Researchers describe unusual Mars rock

The first rock that scientists analyzed on Mars with a pair of chemical instruments aboard the Curiosity rover turned out to be a doozy – a pyramid-shaped volcanic rock called a “mugearite” that is unlike any other Martian igneous rock ever found.

Dubbed “Jake_M” – after Jet Propulsion Laboratory engineer Jake Matijevic – the rock is similar to mugearites found on Earth, typically on ocean islands and in continental rifts. The process through which these rocks form often suggests the presence of water deep below the surface, according to Martin Fisk, an Oregon State University marine geologist and member of the Mars Science Laboratory team.

Results of the analysis were published this week in the journal Science, along with two other papers on Mars’ soils.

“On Earth, we have a pretty good idea how mugearites and rocks like them are formed,” said Fisk, who is a co-author on all three Science articles. “It starts with magma deep within the Earth that crystallizes in the presence of 1-2 percent water. The crystals settle out of the magma and what doesn’t crystallize is the mugearite magma, which can eventually make its way to the surface as a volcanic eruption.”

Fisk, who is a professor in OSU’s College of Earth, Ocean, and Atmospheric Sciences, said the most common volcanic rocks typically crystallize in a specific order as they cool, beginning with olivine and feldspar. In the presence of water, however, feldspar crystallizes later and the magma will have a composition such as mugearite.

Although this potential evidence for water deep beneath the surface of Mars isn’t ironclad, the scientists say, it adds to the growing body of studies pointing to the presence of water on the Red Planet – an ingredient necessary for life.

“The rock is significant in another way,” Fisk pointed out. “It implies that the interior of Mars is composed of areas with different compositions; it is not well mixed. Perhaps Mars never got homogenized the way Earth has through its plate tectonics and convection processes.”

In another study, scientists examined the soil diversity and hydration of Gale Crater using a ChemCam laser instrument. They found hydrogen in all of the sites sampled, suggesting water, as well as the likely presence of sulphates. Mars was thought to have three stages – an early phase with lots of water, an evaporation phase when the water disappeared leaving behind sulphate salts, and a third phase when the surface soils dried out and oxidized – creating the planet’s red hue.

“ChemCam found hydrogen in almost every place we found iron,” Fisk said.

The third study compared grains of rock on the surface with a darker soil beneath at a site called the Rocknest Sand Shadow. Some of the sand grains are almost perfectly round and may have come from space, Fisk said.

Late Cretaceous Period was likely ice-free

In a new study, MacLeod found evidence that a continental ice sheet did not form during the Late Cretaceous Period more than 90 million years ago. This information could help scientists predict changes in earth's climate as our temperatures rise. -  University of Missouri
In a new study, MacLeod found evidence that a continental ice sheet did not form during the Late Cretaceous Period more than 90 million years ago. This information could help scientists predict changes in earth’s climate as our temperatures rise. – University of Missouri

For years, scientists have thought that a continental ice sheet formed during the Late Cretaceous Period more than 90 million years ago when the climate was much warmer than it is today. Now, a University of Missouri researcher has found evidence suggesting that no ice sheet formed at this time. This finding could help environmentalists and scientists predict what the earth’s climate will be as carbon dioxide levels continue to rise.

“Currently, carbon dioxide levels are just above 400 parts per million (ppm), up approximately 120 ppm in the last 150 years and rising about 2 ppm each year,” said Ken MacLeod, a professor of geological sciences at MU. “In our study, we found that during the Late Cretaceous Period, when carbon dioxide levels were around 1,000 ppm, there were no continental ice sheets on earth. So, if carbon dioxide levels continue to rise, the earth will be ice-free once the climate comes into balance with the higher levels.”

In his study, MacLeod analyzed the fossilized shells of 90 million-year-old planktic and benthic foraminifera, single-celled organisms about the size of a grain of salt. Measuring the ratios of different isotopes of oxygen and carbon in the fossils gives scientists information about past temperatures and other environmental conditions. The fossils, which were found in Tanzania, showed no evidence of cooling or changes in local water chemistry that would have been expected if a glacial event had occurred during that time period.

“We know that the carbon dioxide (CO2) levels are rising currently and are at the highest they have been in millions of years. We have records of how conditions have changed as CO2 levels have risen from 280 to 400 ppm, but I believe it also is important to know what could happen when those levels reach 600 to 1000 ppm,” MacLeod said. “At the rate that carbon dioxide levels are rising, we will reach 600 ppm around the end of this century. At that level of CO2, will ice sheets on Greenland and Antarctica be stable? If not, how will their melting affect the planet?”

Previously, many scientists have thought that doubling CO2 levels would cause earth’s temperature to increase as much as 3 degrees Celsius, or approximately 6 degrees Fahrenheit. However, the temperatures MacLeod believes existed in Tanzania 90 million years ago are more consistent with predictions that a doubling of CO2 levels would cause the earth’s temperature could rise an average of 6 degrees Celsius, or approximately 11 degrees Fahrenheit.

“While studying the past can help us predict the future, other challenges with modern warming still exist,” MacLeod said. “The Late Cretaceous climate was very warm, but the earth adjusted as changes occurred over millions of years. We’re seeing the same size changes, but they are happening over a couple of hundred years, maybe 10,000 times faster. How that affects the equation is a big and difficult question.”

MacLeod’s study was published in the October issue of the journal Geology.

Scientists push closer to understanding mystery of deep earthquakes

Scientists broke new ground in the study of deep earthquakes, a poorly understood phenomenon that occurs where the oceanic lithosphere, driven by tectonics, plunges under continental plates – examples are off the coasts of the western United States, Russia and Japan.

This research is a large step toward replicating the full power of these earthquakes to learn what sets them off and how they unleash their violence. It was made possible only by the construction of a one-of-a-kind X-ray facility that can replicate high-pressure and high-temperature while allowing scientists to peer deep into material to trace the propagation of cracks and shock waves.

“We are capturing the physics of deep earthquakes,” said Yanbin Wang, a senior scientist at the University of Chicago who helps run the X-ray facility where the research occurred. “Our experiments show that, for the first time, laboratory-triggered brittle failures during the olivine-spinel (mineral) phase transformation has many similar features to deep earthquakes.”

Wang and a team of scientists from Illinois, California and France simulated deep earthquakes at the U.S. Department of Energy’s Argonne National Laboratory by using pressure of 5 gigapascals, more than double the previous studies of 2 GPa. For comparison, pressure of 5 GPa is 4.9 million times the pressure at sea level.

At this pressure, rock should be squeezed too tight to rapture and erupt into violent earthquakes. But it does. And that has puzzled scientists since the phenomenon of deep earthquakes was discovered nearly 100 years ago. Interest spiked with the May 24 eruption in the waters near Russia of the world’s strongest deep earthquake – roughly five times the power of the great San Francisco quake of 1906.

These deep earthquakes occur in older and colder areas of the oceanic plate that gets pushed into the earth’s mantle. It has been speculated that the earthquakes are triggered when a mineral common in the upper mantle, olivine, undergoes a phase transformation that weakens the whole rock temporarily, causing it to fail.

“Our current goal is to understand why and how deep earthquakes happen. We are not at a stage to predict them yet; it is still a long way to go,” Wang said.

The work was conducted at the GeoSoilEnviroCARS beamline operated by the University of Chicago at Argonne’s Advanced Photon Source.

“GSECARS is the only beamline in the world that has the combined capabilities of in-situ X-ray diffraction and imaging, controlled deformation, in terms of stress, strain and strain rate, at high pressure and temperature, and acoustic emission detection,” Wang said. ” It took us several years to reach this technical capability.”

This new technology is a dream come true for the paper’s coauthor, geologist Harry Green, a distinguished professor of the graduate division at the University of California, Riverside.

More than 20 years ago, he 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 explained how to simulate these earthquakes in a paper titled “Deep-Focus Earthquake Analogs Recorded at High Pressure and Temperature 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 experiment – 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.”

“Our next goal is to study the ‘real’ material, the silicate olivine (Mg,Fe)2SiO4, which requires much higher pressures,” Wang said.

200,000-year environmental history of continental shelf based on a deep-sea core from Okinawa Trough

This shows terrigenous palynomorphs of short-distance transportation: light microscope and scanning electron microscope photos of pollen and spore, plant debris and charcoals. -  © Science China Press
This shows terrigenous palynomorphs of short-distance transportation: light microscope and scanning electron microscope photos of pollen and spore, plant debris and charcoals. – © Science China Press

A new research paper shows that a great number of nearby terrigenous pollen and charcoal have been found from the deep-sea sediments of the last 200 kyrs in Okinawa Trough. It is tesitfied that the continental shelf of the East China Sea was exposed and covered with the huge wetland and grassland ecosystems during the the last two glacial periods. They discovered that the variation of terrestrial sources is concordent with global glacial volume and sea-level changes at orbital-scale since 200 kyrs before present. Their work, entitled “A ~200 ka pollen record from Okinawa Trough: Paleoenvironment reconstruction of glacial-interglacial cycles”, was published in SCIENCE CHINA Earth Sciences.2013, Vol 56 (doi: 10.1007/s11430-013-4619-0)

This research work concerns mainly the Quaternary environment and global chages based on pollen analysis from a deep-sea core in Okinawa Trough. The project was directed by Department of Earth Sciences, Sun Yat-sen University, with colaboration of University Claude Bernard-Lyon 1 and Laboratory of Climate and Environment Sciences in Gif-sur-Yvette. The first author is professor ZHENG Zhuo from Sun Yat-sen University. Their research work was supported by the National Natural Science Foundation of China (grant no. 40772113, 41072128).

The discoreries show that terrestrial-source materials vary greatly during the transition of glacial and interglacial periods, proving the sensitive response on the global ice volume and sea-level changes. This deep-sea record has firstly documented high percentage of sedge, grass and many freshwater algaes in the glacial interval, which indicates that the offshore distance of Okinawa Trough has obviously shortened due to the exposed continental shelf during the glacial stages. The vegetation on the exposed continental shelf was dominated by intrazonal communities such as halophyte grasslands and freshwater wetlands. New evidence demonstrated that the fundamental changes of sediment sources in Okinawa Trough since ~200 ka BP were affected by combine factors including the offshore coastline distance, monsoon variability and sea-level changes.

This new research provides an oldest record of Quaternry environment reconstruction so far in the Okinawa Trough. It has a great scientific significance on highlighting the evolution history of continental shelf extension, the tracing of the sediment source areas of the Okinawa Trough and global climate changes since the last 200 kyrs.

Study to enhance earthquake prediction and mitigation in Pakistani region

This is a sketch map of southeast Asia showing major faults and tectonic blocks, including the Chaman Fault. -  Courtesy of Shuhab Khan
This is a sketch map of southeast Asia showing major faults and tectonic blocks, including the Chaman Fault. – Courtesy of Shuhab Khan

A three-year, $451,000 grant from the United States Agency for International Development to study the Chaman Fault in Western Pakistan will help earthquake prediction and mitigation in this heavily populated region.

The research, part of the Pakistan-U.S. Science and Technology Cooperation Program, will also increase the strength and breadth of cooperation and linkages between Pakistani scientists and institutions with counterparts in the U.S. The National Academy of Sciences implements the U.S. side of the program.

Shuhab Khan, associate professor of geology at University of Houston, will lead the project in the U.S. His counterpart in Pakistan is Abdul Salam Khan of the University of Balochistan.

“The Chaman Fault is a large, active fault around 1,000 kilometers, or 620 miles, long. It crosses back and forth between Afghanistan and Pakistan, ultimately merging with some other faults and going to the Arabian Sea,” Khan said.

The study area is located close to megacities in both countries.

“Seismic activity across this region has caused hundreds of thousands of deaths and catastrophic economic losses,” Khan said. “However, the Chaman Fault is one of the least studied fault systems. Through this research, we will determine how fast the fault is moving and which part is more active.”

The Chaman Fault is the largest, strike-slip fault system in Central Asia. It is a little more than half the size of the San Andreas Fault in California.

“In strike-slip faults, the Earth’s crust moves laterally. Earthquakes along these types of faults are shallow and more damaging,” he said. “Rivers can also be displaced and change course with activity related to this type of fault.”

The study team will gather data using remote sensing satellite technology and field measurements made at various sites along the fault.

“In addition to current movement, the techniques will allow us to go back tens of thousands of years to determine which areas have moved and how much,” Khan said.

Field measurement techniques include sampling and analysis of rocks and sand along the fault system.

“Through cosmogenic age dating, we can determine how much time rocks along the fault have been exposed to sunlight by measuring for cosmic rays and radiation. Those measurements help us determine how much time it took the rocks to move in the area,” Khan said.

Sand buried below the surface will be sampled without being exposed to light. In the lab, measurements using optically stimulated luminescence will reveal how long the sand has been buried.

Three students from the University of Balochistan will come to the U.S. to learn the field techniques. “We will take them to the San Andreas Fault for training because the locations and faults are similar,” Khan said. “They will return to Pakistan and gather samples from designated areas along the fault.”

The samples will be analyzed at the University of Cincinnati lab of geology professor Lewis Owen, co-investigator on the grant. The research team also includes University of Houston geosciences students. Two undergraduate students will help process the rock samples, and a Ph.D. student will work with the remote sensing data.

“Through the data collection, we will learn more about the movement along this fault in the recent past. That information will help with earthquake prediction and mitigation,” Khan said.

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.

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.

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.