Ripples in spacetime travel at the speed of light. That fact, confirmed by the recent detection of a pair of colliding stellar corpses, kills a whole category of theories that mess with the laws of gravity to explain why the universe is expanding as fast as it is.

On October 16, physicists announced that the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, had detected gravitational waves from a neutron star merger (SN Online: 10/16/17). Also, the neutron stars emitted high-energy light shortly after merging. The Fermi space telescope spotted that light coming from the same region of the sky 1.7 seconds after the gravitational wave detection. That observation showed for the first time that gravitational waves, the shivers in spacetime set off when massive bodies move, travel at the speed of light to within a tenth of a trillionth of a percent.
Within a day, five papers were posted at arXiv.org mourning hundreds of expanding universe theories that predicted gravitational waves should travel faster than light — an impossibility without changes to Einstein’s laws of gravity. These theories “are very, very dead,” says the coauthor of one of the papers, cosmologist Miguel Zumalacárregui of the Nordic Institute for Theoretical Physics, or NORDITA, in Stockholm. “We need to go back to our blackboards and start thinking of other alternatives.”

In the 1990s, observations of exploding stars showed that more distant explosions were dimmer than existing theories predicted. That suggested that the universe is expanding at an ever-increasing rate (SN: 10/22/11, p. 13). Cosmologists have struggled ever since to explain why.

The most popular explanation for the speedup is that spacetime is filled with a peculiar entity dubbed dark energy. “You can think of it like a mysterious fluid that pushes everything apart and counteracts gravity,” says cosmologist Jeremy Sakstein of the University of Pennsylvania, coauthor of another new paper.
In the simplest version of this theory, the density of this dark energy has not changed over the history of the universe, so physicists call it a cosmological constant. This doesn’t require any changes to gravity — which is good, because gravity has been well-tested inside the solar system.

The cosmological constant idea matches observations of the wider universe, but it has some theoretical difficulties. Dark energy is about 120 orders of magnitude weaker than theorists calculate it should be (SN Online: 11/18/13), a mismatch that makes scientists uncomfortable.

Also, different methods for measuring the rate of expansion come up with slightly different numbers (SN: 8/6/16, p. 10). Measurements based on exploding stars suggest that distant galaxies are speeding away from each other at 73 kilometers per second for each megaparsec (about 3.3 million light-years) of space between them. But observations based on the cosmic microwave background, ancient light that encodes information about the conditions of the early universe, found that the expansion rate is 67 km/s per megaparsec. The disagreement suggests that either one of the measurements is wrong, or the theory behind dark energy needs a tweak.

So instead of invoking a substance to counteract gravity, theorists tried to explain the expanding universe by weakening gravity itself. Any modifications to gravity need to leave the solar system intact. “It’s quite hard to build a theory that accelerates the universe and also doesn’t mess up the solar system,” says cosmologist Tessa Baker of the University of Oxford, coauthor of still another paper.

These theories take hundreds of forms. “This field of modified gravity theories is a zoo,” says Baker. Some suggest that gravity leaks out into extra dimensions of space and time. Many others account for the universe’s speedy spreading by adding a different mysterious entity — some unknown particle perhaps — that drains gravity’s strength as the universe evolves.

But the new entity would have another crucial effect: It could slow the speed of light waves, similar to the way light travels more slowly through water than through air. That means that the best alternatives to dark energy required gravitational waves to travel faster than light — which they don’t.

Justin Khoury, a theoretical physicist at the University of Pennsylvania who has worked on several of the alternative gravity theories but was not involved in the new papers, was surprised that one gravitational-wave observation ruled out so many theories at once. He’s hardly disappointed, though.

“The fact that we’re learning something about dark energy because of this measurement is incredibly exciting,” he says.

Observing gravitational waves and light waves at the same time offers a third, independent way to measure how fast the universe is expanding. For now, that rate lies frustratingly right between the two clashing measurements scientists already had, at 70 km/s per megaparsec. But it’s still imprecise. Once LIGO and other observatories have seen 10 or 20 more neutron star collisions, researchers should be able to tell which measurement is correct and figure out whether dark energy needs an update, Zumalacárregui says.

“Gravitational waves may kill these models, but eventually they have the potential to tell us if this discrepancy is for real,” he says. “That’s something that is in itself very beautiful.”

Orangutans living in forested foothills on the Indonesian island of Sumatra represent a previously unknown species, researchers say.

Skeletal and genetic evidence puts these apes on a separate evolutionary trajectory from other orangutans in Sumatra (Pongo abelii) and Bornean orangutans (Pongo pygmaeus), says a team led by evolutionary anthropologist Michael Krützen of the University of Zurich. The researchers named the new species Pongo tapanuliensis, or the Tapanuli orangutan. Krützen’s team reports its findings online November 2 in Current Biology.
The name P. tapanuliensis refers to three north Sumatran districts — North, Central and South Tapanuli — where no more than 800 of these orangutans inhabit several forested areas. Tapanuli orangutans live on the brink of extinction due to road construction, illegal forest clearing and killings by villagers and hunters, the scientists say. Estimates vary, but the World Wildlife Fund puts the total number of living orangutans at nearly 120,000.

Researchers observed Tapanuli orangutans in their hilly habitat as early as the 1930s. Yet these apes have long been overlooked in favor of Sumatran orangutans that live in swampy forests north of the Tapanuli population. Bornean orangutans also live in swampy forests.

A chance to explore Tapanuli orangutans’ biology came in 2013. Krützen’s team gained permission to study the museum-held skeleton of an adult male Tapanuli orangutan that had been killed by villagers. Comparisons with skeletons of 33 Sumatran and Bornean male orangutans revealed a range of differences in the skull and teeth of the Tapanuli ape, including a distinctively narrow palate and a relatively short jaw joint.

An analysis of DNA from 37 living orangutans, including two Tapanuli animals, indicated that Tapanuli and Sumatran orangutans diverged from a common ancestor around 3.4 million years ago. Shared gene variants pointed to interbreeding between the two species after their evolutionary split. Cross-species hookups declined sharply around 100,000 years ago and then stopped between 10,000 and 20,000 years ago, the scientists say. Sumatran and Bornean orangutans separated around 674,000 years ago, the team estimates.
Only Tapanuli orangutans appear to be direct descendants of the first mainland Asian orangutan ancestors to reach Sumatra, the investigators find. Later migrations of mainland animals may have led to the evolution of Sumatran and Bornean orangutans

Scenarios in which closely related ape species interbred after evolving into distinctive biological populations probably occurred frequently, Krützen says. DNA studies suggest ancient chimpanzees and bonobos interbred, as did Homo sapiens and Neandertals (SN: 10/15/16, p. 22). Such evidence has fueled a long-standing debate over how to define the term “species” (SN: 11/11/17, p. 22).

Krützen’s team makes a good case for a third orangutan species that interbred for a long time with a closely related species, says biological anthropologist Rebecca Ackermann of the University of Cape Town in South Africa. “I’d go out on a limb and say not only that [interbreeding] played an important role in the evolution of all living apes, but that it shaped the evolution of extinct ones as well.”

Male mammoths really had to watch their steps. More than two-thirds of woolly mammoth specimens recovered from several types of natural traps in Siberia came from males, researchers report November 2 in Current Biology.

Paleogenomicist Patrícia Pečnerová of the Swedish Museum of Natural History in Stockholm and her colleagues examined genomic data recovered from 98 mammoth bone, tooth, tusk and hair shaft specimens and found that 69 percent of their owners were male. Sex biases in fossil preservation are rare, and the sexes were almost certainly balanced at birth. So the researchers considered whether social and behavioral patterns might have meant that male mammoths more often died in such a way that their remains were buried and preserved, such as becoming trapped in a bog or falling through thin ice.

In modern elephants, herds of females and young live together, led by an experienced female, whereas males are more likely to live in bachelor groups or alone. That could result in more risk-taking behavior for those males. Woolly mammoths, the distant cousins of modern elephants, may have had the same social structures, the researchers suggest.

The study, the authors say, highlights how fossil genomic data can help illuminate the past social structures and behavior of extinct animals — and how existing fossils may not fully represent the original population.

Gene-editing tools heralded as hope for fighting invader rats, malarial mosquitoes and other scourges may be too powerful to use in their current form, two new papers warn.

Standard forms of CRISPR gene drives, as the tools are called, can make tweaked DNA race through a population so easily that a small number of stray animals or plants could spread it to new territory, predicts a computer simulation released November 16 at bioRxiv.org. Such an event would have unknown, potentially damaging, ramifications, says a PLOS Biology paper released the same day.
“We need to get out of the ivory tower and have this discussion in the open, because ecological engineering will affect everyone living in the area,” says Kevin Esvelt of MIT, a coauthor of both papers who studies genetic solutions to ecological problems. What’s a pest in one place may be valued in another, so getting consent to use a gene drive could mean consulting people across a species’s whole range, be it several nations or continents.

Researchers have constructed this kind of drive in yeast, a fruit fly and several mosquitoes, but none of the tools have been deployed yet in the wild (SN: 12/12/15, p. 16). Meanwhile, some researchers are already working to add brakes or off-switches into a new generation of gene drives.

The major concern is that current gene drives “are probably too powerful for us to seriously consider deploying in conservation,” says geneticist Neil Gemmell of the University of Otago in Dunedin, New Zealand. Gemmell is a coauthor of the PLOS Biology paper.
This opinion could prove especially controversial in New Zealand. In 2016, the government resolved to protect the nation’s imperiled biodiversity by exterminating invader rats, stoats and possums that are wreaking havoc on native species. Gene drives just might make that possible.

Though warning of perils, the researchers also propose some solutions. A weaker system, which Esvelt calls a daisy drive, splits up components of the drive called guide RNAs. These guide RNAs direct the gene-editing machinery to its DNA target, where molecular scissors then snip and swap genetic material. As genes get inherited or not in the chancy jumbling of sexual reproduction, descendants in later generations become less likely to inherit all the spaced-apart pieces needed to operate the gene drive.

Esvelt’s lab is working to create a daisy drive in two kinds of nematode worms and is looking at other species as well. Other labs are now working on tamer gene drives, too.

Anthony A. James of the University of California, Irvine says that the disease-carrying Anopheles mosquito species that he and his colleagues have equipped with gene drives are self-limiting. When females end up with two of the genes he’s inserting, they don’t “survive very well after they have fed on blood.” Researchers are now raising these mosquitoes to see whether the genes spread and then dwindle away. “We don’t need our genes to last forever,” James says, “only long enough to contribute to getting rid of malaria.”

Another lab’s current version of disease-fighter mosquitoes already has a touch of the daisy. Aedes aegypti mosquitoes engineered with some built-in parts of the gene editor have their guide RNA split into two parts and put on different chromosomes, says molecular biologist Omar Akbari of the University of California, San Diego. Pictures of many weird mosquitoes created this way — all yellow or with three eyes or forked wings — attest to the fact that the drive system works. Akbari’s research appears November 14 in the Proceedings of the National Academy of Sciences.

Akbari is not very worried about the risk of accidentally wiping out disease-carrying mosquitoes. “A thousand children die every day,” he says. It would be unethical not to use a tool that could lessen the loss, he says.

He does recognize that the case for caution could be different for other species. “A lot of pet owners would be sad,” he says, if a gene drive went wrong and escaped worldwide during some future attempt to rid, say, Australia of its terribly destructive feral cats.

Sickness makes some corals lose their glow.

Disease reduces a coral’s overall fluorescence even before any sign of the infection is visible to the naked eye, a new study finds. An imaging technique that illuminates the change could help with efforts to better monitor coral health, researchers report November 6 in Scientific Reports.

Many corals naturally produce fluorescent proteins that glow in a wavelength of light that human eyes can’t see in natural light. Previous studies have shown that heat stress and wounding, among others stressors, can affect coral fluorescence, but the new study is the first to look at the relationship between fluorescence and infectious disease.
Jamie Caldwell, a disease ecologist now at Stanford University, and colleagues used a technique called live-imaging laser scanning confocal microscopy to compare fluorescence in living fragments of healthy and diseased Montipora capitata coral. The reef coral, common in Hawaii, fluoresces in red and cyan, and can contract a bacterial infection called Montipora white syndrome, which causes coral lesions and tissue loss.

The diseased bits looked healthy at the macroscopic level, but under the researchers’ microscope, the sick coral’s pallid complexion was pronounced. Computer analyses of the microscopy images quantified the lost glow (red is the total area of fluorescence, black regions are where fluorescence was lost, and white lines indicate edges between the two zones). Among the samples studied, healthy coral had on average 1.2 times as much fluorescence area as diseased fragments. Diseased coral had disorganized and fragmented patterns of fluorescence — similar to a forest that has been logged extensively, the researchers found.
Such research “is transformative in our struggle to visualize the dance between pathogen attack and host response in the initial attack,” says Drew Harvell, a disease ecologist at Cornell University.
Many coral diseases appear to be increasing around the world, even when accounting for increased research effort, Caldwell says. Along with bleaching events and pollution, disease is considered one of the major contributors to reef declines globally. The new technique could be used for other coral species and diseases, she says.

Hard-boiled eggs are a dish best served cold.

When quickly reheated in a microwave and then pierced, the picnic staple can explode with a loud bang in a shower of hot, rubbery shrapnel. But this blast is far more likely to make a hot mess than hurt your hearing, according to research presented December 6 at the Acoustical Society of America meeting in New Orleans.

That distinction isn’t as odd as it might sound. In a lawsuit, a man claimed to have suffered burns and hearing damage after a microwaved, hard-boiled egg exploded in his mouth at a restaurant. Researchers from Charles M. Salter Associates, Inc. in San Francisco called as expert witnesses couldn’t find scientific papers backing up the claim that an egg could burst with enough vigor to cause hearing loss — just a lot of YouTube videos documenting eggsplosions.
So the researchers microwaved peeled hard-boiled eggs in water on high power for three minutes.

The eggs were “uncooperative,” study coauthor Anthony Nash said in a news conference. Some exploded in the microwave, while others wouldn’t explode at all. But of nearly 100 eggs tested, 28 exploded outside of the microwave after being poked with a meat thermometer. From 30 centimeters away, the sound pressure from those explosions ranged from 86 to 133 decibels.

The median sound pressure level recorded, 108 decibels, is about the same as that at an average rock concert. Continuous exposure to that noise level could damage hair cells inside ears that respond to sound. The National Institute for Occupational Safety and Health sets recommended exposure limits for sound pressures above 85 decibels, says William Murphy, a researcher at NIOSH who wasn’t part of the study. But those limits are based on daily exposure over years, he says.
A burst egg’s boom, on the other hand, lasts just milliseconds — not long enough to do much harm. “The likelihood for hearing damage from a single exploding egg was very low,” Nash said.

The lawsuit was settled out of court before Nash and his colleagues conducted the second phase of the study – considering how sound hits your ears when it’s coming from inside your mouth. An in-mouth explosion might send slightly more sound pressure to the ears, Nash says, but still probably not enough to cause lasting damage as a one-time accident.

A peeled egg probably explodes when pockets of water trapped in the yolk become superheated — hotter than the boiling temperature of water without actually bubbling, Nash suggested. When disrupted, say by a fork or a tooth, the water pockets spontaneously boil, bursting through the squishy egg white and sending bits flying. (It’s the same phenomenon that can occasionally make microwaved coffee spurt out of the mug onto your clean work clothes.)

A bigger risk than the noise might be the heat. Nash and his colleagues measured the temperature of yolks in eggs that didn’t burst. Those temperatures were, on average, 12 degrees Celsius above the surrounding water bath, which was often close to boiling.

This year, gene therapy finally became a clinical reality. The U.S. Food and Drug Administration approved two personalized treatments that engineer a patient’s own immune system to hunt down and kill cancer cells. The treatments, the first gene therapies ever approved by the FDA, work in people with certain blood cancers, even patients whose cancers haven’t responded to other treatments.

Called CAR-T cell immunotherapy (for chimeric antigen receptor T cell), one is for kids and young adults with B cell acute lymphoblastic leukemia, or ALL, approved in August (SN Online: 8/30/17). The other is for adults with non-Hodgkin lymphoma, approved in October. Other CAR-T cell therapies are in testing, including a treatment for multiple myeloma.
“It’s a completely different way of treating cancer,” says pediatric oncologist Stephan Grupp, who directs the Cancer Immunotherapy Program at the Children’s Hospital of Philadelphia. Grupp spearheaded the clinical trials of the newly approved ALL therapy, called Kymriah.

Researchers are developing many different versions of CAR-T cell therapies, but the basic premise is the same: Doctors remove a patient’s T cells (immune system cells that attack invaders) from a blood sample and genetically modify them to produce artificial proteins on their surfaces. Those proteins, called chimeric antigen receptors, recognize the cancer cells in the patient’s body. After the modified T cells make many copies of themselves in the lab, they’re unleashed in the patient’s bloodstream to find and kill cancer cells.
CAR-T cell therapy is particularly exciting because it works well in people whose cancers haven’t responded to other available treatments, says Renier Brentjens, an oncologist at Memorial Sloan Kettering Cancer Center in New York City. Of the 63 kids and young adults treated in a clinical trial of Kymriah, 83 percent had their cancers go into remission within three months.
Now that these therapies have been clinically approved, there’s been an “explosion of interest” in the approach, says Dario Campano, an immunopathologist at the National University Cancer Institute in Singapore. Going forward, he expects to see even more rapid progress in the technology. Fifteen years ago, Campana helped develop the chimeric antigen receptor that’s used in Kymriah today. For now, the treatments are approved for use only when other treatments have failed, but someday CAR-T cell therapy could be the first treatment doctors try, he says.

One drawback is the price. Kymriah costs $475,000 for a onetime treatment, according to Novartis, which makes Kymriah. The non-Hodgkin lymphoma treatment made by Gilead Sciences, called Yescarta, is listed at $373,000. The total price tag for treatment could be higher when the costs of dealing with side effects and complications are factored in.

The approach is approved only for blood cancers. Using CAR-T cell therapy on solid tumors will require finding ways to get the T cells past additional cellular roadblocks, Grupp says.

Have you fallen behind on your reading this year? Or maybe you’ve plowed through your must-reads and are ready for more. Science News has got you covered. Here are the staff’s picks for some of the best science books of 2017. Find detailed reviews from previous issues in the links below or in our Editors pick: Favorite books of 2017.

Against the Grain
James C. Scott

Armed with the latest archaeological research, a political anthropologist argues that the rise of civilization came at a big cost. The initial switch from hunting and gathering to agricultural states brought poor diets, labor-intensive work, outbreaks of infectious diseases and other hardships (SN: 10/14/17, p. 28). Yale Univ., $26

The Great Quake
Henry Fountain

Historical records and interviews with survivors flesh out this tale of how a massive earthquake in Alaska in 1964 provided geologists with key evidence needed to verify the theory of plate tectonics (SN: 9/16/17, p. 32). Crown, $28

Eclipse
Frank Close

More than just a primer on the science of solar eclipses, this memoir chronicles a physicist’s lifetime fascination with the celestial phenomenon and introduces readers to the quirky world of eclipse chasers (SN: 5/13/17, p. 28). Oxford Univ., $21.95

Rise of the Necrofauna
Britt Wray

Resurrecting woolly mammoths, passenger pigeons and other extinct creatures isn’t just a technological problem, as this book explains. “De-extinction” is also rife with ethical dilemmas (SN: 10/28/17, p. 28). Greystone Books, $26.95

Big Chicken
Maryn McKenna

Antibiotics transformed chicken farming, to the detriment of the birds and of human health, a journalist contends. Widespread use of the drugs fueled the industrialization of poultry production and the rise of antibiotic-resistant bacteria (SN: 9/30/17, p. 30). National Geographic, $27

Inferior
Angela Saini

A science writer makes a persuasive case that centuries of biased thinking and flawed scientific research have reinforced sexist stereotypes about women (SN: 9/2/17, p. 27). Beacon Press, $25.95

Caesar’s Last Breath
Sam Kean

Through fun historical anecdotes and lesser-known backstories of scientific greats, this entertaining book profiles the chemical elements that make up the air we breathe and traces the history of Earth’s atmosphere (SN: 7/8/17 & 7/22/17, p. 38). Little, Brown and Co., $28

Cannibalism
Bill Schutt

The grisly practice of eating your own kind turns out to be widespread in the animal kingdom, a zoologist explains in this captivating look at cannibalism (SN: 2/18/17, p. 29). Algonquin Books, $26.95

The Lost City of the Monkey God
Douglas Preston

A journalist tags along on an archaeological expedition to search for the real-life remains of a mythological city in this rainforest adventure tale that morphs into a medical mystery (SN: 2/4/17, p. 28). Grand Central Publishing, $28

The Death and Life of the Great Lakes
Dan Egan

Invasive species, urbanization and other threats have wreaked havoc on the Great Lakes, but this book still finds some glimmers of hope in the scientists who are making headway in resuscitating the ecosystem (SN: 3/18/17, p. 30). W.W. Norton & Co., $27.95

How to Tame a Fox
Lee Alan Dugatkin and Lyudmila Trut

An experiment to replay animal domestication by selectively breeding wild silver foxes is lovingly retold, including by the researcher who has kept the project alive for nearly 60 years (SN: 5/13/17, p. 29). Univ. of Chicago, $26

Making Contact
Sarah Scoles

In the face of numerous obstacles, Jill Tarter still managed to spearhead the search for extraterrestrial intelligence for decades, as this biography recounts (SN: 8/5/17, p. 26). Pegasus Books, $27.95

A Crack in Creation
Jennifer A. Doudna and Samuel H. Sternberg

Two experts, including one of the pioneers of CRISPR/Cas9, discuss the science and ethics of gene editing. Houghton Mifflin Harcourt, $28

These book reviews contain links to Amazon.com. Science News is a participant in the Amazon Services LLC Associates Program. Please see our FAQ for more details.

Life expectancy in the United States has decreased for the second year in a row, the first back-to-back drops in more than 50 years, the U.S. Centers for Disease Control and Prevention reports.

In 2016, life expectancy at birth was 78.6 years for the U.S. population as a whole. That’s 0.1 year less than in 2015. For men, life expectancy decreased from 76.3 years in 2015 to 76.1 years in 2016, while in women it remained the same, at 81.1 years. The new data, from CDC’s National Center for Health Statistics, are published online December 21.
Heart disease was the leading cause of death for 2016, followed by cancer, unintentional injuries such as drug overdoses and car crashes, chronic lower respiratory diseases including asthma, and stroke. Rounding out the top 10 causes of death were Alzheimer’s disease, diabetes, influenza and pneumonia, kidney disease and suicide.

The overall drop in life expectancy is largely a result of an uptick in the age-adjusted death rates for unintentional injuries, Alzheimer’s disease and suicide, the report’s authors say.

The driving force behind Yellowstone’s long and explosive volcanic history may not be as deep as once thought. A new study suggests that instead of a plume of hot mantle that extends down to Earth’s core, the real culprit is a subducting tectonic plate that began sinking beneath North America hundreds of millions of years ago.

Computer simulations show that movement of broken-up remnants of the ancient Farallon Plate could be stirring the mantle in a way that fuels Yellowstone, researchers report December 18 in Nature Geoscience. “The fit is so good,” says study coauthor Lijun Liu, a geodynamicist at the University of Illinois at Urbana-Champaign.
The giant supervolcano now beneath Yellowstone National Park, located mostly in Wyoming, has a 17-million-year history — much of it on the move. In that time, the locus of volcanism has moved northeastward from southwestern Idaho to its current location, where it most recently explosively erupted about 640,000 years ago. These shifting eruptions have created a track of volcanic craters resembling those created by the hot spot that formed the Hawaiian island chain. As a result, scientists have long suspected that a deep plume of magma originating from the core-mantle boundary, similar to the one that fuels Hawaii’s volcanoes, is the source of Yellowstone’s fury.

But the nature of the Yellowstone plume has been the subject of debate. “Usually with plumes, we can trace them to the core-mantle boundary,” says Robert Porritt, a seismologist at the University of Texas at Austin, who was not involved in the new work. To “see” Earth’s structure, seismologists use a technique called seismic tomography, which maps the interior using seismic waves generated by earthquakes. Particularly hot or liquid parts of the mantle slow some seismic waves known as shear waves. Tomographic images of mantle plumes such as the one beneath Hawaii show a low-velocity region that extends all the way down to the boundary between mantle and core, about 2,900 kilometers below Earth’s surface. Such deep plumes are thought to be necessary to provide sufficient heat for the volcanism.

“But at Yellowstone, we don’t have that large low-shear velocity thing at the core-mantle boundary,” Porritt says. Current images suggest a region of low-velocity material extending at least 1,000 kilometers deep — but whether there is a deeper plume is uncertain.

And the region is tectonically complex. About 200 million years ago, a tectonic plate to the west, known as the Farallon Plate, began to slide eastward beneath the North American Plate. The current Juan de Fuca Plate off the Pacific Northwest coast, one of the last remnants of the Farallon Plate, continues to slide beneath the western United States. Some researchers have suggested that, instead of a deep mantle plume, the flexing and melting of the subducting Juan de Fuca Plate are responsible for Yellowstone’s volcanism.
Liu and his colleagues have yet another idea. In 2016, Liu published research suggesting that the sinking ancient Farallon slab was acting like a lid on a deep mantle plume, preventing the plume from rising to the surface (SN Online: 2/3/16). “But we kept in mind that the problem was not solved,” Liu says. “The heat source [for Yellowstone] was still missing.”

The researchers created a sophisticated, supercomputer-driven series of simulations to try to find the best scenario that matches the three known knowns: the current tomographic images of the subsurface beneath the western United States; the volcanic history at Yellowstone as well as in the nearby Basin and Range regions; and the movements of the subducting slab since about 20 million years ago.

Yellowstone’s volcanism is linked not just to the currently subducting young Juan de Fuca Plate, but also to the remnants of its older incarnation, the Farallon Plate, the simulations suggest. Those remnants have continued to slide deeper and now lie beneath the eastern United States. This downward dive dragged hot mantle eastward along with it. As the Juan de Fuca Plate began to break up beneath the western United States, the hot mantle rose through the cracks. Some of that hot mantle circulated back to the west across the top of the Juan de Fuca Plate, fueling volcanism in the Basin and Range region. And some of it flowed eastward, adding heat to Yellowstone’s fire. The study doesn’t rule out the presence of a deep magma plume, but it suggests that such a plume plays little role in Yellowstone’s volcanism.

Porritt says he’s intrigued by the idea that the sinking Farallon slab beneath the central and eastern United States could be driving mantle circulation on such a large scale. However, he says, he isn’t convinced that the authors have truly solved the larger mystery of Yellowstone’s volcanism — or that a yet-to-be-found deep plume still isn’t playing a major role. “It’s an interesting debate that’s going to be raging, hopefully for decades.”