Asteroid impacts on Earth make structurally bizarre diamonds

Diamond grains from the Canyon Diablo meteorite are shown. The tick marks are spaced one-fifth of a millimeter (200 microns) apart. -  Arizona State University/Laurence Garvie
Diamond grains from the Canyon Diablo meteorite are shown. The tick marks are spaced one-fifth of a millimeter (200 microns) apart. – Arizona State University/Laurence Garvie

Scientists have argued for half a century about the existence of a form of diamond called lonsdaleite, which is associated with impacts by meteorites and asteroids. A group of scientists based mostly at Arizona State University now show that what has been called lonsdaleite is in fact a structurally disordered form of ordinary diamond.

The scientists’ report is published in Nature Communications, Nov. 20, by Péter Németh, a former ASU visiting researcher (now with the Research Centre of Natural Sciences of the Hungarian Academy of Sciences), together with ASU’s Laurence Garvie, Toshihiro Aoki and Peter Buseck, plus Natalia Dubrovinskaia and Leonid Dubrovinsky from the University of Bayreuth in Germany. Buseck and Garvie are with ASU’s School of Earth and Space Exploration, while Aoki is with ASU’s LeRoy Eyring Center for Solid State Science.

“So-called lonsdaleite is actually the long-familiar cubic form of diamond, but it’s full of defects,” says Péter Németh. These can occur, he explains, due to shock metamorphism, plastic deformation or unequilibrated crystal growth.

The lonsdaleite story began almost 50 years ago. Scientists reported that a large meteorite, called Canyon Diablo after the crater it formed on impact in northern Arizona, contained a new form of diamond with a hexagonal structure. They described it as an impact-related mineral and called it lonsdaleite, after Dame Kathleen Lonsdale, a famous crystallographer.

Since then, “lonsdaleite” has been widely used by scientists as an indicator of ancient asteroidal impacts on Earth, including those linked to mass extinctions. In addition, it has been thought to have mechanical properties superior to ordinary diamond, giving it high potential industrial significance. All this focused much interest on the mineral, although pure crystals of it, even tiny ones, have never been found or synthesized. That posed a long-standing puzzle.

The ASU scientists approached the question by re-examining Canyon Diablo diamonds and investigating laboratory samples prepared under conditions in which lonsdaleite has been reported.

Using the advanced electron microscopes in ASU’s Center for Solid State Science, the team discovered, both in the Canyon Diablo and the synthetic samples, new types of diamond twins and nanometer-scale structural complexity. These give rise to features attributed to lonsdaleite.

“Most crystals have regular repeating structures, much like the bricks in a well-built wall,” says Peter Buseck. However, interruptions can occur in the regularity, and these are called defects. “Defects are intermixed with the normal diamond structure, just as if the wall had an occasional half-brick or longer brick or row of bricks that’s slightly displaced to one side or another.”

The outcome of the new work is that so-called lonsdaleite is the same as the regular cubic form of diamond, but it has been subjected to shock or pressure that caused defects within the crystal structure.

One consequence of the new work is that many scientific studies based on the presumption that lonsdaleite is a separate type of diamond need to be re-examined. The study implies that both shock and static compression can produce an intensely defective diamond structure.

The new discovery also suggests that the observed structural complexity of the Canyon Diablo diamond results in interesting mechanical properties. It could be a candidate for a product with exceptional hardness.

The School of Earth and Space Exploration is an academic unit of ASU’s College of Liberal Arts and Sciences.

Good vibrations give electrons excitations that rock an insulator to go metallic

Vanadium atoms (blue) have unusually large thermal vibrations that stabilize the metallic state of a vanadium dioxide crystal. Red depicts oxygen atoms. -  ORNL
Vanadium atoms (blue) have unusually large thermal vibrations that stabilize the metallic state of a vanadium dioxide crystal. Red depicts oxygen atoms. – ORNL

For more than 50 years, scientists have debated what turns particular oxide insulators, in which electrons barely move, into metals, in which electrons flow freely. Some scientists sided with Nobel Prize-winning physicist Nevill Mott in thinking direct interactions between electrons were the key. Others believed, as did physicist Rudolf Peierls, that atomic vibrations and distortions trumped all. Now, a team led by the Department of Energy’s Oak Ridge National Laboratory has made an important advancement in understanding a classic transition-metal oxide, vanadium dioxide, by quantifying the thermodynamic forces driving the transformation. The results are published in the Nov. 10 advance online issue of Nature.

“We proved that phonons–the vibrations of the atoms–provide the driving force that stabilizes the metal phase when the material is heated,” said John Budai, who co-led the study with Jiawang Hong, a colleague in ORNL’s Materials Science and Technology Division.

Hong added, “This insight into how lattice vibrations can control phase stability in transition-metal oxides is needed to improve the performance of many multifunctional materials, including colossal magnetoresistors, superconductors and ferroelectrics.”

Today vanadium dioxide improves recording and storage media, strengthens structural alloys, and colors synthetic jewels. Tomorrow it may find its way into nanoscale actuators for switches, optical shutters that turn opaque on satellites to thwart intruding signals, and field-effect transistors to manipulate electronics in semiconductors and spintronics in devices that manipulate magnetic spin.

The next application we see may be energy-efficient “smart windows” coated with vanadium dioxide peppered with an impurity to control the transmission of heat and light. On cool days, windows would be transparent insulators that let in heat. On warm days, they would turn shiny and reflect the outside heat.

Complete thermodynamics


Materials are stabilized by a competition between internal energy and entropy (a measure of disorder that increases with temperature). While Mott and Peierls focused on energy, the ORNL-led team focused on the entropy.

Before the ORNL-led experiments, scientists knew the total amount of heat absorbed during vanadium dioxide’s transition from insulator to metal. But they didn’t know how much entropy was due to electrons and how much was due to atomic vibrations.

“This is the first complete description of thermodynamic forces controlling this archetypical metal-insulator transition,” said Budai.

The team’s current accomplishment was made possible by a novel combination of X-ray and neutron scattering tools, developed within the decade, that enabled lattice dynamics measurements and a calculation technique that Olle Hellman of Linköping University in Sweden recently developed to capture anharmonicity (a measure of nonlinearity in bond forces between atoms). It’s especially important that the calculations, performed by Hong, agree well with experiments because they can now be used to make new predictions for other materials.

The ORNL team came up with the idea to measure “incoherent” neutron scattering (each atom scatters independently) at ORNL’s Spallation Neutron Source (SNS) to determine the phonon spectra at many temperatures, and to measure coherent inelastic and diffuse X-ray scattering at Argonne National Laboratory’s Advanced Photon Source (APS) to probe collective vibrations in pristine crystals. Neutron measurements were enabled by the SNS’s large neutron flux, and X-ray measurements benefited from the high-resolution enabled by the high APS brightness. SNS and APS are DOE Office of Science User Facilities.

Among ORNL collaborators, Robert McQueeney made preliminary X-ray measurements and Lynn Boatner grew crystals for the experiment. Eliot Specht mapped phonon dispersions with diffuse X-ray scattering. Michael Manley and Olivier Delaire determined the phonon spectra using inelastic neutron scattering. Postdoctoral researcher Chen Li helped make experimental measurements and provided neutron expertise. Douglas Abernathy provided expertise with experimental beam lines, as did Argonne’s Ayman Said, Bogdan Leu and Jonathan Tischler.

Their measurements revealed that phonons with unusually large atomic vibrations and strong anharmonicity are responsible for about two-thirds of the total heat that each atom transfers during the lattice’s transition to a metallic phase.

“The entropy of the lattice vibrations competes against and overcomes the electronic energy, and that’s why the metallic phase is stabilized at high temperatures in vanadium dioxide,” Budai summed up. “Using comprehensive measurements and new calculations, we’re the first to close this gap and present convincing arguments for the dominant influence of low-energy, strongly anharmonic phonons.”

Atomic underpinnings


The findings reveal that the vanadium-dioxide lattice is anharmonic in the metal state. Think of atoms connected by bonds in a lattice as masses connected by springs. Pull on a mass and let go; it bounces. If the force is proportional to the distance a mass is pulled, the interaction is harmonic. Vanadium dioxide’s anharmonicity greatly complicates the way the lattice wiggles upon heating.

“A material that only had harmonic connections between atoms would have no thermal expansion; if you heat it up, it would stay the same size,” said Budai. Most materials, it turns out, are somewhat anharmonic. Metals, for example, expand when heated.

When heated to 340 kelvin (just above room temperature), vanadium dioxide turns from insulator to metal. Below 340 K, its lowest-energy lattice configuration is akin to a leaning cardboard box. Above 340 K, where entropy due to phonon vibrations dominates, its preferred state has all bond angles at 90 degrees. The phase change is fully reversible, so cooling a metal below the transition temperature reverts it to an insulator, and heating it past this point turns it metallic.

In metallic vanadium dioxide, each vanadium atom has one electron that is free to roam. In contrast, in insulating vanadium dioxide, that electron gets trapped in a chemical bond that forms vanadium dimers. “For understanding the atomic mechanisms, we needed theory,” Budai said.

That’s where Hong, a theorist at ORNL’s Center for Accelerating Materials Modeling, made critical contributions with quantum molecular dynamics calculations. He ran large-scale simulations at the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility at Lawrence Berkeley National Laboratory, using 1 million computing-core hours to simulate the lattice dynamics of metal and insulator phases of vanadium dioxide. All three types of experiments agreed well with Hong’s simulations. In addition, his calculation further reveals how phonon and electron contributions compete in the different phases.

Predicting new materials


“The theory not only provides us deep understanding of the experimental observations and reveals fundamental principles behind them,” said Hong, “but also gives us predictive modeling, which will accelerate fundamental and technological innovation by giving efficient strategies to design new materials with remarkable properties.”

Many other materials besides vanadium dioxide show a metal-to-insulator transition; however, the detailed role of lattice vibrations in controlling phase stability remains largely unknown. In future studies of other transition metal oxides, the researchers will continue to investigate the impact of anharmonic phonons on physical properties such as electrical conductivity and thermal transport. This fundamental research will help guide the development of improved energy-efficient materials.

Early Earth less hellish than previously thought

Calvin Miller is shown at the Kerlingarfjoll volcano in central Iceland. Some geologists have proposed that the early Earth may have resembled regions like this. -  Tamara Carley
Calvin Miller is shown at the Kerlingarfjoll volcano in central Iceland. Some geologists have proposed that the early Earth may have resembled regions like this. – Tamara Carley

Conditions on Earth for the first 500 million years after it formed may have been surprisingly similar to the present day, complete with oceans, continents and active crustal plates.

This alternate view of Earth’s first geologic eon, called the Hadean, has gained substantial new support from the first detailed comparison of zircon crystals that formed more than 4 billion years ago with those formed contemporaneously in Iceland, which has been proposed as a possible geological analog for early Earth.

The study was conducted by a team of geologists directed by Calvin Miller, the William R. Kenan Jr. Professor of Earth and Environmental Sciences at Vanderbilt University, and published online this weekend by the journal Earth and Planetary Science Letters in a paper titled, “Iceland is not a magmatic analog for the Hadean: Evidence from the zircon record.”

From the early 20th century up through the 1980’s, geologists generally agreed that conditions during the Hadean period were utterly hostile to life. Inability to find rock formations from the period led them to conclude that early Earth was hellishly hot, either entirely molten or subject to such intense asteroid bombardment that any rocks that formed were rapidly remelted. As a result, they pictured the surface of the Earth as covered by a giant “magma ocean.”

This perception began to change about 30 years ago when geologists discovered zircon crystals (a mineral typically associated with granite) with ages exceeding 4 billion years old preserved in younger sandstones. These ancient zircons opened the door for exploration of the Earth’s earliest crust. In addition to the radiometric dating techniques that revealed the ages of these ancient zircons, geologists used other analytical techniques to extract information about the environment in which the crystals formed, including the temperature and whether water was present.

Since then zircon studies have revealed that the Hadean Earth was not the uniformly hellish place previously imagined, but during some periods possessed an established crust cool enough so that surface water could form – possibly on the scale of oceans.

Accepting that the early Earth had a solid crust and liquid water (at least at times), scientists have continued to debate the nature of that crust and the processes that were active at that time: How similar was the Hadean Earth to what we see today?

Two schools of thought have emerged: One argues that Hadean Earth was surprisingly similar to the present day. The other maintains that, although it was less hostile than formerly believed, early Earth was nonetheless a foreign-seeming and formidable place, similar to the hottest, most extreme, geologic environments of today. A popular analog is Iceland, where substantial amounts of crust are forming from basaltic magma that is much hotter than the magmas that built most of Earth’s current continental crust.

“We reasoned that the only concrete evidence for what the Hadean was like came from the only known survivors: zircon crystals – and yet no one had investigated Icelandic zircon to compare their telltale compositions to those that are more than 4 billion years old, or with zircon from other modern environments,” said Miller.

In 2009, Vanderbilt doctoral student Tamara Carley, who has just accepted the position of assistant professor at Layfayette College, began collecting samples from volcanoes and sands derived from erosion of Icelandic volcanoes. She separated thousands of zircon crystals from the samples, which cover the island’s regional diversity and represent its 18 million year history.

Working with Miller and doctoral student Abraham Padilla at Vanderbilt, Joe Wooden at Stanford University, Axel Schmitt and Rita Economos from UCLA, Ilya Bindeman at the University of Oregon and Brennan Jordan at the University of South Dakota, Carley analyzed about 1,000 zircon crystals for their age and elemental and isotopic compositions. She then searched the literature for all comparable analyses of Hadean zircon and for representative analyses of zircon from other modern environments.

“We discovered that Icelandic zircons are quite distinctive from crystals formed in other locations on modern Earth. We also found that they formed in magmas that are remarkably different from those in which the Hadean zircons grew,” said Carley.

Most importantly, their analysis found that Icelandic zircons grew from much hotter magmas than Hadean zircons. Although surface water played an important role in the generation of both Icelandic and Hadean crystals, in the Icelandic case the water was extremely hot when it interacted with the source rocks while the Hadean water-rock interactions were at significantly lower temperatures.

“Our conclusion is counterintuitive,” said Miller. “Hadean zircons grew from magmas rather similar to those formed in modern subduction zones, but apparently even ‘cooler’ and ‘wetter’ than those being produced today.”

Birth of a mineral

<IMG SRC="/Images/904289364.jpg" WIDTH="350" HEIGHT="233" BORDER="0" ALT="An aragonite crystal — with its characteristic 'sheaf of wheat' look — consumed a particle of amorphous calcium carbonate as it formed. – Nielsen et al. 2014/Science“>
An aragonite crystal — with its characteristic ‘sheaf of wheat’ look — consumed a particle of amorphous calcium carbonate as it formed. – Nielsen et al. 2014/Science

One of the most important molecules on earth, calcium carbonate crystallizes into chalk, shells and minerals the world over. In a study led by the Department of Energy’s Pacific Northwest National Laboratory, researchers used a powerful microscope that allows them to see the birth of crystals in real time, giving them a peek at how different calcium carbonate crystals form, they report in September 5 issue of Science.

The results might help scientists understand how to lock carbon dioxide out of the atmosphere as well as how to better reconstruct ancient climates.

“Carbonates are most important for what they represent, interactions between biology and Earth,” said lead researcher James De Yoreo, a materials scientist at PNNL. “For a decade, we’ve been studying the formation pathways of carbonates using high-powered microscopes, but we hadn’t had the tools to watch the crystals form in real time. Now we know the pathways are far more complicated than envisioned in the models established in the twentieth century.”

Earth’s Reserve


Calcium carbonate is the largest reservoir of carbon on the planet. It is found in rocks the world over, shells of both land- and water-dwelling creatures, and pearls, coral, marble and limestone. When carbon resides within calcium carbonate, it is not hanging out in the atmosphere as carbon dioxide, warming the world. Understanding how calcium carbonate turns into various minerals could help scientists control its formation to keep carbon dioxide from getting into the atmosphere.

Calcium carbonate deposits also contain a record of Earth’s history. Researchers reconstructing ancient climates delve into the mineral for a record of temperature and atmospheric composition, environmental conditions and the state of the ocean at the time those minerals formed. A better understanding of its formation pathways will likely provide insights into those events.

To get a handle on mineral formation, researchers at PNNL, the University of California, Berkeley, and Lawrence Berkeley National Laboratory examined the earliest step to becoming a mineral, called nucleation. In nucleation, molecules assemble into a tiny crystal that then grows with great speed. Nucleation has been difficult to study because it happens suddenly and unpredictably, so the scientists needed a microscope that could watch the process in real time.

Come to Order


In the 20th century, researchers established a theory that crystals formed in an orderly fashion. Once the ordered nucleus formed, more molecules added to the crystal, growing the mineral but not changing its structure. Recently, however, scientists have wondered if the process might be more complicated, with other things contributing to mineral formation. For example, in previous experiments they’ve seen forms of calcium carbonate that appear to be dense liquids that could be sources for minerals.

Researchers have also wondered if calcite forms from less stable varieties or directly from calcium and carbonate dissolved in the liquid. Aragonite and vaterite are calcium carbonate minerals with slightly different crystal architectures than calcite and could represent a step in calcite’s formation. The fourth form called amorphous calcium carbonate – or ACC, which could be liquid or solid, might also be a reservoir for sprouting minerals.

To find out, the team created a miniature lab under a transmission electron microscope at the Molecular Foundry, a DOE Office of Science User Facility at LBNL. In this miniature lab, they mixed sodium bicarbonate (used to make club soda) and calcium chloride (similar to table salt) in water. At high enough concentrations, crystals grew. Videos of nucleating and growing crystals recorded what happened [URLs to come].

Morphing Minerals


The videos revealed that mineral growth took many pathways. Some crystals formed through a two-step process. For example, droplet-like particles of ACC formed, then crystals of aragonite or vaterite appeared on the surface of the droplets. As the new crystals formed, they consumed the calcium carbonate within the drop on which they nucleated.

Other crystals formed directly from the solution, appearing by themselves far away from any ACC particles. Multiple forms often nucleated in a single experiment — at least one calcite crystal formed on top of an aragonite crystal while vaterite crystals grew nearby.

What the team didn’t see in and among the many options, however, was calcite forming from ACC even though researchers widely expect it to happen. Whether that means it never does, De Yoreo can’t say for certain. But after looking at hundreds of nucleation events, he said it is a very unlikely event.

“This is the first time we have directly visualized the formation process,” said De Yoreo. “We observed many pathways happening simultaneously. And they happened randomly. We were never able to predict what was going to come up next. In order to control the process, we’d need to introduce some kind of template that can direct which crystal forms and where.”

In future work, De Yoreo and colleagues plan to investigate how living organisms control the nucleation process to build their shells and pearls. Biological organisms keep a store of mineral components in their cells and have evolved ways to make nucleation happen when and where needed. The team is curious to know how they use cellular molecules to achieve this control.




Video
Click on this image to view the .mp4 video
Diamond-shaped crystals of calcite form directly from solution. A round particle that could be either amorphous calcium carbonate or vaterite forms nearby. – Nielsen et al. 2014/Science

How much magma is hiding beneath our feet?

Molten rock (or magma) has a strong influence on our planet and its inhabitants, causing destructive volcanic eruptions and generating some of the giant mineral deposits. Our understanding of these phenomena is, however, limited by the fact that most magma cools and solidifies several kilometres beneath our feet, only to be exposed at the surface, millions of years later, by erosion. Scientists have never been able to track the movements of magma at such great depths? that is, until a team from the University of Geneva (UNIGE) discovered an innovative technique, details of which will be published in the next issue of the journal Nature.

It is a story of three scientists: a modelling specialist, an expert in a tiny mineral known as “zircon”, and a volcanologist. Following a casual conversation, the researchers stumbled upon an idea, and eventually a new method to estimate the volume and flow of magma required for the construction of magma chambers was shaped. The technique they developed makes it possible to refine predictions of future volcanic eruptions as well as identifying areas of the planet that are rich in magma-related natural resources.

Zircon: a valuable mineral for scientists

Professor Urs Schaltegger has been studying zircon for more than ten years in his laboratory at UNIGE, one of the world’s few labs in this field. «The zircon crystals that are found in solidified magma hold key information about the injection of molten rock into a magma chamber before it freezes underground,» explains the professor. Zircon contains radioactive elements that enable researchers to determine its age. As part of the study, the team from the Section of Earth and Environmental Sciences of UNIGE paired data collected using natural samples and numerical simulation. As Guy Simpson, a researcher at UNIGE further explains: «Modelling meant that we could establish how the age of crystallised zircon in a cooled magma reservoir depends on the flow rate of injected magma and the size of the reservoir.»

Applications for society and industry


In the Nature article, the researchers propose a model that is capable of determining with unprecedented accuracy the age, volume and injection rate of magma that has accumulated at inaccessible depths. As a result, they have established that the formation of Earth’s crust, volcanic super eruptions and mineral deposits occur under very specific yet different conditions. Professor Luca Caricchi adds: «When we determine the age of a family of zircons from a small sample of solidified magmatic rock, using results from the mathematical model we have developed, we can tell what the size of the entire magma chamber was, as well as how fast the magma reservoir grew». The professor continues: «This information means that we can determine the probability of an explosive volcanic eruption of a certain size to occur. In addition, the model will be of interest to industry because we will be able to identify new areas of our planet that are home to large amounts of natural resources such as copper and gold.»

New view of Rainier’s volcanic plumbing

This image was made by measuring how the ground conducts or resists electricity in a study co-authored by geophysicist Phil Wannamaker of the University of Utah Energy & Geoscience Institute. It  shows the underground plumbing system that provides molten and partly molten rock to the magma chamber beneath the Mount Rainier volcano in Washington state. The scale at left is miles depth. The scale at bottom is miles from the Pacific Coast. The Juan de Fuca plate of Earth's Pacific seafloor crust and upper mantle is shown in blue on the left half of the image as it dives or 
'subducts' eastward beneath Washington state. The reddish orange and yellow colors represent molten and partly molten rock forming atop the Juan de Fuca plate or 'slab.' The image shows the rock begins to melt about 50 miles beneath Mount Rainier (the red triangle at top). Some is pulled downward and eastward as the slab keeps diving, but other melts move upward to the orange magma chamber shown under but west of Mount Rainier. The line of sensors used to make this image were placed north of the 14,410-foot peak, so the image may be showing a lobe of the magma chamber that extends northwest of the mountain. Red ovals on the left half of the page are the hypocenters of earthquakes. -  R Shane McGary, Woods Hole Oceanographic Institution.
This image was made by measuring how the ground conducts or resists electricity in a study co-authored by geophysicist Phil Wannamaker of the University of Utah Energy & Geoscience Institute. It shows the underground plumbing system that provides molten and partly molten rock to the magma chamber beneath the Mount Rainier volcano in Washington state. The scale at left is miles depth. The scale at bottom is miles from the Pacific Coast. The Juan de Fuca plate of Earth’s Pacific seafloor crust and upper mantle is shown in blue on the left half of the image as it dives or
‘subducts’ eastward beneath Washington state. The reddish orange and yellow colors represent molten and partly molten rock forming atop the Juan de Fuca plate or ‘slab.’ The image shows the rock begins to melt about 50 miles beneath Mount Rainier (the red triangle at top). Some is pulled downward and eastward as the slab keeps diving, but other melts move upward to the orange magma chamber shown under but west of Mount Rainier. The line of sensors used to make this image were placed north of the 14,410-foot peak, so the image may be showing a lobe of the magma chamber that extends northwest of the mountain. Red ovals on the left half of the page are the hypocenters of earthquakes. – R Shane McGary, Woods Hole Oceanographic Institution.

By measuring how fast Earth conducts electricity and seismic waves, a University of Utah researcher and colleagues made a detailed picture of Mount Rainier’s deep volcanic plumbing and partly molten rock that will erupt again someday.

“This is the most direct image yet capturing the melting process that feeds magma into a crustal reservoir that eventually is tapped for eruptions,” says geophysicist Phil Wannamaker, of the university’s Energy & Geoscience Institute and Department of Civil and Environmental Engineering. “But it does not provide any information on the timing of future eruptions from Mount Rainier or other Cascade Range volcanoes.”

The study was published today in the journal Nature by Wannamaker and geophysicists from the Woods Hole Oceanographic Institution in Massachusetts, the College of New Jersey and the University of Bergen, Norway.

In an odd twist, the image appears to show that at least part of Mount Rainier’s partly molten magma reservoir is located about 6 to 10 miles northwest of the 14,410-foot volcano, which is 30 to 45 miles southeast of the Seattle-Tacoma area.

But that could be because the 80 electrical sensors used for the experiment were placed in a 190-mile-long, west-to-east line about 12 miles north of Rainier. So the main part of the magma chamber could be directly under the peak, but with a lobe extending northwest under the line of detectors, Wannamaker says.

The top of the magma reservoir in the image is 5 miles underground and “appears to be 5 to 10 miles thick, and 5 to 10 miles wide in east-west extent,” he says. “We can’t really describe the north-south extent because it’s a slice view.”

Wannamaker estimates the reservoir is roughly 30 percent molten. Magma chambers are like a sponge of hot, soft rock containing pockets of molten rock.

The new image doesn’t reveal the plumbing tying Mount Rainier to the magma chamber 5 miles below it. Instead, it shows water and partly molten and molten rock are generated 50 miles underground where one of Earth’s seafloor crustal plates or slabs is “subducting” or diving eastward and downward beneath the North America plate, and how and where those melts rise to Rainier’s magma chamber.

The study was funded largely by the National Science Foundation’s Earthscope program, which also has made underground images of the United States using seismic or sound-wave tomography, much like CT scans show the body’s interior using X-rays.

The new study used both seismic imaging and magnetotelluric measurements, which make images by showing how electrical and magnetic fields in the ground vary due to differences in how much underground rock and fluids conduct or resist electricity.

Wannamaker says it is the most detailed cross-section view yet under a Cascades volcanic system using electrical and seismic imaging. Earlier seismic images indicated water and partly molten rock atop the diving slab. The new image shows melting “from the surface of the slab to the upper crust, where partly molten magma accumulates before erupting,” he adds.

Wannamaker and Rob L. Evans, of the Woods Hole Oceanographic Institution, conceived the study. First author R Shane McGary – then at Woods Hole and now at the College of New Jersey – did the data analysis. Other co-authors were Jimmy Elsenbeck of Woods Hole and Stéphane Rondenay of the University of Bergen.

Mount Rainier: Hazardous Backdrop to Metropolitan Seattle-Tacoma

Mount Rainier, the tallest peak in the Cascades, “is an active volcano that will erupt again,” says the U.S. Geological Survey. Rainier sits atop volcanic flows up to 36 million years old. An ancestral Rainier existed 2 million to 1 million years ago. Frequent eruptions built the mountain’s modern edifice during the past 500,000 years. During the past 11,000 years, Rainier erupted explosively dozens of times, spewing ash and pumice.

Rainier once was taller until it collapsed during an eruption 5,600 years ago to form a large crater open to the northeast, much like the crater formed by Mount St. Helens’ 1980 eruption. The 5,600-year-old eruption sent a huge mudflow west to Puget Sound, covering parts or all of the present sites of the Port of Tacoma, Seattle suburbs Kent and Auburn, and the towns Puyallup, Orting, Buckley, Sumner and Enumclaw.

Rainier’s last lava flows were 2,200 years ago, the last flows of hot rock and ash were 1,100 years ago and the last big mudflow 500 years ago. There are disputed reports of steam eruptions in the 1800s.

Subduction Made Simple – and a Peek beneath a Peak

The “ring of fire” is a zone of active volcanoes and frequent earthquake activity surrounding the Pacific Ocean. It exists where Earth’s tectonic plates collide – specifically, plates that make up the seafloor converge with plates that carry continents.

From Cape Mendocino in northern California and north past Oregon, Washington state and into British Columbia, an oceanic plate is being pushed eastward and downward – a process called subduction – beneath the North American plate. This relatively small Juan de Fuca plate is located between the huge Pacific plate and the Pacific Northwest.

New seafloor rock – rich with water in cracks and minerals – emerges from an undersea volcanic ridge some 250 miles off the coast, from northern California into British Columbia. That seafloor adds to the western edge of the Juan de Fuca plate and pushes it east-northeast under the Pacific Northwest, as far as Idaho.

The part of the plate diving eastward and downward is called the slab, which ranges from 30 to 60 miles thick as it is jammed under the North American plate. The part of the North American plate above the diving slab is shaped like a wedge.

When the leading, eastern edge of the diving slab descends deep enough, where pressures and temperatures are high, water-bearing minerals such as chlorite and amphibole release water from the slab, and the slab and surrounding mantle rock begin to melt. That is why the Cascade Range of active volcanoes extends north-to-south – above the slab and parallel but about 120 miles inland from the coast – from British Columbia south to Mount Shasta and Lassen Peak in northern California.

In the new image, yellow-orange-red areas correspond to higher electrical conductivity (or lower resistivity) in places where fluids and melts are located.

The underground image produced by the new study shows where water and molten rock accumulate atop the descending slab, and the route they take to the magma chamber that feeds eruptions of Mount Rainier:

– The rock begins to melt atop the slab about 50 miles beneath Mount Rainier. Wannamaker says it is best described as partly molten rock that contains about 2 percent water and “is a mush of crystals within an interlacing a network of molten rock.”

– Some water and partly molten rock actually gets dragged downward atop the descending slab, to depths of 70 miles or more.

– Other partly molten rock rises up through the upper mantle wedge, crosses into the crust at a depth of about 25 miles, and then rises into Rainier’s magma chamber – or at least the lobe of the chamber that crosses under the line of sensors used in the study. Evidence suggests the magma moves upward at least 0.4 inches per year.

– The new magnetotelluric image also shows a shallower zone of fluid perhaps 60 miles west of Rainier and 25 miles deep at the crust-mantle boundary. Wannamaker says it is largely water released from minerals as the slab is squeezed and heated as it dives.

The seismic data were collected during 2008-2009 for other studies. The magnetotelluric data were gathered during 2009-2010 by authors of the new study.

Wannamaker and colleagues placed an east-west line of magnetotelluric sensors: 60 that made one-day measurements and looked as deep as 30 miles into the Earth, and 20 that made measurements for a month and looked at even greater depths.

Deep origins to the behavior of Hawaiian volcanoes

A 300-m-high fountain during episode 8 of the 1959 Kilauea Iki eruption from close to the Byron Ledge overlook.  7 am (HST) on 11 December 1959. -  Hawaiian Volcano Observatory, U.S. Geological Survey
A 300-m-high fountain during episode 8 of the 1959 Kilauea Iki eruption from close to the Byron Ledge overlook. 7 am (HST) on 11 December 1959. – Hawaiian Volcano Observatory, U.S. Geological Survey

Kīlauea volcano, on the Big Island of Hawai’i, typically has effusive eruptions, wherein magma flows to create ropy pāhoehoe lava, for example. However, Kīlauea less frequently erupts more violently, showering scoria and blocks over much of the surface of the island. To explain the variability in Kīlauea’s eruption styles, a team including Bruce Houghton, the Gordon Macdonald Professor of Volcanology in Geology and Geophysics at the University of Hawai’i at Mānoa (UHM) School of Ocean and Earth Science and Technology (SOEST) and colleagues from the University of Cambridge (UC) and Don Swanson from the Hawaiian Volcano Observatory (HVO) of the U.S. Geological Survey analyzed 25 eruptions that have taken place over the past 600 years.

The team’s research shows that the ultimate fate of a magma at Kīlauea, that is if the eruption will be effusive or explosive, is strongly influenced by the variability in composition of the deep magma – with more gas-rich magmas producing more explosive eruptions. “Gas-rich magmas are ‘predisposed’ to rise quickly through the Earth’s mantle and crust and erupt powerfully,” Houghton explained.

One of the biggest challenges in volcanic forecasting is to predict at an early stage the full path that an eruption will follow. Monitoring gives scientists an indication where an eruption will occur but not always the probable form it will take.

“Other statistics like a volcano’s volume, eruption rate, and duration are keys to real-time hazard and risk mapping,” said Houghton. “They are the target of approaches like ours.”

This investigation, published this week in Nature Geoscience, required careful analysis of the physical and chemical properties of eruption products over the last 600 years. Swanson and Houghton supplied a framework of very well-characterized eruptions using a detailed classified scheme for the size and power of the eruptions. UC performed nano-scale measurements of the original gas content of the magmas as ‘frozen’ in tiny packets of chilled melt inside large crystals in the magma.

This new look at the eruption history at Kīlauea has led to new understanding of what causes eruption style there. “Pre-existing wisdom had it that the form of an eruption was principally decided during the last kilometer of rise towards the surface. But now we know the content of dissolved gas at the deep source is a key,” said Houghton.

In the future, Houghton and colleagues hope to offer even more accurate models by estimating just how fast magma does rise at Kīlauea prior to eruption by using the rates at which the trapped original gasses can ‘leak’ out of the trapped magma.

Volcanoes, including Mount Hood in the US, can quickly become active

Researchers have discovered that volcanoes can go from dormant to active very quickly. -  OSU
Researchers have discovered that volcanoes can go from dormant to active very quickly. – OSU

New research results suggest that magma sitting 4-5 kilometers beneath the surface of Oregon’s Mount Hood has been stored in near-solid conditions for thousands of years.

The time it takes to liquefy and potentially erupt, however, is surprisingly short–perhaps as little as a couple of months.

The key to an eruption, geoscientists say, is to elevate the temperature of the rock to more than 750 degrees Celsius, which can happen when hot magma from deep within the Earth’s crust rises to the surface.

It was the mixing of hot liquid lava with cooler solid magma that triggered Mount Hood’s last two eruptions about 220 and 1,500 years ago, said Adam Kent, an Oregon State University (OSU) geologist and co-author of a paper reporting the new findings.

Results of the research, which was funded by the National Science Foundation (NSF), are in this week’s journal Nature.

“These scientists have used a clever new approach to timing the inner workings of Mount Hood, an important step in assessing volcanic hazards in the Cascades,” said Sonia Esperanca, a program director in NSF’s Division of Earth Sciences.

“If the temperature of the rock is too cold, the magma is like peanut butter in a refrigerator,” Kent said. “It isn’t very mobile.

“For Mount Hood, the threshold seems to be about 750 degrees (C)–if it warms up just 50 to 75 degrees above that, it greatly decreases the viscosity of the magma and makes it easier to mobilize.”

The scientists are interested in the temperature at which magma resides in the crust, since it’s likely to have important influence over the timing and types of eruptions that could occur.

The hotter magma from deeper down warms the cooler magma stored at a 4-5 kilometer depth, making it possible for both magmas to mix and be transported to the surface to produce an eruption.

The good news, Kent said, is that Mount Hood’s eruptions are not particularly violent. Instead of exploding, the magma tends to ooze out the top of the peak.

A previous study by Kent and OSU researcher Alison Koleszar found that the mixing of the two magma sources, which have different compositions, is both a trigger to an eruption and a constraining factor on how violent it can be.

“What happens when they mix is what happens when you squeeze a tube of toothpaste in the middle,” said Kent. “Some comes out the top, but in the case of Mount Hood it doesn’t blow the mountain to pieces.”

The study involved scientists at OSU and the University of California, Davis. The results are important, they say, because little was known about the physical conditions of magma storage and what it takes to mobilize that magma.

Kent and UC-Davis colleague Kari Cooper, also a co-author of the Nature paper, set out to discover whether they could determine how long Mount Hood’s magma chamber has been there, and in what condition.

When Mount Hood’s magma first rose up through the crust into its present-day chamber, it cooled and formed crystals.

The researchers were able to document the age of the crystals by the rate of decay of naturally occurring radioactive elements. However, the growth of the crystals is also dictated by temperature: if the rock is too cold, they don’t grow as fast.

The combination of the crystals’ age and apparent growth rate provides a geologic fingerprint for determining the approximate threshold for making the near-solid rock viscous enough to cause an eruption.

“What we found was that the magma has been stored beneath Mount Hood for at least 20,000 years–and probably more like 100,000 years,” Kent said.

“During the time it’s been there, it’s been in cold storage–like peanut butter in the fridge–a minimum of 88 percent of the time, and likely more than 99 percent of the time.”

Although hot magma from below can quickly mobilize the magma chamber at 4-5 kilometers below the surface, most of the time magma is held under conditions that make it difficult for it to erupt.

“What’s encouraging is that modern technology should be able to detect when the magma is beginning to liquefy or mobilize,” Kent said, “and that may give us warning of a potential eruption.

“Monitoring gases and seismic waves, and studying ground deformation through GPS, are a few of the techniques that could tell us that things are warming.”

The researchers hope to apply these techniques to other, larger volcanoes to see if they can determine the potential for shifting from cold storage to potential eruption–a development that might bring scientists a step closer to being able to forecast volcanic activity.

Oldest bit of crust firms up idea of a cool early Earth

A 4.4 billion-year-old zircon crystal is providing new insight into how the early Earth cooled from a ball of magma and formed continents just 160 million years after the formation of our solar system, much earlier than previously believed. The zircon, pictured here, is from the Jack Hills region of Australia and is now confirmed to be the oldest bit of the Earth's crust. -  John Valley
A 4.4 billion-year-old zircon crystal is providing new insight into how the early Earth cooled from a ball of magma and formed continents just 160 million years after the formation of our solar system, much earlier than previously believed. The zircon, pictured here, is from the Jack Hills region of Australia and is now confirmed to be the oldest bit of the Earth’s crust. – John Valley

With the help of a tiny fragment of zircon extracted from a remote rock outcrop in Australia, the picture of how our planet became habitable to life about 4.4 billion years ago is coming into sharper focus.

Writing today (Feb. 23, 2014) in the journal Nature Geoscience, an international team of researchers led by University of Wisconsin-Madison geoscience Professor John Valley reveals data that confirm the Earth’s crust first formed at least 4.4 billion years ago, just 160 million years after the formation of our solar system. The work shows, Valley says, that the time when our planet was a fiery ball covered in a magma ocean came earlier.

“This confirms our view of how the Earth cooled and became habitable,” says Valley, a geochemist whose studies of zircons, the oldest known terrestrial materials, have helped portray how the Earth’s crust formed during the first geologic eon of the planet. “This may also help us understand how other habitable planets would form.”

The new study confirms that zircon crystals from Western Australia’s Jack Hills region crystallized 4.4 billion years ago, building on earlier studies that used lead isotopes to date the Australian zircons and identify them as the oldest bits of the Earth’s crust. The microscopic zircon crystal used by Valley and his group in the current study is now confirmed to be the oldest known material of any kind formed on Earth.

The study, according to Valley, strengthens the theory of a “cool early Earth,” where temperatures were low enough for liquid water, oceans and a hydrosphere not long after the planet’s crust congealed from a sea of molten rock. “The study reinforces our conclusion that Earth had a hydrosphere before 4.3 billion years ago,” and possibly life not long after, says Valley.

The study was conducted using a new technique called atom-probe tomography that, in conjunction with secondary ion mass spectrometry, permitted the scientists to accurately establish the age and thermal history of the zircon by determining the mass of individual atoms of lead in the sample. Instead of being randomly distributed in the sample, as predicted, lead atoms in the zircon were clumped together, like “raisins in a pudding,” notes Valley.

The clusters of lead atoms formed 1 billion years after crystallization of the zircon, by which time the radioactive decay of uranium had formed the lead atoms that then diffused into clusters during reheating. “The zircon formed 4.4 billion years ago, and at 3.4 billion years, all the lead that existed at that time was concentrated in these hotspots,” Valley says. “This allows us to read a new page of the thermal history recorded by these tiny zircon time capsules.”

The formation, isotope ratio and size of the clumps – less than 50 atoms in diameter – become, in effect, a clock, says Valley, and verify that existing geochronology methods provide reliable and accurate estimates of the sample’s age. In addition, Valley and his group measured oxygen isotope ratios, which give evidence of early homogenization and later cooling of the Earth.

“The Earth was assembled from a lot of heterogeneous material from the solar system,” Valley explains, noting that the early Earth experienced intense bombardment by meteors, including a collision with a Mars-sized object about 4.5 billion years ago “that formed our moon, and melted and homogenized the Earth. Our samples formed after the magma oceans cooled and prove that these events were very early.”

Volcanoes, including Mt. Hood, can go from dormant to active quickly

Mount Hood, in the Oregon Cascades, doesn't have a highly explosive history. -  Photo courtesy Alison M Koleszar
Mount Hood, in the Oregon Cascades, doesn’t have a highly explosive history. – Photo courtesy Alison M Koleszar

A new study suggests that the magma sitting 4-5 kilometers beneath the surface of Oregon’s Mount Hood has been stored in near-solid conditions for thousands of years, but that the time it takes to liquefy and potentially erupt is surprisingly short – perhaps as little as a couple of months.

The key, scientists say, is to elevate the temperature of the rock to more than 750 degrees Celsius, which can happen when hot magma from deep within the Earth’s crust rises to the surface. It is the mixing of the two types of magma that triggered Mount Hood’s last two eruptions – about 220 and 1,500 years ago, said Adam Kent, an Oregon State University geologist and co-author of the study.

Results of the research, which was funded by the National Science Foundation, were published this week in the journal Nature.

“If the temperature of the rock is too cold, the magma is like peanut butter in a refrigerator,” Kent said. “It just isn’t very mobile. For Mount Hood, the threshold seems to be about 750 degrees (C) – if it warms up just 50 to 75 degrees above that, it greatly increases the viscosity of the magma and makes it easier to mobilize.”

Thus the scientists are interested in the temperature at which magma resides in the crust, they say, since it is likely to have important influence over the timing and types of eruptions that could occur. The hotter magma from down deep warms the cooler magma stored at 4-5 kilometers, making it possible for both magmas to mix and to be transported to the surface to eventually produce an eruption.

The good news, Kent said, is that Mount Hood’s eruptions are not particularly violent. Instead of exploding, the magma tends to ooze out the top of the peak. A previous study by Kent and OSU postdoctoral researcher Alison Koleszar found that the mixing of the two magma sources – which have different compositions – is both a trigger to an eruption and a constraining factor on how violent it can be.

“What happens when they mix is what happens when you squeeze a tube of toothpaste in the middle,” said Kent, a professor in OSU’s College of Earth, Ocean, and Atmospheric Sciences. “A big glob kind of plops out the top, but in the case of Mount Hood – it doesn’t blow the mountain to pieces.”

The collaborative study between Oregon State and the University of California, Davis is important because little was known about the physical conditions of magma storage and what it takes to mobilize the magma. Kent and UC-Davis colleague Kari Cooper, also a co-author on the Nature article, set out to find if they could determine how long Mount Hood’s magma chamber has been there, and in what condition.

When Mount Hood’s magma first rose up through the crust into its present-day chamber, it cooled and formed crystals. The researchers were able to document the age of the crystals by the rate of decay of naturally occurring radioactive elements. However, the growth of the crystals is also dictated by temperature – if the rock is too cold, they don’t grow as fast.

Thus the combination of the crystals’ age and apparent growth rate provides a geologic fingerprint for determining the approximate threshold for making the near-solid rock viscous enough to cause an eruption. The diffusion rate of the element strontium, which is also sensitive to temperature, helped validate the findings.

“What we found was that the magma has been stored beneath Mount Hood for at least 20,000 years – and probably more like 100,000 years,” Kent said. “And during the time it’s been there, it’s been in cold storage – like the peanut butter in the fridge – a minimum of 88 percent of the time, and likely more than 99 percent of the time.”

In other words – even though hot magma from below can quickly mobilize the magma chamber at 4-5 kilometers below the surface, most of the time magma is held under conditions that make it difficult for it to erupt.

“What is encouraging from another standpoint is that modern technology should be able to detect when magma is beginning to liquefy, or mobilize,” Kent said, “and that may give us warning of a potential eruption. Monitoring gases, utilizing seismic waves and studying ground deformation through GPS are a few of the techniques that could tell us that things are warming.”

The researchers hope to apply these techniques to other, larger volcanoes to see if they can determine their potential for shifting from cold storage to potential eruption, a development that might bring scientists a step closer to being able to forecast volcanic activity.