winds from the Northern and Southern Hemispheres. The convected air forms clusters of thunderstorms characteristic of the ITCZ, releasing heat before moving away from the ITCZ—toward the poles—cooling and descending in the subtropics. This circulation completes the Hadley cells of the ITCZ, which play an important role in balancing Earth’s energy budget—transporting energy between the hemispheres and away from the equator.
However, the position of the ITCZ isn’t static. In order to transport this energy, the ITCZ and Hadley cells shift seasonally between the Northern and Southern Hemispheres, residing in the one that’s most strongly heated from the sun and radiant heat from the Earth’s surface, which on average yearly is the Northern Hemisphere. Accompanying these shifts can be prolonged periods of violent storms or severe drought, which significantly impacts human populations living in its path.
Scientists are therefore keen to understand the climate controls that drive the north-south movement of the ITCZ over the seasonal cycle, as well as on inter-annual to decadal timescales in Earth’s paleoclimatology up through today. Researchers have traditionally approached this issue from the perspective of the atmosphere’s behavior and understanding rainfall, but anecdotal evidence from models with a dynamic ocean has suggested that the ocean’s sensitivity to climate changes could affect the ITCZ’s response. Now, a study from MIT graduate student Brian Green and the Cecil and Ida Green Professor of Oceanography John Marshall from the Program in Atmospheres, Oceans and Climate in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS) published in the American Meteorological Society’s Journal of Climate, investigates the role that the ocean plays in modulating the ITCZ’s position and appreciates its sensitivity when the Northern Hemisphere is heated. In so doing, the work gives climate scientists a better understanding of what causes changes to tropical rainfall.
“In the past decade or so there’s been a lot of research studying controls on the north-south position of the ITCZ, particularly from this energy balance perspective. … And this has normally been done in the context of ignoring the adjustment of the ocean circulation—the ocean circulation is either forcing these [ITCZ] shifts or passively responding to changes in the atmosphere above,” Green says. “But we know, particularly in the tropics, that the ocean circulation is very tightly coupled through the trade winds to atmospheric circulation and the ITCZ position, so what we wanted to do was investigate how that ocean circulation might feedback on the energy balance that controls that ITCZ position, and how strong that feedback might be.”
To examine this, Green and Marshall performed experiments in a global climate model with a coupled atmosphere and ocean, and observed how the ocean circulation’s cross-equatorial energy transport and its associated surface energy fluxes affected the ITCZ’s response when they imposed an inter-hemispheric heating contrast. Using a simplified model that omitted landmasses, clouds, and monsoon dynamics, while keeping a fully circulating atmosphere that interacts with radiation highlighted the ocean’s effect while minimized other confounding variables that could mask the results. The addition of north-south ocean ridges, creating a large and small basin, mimicked the behavior of the Earth’s Atlantic’s meridional overturning circulation and the Pacific Ocean.
Green and Marshall then ran the asymmetrically heated planet simulations in two ocean configurations and compared the ITCZ responses. The first used a stationary “slab ocean,” where the thermal properties were specified so that it mimicked the fully coupled model before perturbation, but was unable to respond to the heating. The second incorporated a dynamic ocean circulation. By forcing the models identically, they could quantify the ocean circulation’s impact on the ITCZ.
“We found in the case where there’s a fully coupled, dynamic ocean, the northward shift of the ITCZ was damped by a factor of four compared to the passive ocean. So that’s hinting that the ocean is one of the leading controls on the position of the ITCZ,” Green says. “It’s a significant damping of the response of the atmosphere, and the reason behind this can just be diagnosed from that energy balance.”
In the dynamic ocean model, they found that when they heat the simulated ocean-covered planet, eddies export some heat into the tropical atmosphere from the extra-tropics, which causes the Hadley cells to respond—the Northern Hemisphere cell to weaken and the Southern Hemisphere cell to strengthen. This transports heat southward through the atmosphere. Concurrently, the ITCZ shifts northward; associated with this are changes in the trade winds—the surface branch of the Hadley cells—and the surface wind stress near the equator. The surface ocean feels this change in winds, which energizes an anomalous ocean circulation and moves water mass southwards across the equator in both hemispheres, carrying heat with it. Once this surface water reaches the extra-tropics, the ocean pumps it downwards where it returns northward across the equator, cooler and at depth. This temperature contrast between the warm surface cross-equatorial flow and the cooler deeper return flow sets the heat transported by the ocean circulation.
“In the slab ocean case, only the atmosphere can move heat across the equator; whereas in our fully coupled case, we see that the ocean is the most strongly compensating component of the system, transporting the majority of the forcing across the equator.” Green says. “So from the atmosphere’s perspective, it doesn’t even feel the full effect of that heating that we’re imposing. And as a result, it has to transport less heat across the equator and shift the ITCZ less.” Green adds that the response of the large basin ocean circulation broadly mimics the Indian Ocean’s yearly average circulation.
Marshall notes that the ability of the wind-driven ocean circulation to damp ITCZ shifts represents a previously unappreciated constraint on the atmosphere’s energy budget: “We showed that the ITCZ cannot move very far away from the equator, even in very ‘extreme’ climates,” indicating that the position of the ITCZ may be much less sensitive to inter-hemispheric heating contrasts than previously thought.”
Green and Marshall are currently expanding upon this work. With the help of David McGee, the Kerr-Mcgee Career Development Assistant Professor in EAPS, and postdoc Eduardo Moreno-Chamarro, the pair are applying this to the paleoclimate record during Heimrich events, when the Earth experiences strong cooling, looking for ITCZ shifts.
They’re also investigating the decomposition of heat and mass transport between the atmosphere and the ocean, as well as between the Earth’s oceans. “The physics that control each of those oceans’ responses are dramatically different, certainly between the Pacific and the Atlantic oceans,” Green says. “Right now, we’re working to understand how the mass transports of the atmosphere and ocean are coupled. While we know that they’re constrained to overturn in the same sense, they’re not actually constrained to transport an identical amount of mass, so you could further enhance or weaken the damping by the ocean circulation by affecting how strongly the mass transports are coupled.”
Hydrogen at elevated temperature creates high electrical conductivity in the Earth’s mantle.
New work by Lawrence Livermore National Laboratory (LLNL) scientists shows the dispersal of water (incorporated as hydrogen in olivine, the most abundant mineral in the upper mantle), could account for high electrical conductivity seen in the asthenosphere (part of the upper mantle just below the lithosphere that is involved in plate tectonic movement). The research appears in Scientific Reports .
The work could lead to a better understanding of present day water distribution in the mantle, which has strong implications for planetary dynamics and evolution. Researchers said such information might provide key evidence as to why Earth is the only known planetary body in our solar system to develop plate tectonics and to retain liquid water oceans on its surface.
“We approached the problem from a different perspective, using new hydrogen diffusion measurements to infer what the contribution of hydrogen would be to electrical conductivity,” said LLNL’s Wyatt Du Frane, the principal investigator on the project. “Our experiments on olivine indicated a larger temperature dependence than previously thought to occur for this phenomenon. The contribution of hydrogen to electrical conductivity, while modest at lower temperatures, becomes quite large at the temperatures expected to occur in the mantle.”
Minerals formed deep in the mantle and transported to the Earth’s surface contain tens to hundreds of parts per million in weight (ppm wt) of water, providing evidence for the presence of dissolved water in the Earth’s interior. Even at these low concentrations, water greatly affects the physico-chemical properties of mantle materials. The diffusion of hydrogen controls the transport of water in the Earth’s upper mantle, but until now was not fully understood for olivine.
Earth’s hydrosphere is a distinctive feature of our planet where massive oceans affect its climate and support its ecosystem. The distribution of water on Earth is not limited to its outermost shell (hydrosphere and hydrated minerals), but extends to great depths within the planet. Downwelling oceanic lithosphere (at subduction zones) and upwelling magmas (at mid ocean ridges, volcanoes and hotspots) are vehicles for transport of H2O between the surface and the Earth’s deep interior.
“The amount of hydrogen required to match geophysical measurements of electrical conductivity inside Earth are in line with the concentrations that are observed in oceanic basalts. This demonstrates that geophysical measurements of electrical conductivity are a promising tool for mapping out water distributions deep inside the Earth,” Du Frane said.
Our Cold War history is now offering scientists a chance to better understand the complex space system that surrounds us. Space weather – which can include changes in Earth’s magnetic environment are usually triggered by the Sun’s activity, but recently declassified data on high-altitude nuclear explosion tests have provided a new look at the mechanisms that set off perturbations in that magnetic system. Such information can help support NASA’s efforts to protect satellites and astronauts from the natural radiation inherent in space.
From 1958 to 1962, the U.S. and U.S.S.R. ran high-altitude tests with exotic code names like Starfish, Argus and Teak. The tests have long since ended, and the goals at the time were military. Today, however, they can provide crucial information on how humans can affect space. The tests, and other human-induced space weather, are the focus of a comprehensive new study published in Space Science Reviews.
“The tests were a human-generated and extreme example of some of the space weather effects frequently caused by the Sun,” said Phil Erickson, assistant director at MIT’s Haystack Observatory, Westford, Massachusetts, and co-author on the paper. “If we understand what happened in the somewhat controlled and extreme event that was caused by one of these man-made events, we can more easily understand the natural variation in the near-space environment.”
By and large, space weather – which affects the region of near-Earth space where astronauts and satellites travel – is typically driven by external factors. The Sun sends out millions of high-energy particles, the solar wind, which races out across the solar system before encountering Earth and its magnetosphere, a protective magnetic field surrounding the planet. Most of the charged particles are deflected, but some make their way into near-Earth space and can impact our satellites by damaging onboard electronics and disrupting communications or navigation signals. These particles, along with electromagnetic energy that accompanies them, can also cause auroras, while changes in the magnetic field can induce currents that damage power grids.
The Cold War tests, which detonated explosives at heights from 16 to 250 miles above the surface, mimicked some of these natural effects. Upon detonation, a first blast wave expelled an expanding fireball of plasma, a hot gas of electrically charged particles. This created a geomagnetic disturbance, which distorted Earth’s magnetic field lines and induced an electric field on the surface.
Some of the tests even created artificial radiation belts, akin to the natural Van Allen radiation belts, a layer of charged particles held in place by Earth’s magnetic fields. The artificially trapped charged particles remained in significant numbers for weeks, and in one case, years. These particles, natural and artificial, can affect electronics on high-flying satellites—in fact some failed as a result of the tests.
Although the induced radiation belts were physically similar to Earth’s natural radiation belts, their trapped particles had different energies. By comparing the energies of the particles, it is possible to distinguish the fission-generated particles and those naturally occurring in the Van Allen belts.
Other tests mimicked other natural phenomena we see in space. The Teak test, which took place on Aug. 1, 1958, was notable for the artificial aurora that resulted. The test was conducted over Johnston Island in the Pacific Ocean. On the same day, the Apia Observatory in Western Samoa observed a highly unusual aurora, which are typically only observed in at the poles. The energetic particles released by the test likely followed Earth’s magnetic field lines to the Polynesian island nation, inducing the aurora. Observing how the tests caused aurora, can provide insight into what the natural auroral mechanisms are too.
Later that same year, when the Argus tests were conducted, effects were seen around the world. These tests were conducted at higher altitudes than previous tests, allowing the particles to travel farther around Earth. Sudden geomagnetic storms were observed from Sweden to Arizona and scientists used the observed time of the events to determine the speed at which the particles from the explosion traveled. They observed two high-speed waves: the first travelled at 1,860 miles per second and the second, less than a fourth that speed. Unlike the artificial radiation belts, these geomagnetic effects were short-lived, lasting only seconds.
Atmospheric nuclear testing has long since stopped, and the present space environment remains dominated by natural phenomena. However, considering such historical events allows scientists and engineers to understand the effects of space weather on our infrastructure and technical systems.
Such information adds to a larger body of heliophysics research, which studies our near-Earth space environment in order to better understand the natural causes of space weather. NASA missions such as Magnetospheric Multiscale (MMS), Van Allen Probes and Time History of Events and Macroscale Interactions during Substorms (THEMIS) study Earth’s magnetosphere and the causes of space weather. Other NASA missions, like STEREO, constantly survey the Sun to look for activity that could trigger space weather. These missions help inform scientists about the complex system we live in, and how to protect the satellites we utilize for communication and navigation on a daily basis.
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.