New theory of why midcontinent faults produce earthquakes

A new theory developed at Purdue University may solve the mystery of why the New Madrid fault, which lies in the middle of the continent and not along a tectonic plate boundary, produces large earthquakes such as the ones that shook the eastern United States in 1811 and 1812.

The theory suggests that the energy necessary to produce the magnitude 7-7.5 earthquakes came from stored stress built up in the Earth’s crust long ago. Rapid erosion from the Mississippi River at the end of the last ice age reduced forces that had kept the New Madrid fault from slipping and triggered the temblors.

Eric Calais, the Purdue professor of earth and atmospheric sciences who led the study, said the theory is the first to explain how a fault could have had large earthquakes in the recent past but today show no signs of accumulating the forces needed to produce another earthquake.

“We understand why earthquakes happen at the contact between tectonic plates, like in California, but it has always been a puzzle as to why earthquakes occur in the middle of the continent as well, and with no visible surface deformation,” Calais said. “Our theory links an external climate-driven process, the melting of the ice sheet, and earthquakes.”

Calais and others have analyzed the fault for more than 10 years using global position system measurements to capture movements of the Earth’s surface that represent a buildup of energy and have traditionally been used to evaluate the potential for an earthquake. As the data was collected, it became evident that such motion was not occurring along the New Madrid fault.

Andrew Freed, co-author of the paper and an associate professor of earth and atmospheric sciences at Purdue, said with no discernable motions at the surface to explain how the requisite crustal stresses could have built up in this area, these stresses must be left over from past tectonic processes that are no longer active.

“The only way to reconcile the fact that this part of the continent is not deforming but is producing earthquakes is for the stresses to have built up long ago, ” Freed said. “Old geologic processes, such as the opening of the Atlantic and the uplift of the Rocky Mountains, may have squeezed the Midwest. The resulting stress remained stored for millions of years until uplift associated with the Mississippi erosion event led to the unclamping of old faults lying beneath.”

If this area of the North American continent is preloaded with the stress that can lead to earthquakes, it will be difficult to assess earthquake risk in the region.

The fault segments that ruptured are unlikely to have future earthquakes as there is no current means to reload them, but there remains a risk that other faults in the region could experience large earthquakes in the future, Calais said.

“Unfortunately, this stored stress is invisible to us, and the usual methods of measuring strain and deformation to evaluate a spot’s potential for an earthquake may not apply to this region,” Calais said. “Under these conditions, once an earthquake occurs on a given fault, it’s done; but this also means that other faults in the region that appear quiet today may still be triggered.”

Details of the team’s work, which was supported by a grant from the U.S. Geological Survey, appear in a paper in the current issue of Nature.

For a period from 16,000 to 10,000 years ago as the ice sheet melted, it steadily rushed water down the Mississippi River. As the river flowed, it washed away sediments and removed weight pressing down on the Earth’s crust. With this relatively rapid removal of weight, the crust rebounded and bulged slightly up from its previous position. This slight arching caused the top layers of the Earth’s crust to be stretched and the bottom layers to be compacted, exerting forces on the preexisting faults sufficient to trigger the earthquakes that began more than 3,000 years ago in the New Madrid region, culminating with the 1811-1812 events, Calais said.

More data needs to be collected to see whether this mechanism applies to similar seismic zones in the world, he said.

Super glaciers leave their mark on the Gondwanan supercontinent

This is a proposed cover illustration, provided by the editors, for GSA Special Paper 468, 'Late Paleozoic Glacial Events and Postglacial Transgressions in Gondwana.' The collection covers state-of-the-art critical topics related to the Late Paleozoic Glaciation and deglaciation-triggered sea-level rise that affected Gondwana. -  Geological Society of America
This is a proposed cover illustration, provided by the editors, for GSA Special Paper 468, ‘Late Paleozoic Glacial Events and Postglacial Transgressions in Gondwana.’ The collection covers state-of-the-art critical topics related to the Late Paleozoic Glaciation and deglaciation-triggered sea-level rise that affected Gondwana. – Geological Society of America

This new Special Paper from The Geological Society of America comprises a wide range of topics related to one of the most extreme paleoclimatic episodes in Earth’s history, the Late Paleozoic Ice Age (LPIA). With over 100 illustrations, chapters paint a broad swath across Gondwana while focusing on specific topics related to the effects of LPIA glaciation and deglaciation-triggered sea-level rise on the supercontinent.

The book’s objective, say editors Oscar R. López-Gamundí of Hess Corporation and Luis A. Buatois of the University of Saskatchewan, is “not to give a state-of-the-art review of the Late Paleozoic Ice Age,” which has been done with competency elsewhere, but, rather, to turn the reader’s attention toward facets of the LPIA that require further study.

Topics include the sedimentologic, paleoenvironmental, and paleoclimatic aspects of the glacial event; the influence of postglacial transgressions on the salinity of coastlines; the nature of glacial and glacially influenced ecosystems, with a look at the faunas (including the Levipustula Fauna) and floras of the time; analysis and illustration of trace fossil assemblies; and discussion of relatively less well-known glacial deposits in some Gondwanan regions. One chapter even challenges the popular interpretation that there was a single massive ice sheet over much of Gondwana during the late Paleozoic glaciation.

Expedition to Mid-Cayman Rise identifies unusual variety of deep sea vents

As storm clouds gather and seas deteriorate, a team recovers the hybrid vehicle Nereus aboard the R/V Cape Hatteras during an expedition to the Mid-Cayman Rise in October 2009. A search for new vent sites along the 110 km ridge, the expedition featured the first use of Nereus in 'autonomous' or free-swimming mode. - Photo Credit: Woods Hole Oceanographic Institution
As storm clouds gather and seas deteriorate, a team recovers the hybrid vehicle Nereus aboard the R/V Cape Hatteras during an expedition to the Mid-Cayman Rise in October 2009. A search for new vent sites along the 110 km ridge, the expedition featured the first use of Nereus in ‘autonomous’ or free-swimming mode. – Photo Credit: Woods Hole Oceanographic Institution

The first expedition to search for deep-sea hydrothermal vents along the Mid-Cayman Rise has turned up three distinct types of hydrothermal venting, reports an interdisciplinary team led by Woods Hole Oceanographic Institution (WHOI) in this week’s Proceedings of the National Academy of Sciences. The work was conducted as part of a NASA-funded effort to search extreme environments for geologic, biologic, and chemical clues to the origins and evolution of life.

Hydrothermal activity occurs on spreading centers all around the world. However, the diversity of the newly discovered vent types, their geologic settings and their relative geographic isolation make the Mid-Cayman Rise a unique environment in the world’s ocean.

“This was probably the highest risk expedition I have ever undertaken,” said chief scientist Chris German, a WHOI geochemist who has pioneered the use of autonomous underwater vehicles (AUVs) to search for hydrothermal vent sites. “We know hydrothermal vents appear along ridges approximately every 100 km. But this ridge crest is only 100 km long, so we should only have expected to find evidence for one site at most. So finding evidence for three sites was quite unexpected – but then finding out that our data indicated that each site represents a different style of venting – one of every kind known, all in pretty much the same place – was extraordinarily cool.”

The Mid-Cayman Rise (MCR) is an ultraslow spreading ridge located in the Cayman Trough – the deepest point in the Caribbean Sea and a part of the tectonic boundary between the North American Plate and the Caribbean Plate. At the boundary where the plates are being pulled apart, new material wells up from Earth’s interior to form new crust on the seafloor.

The team identified the deepest known hydrothermal vent site and two additional distinct types of vents, one of which is believed to be a shallow, low temperature vent of a kind that has been reported only once previously – at the “Lost City” site in the mid-Atlantic ocean.

“Being the deepest, these hydrothermal vents support communities of organisms that are the furthest from the ocean surface and sources of energy like sunlight,” said co-author Max Coleman of NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “Most life on Earth is sustained by food chains that begin with sunlight as their energy source. That’s not an option for possible life deep in the ocean of Jupiter’s icy moon Europa, prioritized by NASA for future exploration. However, organisms around the deep vents get energy from the chemicals in hydrothermal fluid, a scenario we think is similar to the seafloor of Europa, and this work will help us understand what we might find when we search for life there.”

Approach

While vent sites occupy small areas on the sea floor, the plumes formed when hot acidic vent fluids mix with cold deep-ocean seawater can rise hundreds of meters until they reach neutral buoyancy. Because these plumes contain dissolved chemicals, particulate minerals and microbes, they can then be detected for kilometers or more away from their source as they disperse horizontally in the ocean. The chemical signatures of these plumes vary according to the type of vent site from which they originated.

The three known types of vent sites are distinguished by the kinds of rock that host the sites. The first type of vents occur throughout the world’s mid-ocean ridges and are hosted by rocks that are rich in magnesium and iron –called mafic rocks. The second and third types of vent sites are hosted in rocks called ultramafic that form deep below the seafloor and are composed of material similar to the much hotter lavas that erupted on Earth’s very earliest seafloor, thousands of millions of years ago.

The discovery of ultramafic-hosted vent sites such as those on the Mid-Cayman Rise could provide insight into the very earliest life on our planet and the potential for similar life to become established elsewhere,” said German.

For this mission, German and his colleagues used the plumes in the search for hydrothermal vents, employing sensors mounted on equipment and robotic vehicles to track the chemicals back to their source. This expedition used a CTD (conductivity, temperature, and depth) array augmented with sensors to detect suspended particles and anomalous chemical compositions (the latter sensor courtesy of Ko-ichi Nakamura from AIST in Tsukuba, Japan) mounted on both a water sampling rosette and the hybrid vehicle Nereus, a deep-diving robot that can operate in both in tethered and free-swimming modes.

Using the CTDs and Nereus in “autonomous” or free-swimming mode, the team sniffed out deep-sea plumes originating from the seafloor hydrothermal vents. Using a combination of shipboard and shore-based analysis of water samples for both their chemical and microbial contents, the team was then able to track the plumes toward their sources as well as to determine the likely nature of the venting present at each site. The ultimate goal was to switch Nereus into tethered or “remotely operated” (ROV) mode during the latter stages of the cruise and dive on each vent site to collect samples using Nereus’ robotic manipulator arm.

“Part of the excitement of this NASA-funded project was the success of deploying a full-ocean-capable tethered vehicle to search for vents at 5000 m from the R/V Cape Hatteras, which, at 41 meters in length, is one of the smallest ocean-going ships in the national fleet. This is a first,” said Cindy Lee Van Dover, co-author on the study and director of the Duke University Marine Laboratory.

The first two sites the team identified are extremely deep and were named Piccard and Walsh in honor of the only two humans to dive to the Challenger Deep – the deepest part of the world’s ocean. The plume detected at the Piccard site – 800 meters deeper than the previously known deepest vent – was comparable to plumes from the “Type 1″ vent sites, first found in the Pacific Ocean in 1977.

“We were particularly excited to find compelling evidence for high-temperature venting at almost 5000m depth. We have absolutely zero microbial data from high-temperature vents at this depth,” said Julie Huber, a scientist in the Josephine Bay Paul Center at the Marine Biological Laboratory (MBL) in Woods Hole. Huber and MBL postdoctoral scientist Julie Smith participated in this cruise to collect samples, and all of the microbiology work for this paper was carried out in Huber’s laboratory. “With the combination of extreme pressure, temperature, and chemistry, we are sure to discover novel microbes in this environment,” Huber added. “We look forward to returning to the Cayman and sampling these vents in the near future. We are sure to expand the known growth parameters and limits for life on our planet by exploring these new sites.”

The Walsh plume also exhibited signals characteristic of a high temperature site, but with a chemical composition (notably the high methane-to-manganese ratio) typically found at a high temperature, ultramafic hosted “Type 2″ vent site. The third site – which the team have named Europa, after the moon of Jupiter – most closely resembles the “Lost City” vent site in the mid-Atlantic ocean- to date the only confirmed low-temperature “Type 3″ site.

Half way through the six-day leg in which Nereus was converted into ROV mode, tropical storm Ida intervened and stopped the team from viewing or sampling the vent site. Though they had come within <250m of the vents at the seafloor, they had to ride out the storm for the last three days of the cruise and return to port frustrated. Happily, however, all was not lost – the research team shared their findings with an international team led by Jon Copley of the National Oceanography Centre in Southampton, UK, who returned to the MCR in Spring 2010 and imaged active vents at both the Piccard and Europa locations using a deep-towed camera called Hybis.

“Given the range and diversity of systems present, and now that we have established exactly where the sites are and what they look like, we really can’t wait to get back and collect first samples with our ROV Jason,” said German. “This region has the potential to develop into an exciting natural laboratory with plenty of potential for repeat visits and long-term experiments over the decade ahead.”

By exploring this extreme and previously uninvestigated section of the Earth’s deep seafloor, the researchers seek to extend our understanding of the limits to which life can exist on Earth and to help prepare for future efforts to explore for life on other planets.

Breakthrough achieved in explaining why tectonic plates move the way they do

The sinking of the Farallon plate beneath the North American continent over 30 million years created the geologic feature known as the Basin and Range Province, an area of the western United States that encompasses much of Nevada, seen here in a topographic model. -  Mike Sandiford/University of Melbourne
The sinking of the Farallon plate beneath the North American continent over 30 million years created the geologic feature known as the Basin and Range Province, an area of the western United States that encompasses much of Nevada, seen here in a topographic model. – Mike Sandiford/University of Melbourne

A team of researchers including Scripps Institution of Oceanography, UC San Diego geophysicist Dave Stegman has developed a new theory to explain the global motions of tectonic plates on the earth’s surface.

The new theory extends the theory of plate tectonics – a kinematic description of plate motion without reference to the forces behind it – with a dynamical theory that provides a physical explanation for both the motions of tectonic plates as well as motion of plate boundaries. The new findings have implications for how scientists understand the geological evolution of Earth, and in particular, the tectonic evolution of western North America, in the past 50 million years.

The research, led by Monash University’s Wouter Schellart, is published in the July 16 issue of the journal Science.

These findings provide a new explanation as to why tectonic plates move along the Earth’s surface at the speeds that are observed, the details of which were previously not well-understood.

“The earth’s surface is covered with tectonic plates that move with respect to one another at centimeters per year,” Schellart said. “These plates converge at deep-sea trenches, plate boundaries where one plate sinks (subducts) below the other at so-called subduction zones. The velocities of these plates and the velocities of the boundaries between these plates vary significantly on Earth.”

Schellart and his team, including Stegman and Rebecca Farrington, Justin Freeman and Louis Moresi from Monash University, used observational data and advanced computer models to develop a new mathematical scaling theory, which demonstrates that the velocities of the plates and the plate boundaries depend on the size of subduction zones and the presence of subduction zone edges.

“The scalings for how subducted plates sink in the earth’s mantle are based on essentially the same fluid dynamics that describe how a penny sinks through a jar of honey,” said Stegman, who developed the computer models that helped the team reenact tens of millions of years of tectonic movement. “The computer models demonstrate that the subducted portion of a tectonic plate pulls on the portion of the plate that remains on the earth’s surface. This pull results in either the motion of the plate, or the motion of the plate boundary, with the size of the subduction zone determining how much of each.”

“In some ways, plate tectonics is the surface expression of dynamics in the earth’s interior but now we understand the plates themselves are controlling the process more than the mantle underneath. It means Earth is really more of a top-down system than the predominantly held view that plate motion is being driven from the bottom-up.”

This discovery explains why the Australian, Nazca and Pacific plates move up to four times faster than the smaller African, Eurasian and Juan de Fuca plates.

“It also provides explanations for the motions of the ancient Farallon plate that sank into the mantle below North and South America. This plate slowed down during eastward motion from about 10 centimeters (four inches) per year some 50 million years ago to only 2 centimeters (0.8 inches) per year at present,” Schellart said.

The decrease in plate velocity resulted from the decrease in subduction zone size, which decreased from 14,000 kilometers (8,700 miles) to only 1,400 kilometers (870 miles).

“This had a dramatic effect on the topography and the structure of the North American continent,” said Schellart. “Until 50 million years ago, the west coast of North America was characterized by a massive mountain chain similar to the present day Andes in South America, and ran from Canada in the north to southern Mexico in the south.”

As the subduction zone decreased in size, the compressive stresses along the west coast of North America decreased, resulting in destruction of the mountain range and formation of the Basin and Range province, a 2 million-square-kilometer (772,000-square-mile) area of elongated basins and ridges that characterizes the present-day western North American landscape.

Indian Ocean sea-level rise threatens coastal areas

Indian Ocean sea levels are rising unevenly and threatening coastal areas and islands. Sea-level rise is especially high along the coastlines of the Bay of Bengal, the Arabian Sea, Sri Lanka, Sumatra and Java. - Credit: NASA
Indian Ocean sea levels are rising unevenly and threatening coastal areas and islands. Sea-level rise is especially high along the coastlines of the Bay of Bengal, the Arabian Sea, Sri Lanka, Sumatra and Java. – Credit: NASA

Indian Ocean sea levels are rising unevenly and threatening residents in some densely populated coastal areas and islands, a new study concludes.

The study, led by scientists at the University of Colorado at Boulder (CU) and the National Center for Atmospheric Research (NCAR) in Boulder, Colo., finds that the sea-level rise is at least partly a result of climate change.

Sea-level rise is particularly high along the coastlines of the Bay of Bengal and the Arabian Sea, as well as the islands of Sri Lanka, Sumatra and Java, the authors found.

The rise–which may aggravate monsoon flooding in Bangladesh and India–could have future impacts on both regional and global climate.

The key player in the process is the Indo-Pacific warm pool, an enormous, bathtub-shaped area spanning a huge area of the tropical oceans stretching from the east coast of Africa west to the International Date Line in the Pacific.

The warm pool has heated by about 1 degree Fahrenheit, or 0.5 degrees Celsius, in the past 50 years, primarily because of human-generated emissions in greenhouses gases.

“Our results from this study imply that if future anthropogenic warming effects in the Indo-Pacific warm pool dominate natural variability, mid-ocean islands such as the Mascarenhas Archipelago, coasts of Indonesia, Sumatra and the north Indian Ocean may experience significantly more sea-level rise than the global average,” says scientist Weiqing Han of CU and lead author of a paper published this week in the journal Nature Geoscience.

While several areas in the Indian Ocean region are experiencing sea-level rise, sea level is lowering in other areas. The study indicated that the Seychelles Islands and Zanzibar off Tanzania’s coastline show the largest sea level drop.

“Global sea-level patterns are not geographically uniform,” says NCAR scientist Gerald Meehl, a co-author of the paper. “Sea-level rise in some areas correlates with sea-level fall in other areas.”

Funding for the research came from the National Science Foundation (NSF), NCAR’s sponsor, as well as the Department of Energy and NASA.

“This work is a step forward towards getting improved estimates of sea-level changes in one of the most heavily populated regions of the globe,” says Eric Itsweire, director of NSF’s physical oceanography program.

“Quantifying the heat and fresh water balance, as well as the large-scale circulation changes, in the Indo-Pacific warm pool through the use of observations and numerical models is crucial to understanding the subtle sea-level changes occurring in that region,” Itsweire say.

The patterns of sea-level change are driven by the combined enhancement of two primary atmospheric wind patterns known as the Hadley circulation and the Walker circulation.

The Hadley circulation in the Indian Ocean is dominated by air currents rising above strongly heated tropical waters near the equator and flowing poleward at upper levels, then sinking to the ocean in the subtropics and causing surface air to flow back toward the equator.

The Indian Ocean’s Walker circulation causes air to rise and flow westward at upper levels, sink to the surface and then flow eastward back toward the Indo-Pacific warm pool.

“The combined enhancement of the Hadley and Walker circulation form a distinct surface wind pattern that drives specific sea-level patterns,” Han says.

In their paper, the authors write that “our new results show that human-caused changes of atmospheric and oceanic circulation over the Indian Ocean region–which have not been studied previously–are the major cause for the regional variability of sea-level change.”

The study indicates that in order to anticipate global sea-level change, researchers also need to know the specifics of regional sea-level changes.

“It is important for us to understand the regional changes of the sea level, which will have effects on coastal and island regions,” says NCAR scientist Aixue Hu.

The research team used several sophisticated ocean and climate models for the study, including the Parallel Ocean Program–the ocean component of the widely used Community Climate System Model, which is supported by NCAR and the U.S. Department of Energy (DOE).

In addition, the team used a wind-driven, linear ocean model for the study.

The complex circulation patterns in the Indian Ocean may also affect precipitation by forcing even more atmospheric air down to the surface in Indian Ocean subtropical regions than normal, Han speculates.

“This may favor a weakening of atmospheric convection in subtropics, which may increase rainfall in the eastern tropical regions of the Indian Ocean and drought in the western equatorial Indian Ocean region, including east Africa,” Han says.

Footloose glaciers crack up

This is a view of Columbia Glacier terminus from sea level. The calving front is approximately five kilometers (3.1 miles) wide and between 20 and 70 meters (66 and 230 feet) tall.  This image was taken after the calving front came afloat. -  Shad O'Neel, USGS
This is a view of Columbia Glacier terminus from sea level. The calving front is approximately five kilometers (3.1 miles) wide and between 20 and 70 meters (66 and 230 feet) tall. This image was taken after the calving front came afloat. – Shad O’Neel, USGS

Glaciers that lose their footing on the seafloor and begin floating behave very erratically, according to a new study led by a Scripps Institution of Oceanography, UC San Diego researcher.

Floating glaciers produce larger icebergs than their grounded cousins and do so at unpredictable intervals, according to Scripps glaciologist Fabian Walter and colleagues in a paper to be published in the journal Geophysical Research Letters.

This study presents the first detailed observation of the transition from grounded to floating glaciers. Such a transition is currently taking place at Columbia Glacier, one of Alaska’s many tidewater glaciers. Tidewater glaciers flow directly into the ocean, ending at a cliff in the sea, where icebergs are formed. Prior to this study, Alaskan tidewater glaciers were believed to be exclusively “grounded” (resting on the ocean floor), and unable to float without disintegrating.

However, Columbia Glacier unexpectedly developed a floating extension in 2007 that has endured far longer than researchers expected. The research team believes that this floating section may have been caused by the speed at which the glacier is receding. Columbia is one of the fastest receding glaciers in the world, having retreated 4 kilometers (2.49 miles) since 2004, and nearly 20 kilometers (12.43 miles) since 1980.

“We’re seeing more tidewater glaciers retreat,” Walter said. “As they retreat, they thin and that increases the likelihood that they’ll come afloat.”

The study, co-authored by U.S. Geological Survey (USGS) glaciologist and Scripps alumnus Shad O’Neel, is part of a larger effort to understand and include calving in large-scale glacier models, which are essential in producing accurate forecasts of sea-level rise. The research team conducted its study on Columbia Glacier by installing a seismometer, a sensor that measures seismic waves that are produced by shifts in geologic formations, including earthquakes, landslides, and glacier calving. They studied data collected from 2004-2005 and 2008-2009 that allowed them to compare the glacier’s activity before and after it began floating.

The formation of icebergs, through a process known as “calving,” is a leading source of additional water for the global ocean basin. As this study confirms, grounded glaciers and floating glaciers often show fundamentally different calving mechanics. However, iceberg calving is also one of the least understood processes involved in ice mass loss and consequential sea level rise. This study, which is funded by the National Science Foundation, sheds light on the process by comparing the size and frequency of icebergs calved by a glacier during both floating and grounded conditions.

Calving occurs when fractures in the ice join up and cause a piece of ice to completely separate from the main glacier to form an iceberg. Unlike the floating glaciers, grounded glaciers calve icebergs nearly continuously, but they are generally quite small.

Through this study, scientists can begin to analyze the mechanics of the calving process in glaciers (both floating and grounded) and ice shelves, which will allow them to better understand and predict iceberg production from glaciers and ice sheets. These predictions, in turn, will provide a more accurate estimate of sea-level rise in the coming years.

Scientists’ work improves odds of finding diamonds

While prospectors and geologists have been successful in finding diamonds through diligent searching, one University of Houston professor and his team’s work could help improve the odds by focusing future searches in particular areas. Kevin Burke, professor of geology and tectonics at UH, and his fellow researchers describe these findings in a paper titled “Diamonds Sampled by Plumes from the Core-Mantle Boundary,” appearing July 15 in Nature, the weekly scientific research journal.

While prospectors and geologists have been successful in finding diamonds through diligent searching, one University of Houston professor and his team’s work could help improve the odds by focusing future searches in particular areas.

Kevin Burke, professor of geology and tectonics at UH, and his fellow researchers describe these findings in a paper titled “Diamonds Sampled by Plumes from the Core-Mantle Boundary,” appearing July 15 in Nature, the weekly scientific research journal.

Burke’s team found that kimberlites, which are rare volcanic rocks that include diamonds, owe their origin to occasional pulses of hot mantle rock – called mantle plumes – that have risen through the entire thickness of the Earth’s mantle from deep down next to the core, or innermost part, of the planet. This core/mantle boundary lies at a depth of about 2,000 miles. While the idea there might be mantle plumes rising from the core/mantle boundary was first suggested about 40 years ago, it is only within the past few years that evidence of plumes coming all the way from this boundary to the Earth’s surface has been clearly demonstrated by Burke’s group.

“Our approach is new, because it combines observations of the Earth’s deep interior from seismology with evidence of how tectonic plates have moved about on the Earth’s surface during the past 500 million years,” Burke said. “I have been interested in mantle plumes from the core/mantle boundary since they were first hypothesized in 1971. About 10 years ago, I realized there might be a link between the seismically defined structure at the core/mantle boundary and volcanic rocks at the Earth’s surface that had been suggested to be linked to mantle plumes. I immediately realized how the existence of that link could be tested, and it was then that I came in contact with Trond Torsvik in Norway, who proved to be uniquely qualified to carry out the required tests.”

Torsvik, a professor at the University of Oslo in Norway, and Burke developed the conceptual ideas for this research. Additional members of the team were Bernhard Steinberger at the Helmholtz Centre Potsdam in Germany, and Lew Ashwal and Sue Webb from the University of the Witwatersrand in South Africa. The research consisted of applying and interpreting the results of mathematical analysis, much of it applying spherical geometry to the Earth’s surface, to publicly available data-sets put together mainly by Ashwal, Webb and Torsvik.

The present structure of the Earth’s mantle has been increasingly understood by researchers in seismology during the past 25 years, and Burke and his colleagues’ work has helped confirm the seismologists’ results. The work of the Burke group, however, also describes the structure as it was in the past, revealing the history of deep mantle structure over the geologically long period of 500 million years. That, Burke said, is new.

“Establishing the history of deep mantle structure has shown, unexpectedly, that two large volumes lying just above the core/mantle boundary have been stable in their present positions for the past 500 million years,” he said. “The reason this result was not expected is that those of us who study the Earth’s deep interior have assumed that, although the deep mantle is solid, the material making it up would all be in motion all the time, because the deep mantle is so hot and under such high pressure from the weight of rock above it.”

As for how this improves the odds of finding these precious gems, Burke explained that geologists interested in diamonds have known for more than 50 years that rare diamond-bearing kimberlite volcanic rocks are highly concentrated in ancient cratons within areas of the Earth’s continents. This has concentrated the search for diamond-bearing rocks within an area amounting to no more than about 10 percent of the entire area of the world’s continents. The new work has shown that most of the kimberlites have been erupted into one or the other of those old cratons only under certain conditions. These findings will enable the search for diamonds to be further concentrated.

Ultimately aiming for a better integrated understanding of how the solid Earth of the crust and mantle works, the group hopes to obtain further results within months. They hope to better establish how plate motions at the Earth’s surface have evolved over the last 500 million years and how to work out just how those movements have related to both the stable and the moving parts of the Earth’s mantle during the same interval.

Noninvasive probing of geological core samples

The directionality of the coil-generated magnetic fields is shown, along with the induced currents (influenced by the directional conductivity) when the transmitter coils for measuring the axial Z-directional conductivity (middle) and for measuring the X- and Y-directional conductivity (right) are energized. -  No Credit is given
The directionality of the coil-generated magnetic fields is shown, along with the induced currents (influenced by the directional conductivity) when the transmitter coils for measuring the axial Z-directional conductivity (middle) and for measuring the X- and Y-directional conductivity (right) are energized. – No Credit is given

Oil and natural gas companies rely upon geological core analysis to help them understand and evaluate oil and gas reserves. A rock sample can reveal myriad details about a geological structure’s formation, content, and history.

Conductivity, a material’s ability to carry an electrical current, is one of the most useful measurements of core samples. The conductivity of a geological formation depends on the direction of measurement, so it’s considered “anisotropic.”

“Anisotropy is usually the result of many thin layers of oil-bearing sandstone rocks sandwiched between thin shale layers, which form a laminate structure that may be tens to thousands of feet thick and contain a large amount of oil,” explains John Kickhofel, a research scientist. In anisotropic layers of the Earth, the conductivity in one direction is different from that of others.

Tools to measure a core sample’s electrical anisotropy have been sadly lacking, says Kickhofel. To solve this problem, he and colleagues at the company Schlumberger found inspiration in a type of logging technology currently used by the modern oil industry. They created a device capable of noninvasively measuring electrical conductivity — a device they describe in the journal Review of Scientific Instruments, which is published by the American Institute of Physics.

“We designed a probe that uses a core from the formations and measures its electrical anisotropy without destroying the core,” Kickhofel says. “This device can make continuous measurements on cores that are hundreds or thousands of feet long.”

Cores are valuable because they provide firsthand information about the structure and nature of rock layers and need to be preserved for future reference. “It’s very important to preserve the cores, and the new device provides a way to do this. We can’t overstate the value of such a measurement to the oil industry in an era when most fields are in decline — yet the world’s demand for oil and gas continues to skyrocket,” adds Kickhofel.

Sea levels rising in parts of Indian Ocean, according to new study

Rising sea levels in parts of the Indian Ocean appear to be at least partly the result of rising greenhouse emissions. -  University of Colorado
Rising sea levels in parts of the Indian Ocean appear to be at least partly the result of rising greenhouse emissions. – University of Colorado

Newly detected rising sea levels in parts of the Indian Ocean, including the coastlines of the Bay of Bengal, the Arabian Sea, Sri Lanka, Sumatra and Java, appear to be at least partly a result of human-induced increases of atmospheric greenhouse gases, says a study led by the University of Colorado at Boulder.

The study, which combined sea surface measurements going back to the 1960s and satellite observations, indicates anthropogenic climate warming likely is amplifying regional sea rise changes in parts of the Indian Ocean, threatening inhabitants of some coastal areas and islands, said CU-Boulder Associate Professor Weiqing Han, lead study author. The sea level rise — which may aggravate monsoon flooding in Bangladesh and India — could have far-reaching impacts on both future regional and global climate.

The key player in the process is the Indo-Pacific warm pool, an enormous, bathtub-shaped area of the tropical oceans stretching from the east coast of Africa west to the International Date Line in the Pacific. The warm pool has heated by about 1 degree Fahrenheit, or 0.5 degrees Celsius, in the past 50 years, primarily caused by human-generated increases of greenhouse gases, said Han.

“Our results from this study imply that if future anthropogenic warming effects in the Indo-Pacific warm pool dominate natural variability, mid-ocean islands such as the Mascarenhas Archipelago, coasts of Indonesia, Sumatra and the north Indian Ocean may experience significantly more sea level rise than the global average,” said Han of CU-Boulder’s atmospheric and oceanic sciences department.

A paper on the subject was published in this week’s issue of Nature Geoscience. Co-authors included Balaji Rajagopalan, Xiao-Wei Quan, Jih-wang Wang and Laurie Trenary of CU-Boulder, Gerald Meehl, John Fasullo, Aixue Hu, William Large and Stephen Yeager of the National Center for Atmospheric Research in Boulder, Jialin Lin of Ohio State University, and Alan Walcraft and Toshiaki Shinoda of the Naval Research Laboratory in Mississippi.

While a number of areas in the Indian Ocean region are showing sea level rise, the study also indicated the Seychelles Islands and Zanzibar off Tanzania’s coastline show the largest sea level drop. Global sea level patterns are not geographically uniform, and sea rise in some areas correlate with sea level fall in other areas, said NCAR’s Meehl.

The Indian Ocean is the world’s third largest ocean and makes up about 20 percent of the water on Earth’s surface. The ocean is bounded on the west by East Africa, on the north by India, on the east by Indochina and Australia, and on the south by the Southern Ocean off the coast of Antarctica.

The patterns of sea level change are driven by the combined enhancement of two primary atmospheric wind patterns known as the Hadley circulation and the Walker circulation. The Hadley circulation in the Indian Ocean is dominated by air currents rising above strongly heated tropical waters near the equator and flowing poleward, then sinking to the ocean in the subtropics and causing surface air to flow back toward the equator.

The Indian Ocean’s Walker circulation causes air to rise and flow westward at upper levels, sink to the surface and then flow eastward back toward the Indo-Pacific warm pool. “The combined enhancement of the Hadley and Walker circulation form a distinct surface wind pattern that drives specific sea level patterns,” said Han.

The international research team used several different sophisticated ocean and climate models for the study, including the Parallel Ocean Program — the ocean component of NCAR’s widely used Community Climate System Model. In addition, the team used a wind-driven, linear ocean model for the study.

“Our new results show that human-caused changes of atmospheric and oceanic circulation over the Indian Ocean region — which have not been studied previously — are the major cause for the regional variability of sea level change,” wrote the authors in Nature Geoscience.

Han said that based on all-season data records, there is no significant sea level rise around the Maldives. But when the team looked at winter season data only, the Maldives show significant sea level rise, a cause for concern. The smallest Asian country, the Maldives is made up of more than 1,000 islands — about 200 of which are inhabited by about 300,000 people — and are on average only about five feet above sea level.

The complex circulation patterns in the Indian Ocean may also affect precipitation by forcing even more atmospheric air down to the surface in Indian Ocean subtropical regions than normal, Han speculated. “This may favor a weakening of atmospheric convection in the subtropics, which may increase rainfall in the eastern tropical regions of the Indian Ocean and increase drought in the western equatorial Indian Ocean region, including east Africa,” Han said.

The new study indicates that in order to document sea level change on a global scale, researchers also need to know the specifics of regional sea level changes that will be important for coastal and island regions, said NCAR’s Hu. Along the coasts of the northern Indian Ocean, seas have risen by an average of about 0.5 inches, or 13 millimeters, per decade.

“It is important for us to understand the regional changes of the sea level, which will have effects on coastal and island regions,” said Hu.

Researchers witness overnight breakup, retreat of Greenland glacier

Images courtesy of DigitalGlobe
Images courtesy of DigitalGlobe

NASA-funded researchers monitoring Greenland’s Jakobshavn Isbrae glacier report that a 7 square kilometer (2.7 square mile) section of the glacier broke up on July 6 and 7, as shown in the image above. The calving front – where the ice sheet meets the ocean – retreated nearly 1.5 kilometers (a mile) in one day and is now further inland than at any time previously observed. The chunk of lost ice is roughly one-eighth the size of Manhattan Island, New York.

Research teams led by Ian Howat of the Byrd Polar Research Center at Ohio State University and Paul Morin, director of the Antarctic Geospatial Information Center at the University of Minnesota have been monitoring satellite images for changes in the Greenland ice sheet and its outlet glaciers. While this week’s breakup itself is not unusual, Howat noted, detecting it within hours and at such fine detail is a new phenomenon for scientists.

“While there have been ice breakouts of this magnitude from Jakonbshavn and other glaciers in the past, this event is unusual because it occurs on the heels of a warm winter that saw no sea ice form in the surrounding bay,” said Thomas Wagner, cryospheric program scientist at NASA Headquarters. “While the exact relationship between these events is being determined, it lends credence to the theory that warming of the oceans is responsible for the ice loss observed throughout Greenland and Antarctica.”

The researchers relied on imagery from several satellites, including Landsat, Terra, and Aqua, to get a broad view of ice changes at both poles. Then, in the days leading up to the breakup, the team received images from DigitalGlobe’s WorldView 2 satellite showing large cracks and crevasses forming.

DigitalGlobe Inc. provides the images as part of a public-private partnership with U.S. scientists. Howat and Morin are receiving near-daily satellite updates from the Jakobshavn, Kangerlugssuaq, and Helheim glaciers (among the islands largest) and weekly updates on smaller outlet glaciers.

Jakobshavn Isbrae is located on the west coast of Greenland at latitude 69°N and has been retreated more than 45 kilometers (27 miles) over the past 160 years, 10 kilometers (6 miles) in just the past decade. As the glacier has retreated, it has broken into a northern and southern branch. The breakup this week occurred in the north branch.

Scientists estimate that as much as 10 percent of all ice lost from Greenland is coming through Jakobshavn, which is also believed to be the single largest contributor to sea level rise in the northern hemisphere. Scientists are more concerned about losses from the south branch of the Jakobshavn, as the topography is flatter and lower than in the northern branch.

In addition to the remote sensing work, Howat, Morin, and other researchers have been funded by NASA and the National Science Foundation to plant GPS sensors, cameras, and other scientific equipment on top of the ice sheet to monitor changes and understand the fundamental workings of the ice. NASA also has been conducting twice-yearly airborne campaigns to the Arctic and Antarctic through the IceBridge program and measuring ice loss with the ICESat and GRACE satellites.