Earthquakes generate big heat in super-small areas

Geophysicists Terry Tullis, left, and David Goldsby have shown that rock surfaces sliding past each other in an earthquake can create intense heat but only at the pinpoint places where their surfaces actually touch. -  Mike Cohea, Brown University
Geophysicists Terry Tullis, left, and David Goldsby have shown that rock surfaces sliding past each other in an earthquake can create intense heat but only at the pinpoint places where their surfaces actually touch. – Mike Cohea, Brown University

Most earthquakes that are seen, heard, and felt around the world are caused by fast slip on faults. While the earthquake rupture itself can travel on a fault as fast as the speed of sound or better, the fault surfaces behind the rupture are sliding against each other at about a meter per second.

But the mechanics that underlie fast slip during earthquakes have eluded scientists, because it’s difficult to replicate those conditions in the laboratory. “We still largely don’t understand what is going at earthquake slip speeds,” said David Goldsby, a geophysicist at Brown, “because it’s difficult to do experiments at these speeds.”

Now, in experiments mimicking earthquake slip rates, Goldsby and Brown geophysicist Terry Tullis show that fault surfaces in earthquake zones come into contact only at microscopic points between scattered bumps, called asperities, on the fault. These tiny contacts support all the force across the fault. The experiments show that when two fault surfaces slide against other at fast slip rates, the asperities may reach temperatures in excess of 2,700 degrees Fahrenheit, lowering their friction, the scientists write in a paper published in Science. The localized, intense heating can occur even while the temperature of the rest of the fault remains largely unaffected, a phenomenon known as flash heating.

“This study could explain a lot of the questions about the mechanics of the San Andreas Fault and other earthquakes,” said Tullis, professor emeritus of geological sciences, who has studied earthquakes for more than three decades.

The experiments simulated earthquake speeds of close to half a meter per second. The rock surfaces touched only at the asperities, each with a surface area of less than 10 microns – a tiny fraction of the total surface area. When the surfaces move against each other at high slip rates, the experiments revealed, heat is generated so quickly at the contacts that temperatures can spike enough to melt most rock types associated with earthquakes. Yet the intense heat is confined to the contact flashpoints; the temperature of the surrounding rock remained largely unaffected by these microscopic hot spots, maintaining a “room temperature” of around 77 degrees Fahrenheit, the researchers write.

“You’re dumping in heat extremely quickly into the contacts at high slip rates, and there’s simply no time for the heat to get away, which causes the dramatic spike in temperature and decrease in friction,” Goldsby said.

“The friction stays low so long as the slip rate remains fast,” said Goldsby, associate professor of geological sciences (research). “As slip slows, the friction immediately increases. It doesn’t take a long time for the fault to restrengthen after you weaken it. The reason is the population of asperities is short-lived and continually being renewed, and therefore at any given slip rate, the asperities have a temperature and therefore friction appropriate for that slip rate. As the slip rate decreases, there is more time for heat to diffuse away from the asperities, and they therefore have lower temperature and higher friction.”

Flash heating and other weakening processes that lead to low friction during earthquakes may explain the lack of significant measured heat flows along some active faults like the San Andreas Fault, which might be expected if friction was high on faults during earthquakes. Flash heating in particular may also explain how faults rupture as “slip pulses,” wrinkle-like zones of slip on faults, which would also decrease the amount of heat generated.

If that is the case, then many earthquakes have been misunderstood as high-friction events. “It’s a new view with low dynamic friction. How can it be compatible with what we know?” asked Tullis, who chairs the National Earthquake Prediction Evaluation Council, an advisory body for the U.S. Geological Survey.

“Flash heating may explain it,” Goldsby replied.

Metal shortages alert from leading geologists

Geologists are warning of shortages and bottlenecks of some metals due to an insatiable demand for consumer products.

A meeting of leading geologists, reported in the scientific journal Nature Geoscience, highlights the dangers in the inexorable surge in demand for metals.

Dr. Gawen Jenkin, of the Department of Geology, University of Leicester, is the lead convenor of the Fermor Meeting of the Geological Society of London which met to discuss this issue.

Dr Jenkin said: “Mobile phones contain copper, nickel, silver and zinc, aluminium, gold, lead, manganese, palladium, platinum and tin. More than a billion people will buy a mobile in a year – so that’s quite a lot of metal. And then there’s the neodymium in your laptop, the iron in your car, the aluminium in that soft drinks can – the list goes on…

“With ever-greater use of these metals, are we running out? That was one of the questions we addressed at our meeting. It is reassuring that there’s no immediate danger of ‘peak metal’ as there’s quite a lot in the ground, still – but there will be shortages and bottlenecks of some metals like indium due to increased demand.

“That means that exploration for metal commodities is now a key skill. It’s never been a better time to become an economic geologist, working with a mining company. It’s one of the better-kept secrets of employment in a recession-hit world.

“And a key factor in turning young people away from the large mining companies – their reputation for environmental unfriendliness – is being turned around as they make ever-greater efforts to integrate with local communities for their mutual benefit.”

So, our appetite for technological goodies will be satisfied for some time to come still – as long as sufficient people with the skills to seek out the metals emerge into the marketplace.

Scientists find possible trigger for volcanic ‘super-eruptions’

The “super-eruption” of a major volcanic system occurs about every 100,000 years and is considered one of the most catastrophic natural events on Earth, yet scientists have long been unsure about what triggers these violent explosions.

However, a new model presented this week by researchers at Oregon State University points to a combination of temperature influence and the geometrical configuration of the magma chamber as a potential cause for these super-eruptions.

Results of the research, which was funded by the National Science Foundation, were presented at the annual meeting of the Geological Society of America in Minneapolis, Minn.

Patricia “Trish” Gregg, a post-doctoral researcher at OSU and lead author on the modeling study, says the creation of a ductile halo of rock around the magma chamber allows the pressure to build over tens of thousands of years, resulting in extensive uplifting in the roof above the magma chamber. Eventually, faults from above trigger a collapse of the caldera and subsequent eruption.

“You can compare it to cracks forming on the top of baking bread as it expands,” said Gregg, a researcher in OSU’s College of Oceanic and Atmospheric Sciences. “As the magma chamber pressurizes at depth, cracks form at the surface to accommodate the doming and expansion. Eventually, the cracks grow in size and propagate downward toward the magma chamber.

“In the case of very large volcanoes, when the cracks penetrate deep enough, they can rupture the magma chamber wall and trigger roof collapse and eruption,” Gregg added.

The eruption of super-volcanoes dwarfs the eruptions of recent volcanoes and can trigger planetary climate change by inducing Ice Ages and other impacts. One such event was the Huckleberry Ridge eruption of present-day Yellowstone Park about two million years ago, which was more than 2,000 times larger than the 1980 eruption of Mount St. Helens in Washington.

“Short of a meteor impact, these super-eruptions are the worst environmental hazards our planet can face,” Gregg said. “Huge amounts of material are expelled, devastating the environment and creating a gas cloud that covers the globe for years.”

Previous modeling efforts have focused on an eruption trigger from within the magma chamber, which scientists thought would leave a visible trace in the form of a precursor eruption deposits, according to Shanaka “Shan” de Silva, an OSU geologist and co-author on the study. Yet there has been a distinct lack of physical evidence for a pre-cursor eruption at the site of these super-volcanoes.

The model suggests the reason there may be no precursor eruption is that the trigger comes from above, not from within, de Silva pointed out.

“Instead of taking the evidence in these eruptions at face value, most models have simply taken small historic eruptions and tried to scale the process up to super-volcanic proportions,” de Silva said. “Those of us who actually study these phenomena have known for a long time that these eruptions are not simply scaled-up Mt. Mazamas or Krakataus – the scaling is non-linear. The evidence is clear.”

It takes a “perfect storm” of conditions to grow an eruptible magma chamber of this size, Gregg says, which is one reason super-volcano eruptions have occurred infrequently throughout history. The magma reservoirs feeding the eruptions could be as large as 10,000- to 15,000-square cubic kilometers, and the chamber requires repeated intrusions of magma from below to heat the surrounding rock and make it malleable. It is that increase in ductility that allows the chamber to grow without magma evacuation in a more conventional manner.

When magma chambers are smaller, they may expel magma before maximum pressure is reached through frequent small eruptions.

The Yellowstone eruption is one of the largest super-volcano events in history and it has happened several times. Other super-volcano sites include Lake Toba in Sumatra, the central Andes Mountains, New Zealand and Japan.

Gregg said that despite its explosive history, it doesn’t appear that Yellowstone is primed for another super-eruption anytime soon, though the slow process of volcanic uplift is taking place every day.

“The uplift of the surface at Yellowstone right now is on the order of millimeters,” she explained. “When the Huckleberry Ridge eruption took place, the uplift of the whole Yellowstone region would have been hundreds of meters high, and perhaps as much as a kilometer.”

The strange rubbing boulders of the Atacama

These are huge boulders in Chile's Atacama desert which appear to be rubbed very smooth about their midsections, leading University of Arizona geologist Jay Quade to wonder what could cause this in a place where water, Earth's most common agent of erosion, is as almost nonexistent. -  Image courtesy of Jay Quade.
These are huge boulders in Chile’s Atacama desert which appear to be rubbed very smooth about their midsections, leading University of Arizona geologist Jay Quade to wonder what could cause this in a place where water, Earth’s most common agent of erosion, is as almost nonexistent. – Image courtesy of Jay Quade.

A geologist’s sharp eyes and upset stomach has led to the discovery, and almost too-close encounter, with an otherworldly geological process operating in a remote corner of northern Chile’s Atacama Desert.

The sour stomach belonged to University of Arizona geologist Jay Quade. It forced him and his colleagues Peter Reiners and Kendra Murray to stop their truck at a lifeless expanse of boulders which they had passed before without noticing anything unusual.

“I had just crawled underneath the truck to get out of the sun,” Quade said. The others had hiked off to look around, as geologists tend to do. That’s when Quade noticed something very unusual about the half-ton to 8-ton boulders near the truck: they appeared to be rubbed very smooth about their midsections. What could cause this in a place where Earth’s most common agent of erosion — water — is as almost nonexistent?

About the only thing that came to mind was earthquakes, said Quade. Over the approximately two million years that these rocks have been sitting on their sandy plain perhaps they were jostled by seismic waves. They caused them gradually grind against each other and smooth their sides. It made sense, but Quade never thought he’d be able to prove it.

Then, on another trip to the Atacama, Quade was standing on one of these boulders, pondering their histories when a 5.3 magnitude earthquake struck. The whole landscape started moving and the sound of the grinding of rocks was loud and clear.

“It was this tremendous sound, like the chattering of thousands of little hammers,” Quade said. He’d probably have made a lot more observations about the minute-long event, except he was a bit preoccupied by the boulder he was standing on, which he had to ride like a surfboard.”The one I was on rolled like a top and bounced off another boulder. I was afraid I would fall off and get crushed.”

He managed to stay atop his boulder, of course, and became thoroughly convinced that the earlier hypothesis about the boulders was correct.

“I was just astonished when this earthquake came along and showed us how it worked,” Quade said. Quade will explain the phenomenon on Tuesday, 11 Oct. at the annual meeting of the Geological Society of America in Minneapolis.

The whole story appears to be that the boulders tumbled down from the hills above — probably dislodged by earthquakes. They accumulated on the sand flat, with no place else to go. Quade compares the situation to a train station where people are crowded together closely, rubbing shoulders as they waiting for a train. In this case the boulders have been stuck at the station for hundreds of millennia and the train never comes. So they just get more crowded and rub shoulders more over time.

Analyses of the boulder top surfaces suggest that they have been there one to two million years. That age, combined with the fact that seismic activity in the area generates a quake like that Quade witnessed on the average of once every four months, suggests that the average boulder has experienced 50,000 to 100,000 hours of bumping and grinding while waiting for that nonexistent train.

“It also answers a mystery that had been eating at me for years: How do the boulders get transported off the hills when there is so little rain,” Quade said. “How do you erode a landscape that is rainless?”

Again the answer is seismic activity.

“It raises the question in my mind of other planets like Mars.” If there is seismic activity, even from meteor impacts, might it also be creating similar landscapes? “I would predict that these kinds of crowds of boulders might be found on Mars as well, if people look for them.”

New technique unlocks secrets of ancient ocean

ASU graduate student Greg Brennecka stands in the W.M. Keck Foundation Laboratory for Environmental Biogeochemistry at ASU in front of the powdered carbonate rock samples collected in Dawen, Southern China. These samples are prior to chemical processing to ready for measurement on the mass spectrometer. There were about 40 samples total for this study. -  P.S. Noonan
ASU graduate student Greg Brennecka stands in the W.M. Keck Foundation Laboratory for Environmental Biogeochemistry at ASU in front of the powdered carbonate rock samples collected in Dawen, Southern China. These samples are prior to chemical processing to ready for measurement on the mass spectrometer. There were about 40 samples total for this study. – P.S. Noonan

Earth’s largest mass extinction event, the end-Permian mass extinction, occurred some 252 million years ago. An estimated 90 percent of Earth’s marine life was eradicated. To better understand the cause of this “mother of all mass extinctions,” researchers from Arizona State University and the University of Cincinnati used a new geochemical technique. The team measured uranium isotopes in ancient carbonate rocks and found that a large, rapid shift in the chemistry of the world’s ancient oceans occurred around the extinction event.

The mechanism of the end-Permian mass extinction has been much debated. One proposed cause for the extinction, the release of toxic hydrogen sulfide gas, is directly related to oceanic anoxia, which is a depletion of dissolved oxygen from the ocean.

Widespread evidence exists for oceanic anoxia before the extinction, but the timing and extent of anoxia remain unknown. Previous hypotheses posited that the deep ocean was depleted of oxygen for millions of years before the end-Permian extinction. The new research using measurements of uranium isotopes in ancient carbonate rocks indicates that the period of ocean-wide anoxia was much shorter.

“Our study shows that the ocean was anoxic for at most tens of thousands of years before the extinction event. That’s much shorter than prior estimates,” says Gregory Brennecka, the lead author of the study and a graduate student in ASU’s School of Earth and Space Exploration in the College of Liberal Arts and Sciences.

Brennecka, working in Professor Ariel Anbar’s research group, conducted the analysis of the samples. Anbar is a professor in ASU’s School of Earth and Space Exploration and the Department of Chemistry and Biochemistry. Achim Herrmann, a senior lecturer at Barrett, the Honors College at ASU, and Thomas Algeo of the University of Cincinnati, who collected the samples in China, helped guide the selection of samples and interpretation of data.

The team studied samples of carbonate rock from Dawen in southern China for uranium isotope ratios (238U/235U) and thorium to uranium ratios (Th/U). The study presumes that carbonate rocks capture 238U/235U and Th/U of the seawater in which they were deposited. If so, they can be used to study changes in the chemistry of ancient oceans. In separate, related work, the team is testing the limits of this assumption.

In a section of rock spanning the time of the extinction, the team found a marked shift in 238U/235U in the carbonate rocks immediately prior to the mass extinction, which signals an increase in oceanic anoxia. The team also found higher Th/U ratios in the same interval, which indicate a decrease in the uranium content of seawater. Lower concentrations of uranium in seawater also serve as signals of oceanic anoxia.

These decreases in 238U/235U and increases in Th/U only occur at the section of rock that contains the end-Permian extinction horizon. This shows that a period of oceanic anoxia existed only briefly prior to the mass extinction, rather than the previously hypothesized much longer timeframe.

The team’s findings represent an increase in knowledge about the ocean’s chemistry at a critical period of the Earth’s history. “This technique gives us a better understanding of how ocean chemistry can change over time, and how sensitive it is to certain environmental factors,” says Brennecka.

The implications of the new geochemical tool the researchers developed are just as important as the study’s findings.

Uranium isotope ratios have been utilized to study the ocean’s chemistry before, but only in black shale, a different and less common type of rock. This study represents the first time uranium isotope ratios have been studied in carbonates for paleo-redox purposes, which is a promising new geochemical tool for future research.

“One of the important outcomes of this study is that we were able to quantify the relative change in the amount of oceanic anoxia across the extinction event in the global ocean. Previous studies were only able to show whether anoxic conditions existed or not. We can now compare this event to other events in Earth history and develop a better understanding of how the amount of oxygen in the Earth’s ocean has changed through time and how this might have affected marine diversity,” says Herrmann.

Carbonates are much more widespread than black shales on Earth through space and time. “By focusing on carbonates we can study ancient anoxic events in many more places and times,” says Anbar. “This was our major motivation in developing the uranium isotope technique.”

It is only recently that researchers have developed the ability to precisely measure slight variations in uranium ratios, largely due to research completed at ASU. Most of the team’s research in this study was conducted at ASU. The study samples were analyzed at ASU’s W. M. Keck Foundation Laboratory for Environmental Biogeochemistry.

“Over the past decade, my research group has worked with many collaborators to develop new techniques to study changes in oxygen in the Earth’s ocean through time,” says Anbar. “We are especially interested in the connections between ocean oxygenation and biological evolution. The uranium isotope technique is the newest method. We expect it will be very useful. This study shows that it is yielding insights pretty quickly.”

“It is exciting to be here, because most of the development work to measure uranium isotopes was done at ASU over the past five years. It is exciting to be at the forefront of these advancements,” says Brennecka.

The team’s results will be published in the Proceedings of National Academy of Sciences Oct. 10 in a paper titled, “Rapid expansion of oceanic anoxia immediately before the end-Permian mass extinction.”

Luminous grains of sand determine year of historic storm flood

Scientists at Delft University of Technology (TU Delft, The Netherlands) have successfully matched a layer of sediment from the dunes near Heemskerk to a severe storm flood that occurred in either 1775 or 1776. This type of information helps us gain more insight into past storm floods and predict future surges more accurately. The scientists’ findings have been be published in the online edition of the scientific magazine Geology, and will be cover story of the November paper edition.

Historic knowledge


Our historic knowledge about storm floods (and water levels) on the Dutch coast is relatively limited. Records were not kept consistently until the late nineteenth century. This is unfortunate, because the limited historical archive makes it difficult to formulate statistical conclusions and predictions about future storm floods. It is also harder for us to establish whether storm floods are becoming more severe over the years.

Heemskerk


With the support of Technology Foundation STW and in cooperation with scientists from the Geological Survey of the Netherlands (TNO) and Deltares, scientists at TU Delft have now shown that historic storm-flood data can be augmented using luminescence dating. The team, led by Dr Jakob Wallinga, published their findings last week in the scientific magazine Geology.

The method was applied to a layer of sediment in the dunes near Heemskerk, created during a storm flood centuries ago and exposed by a storm in 2007. The level to which the storms and waves pushed the water can be deduced from the height of this layer. During the storm-surge in question, the water was higher than the catastrophic flood of 1953.

In order to put these data on a historical timeline, however, it is essential to know when the storm occurred. The scientists have now been able to show that in all probability the layer of sediment was deposited in 1775 or 1776. Historical sources indicate that severe storm floods took place in both years.

Grains of sand


Optical stimulated luminescence was used for the dating procedure. It simply requires a sample of sand from the sediment layer. The technique is based on the phenomenon that grains of sand can emit a faint light signal when they are illuminated with a certain frequency of light. The strength of the luminescence signal grows stronger over time as a result of natural radioactivity (background radiation) from the surroundings. However, the signal is reset to zero when the grains of sand are exposed to sunlight.

The strength of the luminescence signal (and the local strength of the background radiation) indicates the length of time since the grains were last exposed to light; in other words, the moment when they were ‘buried’. Using luminescence dating, a precision of 5% is achievable.

C-14


Luminescence dating requires nothing more than grains of sand, which means it can be used instead of the popular C-14 method at different sites and in different situations. After all, C-14 dating requires organic material. Luminescence dating can be used to date sediment from anywhere between just a few years to over 150,000 years old. It is also used in other disciplines, such as archaeology and art history.

Critical minerals ignite geopolitical storm

The clean energy economy of the future hinges on a lot of things, chief among them the availability of the scores of rare earth elements and other elements used to make everything from photovoltaic panels and cellphone displays to the permanent magnets in cutting edge new wind generators. And right out of the gate trouble is brewing over projected growth in demand for these minerals and the security of their supplies.

Last year, for instance, China restricted the export of neodymium, which is used in wind energy generators. The move was ostensibly to direct the supplies to toward a massive wind generation project within China. The effect, however, is to create a two-tiered price for neodymium: one inside China and another, higher price, for the rest of the world, explained economics professor Roderick Eggert of the Colorado School of Mines. The result could be that China not only will control the neodymium supply, but the manufacture of neodymium technology as well.

The geopolitical implications of critical minerals have started bringing together scientists, economists and policy makers who are trying to cut a path through the growing thicket of challenges. In that spirit, on Monday, 10 October, 2011, Eggert and other professors will be presenting their research alongside senior staff from the U.S. House of Representatives and Senate, the Executive Office of the President of the U.S., the U.S. Geological Survey, in a session at the meeting of The Geological Society of America in Minneapolis.

Among the basics that need to be grasped to understand the current state of affairs is how rare these minerals and elements really are. Some are plentiful, but only found in rare places or are difficult to extract. Indium, for instance, is a byproduct of zinc mining and extraction. It is not economically viable to extract unless zinc is being sought in the same ore, Eggert explained, Others are just plain scarce, like rhenium and tellurium, which only exist in very small amounts in the Earth’s crust.

There are basically two responses to this sort of situation: use less of these minerals or improve the extraction of them from other ores in other parts of the world. The latter would seem to be where most people are heading.

“China’s efforts to restrict exports of mineral commodities garnered the attention of Congress and highlighted the need for the United States to assess the state of the Nation’s mineral policies and examine opportunities to produce rare earths and other strategic and critical minerals domestically,” reads the session abstract of Kathleen Benedetto of the Subcommittee on Energy and Mineral Resources, Committee on Natural Resources, U.S. House of Representatives. “Nine bills have been introduced in the House and Senate to address supply disruptions of rare earths and other important mineral commodities.”

Benedetto will be explaining the meaning and status of those bills, and what it will take to get them signed into law.

“Deposits of rare earth elements and other critical minerals occur throughout the Nation,” reads the abstract for another prominent session presenter: Marcia McNutt, director of the U.S. Geological Survey. She will be putting the current events in the larger historical perspective of mineral resource management, which has been the USGS’s job for more than 130 years. “The definition of ‘a critical mineral or material’ is extremely time dependent, as advances in materials science yield new products and the adoption of new technologies result in shifts in both supply and demand.”

The White House Office of Science and Technology Policy (OSTP) has answered the call as well. An abstract by OSTP Assistant Director Cyrus Wadia provides a five-point strategy to begin addressing the matter. The first point is to mitigating long term risks associated with the use of critical materials. The second, diversify supplies of raw materials. Third, to promote a domestic supply chain for areas of strategic importance like clean energy. Fourth, inform decision makers; and fifth, prepare the workforce of the next generation.

Southern California’s tectonic plates revealed in detail

The geologic forces that shape the Earth's surface do their work in the lithosphere, often out of sight and far below the surface. Researchers have now measured the lithosphere's thickness in southern California. It varies widely, from less than 25 miles to nearly 60 miles. -  Fischer Lab, Brown University
The geologic forces that shape the Earth’s surface do their work in the lithosphere, often out of sight and far below the surface. Researchers have now measured the lithosphere’s thickness in southern California. It varies widely, from less than 25 miles to nearly 60 miles. – Fischer Lab, Brown University

Rifting is one of the fundamental geological forces that have shaped our planet. Were it not for the stretching of continents and the oceans that filled those newly created basins, Earth would be a far different place. Yet because rifting involves areas deep below the Earth’s surface, scientists have been unable to understand fully how it occurs.

What is known is that with rifting, the center of the action lies in the lithosphere, which makes up the tectonic plates and includes the crust and part of the upper mantle. In a paper in Science, researchers at Brown University produce the highest-resolution picture of the bottom of the lithosphere in southern California, one of the most complex, captivating geologic regions in the world. The team found the lithosphere’s thickness differs markedly throughout the region, yielding new insights into how rifting shaped the southern California terrain.

“What we’re getting at is how (continental) plates break apart,” said Vedran Lekic, a postdoctoral researcher at Brown University and first author on the paper. “What happens below the surface is just not known.”

The team measured the boundary separating the lithosphere from the more ductile layer just below it known as the asthenosphere in a 400-by-300-mile grid, an area that includes Santa Barbara, Los Angeles, San Diego and the Salton Trough. The lithosphere’s thickness varies surprisingly from less than 25 miles to nearly 60 miles, the researchers write.

“We see these really dramatic changes in lithosphere thickness, and these occur over very small horizontal distances,” said Karen Fischer, professor of geological sciences at Brown and a paper author. “That means that the deep part of the lithosphere, the mantle part, has to be strong enough to maintain relatively steep sides.”

“This approach provides a new way to put observational constraints on how strong the rocks are at these depths,” she added.

Specifically, the researchers found two areas of particular interest. One is the Western Transverse Range Block. The plate lies below Santa Barbara, yet some 18 million years ago, it was located some 125 miles to the south and hugged the coastline. At some point, this plate swung clockwise, rotating more than 90 degrees and journeyed northward, like a mobile, swinging door. Interestingly, the lithosphere remained intact, while the area left behind the swinging plate, called the Inner Continental Borderland and which lies off the coast of Los Angeles, was stretched, the Brown geophysicists believe. Indeed, the lithosphere is nearly 30 percent thinner in the area left behind than the range block.

“The fact that the Western Tranverse Range Block retained its lithosphere along its journey tells us the mantle-lithosphere (of the block) must be very strong,” Lekic said.

Another interesting feature noted by the researchers is the Salton Trough, which encompasses the Salton Sea and the city of Palm Springs, and “is a classic example of rifting,” according to Fischer. Some 6 million years ago, the continental plate at this location was stretched, but the question remains whether it simply thinned or whether it actually broke apart, creating new lithosphere in between. In the paper, the researchers confirm that the lithosphere is thin, but “we can’t tell which of these scenarios happened,” Fischer said. However, the thickness of the mantle part of the lithosphere and the fact that deformation at the surface runs all the way to the base of the lithosphere in roughly the same geographical location are new constraints against which modelers can test their predictions, she added.

The team made use of permanent seismic recording stations set up by the Southern California Seismic Network and other networks, as well as seismometers from the EarthScope USarray Transportable Array, a grid of National Science Foundation-funded stations that is gathering earthquake information as it moves west to east across the nation. To measure the lithosphere’s depth, the authors looked at how waves generated by earthquakes – called S waves and P waves – convert from type S to type P across the boundary between the lithosphere and the asthenosphere.

The team will compare its results with those of another famous rift system in East Africa, from a study at the University of Bristol led by Kate Rychert, who earned her doctorate at Brown in 2007.

Scott French, who earned his baccalaureate at Brown and is now a doctoral student at Berkeley Seismological Laboratory in California, is an author on the paper. The National Science Foundation funded the study, through its Earthscope program and an Earth Sciences postdoctoral fellowship to Lekic.

Multibeam sonar can map undersea gas seeps

This is a perspective of the seafloor showing preliminary results of gas seeps detected by multibeam sonar in vicinity of Biloxi Dome in Northern Gulf of Mexico. Gas seep locations are shown as blue dots and are overlaid on the seafloor bathymetry that was collected -  Image produced by the University of New Hampshire Center for Coastal and Ocean Mapping/Joint Hydrographic Center using IVS Fledermaus software.
This is a perspective of the seafloor showing preliminary results of gas seeps detected by multibeam sonar in vicinity of Biloxi Dome in Northern Gulf of Mexico. Gas seep locations are shown as blue dots and are overlaid on the seafloor bathymetry that was collected – Image produced by the University of New Hampshire Center for Coastal and Ocean Mapping/Joint Hydrographic Center using IVS Fledermaus software.

A technology commonly used to map the bottom of the deep ocean can also detect gas seeps in the water column with remarkably high fidelity, according to scientists from the University of New Hampshire and the National Oceanic and Atmospheric Administration (NOAA). This finding, made onboard the NOAA ship Okeanos Explorer in the Gulf of Mexico, will lead to more effective mapping of these gas seeps and, ultimately, enhanced understanding of our ocean environments.

The mapping technology, multibeam sonar, is an echo-sounding technology that surveys a wide, fan-shaped swath of the seafloor, providing much greater coverage than the single-beam sonar systems previously used to map seeps. “We wanted to see whether we could map a large area of gaseous seeps effectively using this technology, and how well the multibeam sonar compared to our very sensitive single-beam sonars,” says Tom Weber of UNH’s Center for Coastal Mapping, who was lead scientist of this mission. “It turns out it works wonderfully.” The multibeam sonar on the Okeanos Explorer produced data to make high-resolution maps of gas in the water column in depths ranging from 3,000 to 7,000 feet.

Working jointly with scientists and technicians from NOAA’s Office of Ocean Exploration and Research (OER) and the Bureau of Ocean Energy Management (BOEM), Weber and colleagues mapped more than 17,000 square kilometers of the Gulf of Mexico from Aug. 22 through Sept. 10, 2011.

Sonar finds features on the ocean floor much the way a bat tracks its dinner: “It’s an acoustic wave hitting the target and reflecting back,” says Weber. Multibeam sonar sends those sound waves in many directions at the same time, enabling it to “see” a swath of targets that is much wider than what would be observed with a single-beam sonar. While it’s known to be an effective tool for mapping large, stable items like the bottom of the ocean, it wasn’t designed to detect targets within the water column.

Gas seeps – primarily but not exclusively methane – are numerous in the Gulf of Mexico, emanating from natural fissures in the seafloor. They can be associated with oil, but oil was not the focus for Weber and his collaborators. Finding and mapping gaseous seeps, says Weber, helps scientists better understand the ocean: its methane fluxes, carbon cycle, and deep-water marine environments.

Further, the Gulf of Mexico is home to many active oil-drilling sites, and mapping the gaseous seeps in the water column will inform scientific as well as regulatory decisions. “In the deep ocean, there are life forms like tubeworms and clams associated with gas seeps, and they’re treated as protected resources,” Weber says.

Further, mapping these seeps will give researchers baseline data on what exists in the water column, helping them determine whether future seeps are natural or unwanted byproducts of drilling.

“Mapping the seafloor and the water column are essential first steps in exploring our largely unknown ocean,” says Weber. “This expedition confirms earlier indications that multibeam technology provides a valuable new tool in the inventory to detect plumes of gas in the water column, and especially in deep water.”

Also on the mission from UNH were CCOM research scientist Jonathan Beaudoin and graduate students Kevin Jerram (pursuing an M.S. in ocean engineering) and Maddie Schroth-Miller (pursuing an M.S. in applied mathematics). NOAA’s expedition coordinator and lead NOAA scientist on the mission was Mashkoor Malik, who graduated from UNH in 2005 with a M.S. in ocean mapping.

Long-lost Lake Agassiz offers clues to climate change

Not long ago, geologically speaking, a now-vanished lake covered a huge expanse of today’s Canadian prairie. As big as Hudson Bay, the lake was fed by melting glaciers as they receded at the end of the last ice age. At its largest, Glacial Lake Agassiz, as it is known, covered most of the Canadian province of Manitoba, plus a good part of western Ontario. A southern arm straddled the Minnesota-North Dakota border.

Not far from the ancient shore of Lake Agassiz, University of Cincinnati Professor of Geology Thomas Lowell will present a paper about the lake to the Geological Society of America annual meeting in Minneapolis. Lowell’s paper is one of 14 to be presented Oct. 10 in a session titled: “Glacial Lake Agassiz — Its History and Influence on North America and on Global Systems: In Honor of James T. Teller.”

Although Lake Agassiz is gone, questions about its origin and disappearance remain. Answers to those questions may provide clues to our future climate. One question involves Lake Agassiz’ role in a thousand-year cold snap known as the Younger Dryas.

As the last ice age ended, thousands of years of warming temperatures were interrupted by an abrupt shift to cold. Tundra conditions expanded southward, to cover the land exposed as the forests retreated. This colder climate is marked in the fossil record by a flowering plant known as Dryas, which gives the period its name.

“My work focuses on abrupt or rapid climate change,” Lowell said. “The Younger Dryas offers an opportunity to study such change. The climate then went from warming to cooling very rapidly, in less than 30 years or so.”

Scientists noted that the Younger Dryas cold spell seemed to coincide with lower water levels in Lake Agassiz. Had the lake drained? And, if so, had the fresh water of the lake caused this climate change by disrupting ocean currents? This is the view of many scientists, Lowell said.

Lowell investigated a long-standing mystery involving Lake Agassiz — a significant drop in water level known as the Moorhead Low. It has long been believed that the Moorehead Low when water drained from Lake Agassiz through a new drainage pathway. Could this drainage have flowed through the St. Lawrence Seaway into the North Atlantic Ocean?

“The most common hypothesis for catastrophic lowering is a change in drainage pathways,” Lowell said.

The problem is, better dating of lake levels and associated organic materials do not support a rapid outflow at the right time.

“An alternative explanation is needed,” he said.

Lowell’s research shows that, although water levels did drop, the surface area of the lake increased more than seven-fold at the same time. His research suggests that the lower water levels were caused by increased evaporation, not outflow. While the melting glacier produced a lot of water, Lowell notes that the Moorhead Low was roughly contemporaneous with the Younger Dryas cold interval, when the atmosphere was drier and there was increased solar radiation.

“The dry air would reduce rainfall and enhance evaporation,” Lowell said. “The cold would reduce meltwater production, and shortwave radiation would enhance evaporation when the lake was not frozen and sublimation when the lake was ice-covered.”

Further research will attempt a clearer picture of this ancient episode, but researchers will have to incorporate various factors including humidity, yearly duration of lake ice, annual temperature, and a better understanding of how and where meltwater flowed from the receding glaciers.

Lowell’s efforts to understand changes in ancient climates have taken him from Alaska to Peru, throughout northern Canada and Greenland.

In Greenland, Lowell and a team of graduate students pulled cores of sediment from lakes that are still ice-covered for most of the year. Buried in those sediments are clues to long-ago climate.

“We look at the mineralogy of the sediments,” Lowell said, “and also the chironomids. They’re a type of midge and they’re very temperature sensitive. The exact species and the abundance of midges in our cores can help pinpoint temperature when these sediments were deposited.”