Geologists simulate deep earthquakes in the laboratory

Geologist Harry Green is a distinguished professor of the graduate division at the University of California, Riverside. -  Green Lab, UC Riverside.
Geologist Harry Green is a distinguished professor of the graduate division at the University of California, Riverside. – Green Lab, UC Riverside.

More than 20 years ago, geologist Harry Green, now a distinguished professor of the graduate division at the University of California, Riverside, and colleagues discovered a high-pressure failure mechanism that they proposed then was the long-sought mechanism of very deep earthquakes (earthquakes occurring at more than 400 km depth).

The result was controversial because seismologists could not find a seismic signal in the Earth that could confirm the results.

Seismologists have now found the critical evidence. Indeed, beneath Japan, they have even imaged the tell-tale evidence and showed that it coincides with the locations of deep earthquakes.

In the Sept. 20 issue of the journal Science, Green and colleagues show just how such deep earthquakes can be simulated in the laboratory.

“We confirmed essentially all aspects of our earlier experimental work and extended the conditions to significantly higher pressure,” Green said. “What is crucial, however, is that these experiments are accomplished in a new type of apparatus that allows us to view and analyze specimens using synchrotron X-rays in the premier laboratory in the world for this kind of experiments – the Advanced Photon Source at Argonne National Laboratory.”

The ability to do such experiments has now allowed scientists like Green to simulate the appropriate conditions within the Earth and record and analyze the “earthquakes” in their small samples in real time, thus providing the strongest evidence yet that this is the mechanism by which earthquakes happen at hundreds of kilometers depth.

The origin of deep earthquakes fundamentally differs from that of shallow earthquakes (earthquakes occurring at less than 50 km depth). In the case of shallow earthquakes, theories of rock fracture rely on the properties of coalescing cracks and friction.

“But as pressure and temperature increase with depth, intracrystalline plasticity dominates the deformation regime so that rocks yield by creep or flow rather than by the kind of brittle fracturing we see at smaller depths,” Green explained. “Moreover, at depths of more than 400 kilometers, the mineral olivine is no longer stable and undergoes a transformation resulting in spinel. a mineral of higher density”

The research team focused on the role that phase transformations of olivine might play in triggering deep earthquakes. They performed laboratory deformation experiments on olivine at high pressure and found the “earthquakes” only within a narrow temperature range that simulates conditions where the real earthquakes occur in Earth.

“Using synchrotron X-rays to aid our observations, we found that fractures nucleate at the onset of the olivine to spinel transition,” Green said. “Further, these fractures propagate dynamically so that intense acoustic emissions are generated. These phase transitions in olivine, we argue in our research paper, provide an attractive mechanism for how very deep earthquakes take place.”

Green was joined in the study by Alexandre Schubnel at the Ecole Normale Supérieure, France; Fabrice Brunet at the Université de Grenoble, France; and Nadège Hilairet, Julian Gasc and Yanbin Wang at the University of Chicago, Ill.

Geologists simulate deep earthquakes in the laboratory

Geologist Harry Green is a distinguished professor of the graduate division at the University of California, Riverside. -  Green Lab, UC Riverside.
Geologist Harry Green is a distinguished professor of the graduate division at the University of California, Riverside. – Green Lab, UC Riverside.

More than 20 years ago, geologist Harry Green, now a distinguished professor of the graduate division at the University of California, Riverside, and colleagues discovered a high-pressure failure mechanism that they proposed then was the long-sought mechanism of very deep earthquakes (earthquakes occurring at more than 400 km depth).

The result was controversial because seismologists could not find a seismic signal in the Earth that could confirm the results.

Seismologists have now found the critical evidence. Indeed, beneath Japan, they have even imaged the tell-tale evidence and showed that it coincides with the locations of deep earthquakes.

In the Sept. 20 issue of the journal Science, Green and colleagues show just how such deep earthquakes can be simulated in the laboratory.

“We confirmed essentially all aspects of our earlier experimental work and extended the conditions to significantly higher pressure,” Green said. “What is crucial, however, is that these experiments are accomplished in a new type of apparatus that allows us to view and analyze specimens using synchrotron X-rays in the premier laboratory in the world for this kind of experiments – the Advanced Photon Source at Argonne National Laboratory.”

The ability to do such experiments has now allowed scientists like Green to simulate the appropriate conditions within the Earth and record and analyze the “earthquakes” in their small samples in real time, thus providing the strongest evidence yet that this is the mechanism by which earthquakes happen at hundreds of kilometers depth.

The origin of deep earthquakes fundamentally differs from that of shallow earthquakes (earthquakes occurring at less than 50 km depth). In the case of shallow earthquakes, theories of rock fracture rely on the properties of coalescing cracks and friction.

“But as pressure and temperature increase with depth, intracrystalline plasticity dominates the deformation regime so that rocks yield by creep or flow rather than by the kind of brittle fracturing we see at smaller depths,” Green explained. “Moreover, at depths of more than 400 kilometers, the mineral olivine is no longer stable and undergoes a transformation resulting in spinel. a mineral of higher density”

The research team focused on the role that phase transformations of olivine might play in triggering deep earthquakes. They performed laboratory deformation experiments on olivine at high pressure and found the “earthquakes” only within a narrow temperature range that simulates conditions where the real earthquakes occur in Earth.

“Using synchrotron X-rays to aid our observations, we found that fractures nucleate at the onset of the olivine to spinel transition,” Green said. “Further, these fractures propagate dynamically so that intense acoustic emissions are generated. These phase transitions in olivine, we argue in our research paper, provide an attractive mechanism for how very deep earthquakes take place.”

Green was joined in the study by Alexandre Schubnel at the Ecole Normale Supérieure, France; Fabrice Brunet at the Université de Grenoble, France; and Nadège Hilairet, Julian Gasc and Yanbin Wang at the University of Chicago, Ill.

Undersea mountains provide crucial piece in climate prediction puzzle

A mystery in the ocean near Antarctica has been solved by researchers who have long puzzled over how deep and mid-depth ocean waters are mixed. -  Alan Homer and British Antarctic Survey
A mystery in the ocean near Antarctica has been solved by researchers who have long puzzled over how deep and mid-depth ocean waters are mixed. – Alan Homer and British Antarctic Survey

A mystery in the ocean near Antarctica has been solved by researchers who have long puzzled over how deep and mid-depth ocean waters are mixed. They found that sea water mixes dramatically as it rushes over undersea mountains in Drake Passage – the channel between the southern tip of South America and the Antarctic continent. Mixing of water layers in the oceans is crucial in regulating the Earth’s climate and ocean currents.

The research provides insight for climate models which until now have lacked the detailed information on ocean mixing needed to provide accurate long-term climate projections. The study was carried out by the University of Exeter, the University of East Anglia, the University of Southampton, the Woods Hole Oceanographic Institution, the British Antarctic Survey and the Scottish Association for Marine Science and is published in the journal Nature.

Working in some of the wildest waters on the planet, researchers measured mixing in the Southern Ocean by releasing tiny quantities of an inert chemical tracer into the Southeast Pacific. They tracked the tracer for several years as it went through Drake Passage to observe how quickly the ocean mixed.

The tracer showed almost no vertical mixing in the Pacific but as the water passed over the mountainous ocean floor in the relatively narrow continental gap that forms the Drake Passage it began to mix dramatically.

Professor Andrew Watson from the University of Exeter (previously at the University of East Anglia) said: “A thorough understanding of the process of ocean mixing is crucial to our understanding of the overall climate system. Our study indicates that virtually all the mixing in the Southern Ocean occurs in Drake Passage and at a few other undersea mountain locations. Our study will provide climate scientists with the detailed information about the oceans that they currently lack.”

Ocean mixing transfers carbon dioxide from the atmosphere to the deep sea, and ultimately controls the rate at which the ocean takes up carbon dioxide. Over several hundred years this process will remove much of the carbon dioxide that we release into the atmosphere, storing it in the deep ocean. Ocean mixing also affects climate, for example an increase in the rate of deep sea mixing would enable the ocean to transfer more heat towards the poles.

Scientists believe that the lower concentrations of atmospheric carbon dioxide present during the ice ages may have been the result of slower ocean mixing between the surface and the deep sea. Although the reasons for this are not yet clear, this further emphasizes the link between ocean mixing and climate.

Undersea mountains provide crucial piece in climate prediction puzzle

A mystery in the ocean near Antarctica has been solved by researchers who have long puzzled over how deep and mid-depth ocean waters are mixed. -  Alan Homer and British Antarctic Survey
A mystery in the ocean near Antarctica has been solved by researchers who have long puzzled over how deep and mid-depth ocean waters are mixed. – Alan Homer and British Antarctic Survey

A mystery in the ocean near Antarctica has been solved by researchers who have long puzzled over how deep and mid-depth ocean waters are mixed. They found that sea water mixes dramatically as it rushes over undersea mountains in Drake Passage – the channel between the southern tip of South America and the Antarctic continent. Mixing of water layers in the oceans is crucial in regulating the Earth’s climate and ocean currents.

The research provides insight for climate models which until now have lacked the detailed information on ocean mixing needed to provide accurate long-term climate projections. The study was carried out by the University of Exeter, the University of East Anglia, the University of Southampton, the Woods Hole Oceanographic Institution, the British Antarctic Survey and the Scottish Association for Marine Science and is published in the journal Nature.

Working in some of the wildest waters on the planet, researchers measured mixing in the Southern Ocean by releasing tiny quantities of an inert chemical tracer into the Southeast Pacific. They tracked the tracer for several years as it went through Drake Passage to observe how quickly the ocean mixed.

The tracer showed almost no vertical mixing in the Pacific but as the water passed over the mountainous ocean floor in the relatively narrow continental gap that forms the Drake Passage it began to mix dramatically.

Professor Andrew Watson from the University of Exeter (previously at the University of East Anglia) said: “A thorough understanding of the process of ocean mixing is crucial to our understanding of the overall climate system. Our study indicates that virtually all the mixing in the Southern Ocean occurs in Drake Passage and at a few other undersea mountain locations. Our study will provide climate scientists with the detailed information about the oceans that they currently lack.”

Ocean mixing transfers carbon dioxide from the atmosphere to the deep sea, and ultimately controls the rate at which the ocean takes up carbon dioxide. Over several hundred years this process will remove much of the carbon dioxide that we release into the atmosphere, storing it in the deep ocean. Ocean mixing also affects climate, for example an increase in the rate of deep sea mixing would enable the ocean to transfer more heat towards the poles.

Scientists believe that the lower concentrations of atmospheric carbon dioxide present during the ice ages may have been the result of slower ocean mixing between the surface and the deep sea. Although the reasons for this are not yet clear, this further emphasizes the link between ocean mixing and climate.

Warming ocean thawing Antarctica glacier, researchers say

For the first time, researchers completed an extensive exploration of how quickly ice is melting underneath a rapidly changing Antarctic glacier, possibly the biggest source of uncertainty in global sea level projections.

Martin Truffer, a physics professor at the University of Alaska Fairbanks, and Tim Stanton, an oceanographer with the Naval Postgraduate School, were able to look underneath the Pine Island Glacier on the West Antarctic Ice Sheet and take exact measurements of the undersea melting process.

“This particular site is crucial, because the bottom of the ice in that sector of Antarctica is grounded well below sea level and is particularly vulnerable to melt from the ocean and break up,” said Truffer, a researcher with UAF’s Geophysical Institute. “I think it is fair to say that the largest potential sea level rise signal in the next century is going to come from this area.”

Their measurements show that, at some locations, warm ocean water is eating away at the underside of the ice shelf at more than two inches per day. This leads to a thinning of the ice shelf and the eventual production of huge icebergs, one of which just separated from the ice shelf a few months ago.

Their work was highlighted in a recent issue of Science. Both Truffer and Stanton, with other scientists from around the world, have spent years studying the underside of the Antarctic ice shelf and glacier, but the recent research took place in early 2013.

“UAF’s part was to accomplish the drilling,” Truffer said, crediting Dale Pomraning, with the GI’s machine shop.

“We have a hot water drill that is modular enough to be deployed by relatively small airplanes and helicopters, and we have the expertise to carry this out.”

The drilling allowed the team to measure an undersea current of warm water, driven by fresh water from the melting glacier. The measurements will be used with both physical and computer models of ocean and glacier systems, said Stanton.

“These improved models are critical to our improved ability to predict future changes in the ice shelf and glacial melt rates of the potentially unstable Western Antarctic Ice Shelf in response to changing ocean forces,” Stanton said.

Warming ocean thawing Antarctica glacier, researchers say

For the first time, researchers completed an extensive exploration of how quickly ice is melting underneath a rapidly changing Antarctic glacier, possibly the biggest source of uncertainty in global sea level projections.

Martin Truffer, a physics professor at the University of Alaska Fairbanks, and Tim Stanton, an oceanographer with the Naval Postgraduate School, were able to look underneath the Pine Island Glacier on the West Antarctic Ice Sheet and take exact measurements of the undersea melting process.

“This particular site is crucial, because the bottom of the ice in that sector of Antarctica is grounded well below sea level and is particularly vulnerable to melt from the ocean and break up,” said Truffer, a researcher with UAF’s Geophysical Institute. “I think it is fair to say that the largest potential sea level rise signal in the next century is going to come from this area.”

Their measurements show that, at some locations, warm ocean water is eating away at the underside of the ice shelf at more than two inches per day. This leads to a thinning of the ice shelf and the eventual production of huge icebergs, one of which just separated from the ice shelf a few months ago.

Their work was highlighted in a recent issue of Science. Both Truffer and Stanton, with other scientists from around the world, have spent years studying the underside of the Antarctic ice shelf and glacier, but the recent research took place in early 2013.

“UAF’s part was to accomplish the drilling,” Truffer said, crediting Dale Pomraning, with the GI’s machine shop.

“We have a hot water drill that is modular enough to be deployed by relatively small airplanes and helicopters, and we have the expertise to carry this out.”

The drilling allowed the team to measure an undersea current of warm water, driven by fresh water from the melting glacier. The measurements will be used with both physical and computer models of ocean and glacier systems, said Stanton.

“These improved models are critical to our improved ability to predict future changes in the ice shelf and glacial melt rates of the potentially unstable Western Antarctic Ice Shelf in response to changing ocean forces,” Stanton said.

Birth of Earth’s continents

New research led by a University of Calgary geophysicist provides strong evidence against continent formation above a hot mantle plume, similar to an environment that presently exists beneath the Hawaiian Islands.

The analysis, published this month in Nature Geoscience, indicates that the nuclei of Earth’s continents formed as a byproduct of mountain-building processes, by stacking up slabs of relatively cold oceanic crust. This process created thick, strong ‘keels’ in the Earth’s mantle that supported the overlying crust and enabled continents to form.

The scientific clues leading to this conclusion derived from computer simulations of the slow cooling process of continents, combined with analysis of the distribution of diamonds in the deep Earth.

The Department of Geoscience’s Professor David Eaton developed computer software to enable numerical simulation of the slow diffusive cooling of Earth’s mantle over a time span of billions of years.

Working in collaboration with former graduate student, Assistant Professor Claire Perry from the Universite du Quebec a Montreal, Eaton relied on the geological record of diamonds found in Africa to validate his innovative computer simulations.

“For the first time, we are able to quantify the thermal evolution of a realistic 3D Earth model spanning billions of years from the time continents were formed,” states Perry.

Mantle plumes consist of an upwelling of hot material within Earth’s mantle. Plumes are thought to be the cause of some volcanic centres, especially those that form a linear volcanic chain like Hawaii. Diamonds, which are generally limited to the deepest and oldest parts of the continental mantle, provide a wealth of information on how the host mantle region may have formed.

“Ancient mantle keels are relatively strong, cold and sometimes diamond-bearing material. They are known to extend to depths of 200 kilometres or more beneath the ancient core regions of continents,” explains Professor David Eaton. “These mantle keels resisted tectonic recycling into the deep mantle, allowing the preservation of continents over geological time and providing suitable environments for the development of the terrestrial biosphere.”

His method takes into account important factors such as dwindling contribution of natural radioactivity to the heat budget, and allows for the calculation of other properties that strongly influence mantle evolution, such as bulk density and rheology (mechanical strength).

“Our computer model emerged from a multi-disciplinary approach combining classical physics, mathematics and computer science,” explains Eaton. “By combining those disciplines, we were able to tackle a fundamental geoscientific problem, which may open new doors for future research.”

This work provides significant new scientific insights into the formation and evolution of continents on Earth.




Video
Click on this image to view the .mp4 video
This computer simulation spanning 2.5 billion years of Earth history is showing density difference of the mantle, compared to an oceanic reference, starting from a cooler initial state. Density is controlled by mantle composition as well as slowly cooling temperature; a keel of low-density material extending to about 260 km depth on the left side (x < 600 km) provides buoyancy that prevents continents from being subducted ('recycled' into the deep Earth). Graph on the top shows a computed elevation model. – David Eaton, University of Calgary.

Birth of Earth’s continents

New research led by a University of Calgary geophysicist provides strong evidence against continent formation above a hot mantle plume, similar to an environment that presently exists beneath the Hawaiian Islands.

The analysis, published this month in Nature Geoscience, indicates that the nuclei of Earth’s continents formed as a byproduct of mountain-building processes, by stacking up slabs of relatively cold oceanic crust. This process created thick, strong ‘keels’ in the Earth’s mantle that supported the overlying crust and enabled continents to form.

The scientific clues leading to this conclusion derived from computer simulations of the slow cooling process of continents, combined with analysis of the distribution of diamonds in the deep Earth.

The Department of Geoscience’s Professor David Eaton developed computer software to enable numerical simulation of the slow diffusive cooling of Earth’s mantle over a time span of billions of years.

Working in collaboration with former graduate student, Assistant Professor Claire Perry from the Universite du Quebec a Montreal, Eaton relied on the geological record of diamonds found in Africa to validate his innovative computer simulations.

“For the first time, we are able to quantify the thermal evolution of a realistic 3D Earth model spanning billions of years from the time continents were formed,” states Perry.

Mantle plumes consist of an upwelling of hot material within Earth’s mantle. Plumes are thought to be the cause of some volcanic centres, especially those that form a linear volcanic chain like Hawaii. Diamonds, which are generally limited to the deepest and oldest parts of the continental mantle, provide a wealth of information on how the host mantle region may have formed.

“Ancient mantle keels are relatively strong, cold and sometimes diamond-bearing material. They are known to extend to depths of 200 kilometres or more beneath the ancient core regions of continents,” explains Professor David Eaton. “These mantle keels resisted tectonic recycling into the deep mantle, allowing the preservation of continents over geological time and providing suitable environments for the development of the terrestrial biosphere.”

His method takes into account important factors such as dwindling contribution of natural radioactivity to the heat budget, and allows for the calculation of other properties that strongly influence mantle evolution, such as bulk density and rheology (mechanical strength).

“Our computer model emerged from a multi-disciplinary approach combining classical physics, mathematics and computer science,” explains Eaton. “By combining those disciplines, we were able to tackle a fundamental geoscientific problem, which may open new doors for future research.”

This work provides significant new scientific insights into the formation and evolution of continents on Earth.




Video
Click on this image to view the .mp4 video
This computer simulation spanning 2.5 billion years of Earth history is showing density difference of the mantle, compared to an oceanic reference, starting from a cooler initial state. Density is controlled by mantle composition as well as slowly cooling temperature; a keel of low-density material extending to about 260 km depth on the left side (x < 600 km) provides buoyancy that prevents continents from being subducted ('recycled' into the deep Earth). Graph on the top shows a computed elevation model. – David Eaton, University of Calgary.

New insights solve 300-year-old problem: The dynamics of the Earth’s core

Scientists at the University of Leeds have solved a 300-year-old riddle about which direction the centre of the earth spins.

The Earth’s inner core, made up of solid iron, ‘superrotates’ in an eastward direction – meaning it spins faster than the rest of the planet – while the outer core, comprising mainly molten iron, spins westwards at a slower pace.

Although Edmund Halley – who also discovered the famous comet – showed the westward-drifting motion of the Earth’s geomagnetic field in 1692, it is the first time that scientists have been able to link the way the inner core spins to the behavior of the outer core. The planet behaves in this way because it is responding to the Earth’s geomagnetic field.

The findings, published today in Proceedings of the National Academy of Sciences, help scientists to interpret the dynamics of the core of the Earth, the source of our planet’s magnetic field.

In the last few decades, seismometers measuring earthquakes travelling through the Earth’s core have identified an eastwards, or superrotation of the solid inner core, relative to Earth’s surface.

“The link is simply explained in terms of equal and opposite action”, explains Dr Philip Livermore, of the School of Earth and Environment at the University of Leeds. “The magnetic field pushes eastwards on the inner core, causing it to spin faster than the Earth, but it also pushes in the opposite direction in the liquid outer core, which creates a westward motion.”

The solid iron inner core is about the size of the Moon. It is surrounded by the liquid outer core, an iron alloy, whose convection-driven movement generates the geomagnetic field.

The fact that the Earth’s internal magnetic field changes slowly, over a timescale of decades, means that the electromagnetic force responsible for pushing the inner and outer cores will itself change over time. This may explain fluctuations in the predominantly eastwards rotation of the inner core, a phenomenon reported for the last 50 years by Tkalčić et al. in a recent study published in Nature Geoscience.

Other previous research based on archeological artefacts and rocks, with ages of hundreds to thousands of years, suggests that the drift direction has not always been westwards: some periods of eastwards motion may have occurred in the last 3,000 years. Viewed within the conclusions of the new model, this suggests that the inner core may have undergone a westwards rotation in such periods.

The authors used a model of the Earth’s core which was run on the giant super-computer Monte Rosa, part of the Swiss National Supercomputing Centre in Lugano, Switzerland. Using a new method, they were able to simulate the Earth’s core with an accuracy about 100 times better than other models.

New insights solve 300-year-old problem: The dynamics of the Earth’s core

Scientists at the University of Leeds have solved a 300-year-old riddle about which direction the centre of the earth spins.

The Earth’s inner core, made up of solid iron, ‘superrotates’ in an eastward direction – meaning it spins faster than the rest of the planet – while the outer core, comprising mainly molten iron, spins westwards at a slower pace.

Although Edmund Halley – who also discovered the famous comet – showed the westward-drifting motion of the Earth’s geomagnetic field in 1692, it is the first time that scientists have been able to link the way the inner core spins to the behavior of the outer core. The planet behaves in this way because it is responding to the Earth’s geomagnetic field.

The findings, published today in Proceedings of the National Academy of Sciences, help scientists to interpret the dynamics of the core of the Earth, the source of our planet’s magnetic field.

In the last few decades, seismometers measuring earthquakes travelling through the Earth’s core have identified an eastwards, or superrotation of the solid inner core, relative to Earth’s surface.

“The link is simply explained in terms of equal and opposite action”, explains Dr Philip Livermore, of the School of Earth and Environment at the University of Leeds. “The magnetic field pushes eastwards on the inner core, causing it to spin faster than the Earth, but it also pushes in the opposite direction in the liquid outer core, which creates a westward motion.”

The solid iron inner core is about the size of the Moon. It is surrounded by the liquid outer core, an iron alloy, whose convection-driven movement generates the geomagnetic field.

The fact that the Earth’s internal magnetic field changes slowly, over a timescale of decades, means that the electromagnetic force responsible for pushing the inner and outer cores will itself change over time. This may explain fluctuations in the predominantly eastwards rotation of the inner core, a phenomenon reported for the last 50 years by Tkalčić et al. in a recent study published in Nature Geoscience.

Other previous research based on archeological artefacts and rocks, with ages of hundreds to thousands of years, suggests that the drift direction has not always been westwards: some periods of eastwards motion may have occurred in the last 3,000 years. Viewed within the conclusions of the new model, this suggests that the inner core may have undergone a westwards rotation in such periods.

The authors used a model of the Earth’s core which was run on the giant super-computer Monte Rosa, part of the Swiss National Supercomputing Centre in Lugano, Switzerland. Using a new method, they were able to simulate the Earth’s core with an accuracy about 100 times better than other models.