Nuclear test at Bikini Atoll, 1946. Image via U.S. Dept. of Energy.
Has humanity become so prevalent and powerful on Earth that we’re now globally affecting the geologic record, the actual rock record used by geologists to divide the past into named blocks? If the answer is yes, should scientists declare we’ve entered a new geologic epoch? This week, a group of 35 scientists said, yes, we are globally affecting the rock record and, yes, we should officially consider a new epoch. They would name it the Anthropocene, meaning Age of Humans, a word first introduced by two scientists in the year 2000 that’s now gaining wider scientific acceptance. The Anthropocene Work Group reported this conclusion on Monday (August 29, 2016) to the 35th International Geological Congress going on this week in Cape Town, South Africa.
If scientists decide to accept the Anthropocene into the Geologic Time Scale, they’ll have to decide when it began. Scientists speak of golden spikes in Earth’s sediment layers, events laid down in the rocks that clearly demarcate one geologic epoch from another.
A widely known example of a golden spike occurred with the demise of the dinosaurs, 65 million years ago. Most scientists believe an asteroid strike ended their dominance, due to the discovery in the late 1970s of iridium in the rock record on all parts of Earth. Iridium is rare on Earth (found mostly in Earth’s core), but common in the rest of the solar system. This layer of iridium in the rock record is said to mark the time of the asteroid impact; it’s the golden spike that marks the end of the Cretaceous epoch.
What would be the golden spike separating the Anthropocene – the Age of Humans – from the rest of history? The answer is arbitrary, and members of the Anthropocene Work Group do not entirely agree.
But 28 of the 35 scientists do agree that the golden spike for the Anthropocene comes around the 1950s. That’s when the great acceleration began on Earth, when our human impacts intensified and began to happen globally, not just locally, scientists say.
About 10 members of the Anthropocene Work Group said they felt the start of the Anthropocene would coincide with the beginning of nuclear bomb testing. It started in the late 1940s and caused radioactive elements to be dispersed across Earth and thus laid down in the rock record.
Other group members pointed to other ongoing signs of the Age of Humans, however, which will ultimately find their way into the rock record, including plastic pollution, soot from power stations, aluminium and concrete particles and high levels of nitrogen and phosphate in soils, derived from artificial fertilizers.
And so defining when and how the Anthropocene began – assuming scientists do accept it and include it in the Geologic Time Scale – is a task that lies ahead.
Plastics aren’t permanent. They’ll eventually break down into fragments that’ll become buried in Earth’s sediments. When future geologists uncover these fragments, they might point to the start of the Anthropocene. Image via Plastic Ocean Gyre blog.
Colin Waters from the British Geological Survey is secretary to the Anthropocene Work Group. He told the BBC:
“This is an update on where we are in our discussions.
We’ve got to a point where we’ve listed what we think the Anthropocene means to us as a working group.
The majority of us think it is real; that there is clearly something happening; that there are clearly signals in the environment that are recognizable and make the Anthropocene a distinct unit; and the majority of us think it would be justified to formally recognise it.
That doesn’t mean it will be formalized, but we’re going to go through the procedure of putting in a submission.”
If the Anthropocene were formally defined as a geological epoch beginning in 1945, then newer structures – such as the Grant Marsh Interstate 94 bridge over the Missouri River in Bismarck, N.D. (foreground) – would be classified as Anthropocene. Older structures with or without recent updates, such as the Bismarck railroad bridge (center) would be classified as Holocene and Anthropocene. Photo via Joel M. Galloway, USGS.
So the word Anthropocene, though not a part of the official scientific lexicon yet, is gaining acceptance among scientists. You can read more about the history of the word, which was coined in the year 2000 by atmospheric chemist Paul Crutzen and ecologist Eugene Stoermer in this article: What is the Anthropocene?
By the way, scientists now speak of our geologic age – basically everything since the end of the last major Ice Age, corresponding to the rise of complex human civilizations – as the Holocene. Holo is from a Greek root meaning whole or entire. You sometimes hear the Holocene called the Recent age.
Some have argued that the word Holocene is good enough to describe our human impact and that we don’t need the new term Anthropocene. There are arguments for and against including the Anthropocene in the Geologic Time Scale under the subheads multiple meanings and contrasting philosophies – and also hierarchy – in this article.
In the meantime, just remember the word Anthropocene.
You’ll be hearing more about it in the years ahead.
The Geologic Time Spiral from the U.S. Geologic Survey.
Bottom line: The Anthropocene Work Group reported their conclusions on August 29, 2016 to the 35th International Geological Congress in Cape Town, South Africa. The group said that the new epoch Anthropocene should be considered for official inclusion in the Geological Time Scale.
A beach in Juneau, Alaska. Sea levels in Alaska are not rising, but dropping precipitously due to a phenomenon known as glacial isostatic adjustment. (Joseph, Flickr CC BY-SA)
You’ve no doubt by now been inundated with the threat of global sea level rise. At the current estimated rate of one-tenth of an inch each year, sea level rise could cause large swaths of cities like New York, Galveston and Norfolk to disappear underwater in the next 20 years. But a new study out in the Journal of Geophysical Research shows that in places like Juneau, Alaska, the opposite is happening: sea levels are dropping about half an inch every year.
How could this be? The answer lies in a phenomenon of melting glaciers and seesawing weight across the earth called “glacial isostatic adjustment.” You may not know it, but the Last Ice Age is still quietly transforming the Earth’s surface and affecting everything from the length of our days to the topography of our countries.
During the glacier heyday 19,000 years ago, known as the Last Glacial Maximum, the Earth groaned under the weight of heavy ice sheets thousands of feet thick, with names that defy pronunciation: the Laurentide Ice Sheet, the Cordilleran Ice Sheet, the Fennoscandian Ice Sheet, and many more. These enormous hunks of frozen water pressed down on the Earth’s surface, displacing crustal rock and causing malleable mantle substance underneath to deform and flow out, changing the Earth’s shape—the same way your bottom makes a depression on a couch if you sit on it long enough. Some estimates suggest that an ice sheet about half a mile thick could cause a depression 900 feet deep—about the of an 83-story building.
The displaced mantle flows into areas surrounding the ice sheet, causing that land to rise up, the way stuffing inside a couch will bunch up around your weight. These areas, called “forebulges,” can be quite small, but can also reach more than 300 feet high. The Laurentide Ice Sheet, which weighed down most of Canada and the northern United States, for example, caused an uplift in the central to southern parts of the U.S. Elsewhere, ancient glaciers created forebulges around the Amazon delta area that are still visible today even though the ice melted long ago.
As prehistoric ice sheets began to melt around 11,700 years ago, however, all this changed. The surface began to spring back, allowing more space for the mantle to flow back in. That caused land that had previously been weighed down, like Glacier Bay Park in Alaska and the Hudson Bay in Canada, to rise up. The most dramatic examples of uplift are found in places like Russia, Iceland and Scandinavia, where the largest ice sheets existed. In Sweden, for example, scientists have found that the rising land severed an ancient lake called Malaren from the sea, turning it into a freshwater lake.
At the same time, places that were once forebulges are now sinking, since they are no longer being pushed up by nearby ice sheets. For example, as Scotland rebounds, England sinks approximately seven-tenths of an inch into the North Sea each year. Similarly, as Canada rebounds about four inches each decade, the eastern coast of the U.S. sinks at a rate of approximately three-tenths of an inch each year—more than half the rate of current global sea level rise. A study published in 2015 predicted that Washington, D.C. would drop by six or more inches in the next century due to forebulge collapse, which might put the nation’s monuments and military installations at risk.
Some of the most dramatic uplift is found in Iceland. (Martin De Lusenet, Flickr CC BY).
Recent estimates suggest that land in southeast Alaska is rising at a rate of 1.18 inches per year, a rate much faster than previously suspected. Residents already feel the dramatic impacts of this change. On the positive side, some families living on the coast have doubled or tripled their real estate: As coastal glaciers retreat and land once covered by ice undergoes isostatic rebound, lowland areas rise and create “new” land, which can be an unexpected boon for families living along the coast. One family was able to build a nine-hole golf course on land that has only recently popped out of the sea, a New York Times article reported in 2009. Scientists have also tracked the gravitational pull on Russell Island, Alaska, and discovered that it’s been weakening every year as the land moves farther from the Earth’s center.
Uplift will increase the amount of rocky sediment in areas previously covered in water. For example, researchers predict that uplift will cause estuaries in the Alaskan town of Hoonah to dry up, which will increase the amount of red algae in the area, which in turn, could damage the fragile ecosystems there. In addition, some researchers worry that the rapid uplift in Alaska will also change the food ecosystem and livelihood for salmon fishers.
At the same time, there are a lot of new salmon streams opening up in Glacier Bay, says Eran Hood, professor of environmental science at the University of Alaska. “As glaciers are melting and receding, the land cover is changing rapidly,” he says. “A lot of new areas becoming forested. As the ice recedes, salmon is recolonizing. It’s not good or bad, just different.”
The rate of uplift due to glacial isostatic adjustment around the world; Antarctica and Canada are expected to rise the most. (By Erik Ivins, JPL. [Public domain], via Wikimedia Commons)
Although not as visible, all the changes caused by glacier melt and shifting mantle is also causing dramatic changes to the Earth’s rotation and substances below the earth’s surface.
As our gargantuan glaciers melted, the continents up north lost weight quickly, causing a rapid redistribution of weight. Recent research from NASA scientists show that this causes a phenomenon called “true polar wander” where the lopsided distribution of weight on the Earth causes the planet to tilt on its axis until it finds its balance. Our north and south poles are moving towards the landmasses that are shrinking the fastest as the Earth’s center of rotation shifts. Previously, the North Pole was drifting towards Canada; but since 2000, it’s been drifting towards the U.K. and Europe at about four inches per year. Scientists haven’t had to change the actual geographic location of the North Pole yet, but that could change in a few decades.
Redistribution of mass is also slowing down the Earth’s rotation. In 2015, Harvard geophysicist Jerry Mitrovica published a study in Science Advances showing that glacial melt was causing ocean mass to pool around the Earth’s center, slowing down the Earth’s rotation. He likened the phenomenon to a spinning figure skater extending their arms to slow themselves down.
Glacial melt may also be re-awakening dormant earthquakes and volcanoes. Large glaciers suppressed earthquakes, but according to a study published in 2008 in the journal Earth and Planetary Science Letters, as the Earth rebounds, the downward pressure on the plates is released and shaky pre-existing faults could reactivate. In Southeast Alaska, where uplift is most prevalent, the Pacific plate slides under the North American plate, causing a lot of strain. Researchers say that glaciers had previously quelled that strain, but the rebound is allowing those plates to grind up against each other again. “The burden of the glaciers was keeping smaller earthquakes from releasing tectonic stress,” says Erik Ivins, a geophysicist at NASA’s Jet Propulsion Laboratory.
Melting glaciers may also make way for earthquakes in the middle of plates. One example of that phenomenon is the series of New Madrid earthquakes that rocked the Midwestern United States in the 1800s. While many earthquakes occur on fault lines where two separate plates slide on top of each other, scientists speculate that the earthquakes in the New Madrid area occurred at a place where hot, molten rock underneath the Earth’s crust once wanted to burst through, but was quelled by the weight of massive ice sheets. Now that the ice sheets have melted, however, the mantle is free to bubble up once again.
Scientists have also found a link between deglaciation and outflows of magma from the Earth, although they’re not sure why one causes the other. In the past five years, Iceland has suffered three major volcanic eruptions, which is unusual for the area. Some studies suggest that the weight of the glaciers suppressed volcanic activity and the recent melting is 20-30 times more likely to trigger volcanic eruptions in places like Iceland and Greenland.
The wandering poles: Until recently earth’s axis had been slowly moving toward Canada, as shown in this graphic; now, melting ice and other factors are shifting Earth’s axis toward Europe. (NASA/JPL-Caltech)
Much of the mystery pertaining to ancient glaciers is still unsolved. Scientists are still trying to create an accurate model of glacial isostatic adjustment, says Richard Snay, the lead author of the most recent study in the Journal of Geophysical Research. “There’s been such software since the early ’90s for longitude and latitude measurements but vertical measurements have always been difficult,” says Snay. He and colleagues have developed new equations for measuring isostatic adjustment based off of a complex set of models first published by Dick Peltier, a professor at the University of Toronto. Peltier’s models don’t only take into account mantle viscosity, but also past sea level histories, data from satellites currently orbiting the Earth and even ancient records translated from Babylonian and Chinese texts. “We’re trying to look at glaciation history as a function of time and elasticity of the deep earth,” says Peltier. “The theory continues to be refined. One of the main challenges of this work is describing the effects that are occurring in the earth’s system today, that are occurring as a result of the last Ice Age thousands of years ago.”
Added on to all the unknowns, researchers also don’t know exactly how this prehistoric process will be affected by current patterns of global warming, which is accelerating glacial melt at an unprecedented rate. In Alaska, global warming means less snow in the wintertime, says Hood.
“There is a much more rapid rate of ice loss here compared to many regions of the world,” he says. “The human fingerprint of global warming is just exacerbating issues and increasing the rate of glacial isostatic adjustment.”
And while the effects may vary from city to city—local sea levels may be rising or dropping—it’s clear that the effects are dramatic, wherever they may be. Although many of glaciers have long gone, it’s clear that the weight of their presence still lingers on the Earth, and on our lives.
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Kleine Gaisl (Piccola Croda Rossa), is a large (2859 m) mountain in the Braies Valley in the South Tyrol in the northern Italian Dolomites. At the end of last week a large rockfall occurred in a series of stages over two days between 18th and 20th August. There is a good report on Planet Mountain, although they have the volume wrong by three orders of magnitude. From other sources the estimated volume is 600,000 to 700,000 cubic metres.
Mountain guide Roman Valentini captured a part of the rockfall in a video that has been uploaded to Youtube. But note that this is not the main collapse event, as Planet Mountain notes:
“The footage below was filmed by Roman Valentini, a mountain guide working for Alta Badia Guides, who was in the area on Thursday, August 18 at around 12:30. Although this is only the first, smaller part of the landslide, Valentini told planetmountain.com “It was ‘spectacular’ … I’ve never seen anything quite like it. It looked like a river in spate, with rocks half the size of houses tumbling down.”
Although the main rockfall event occurred later (the seismic data will be interesting here in order to understand the sequence of events), there is a significant collapse event at about three minutes into the video:
The rockfall had been anticipated as a large tension crack had been observed prior to the collapse event. Stol.it has a nice article, in German, though Google Translate does a good job, that includes an interview with the Deputy Mayor, Erwin Steiner, which also includes this good image of the source area of the rockfall:
The rockfall scar on Kleine Gaisl, image by Erwin Steiner
Whilst another article on the same site has another view of the source zone that also captures some of the rockfall deposit::
Image of the Kleine Gaisl rockfall zone, including a part of the deposit. Image by Tourismusbüro Prags
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A new model of the impact that created the moon might upend theories about earth, too
Visualization of the giant impact that formed the moon (William Hartmann)
A new theory about how the moon formed might also tweak our understanding of early life on Earth.
The presence of gold and platinum in Earth’s mantle has previously been assumed to be the result of a heavy shower of meteors raining down on early Earth, but new research suggests another source—one enormous impact with the object that crashed into the planet to create the moon.
Around 4 billion years ago the Earth was under constant attack, according to geophysicists. Asteroids and meteors continuously smashed into the planet for about 100 million years, a period known as the Late Heavy Bombardment. Any life on the planet at that time would be in constant peril.
We know about these impacts not because of the craters they left—erosion and plate tectonics have long spirited those away—but because of the presence of certain metals in the Earth’s mantle. The pockmarked surface of the moon, which is not tectonically active, also helps bolster this theory.
But new research suggests that the bombardment may have been milder than expected, because the metals found in Earth’s mantle could instead be from the moon-forming impact, about 500 million years earlier.
Early in the life of the solar system, a growing world known to scientists as Theia collided with the young Earth. The violent impact liquefied Earth’s outer layers and pulverized Theia, creating a ring of debris that swirled around the scarred world. Iron from Theia’s core drew together to form the heart of the moon. The remaining heavy material rained back down on Earth, and gravity drew the lighter components together to create the moon.
But new research suggests not all of Theia’s iron built the lunar core. Instead, some may have settled on Earth’s crust, and was later drawn into the mantle through plate tectonics. Elements such as gold and platinum, which are drawn to iron, may have been pulled into the mantle along with it. Such elements are sparse in the lunar mantle, presumably because all of the iron delivered to the moon created its core while Earth’s original core remained intact after the collision.
That could mean good news for life on the early Earth. If Theia’s core brought in traces of iron that attracted scarcer, iron-loving elements, the rain of asteroids and meteors couldn’t have been as heavy as previously estimated.
“The Earth is not going to be completely unhabitable for a long period of time because the bombardment is relatively benign,” says Norman Sleep, a geophysicist at Stanford University. Sleep investigated the idea that Theia could have brought platinum and similar elements to Earth’s mantle, comparing it with previous suggestions that meteors delivered the material. In a recent paper published in the journal Geochemistry, Geophysics, Geosystems, he found that Theia could have brought in enough iron-loving elements to suggest later bombardment was milder than previously considered.
“It was certainly not anything we would survive, but we’re dealing with microbes,” he says.
However, without a heavy bombardment of meteorites, a new problem arises. The collision between Theia and the young Earth would have vaporized any water on the planet. The leading theory for how Earth got its water back is via collisions with water-carrying meteorites, but meteorites would also have delivered more iron-loving elements along with iron, leaving behind too much gold and platinum than measured. That means Sleep’s calculations would require another method of bringing water to the planet.
That doesn’t make the theory a deal-breaker. “There’s no guarantee that there’s one event that solves every problem,” says Tim Swindle, who studies planetary materials at the University of Arizona. Water could have come from another source unrelated to Theia.
Figuring out exactly what happened in the early life of Earth and its moon may require a return to our satellite. “We’ve got to go back to the moon and get a better handle on the age of the basins,” Swindle says, especially those on the back side of the moon. “We might be able to get an age with a rover that could answer the questions, but I think we’d do better to bring the samples back.” That doesn’t necessarily mean humans have to be onboard the lunar mission, but, as Swindle points out, people do a great job.
Sleep agrees, calling for a visit to the South Pole Aiken basin, the largest and oldest of those on the moon. That basin has never been sampled, and should provide insight into the timing of the bombardment, which would give clues into how much material rained down on Earth.
According to Edward Young, a planetary scientist at the University of California at Los Angeles, the biggest result of Sleep’s research is the mental shift it requires for the scientists studying Earth and the moon. “I think what he’s doing is exposing the soft underbelly of what we do,” Young says, adding that geochemical arguments are filled with basic assumptions of the processes that go into building the Earth and moon. “He’s challenging some of those assumptions.”
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A view of the Salar Grande basin, with pre-Andean ranges in the background. Techniques tested within this basin could help rovers on Mars search for life. Credit: David Fernández-Remolar
Robots like Viking, Phoenix, and Curiosity have found indirect evidence of oxalate minerals on Mars. New techniques tested in the Chile’s Atacama Desert could help future Mars rovers analyze these minerals for possible by-products of life.
Organic oxalates tie up carbon, creating carbon sinks that are extremely resistant to most physical and chemical weathering. On Earth, oxalates are primarily broken down by biological weathering; fungus and bacteria consume oxalate, recycling carbon back into the environment. But Mars’s carbon cycle is not as well understood. Developing better techniques to identify trace concentrations of oxalates may provide insights into the processes at play on the Red Planet.
A close-up of cemented deposits within Chile’s Salar Grande basin, which hosts hardy lichen. Oxalate minerals found here were formed by biological activity, and different techniques tested whether instruments could detect such minerals at low concentrations. Successful detections could help future Mars rovers search for traces of life. Credit: Guillermo Chong
In a new paper, Cheng et al. used samples from a terrestrial analogue of Mars—the Salar Grande basin in the Atacama Desert—to test out various techniques for detecting and characterizing oxalate-bearing rocks. They found that although X-ray diffraction could detect insoluble oxalate concentrates of approximately 2%–4% or higher, only ion chromatography could detect lower concentrations of soluble oxalates.
The team specifically tuned their instruments to search for weddellite and whewellite oxalate minerals formed by biological activity. They found concentrations of weddellite and whewellite in samples collected in the sediment record infilling the Salar Grande basin, which demonstrates the great preservation potential for oxalate compounds sourced in biological activity. This suggests that any similar oxalate-bearing deposits on Mars are worth closer investigation in the ongoing search for life.
Oxalate minerals can be formed by meteoritic, primordial, geothermal, and biological processes and so are not inherently indicators of life, but their resistance to most forms of nonbiological weathering make them a noteworthy aspect of the carbon cycle. Future research could use the techniques tested in Chile for more sensitive detection of oxalates on Mars. This in turn could lead to insights into Martian carbon sinks and sources. (Journal of Geophysical Research: Biogeosciences, doi:10.1002/2016JG003439, 2016)
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A paper published this week in Science finds evidence to support stories that a huge flood took place in China about 4,000 years ago, during the reign of Emperor Yu. The study, led by Chinese researcher Qinglong Wu, finds evidence for a massive landslide dam break that could have redirected the course of the Yellow River, giving rise to the legendary flood that Emperor Yu is credited with controlling.
Emperor Yu is famous through Chinese legends in which he worked to control persistent flooding.Science/AAAS
An accompanying commentary by David Montgomery, a UW professor of Earth and space sciences, discusses how this finding supports the historical basis for traditional tales about China’s Great Flood. It even explains some details of the classic folk story.
“A telling aspect of the story — that it took Yu and his followers decades to control the floodwaters — makes sense in light of geological evidence that Wu et al. present,” Montgomery writes.
An accompanying commentary by David Montgomery, a UW professor of Earth and space sciences, discusses how this finding supports the historical basis for traditional tales about China’s Great Flood. It even explains some details of the classic folk story.
“A telling aspect of the story — that it took Yu and his followers decades to control the floodwaters — makes sense in light of geological evidence that Wu et al. present,” Montgomery writes.
“A telling aspect of the story — that it took Yu and his followers decades to control the floodwaters — makes sense in light of geological evidence that Wu et al. present,” Montgomery writes.
The study showed that an ancient landslide dammed the Yellow River on the edge of the Tibetan Plateau. When the dam broke in about 1922 B.C., the authors found, it created an enormous flood that coincided with a period of major social disruption, suggesting that the Yellow River overflowed its banks and had to set a new course.
“It would have taken considerable time for a large river to adjust to such a change and the associated sustained flooding would fall in the right time and place to account for Yu’s story — including the long time it took to control the floodwaters,” Montgomery commented.
UW geologist David Montgomery is the author of a 2013 book that looks for the geological basis for Noah’s flood and other traditional stories.
The discovery is the latest in a series of efforts to link geologic and oral histories, including the biblical tale of Noah’s flood.
“Great floods figure prominently in some of humanity’s oldest stories,” Montgomery said. “In researching my book, ‘The Rocks Don’t Lie,’ I found that while the idea of a global flood was soundly refuted almost 200 years ago, many of the world’s flood stories have their roots in real catastrophic events — like tsunamis, glacial dam-break floods and disastrous flooding of lowland valleys and areas along major rivers.”
The Pacific Northwest is home to one prominent example. Montgomery notes UW research that has linked Native American tales about shaking and flooding to the 1700 earthquake and tsunami along Washington’s coast, for which no written records exist.
“Now it appears that we can add China’s story of a great flood to the growing list of legends of ancient catastrophes that may be rooted in real events,” Montgomery said.
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Study of natural-occurring 100,000 year-old CO2 reservoirs shows no significant corroding of ‘cap rock’, suggesting the greenhouse gas hasn’t leaked back out – one of the main concerns with greenhouse gas reduction proposal of carbon capture and storage.
New research shows that natural accumulations of carbon dioxide (CO2) that have been trapped underground for around 100,000 years have not significantly corroded the rocks above, suggesting that storing CO2 in reservoirs deep underground is much safer and more predictable over long periods of time than previously thought.
These findings, published today in the journal Nature Communications, demonstrate the viability of a process called carbon capture and storage (CCS) as a solution to reducing carbon emissions from coal and gas-fired power stations, say researchers.
CCS involves capturing the carbon dioxide produced at power stations, compressing it, and pumping it into reservoirs in the rock more than a kilometre underground.
The CO2 must remain buried for at least 10,000 years to avoid the impacts on climate. One concern is that the dilute acid, formed when the stored CO2 dissolves in water present in the reservoir rocks, might corrode the rocks above and let the CO2 escape upwards.
By studying a natural reservoir in Utah, USA, where CO2 released from deeper formations has been trapped for around 100,000 years, a Cambridge-led research team has now shown that CO2 can be securely stored underground for far longer than the 10,000 years needed to avoid climatic impacts.
Their new study shows that the critical component in geological carbon storage, the relatively impermeable layer of “cap rock” that retains the CO2, can resist corrosion from CO2-saturated water for at least 100,000 years.
“Carbon capture and storage is seen as essential technology if the UK is to meet its climate change targets,” says principle investigator Professor Mike Bickle, Director of the Cambridge Centre for Carbon Capture and Storage at the University of Cambridge.
“A major obstacle to the implementation of CCS is the uncertainty over the long-term fate of the CO2 which impacts regulation, insurance, and who assumes the responsibility for maintaining CO2 storage sites. Our study demonstrates that geological carbon storage can be safe and predictable over many hundreds of thousands of years.”
The key component in the safety of geological storage of CO2 is an impermeable cap rock over the porous reservoir in which the CO2 is stored. Although the CO2 will be injected as a dense fluid, it is still less dense than the brines originally filling the pores in the reservoir sandstones, and will rise until trapped by the relatively impermeable cap rocks.
“Some earlier studies, using computer simulations and laboratory experiments, have suggested that these cap rocks might be progressively corroded by the CO2-charged brines, formed as CO2 dissolves, creating weaker and more permeable layers of rock several metres thick and jeopardising the secure retention of the CO2,” explains lead author Dr Niko Kampman.
“However, these studies were either carried out in the laboratory over short timescales or based on theoretical models. Predicting the behaviour of CO2 stored underground is best achieved by studying natural CO2 accumulations that have been retained for periods comparable to those needed for effective storage.”
To better understand these effects, this study, funded by the UK Natural Environment Research Council and the UK Department of Energy and Climate Change, examined a natural reservoir where large natural pockets of CO2 have been trapped in sedimentary rocks for hundreds of thousands of years. Sponsored by Shell, the team drilled deep down below the surface into one of these natural CO2 reservoirs to recover samples of the rock layers and the fluids confined in the rock pores.
The team studied the corrosion of the minerals comprising the rock by the acidic carbonated water, and how this has affected the ability of the cap rock to act as an effective trap over geological periods of time. Their analysis studied the mineralogy and geochemistry of cap rock and included bombarding samples of the rock with neutrons at a facility in Germany to better understand any changes that may have occurred in the pore structure and permeability of the cap rock.
They found that the CO2 had very little impact on corrosion of the minerals in the cap rock, with corrosion limited to a layer only 7cm thick. This is considerably less than the amount of corrosion predicted in some earlier studies, which suggested that this layer might be many metres thick.
The researchers also used computer simulations, calibrated with data collected from the rock samples, to show that this layer took at least 100,000 years to form, an age consistent with how long the site is known to have contained CO2.
The research demonstrates that the natural resistance of the cap rock minerals to the acidic carbonated waters makes burying CO2 underground a far more predictable and secure process than previously estimated.
“With careful evaluation, burying carbon dioxide underground will prove very much safer than emitting CO2 directly to the atmosphere,” says Bickle.
The Cambridge research into the CO2 reservoirs in Utah was funded by the Natural Environment Research Council (CRIUS consortium of Cambridge, Manchester and Leeds universities and the British Geological Survey) and the Department of Energy and Climate Change.
The project involved an international consortium of researchers led by Cambridge, together with Aarchen University (Germany), Utrecht University (Netherlands), Utah State University (USA), the Julich Centre for Neutron Science, (Garching, Germany), Oak Ridge National Laboratory (USA), the British Geological Survey, and Shell Global Solutions International (Netherlands).
Reference:
N. Kampman, et al. “Observational evidence confirms modelling of the long-term integrity of CO2-reservoir caprocks” Nature Communications 28 July 2016.
The University of Cambridge (abbreviated as Cantab in post-nominal letters) is a collegiate public research university in Cambridge, England. Founded in 1209, Cambridge is the second-oldest university in the English-speaking world and the world’s fourth-oldest surviving university. It grew out of an association of scholars who left the University of Oxford after a dispute with townsfolk. The two ancient universities share many common features and are often jointly referred to as “Oxbridge”.
Cambridge is formed from a variety of institutions which include 31 constituent colleges and over 100 academic departments organised into six schools. The university occupies buildings throughout the town, many of which are of historical importance. The colleges are self-governing institutions founded as integral parts of the university. In the year ended 31 July 2014, the university had a total income of £1.51 billion, of which £371 million was from research grants and contracts. The central university and colleges have a combined endowment of around £4.9 billion, the largest of any university outside the United States. Cambridge is a member of many associations and forms part of the “golden triangle” of leading English universities and Cambridge University Health Partners, an academic health science centre. The university is closely linked with the development of the high-tech business cluster known as “Silicon Fen”.
8.4.16
Seth Stein, Carol Stein, Jonas Kley, Randy Keller, Miguel Merino, Emily Wolin, Douglas Wiens, Michael E. Wysession, Ghassan Al-Equabi, Weisen Shen, Andrew Frederiksen, Fiona Darbyshire, Donna Jurdy, Greg Waite, William Rose, Erika Vye, Tyrone Rooney, Robert Moucha, and Eric Brown
Cliffs of 1.1-billion-year-old volcanic rocks from the Midcontinent Rift in Tettegouche State Park, Minn., tower above the brilliant blue waters of Lake Superior. Credit: Seth Stein
Some of the Midwest’s most scenic vistas are the black volcanic cliffs that tower above the brilliant blue waters of Lake Superior’s north shore. How these formed more than a billion years ago is an amazing story, illustrating one of plate tectonics’ most important processes—how continents form and break up.
Over geologic time, continents collide and fuse together. They also split apart, along rifts.
One of the best exposures of the Midcontinent Rift’s 1.1-billion-year-old volcanic rocks is in Interstate Park, along the border between Minnesota and Wisconsin, where the St. Croix River cuts through a series of lava flows. Credit: Seth Stein
Rifts are linear features along which continents stretch. When a rift succeeds, the continent splits, and a new ocean basin forms between the two parts of the continent. Some rifts, however, fail to develop into seafloor spreading centers and instead leave major relict structures within continents—“fossils” preserving the geologic environments in which they formed.
The cliffs on the shores of Lake Superior—and the lake itself—are part of such a fossilized rift. Called the Midcontinent Rift (MCR), this 3000-kilometer-long feature, made of 1.1-billion-year-old igneous and sedimentary rocks, extends underground across the central United States. It stands as one of the best examples of a failed rift (Figure 1).
However, a puzzle remains. The MCR’s igneous rocks tell of magma pools far more vast than what one would expect in a failed rift. Why was the MCR so magma rich? Scientists are beginning to tease out answers.
Formation at a Plate Boundary
The traditional view of the formation and evolution of the MCR involved two premises. First, the MCR formed within Laurentia, the core of North America that assembled in the Precambrian era. Second, the MCR failed to split the continent because compression from a mountain-building episode ended the extension and volcanism. This mountain building, called the Grenville orogeny, finished about 980 million years ago and was associated with the assembly of Laurentia and other continental blocks into the supercontinent of Rodinia.
However, a new view of the MCR emerges from studies catalyzed by the National Science Foundation’s EarthScope program. One of this program’s goals is “to integrate geological and geophysical data to understand the growth and modification of North America over billion-year time scales.”
The new view begins from reanalysis of gravity data, which show that the MCR extends farther south than previously thought. The east arm (Figure 1) had been assumed to end in Michigan at the Grenville Front, the westernmost deformation associated with the Grenville orogeny. However, this arm now appears to extend along the previously enigmatic East Continent Gravity High, which now seems to be part of the MCR. The arm stops at what scientists believe to be a fossil continental margin, where the Amazonia craton (Precambrian rock now found in northeastern South America) rifted from Laurentia (Figure 1b).
Fig. 1. (a) Gravity map showing the Midcontinent Rift (MCR). The west arm extends southward from Lake Superior at least to Oklahoma along the Southern Oklahoma Aulacogen (SOA). The east arm goes through Michigan and extends along the Fort Wayne rift (FWR) and East Continent Gravity High (ECGH) to Alabama [C. A. Stein et al., 2015]. The Grenville Front is shown by a solid line where observed and by a dashed lined where inferred. “S” shows the location of the seismic line across Lake Superior in Figure 2a. The box shows the approximate area of the Superior Province Rifting Earthscope Experiment (SPREE) seismic experiment. (b) Schematic illustration of MCR forming as part of the rifting of Amazonia from Laurentia. (c) Africa rifting into major plates and microplates [Stein et al., 2014].
Hence, the MCR probably formed during this rifting and failed once seafloor spreading between the major plates was established [Stein et al., 2014]. In this view, the MCR’s arms were boundaries of a microplate within a plate boundary zone [Merino et al., 2013], similar to the arms of today’s East African Rift, where microplates exist in the zone where Africa is splitting into two major plates (Figure 1c). Once seafloor spreading between the major plates is fully established in the East African Rift, some of the microplate boundaries should stop spreading and remain within the continents as failed rifts.
The MCR formed in a similar way, as Laurentia and Amazonia split apart.
Paleomagnetic studies show that Laurentia’s motion changed dramatically when the MCR formed—as volcanic rocks cooled, magnetic minerals aligned with Earth’s magnetic field, recording changes in the plate’s motion. Such changes are often found in the paleomagnetic record when continents rifted apart. As part of the rifting, the MCR formed and began extending. The region between the MCR’s two spreading arms was forced to move away from the rest of Laurentia as a microplate.
A Hybrid Rift
Dense volcanic material fills the MCR. This large, concentrated mass exerts a greater gravitational pull than the areas around it—resulting in what’s known as a positive gravity anomaly. In contrast, typical continental rifts have negative anomalies because they are filled primarily with low-density sedimentary rock.
Tracing the gravity anomaly shows that the MCR’s arms have characteristic geometries of rifts—linear depressions that formed as plates pulled apart and that filled with sedimentary and igneous rocks. These depressions often evolve into plate boundaries.
Calculating the volume of volcanic rock causing this positive gravity anomaly reveals something interesting about the MCR. It is also a large igneous province (LIP), a region of extensive volcanism associated with upwelling and melting of deep mantle materials. MCR volcanics are significantly thicker than those at other LIPs because magma was deposited within a narrow rift rather than across a broad surface. Hence, the MCR is a hybrid with the geometry of a rift but the igneous rock volume of a LIP [C. A. Stein et al., 2015].
How Did This Hybrid Evolve?
Reflection seismic data (Figure 2a) across Lake Superior [Green et al., 1989] show how this unusual combination evolved. The MCR is a basin between dipping faults that contains volcanic rocks up to 20 kilometers thick, overlain by about 5–8 kilometers of sedimentary rocks. The lower volcanic layers truncate toward the basin’s north side, indicating that they were deposited while the rift was extending by motion on the northern fault. The upper volcanic layers and overlying sedimentary rocks dip from both sides and thicken toward the basin center, indicating that they were deposited after extension ended, as the basin continued to subside. Radiometric dating shows that the volcanic rocks are about 1.1 billion years old.
Another seismic line to the east shows a similar sequence, but with extension on the southern fault. Basins produced by extension on only one side are called half grabens (Figure 2a), so the MCR is a sequence of alternating half grabens, similar to what’s found today along the arms of the East African Rift.
Fig. 2. (a) Interpreted seismic reflection section across Lake Superior, showing MCR structure. GLIMPCE = Great Lakes International Multidisciplinary Program on Crustal Evolution. (b) Model of MCR evolution. The crust thinned in the rifting stage, rethickened during the postrift phase, and thickened further because of the later basin inversion [C. A. Stein et al., 2015].
Working backward from the geometry of the volcanic and sedimentary rock layers seen today provides an evolutionary model of the feature (Figure 2b). The MCR began as a half graben filled by flood basalts. After rifting (extension) stopped, the basin further subsided, accommodating more flood basalts. After volcanism ended, subsidence continued, accompanied by sedimentation. The crust depressed and flexed under the load of the basalt and sedimentary rocks.
Long after subsidence ended, the area was compressed—a process called basin inversion—which reversed motion on the faults and activated new ones. The original crust thinned during rifting as the crust stretched, rethickened during the postrift phase because of the added volcanics and sediments, and thickened further because of the compression when the basin inverted. If the model is correct, then crustal thickness along the MCR should vary with the amounts of extension and volcanism and the amount and direction of compression applied to that portion of the MCR.
Surprises from Seismic Imaging
New insights on MCR come from data from Earthscope seismometers in the transportable array that “rolled” across the United States, the Superior Province Rifting Earthscope Experiment (SPREE) flexible array [Stein et al., 2011; Wolin et al., 2015], and Canadian seismic stations.
Seismic models covering much of the rift are being developed with these data [Shen et al., 2013; Bollmann et al., 2014; Al-Eqabi et al., 2015; Zhang et al., 2015; Fredriksen et al., 2015]. As expected from the Lake Superior data, crust in the MCR’s west arm is thicker than its surroundings (Figures 3a and 3b) and is composed of sedimentary rocks underlain by layered volcanic rocks. Combining the results with gravity and seismic reflection data gives views of the MCR’s crustal thickness and structure that will better constrain models of its evolution [e.g., Levandowski et al., 2015].
Fig. 3. Different seismological data show various aspects of MCR’s structure. Crustal thickening beneath the MCR’s west arm is shown by (a) receiver functions [Moidaki et al., 2013] and (b) surface wave tomography [Shen et al., 2013]. The line in Figure 3b shows the location of the profile in Figure 3a. Shear-wave velocities (Vs) show that the surface waves capture (c) the MCR’s low velocity sedimentary rocks but not (d) the underlying volcanic rocks.
These studies have already yielded surprising results. The MCR’s dense volcanic rocks appear clearly on gravity, seismic reflection, and receiver function data, which are sensitive to variations in density. However, surface wave tomography (Figures 3c and 3d) “sees” the rift’s sedimentary rocks through which seismic waves travel slowly but does not see high velocity in the rift-filling volcanics. The basalt rift fill is presumably denser than the surrounding crust, but the velocities of shear waves traveling through it are similar or slightly lower than those of the surrounding rocks.
Another surprise is that the formation of the MCR left little signature in the upper mantle. Although vast quantities of melt were extracted to fill the MCR, the mantle shows no significant seismic velocity anomalies [Shen et al., 2013; Bollmann et al., 2014]. Unless the mantle beneath the MCR was replaced after the MCR formed, melt depletion seems to have had little effect on seismic velocities. The idea that extracting melt from the mantle would not change the mantle’s seismic velocity significantly is surprising but consistent with a model for melt extraction by Schutt and Lesher [2006].
Moreover, mantle flows or oriented magma bodies usually make seismic wave speeds vary depending on the direction the waves travel, so waves tend to go faster along currently extending rifts than across them. However, Ola et al. [2016] find no such anisotropy below the MCR, perhaps because much of the volcanism occurred after extension ended.
Current and future studies combining petrology, geochemistry, geochronology, paleomagnetism, plate motions, and magnetotellurics will continue to explore what these surprises mean for the evolution of the rift.
Did a Hot Spot Supply the Excess Magma?
The MCR is an extraordinary feature that arose from an unusual combination of a continental rift and a LIP, illustrating that over a billion years of Earth history, even unlikely events can happen. Rifting can be classified into two types: “passive” rifting in which forces pull the lithosphere in opposite directions, extending it [Sengor and Burke, 1978], and “active” rifting, where a mantle plume or hot spot thermally uplifts and stretches the crust above it [e.g., Nicholson et al., 1997]. For the MCR, we suspect that the rifting continent by chance overrode a plume or a region of anomalously hot upper mantle, so both active and passive rifting may have been at play.
Initial modeling [Moucha et al., 2013] implies that the MCR’s magma volume cannot have been generated by passive upwelling beneath an extending rift, even though the Precambrian mantle was hotter than today’s. Bowles-Martinez and Schultz [2015] find a highly conductive anomaly below western Lake Superior and northwestern Wisconsin, extending to depths below 200 kilometers, that may have been formed by a mantle plume and somehow persisted. A question currently being discussed involves how the magma source operated over a long period of rapid plate motion [Swanson-Hysell et al., 2014].
MCR Failure
The MCR was previously thought to have failed—stopped extending—because of regional compression associated with the Grenville orogeny [Cannon, 1994]. However, new age dating shows that most of the compression recorded by reverse faulting occurred long after extension and volcanism ended. Thus, MCR’s failure was not due to Grenville compression [Malone et al., 2016]. Instead, it stopped spreading much earlier, once seafloor spreading between Amazonia and Laurentia was fully established.
Insights into Other Rifts and Continental Margins
The MCR results help place other rifts worldwide in their evolutionary sequence via “comparative riftology”—comparing rifts at different stages in their evolution. Today’s East African rift looks like what we envision for the MCR during rifting—a gravity low, low velocities due to the high temperatures, and thin crust below the extending arms. The Southern Oklahoma Aulacogen, a failed rift that opened in the Cambrian breakup of Rodinia and was inverted in the late Paleozoic, appears similar to today’s MCR, with a gravity high due to the igneous rocks filling the rift.
By analogy, failed rifts similar to the MCR will have thick crust even if they have not been inverted, and inverted ones will have the thickest crust. Similarly, the gravity anomaly should change from a low to become progressively more positive as the rift fails and is later inverted.
The MCR has many features similar to those observed at volcanic passive continental margins. Volcanic margins arise where continental rifting is associated with large-scale melting that gives rise to thick igneous crust. Hence, the MCR shows what a passive margin looked like in its early stages.
MCR’s Legacy Showcases Geology’s Effect on Culture
The MCR highlights geoheritage: geology’s role in an area’s culture and growth [S. Stein et al., 2015]. Lake Superior and the surrounding spectacular scenery in national, state, and provincial parks are underlain by the MCR. The lake lies above the rift because soft sedimentary rocks within the MCR were easier for ice age glaciers to erode than the volcanic rocks. Thus, the rift provided the region’s first transportation system—Native Americans and Europeans used the lake to import and export trade goods. The lake remains an economic engine and tourist attraction.
Also, the MCR’s mineral deposits shaped the region’s settlement and growth. Water flowing through the volcanic rocks dissolved copper and deposited it in concentrations that became sources of valuable ore in many places around Lake Superior. For at least the past 7000 years, Native Americans have mined copper and traded it as far south as Illinois. The discovery of commercially viable copper deposits during the 1840s led to a mining boom that shaped the area’s economy.
Watch this video for more on MCR’s geologic and cultural history.
The MCR is a place where visitors can interact with geological features more than 1 billion years old. Its evolution—from extensional beginnings to rift failure, from ice age glaciers scouring it to lake infill, from ancient copper miners to modern boaters enjoying a lazy summer day—shows how geology weaves the fabric not only of our continents but also of our lives.
Acknowledgments
This work was supported by National Science Foundation grants EAR-1148088 and EAR-0952345.
References
Al-Eqabi, G., et al. (2015), Rayleigh wave tomography of Mid-Continent Rift, Abstract T11C-2904 presented at 2015 Fall Meeting, AGU, San Francisco, Calif., 14–18 Dec.
Bollmann, T., S. van der Lee, A. W. Frederiksen, E. Wolin, G. I. Aleqabi, J. Revenaugh, D. A. Wiens, and F. A. Darbyshire (2014), Teleseismic P-wave tomography of the Superior Province and Midcontinent Rift region, Abstract S13B-4455 presented at 2014 Fall Meeting, AGU, San Francisco, Calif., 15–19 Dec.
Bowles-Martinez, E., and A. Schultz (2015), Midcontinent Rift and remnants of initiating mantle plume imaged with magnetotellurics, Abstract T11D-2922 presented at 2015 Fall Meeting, AGU, San Francisco, Calif., 14–18 Dec.
Cannon, W. F. (1994), Closing of the Midcontinent Rift—A far-field effect of Grenvillian compression, Geology, 22, 155–158.
Frederiksen, A., et al. (2015), Crustal properties across the Mid-Continent Rift via transfer function analysis, Abstract T11C-2905 presented at 2015 Fall Meeting, AGU, San Francisco, Calif., 14–18 Dec.
Green, A. G., W. F. Cannon, B. Milkereit, D. R. Hutchinson, A. Davidson, J. C. Behrendt, C. Spencer, M. W. Lee, P. Morel-á-LáHuissier, and W. F. Agena (1989), “GLIMPCE” of the deep crust beneath the Great Lakes, in Properties and Processes of Earth’s Lower Crust, Geophys. Monogr. Ser., vol. 51, edited by R. F. Mereu, S. Mueller, and D. M. Fountain, pp. 65–80, AGU, Washington, D. C.
Levandowski, W., O. S. Boyd, R. W. Briggs, and R. D. Gold (2015), A random‐walk algorithm for modeling lithospheric density and the role of body forces in the evolution of the Midcontinent Rift, Geochem. Geophys. Geosyst., 16, 4084–4107, doi:10.1002/2015GC005961.
Malone, D. H., C. A. Stein, J. P. Craddock, J. Kley, S. Stein, and J. E. Malone (2016), Maximum depositional age of the Neoproterozoic Jacobsville Sandstone, Michigan: Implications for the evolution of the Midcontinent Rift, Geosphere, doi:10.1130/GES01302.1.
Merino, M., G. R. Keller, S. Stein, and C. Stein (2013), Variations in Mid-Continent Rift magma volumes consistent with microplate evolution, Geophys. Res. Lett., 40, 1513–1516.
Moidaki, M., S. S. Gao, K. H. Liu, and E. Atekwana (2013), Crustal thickness and Moho sharpness beneath the Midcontinent Rift, Res. Geophys., 3, e1.
Moucha, R., et al. (2013), Geodynamic modeling of the Mid-Continental Rift System: Is a mantle plume required?, Abstract T21B-2545 presented at 2013 Fall Meeting, AGU, San Francisco, Calif., 9–13 Dec.
Nicholson, S. W., S. B. Shirey, K. J. Schulz, and J. C. Green (1997), Rift-wide correlation of 1.1 Ga Midcontinent Rift system basalts: Implications for multiple mantle sources during rift development, Can. J. Earth Sci., 34, 504–520.
Ola, O., A. W. Frederiksen, T. Bollmann, S. van der Lee, F. Darbyshire, E. Wolin, J. Revenaugh, C. Stein, S. Stein, and M. Wysession (2016), Anisotropic zonation in the lithosphere of central North America: Influence of a strong cratonic lithosphere on the Mid-Continent Rift, Tectonophysics, in press.
Schutt, D. L., and C. E. Lesher (2006), Effects of melt depletion on the density and seismic velocity of garnet and spinel lherzolite, J. Geophys. Res., 111, B05401, doi:10.1029/2003JB002950.
Sengor, A. M. C., and K. Burke (1978), Relative timing of rifting and volcanism and its tectonic implications, Geophys. Res. Lett., 5, 419–421.
Shen, W., M. H. Ritzwoller, and V. Schulte-Pelkum (2013), Crustal and uppermost mantle structure in the central U.S. encompassing the Midcontinent Rift, J. Geophys. Res., 118, 4325–4344.
Stein, C. A., S. Stein, M. Merino, G. Randy Keller, L. M. Flesch, and D. M. Jurdy (2014), Was the Midcontinent Rift part of a successful seafloor-spreading episode?, Geophys. Res. Lett., 41, 1465–1470, doi:10.1002/2013GL059176.
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Stein, S., et al. (2011), Learning from failure: The SPREE Mid-Continent Rift Experiment, GSA Today, 21, 5–7.
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Wolin, E., S. van der Lee, T. A. Bollmann, D. A. Wiens, J. Revenaugh, F. A. Darbyshire, A. W. Frederiksen, S. Stein, and M. E. Wysession (2015), Seasonal and diurnal variations in long-period noise at SPREE stations, Bull. Seismol. Soc. Am, 105, 2433–2452.
Zhang, H., et al. (2015), Crustal structure of and near the Mid-Continent Rift System from receiver function studies, Abstract S13B-4454 presented at 2015 Fall Meeting, AGU, San Francisco, Calif., 14–18 Dec.
Eos is the leading source for trustworthy news and perspectives about the Earth and space sciences and their impact. Its namesake is Eos, the Greek goddess of the dawn, who represents the light shed on understanding our planet and its environment in space by the Earth and space sciences.
This low-angle self-portrait of NASA’s Curiosity Mars rover shows the vehicle at the site from which it reached down to drill into a rock target called “Buckskin.” Bright powder from that July 30, 2015, drilling is visible in the foreground. Credit: NASA/JPL-Caltech/MSSS
Scientists have discovered an unexpected mineral in a rock sample at Gale Crater on Mars, a finding that may alter our understanding of how the planet evolved.
NASA’s Mars Science Laboratory rover, Curiosity, has been exploring sedimentary rocks within Gale Crater since landing in August 2012. In July 2015, on Sol 1060 (the number of Martian days since landing), the rover collected powder drilled from rock at a location named “Buckskin.” Analyzing data from an X-ray diffraction instrument on the rover that identifies minerals, scientists detected significant amounts of a silica mineral called tridymite.
A common form of Tridymite, ultra thin colorless tabulars (image width: 1,1 mm) – Locality: Wannenköpfe, Ochtendung, Eifel region, Germany 2005
This detection was a surprise to the scientists, because tridymite is generally associated with silicic volcanism, which is known on Earth but was not thought to be important or even present on Mars.
The discovery of tridymite might induce scientists to rethink the volcanic history of Mars, suggesting that the planet once had explosive volcanoes that led to the presence of the mineral.
Scientists in the Astromaterials Research and Exploration Science (ARES) Division at NASA’s Johnson Space Center in Houston led the study. A paper on the team’s findings has been published in the Proceedings of the National Academy of Sciences.
“On Earth, tridymite is formed at high temperatures in an explosive process called silicic volcanism. Mount St. Helens, the active volcano in Washington State, and the Satsuma-Iwojima volcano in Japan are examples of such volcanoes. The combination of high silica content and extremely high temperatures in the volcanoes creates tridymite,” said Richard Morris, NASA planetary scientist at Johnson and lead author of the paper. “The tridymite was incorporated into ‘Lake Gale’ mudstone at Buckskin as sediment from erosion of silicic volcanic rocks.”
The paper also will stimulate scientists to re-examine the way tridymite forms. The authors examined terrestrial evidence that tridymite could form at low temperatures from geologically reasonable processes and not imply silicic volcanism. They found none. Researchers will need to look for ways that it could form at lower temperatures.
“I always tell fellow planetary scientists to expect the unexpected on Mars,” said Doug Ming, ARES chief scientist at Johnson and co-author of the paper. “The discovery of tridymite was completely unexpected. This discovery now begs the question of whether Mars experienced a much more violent and explosive volcanic history during the early evolution of the planet than previously thought.”
Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. Although the facility has a Pasadena postal address, it is actually headquartered in the city of La Cañada Flintridge [1], on the northwest border of Pasadena. JPL is managed by the nearby California Institute of Technology (Caltech) for the National Aeronautics and Space Administration. The Laboratory’s primary function is the construction and operation of robotic planetary spacecraft, though it also conducts Earth-orbit and astronomy missions. It is also responsible for operating NASA’s Deep Space Network.
The Devil City, a wind erosion landform near the town of Urho in the western Junggar region of northwestern China. Deep below this landscape and surrounding areas lie remnants of an ancient subduction zone, researchers report. Credit: Yixian Xu
Practically every geological feature on the face of the Earth is a result of the steady movement of tectonic plates. Mammoth slabs of oceanic and continental crust jostle and leapfrog each other, and the traces of these plate motions are sometimes preserved in the rock record. Reconstructing the history of tectonic plate movement is essential to better understand the history of the planet.
However, identifying ancient subduction zones remains a challenge because scientists rarely see traces of collisions between oceanic slabs and continental crust. Now Xu et al. have found new evidence of a fossil intraoceanic subduction zone in northwest China.
The team focused on the geology of the Darbut belt, a part of the western Junggar region of northwest China, and collected magnetotelluric data using 60 broadband stations located along a 182-kilometer stretch running northwest to southeast. This technique allows scientists to measure the electrical conductivity of geological materials and recognize features of both modern and fossil subduction zones—for example, spotting a resistive oceanic plate under conductive oceanic upper mantle.
Using this method, the researchers were able to obtain a resistivity model of the area, which they interpreted alongside geological and geophysical observational data, mineral physics experiments, and geodynamic modeling from previous studies.
Hidden beneath the accretionary layers of the western Junggar basin, they spotted the remains of a late Carboniferous intraoceanic subduction system. It is similar to a modern intraoceanic convergent boundary and unusually well preserved—no major tectonic events took place in the millennia since to destroy these geologic features. The authors believe this discovery provides valuable insight into the formation of collision belts and the potential exploration of mineral resources in the area. (Journal of Geophysical Research: Solid Earth, doi:10.1002/2015JB012394, 2016)
Eos is the leading source for trustworthy news and perspectives about the Earth and space sciences and their impact. Its namesake is Eos, the Greek goddess of the dawn, who represents the light shed on understanding our planet and its environment in space by the Earth and space sciences.
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