From Gowanda Research via Science Alert: “This Amazing Map Fills a 500-Million Year Gap in Earth’s History”

Science Alert

28 JUN 2017

ALAN COLLINS
ANDREW MERDITH

Earth is estimated to be around 4.5 billion years old, with life first appearing around 3 billion years ago. To unravel this incredible history, scientists use a range of different techniques to determine when and where continents moved, how life evolved, how climate changed over time, when our oceans rose and fell, and how land was shaped.

Tectonic plates – the huge, constantly moving slabs of rock that make up the outermost layer of the Earth, the crust – are central to all these studies.

Along with our colleagues, we have published the first whole-Earth plate tectonic map of half a billion years of Earth history, from 1,000 million years ago to 520 million years ago.

In the visualisation above, the colours refer to where the continents lie today. Light blue = India, Madagascar and Arabia, magenta = Australia and Antarctica, white = Siberia, red = North America, orange = Africa, dark blue = South America, yellow = China, green = northeast Europe.

The time range is crucial. It’s a period when the Earth went through the most extreme climate swings known, from “Snowball Earth” icy extremes to super-hot greenhouse conditions, when the atmosphere got a major injection of oxygen and when multicellular life appeared and exploded in diversity.

Now with this first global map of plate tectonics through this period, we (and others) can start to assess the role of plate tectonic processes on other Earth systems and even address how movement of structures deep in our Earth may have varied over a billion year cycle.

Below you can see: (a) map of Precambrian cratonic crust used in the reconstruction in their present-day locations; (b) present day geographical map of the world with Precambrian cratonic crust used in the reconstruction in grey.

Fig. 2, Mueller et al., Gondwana Research (2017)

The Earth moves under our feet

Modern plate tectonic boundaries. But how do we map the Earth like this in the past? NASA’s Earth Observatory

The modern Earth’s tectonic plate boundaries are mapped in excruciating detail.

In the modern Earth, global positioning satellites are used to map how the Earth changes and moves. We know that up-welling plumes of hot rock from over 2,500 km (1,553 miles) deep in the planet’s mantle (the layer beneath the Earth’s crust) hit the solid carapace of the planet (the crust and the top part of the mantle). This forces rigid surface tectonic plates to move at the tempo of a fingernail’s growth.

On the other side of the up-welling hot rock plumes are areas known as subduction zones, where vast regions of the ocean floor plunge down into the deep Earth. Eventually these down-going oceanic plates hit the boundary between the core and mantle layers of Earth, about 2,900 km (1,801 miles) down. They come together, forming thermal or chemical accumulations that eventually source these up-welling zones.

It’s fascinating stuff, but these processes also create problems for scientists trying to look back in time. The planet can only be directly mapped over its last 200 million years. Before that, back over the preceding four billion years, the majority of the planet’s surface is missing, as all the crust that lay under the oceans has been destroyed through subduction.

Oceanic crust just doesn’t last: it’s constantly being pulled back deep into the Earth, where it’s inaccessible to science.

Mapping the Earth in deep time

So what did we do to map the Earth in deep time? To get at where plate margins were and how they changed, we looked for proxies – or alternative representations – of plate margins in the geological record.

We found rocks that formed above subduction zones, in continental collisions, or in the fissures where plates ripped apart. Our data came from rocks found in locations including Madagascar, Ethiopia and far west Brazil. The new map and associated work is the result of a couple of decades of work by many excellent PhD students and colleagues from all over the world.

We now have more details, and a view to way further back in geological time, than were previously available for those studying the Earth.

Using other methods, the latitudes of continents in the past can be worked out, as some iron-bearing rocks freeze the magnetic field in them as they form.

This is like a fossil compass, with the needle pointing into the ground at an angle related to the latitude where it formed – near the equator the magnetic field is roughly parallel to the Earth’s surface, at the poles it plunges directly down. You can see this today if you buy a compass in Australia and take it to Canada: the compass won’t work very well, as the needle will want to point down into the Earth.

Compass needles are always balanced to remain broadly horizontal in the region that they are designed to work in.

But, these so-called “palaeomagnetic” measurements are hard to do, and it is not easy finding rocks that preserve these records. Also, they only tell us about the continents and not about plate margins or the oceans.

Why map ancient plate tectonics?

The lack of ancient tectonic maps has posed quite a problem for how we understand our Earth.

Tectonic plates influence many processes on Earth, including the climate, the biosphere (the sphere of life on the outer part of the planet), and the hydrosphere (the water cycle and how it circulates around the planet and how its chemistry varies).

By simply redistributing tectonic plates, and thereby moving the positions (the latitudes and longitudes) of continents and oceans, controls are placed on where different plants and animals can live and migrate.

Plate boundary locations also govern how ocean currents redistribute heat and water chemistry. Different water masses in the ocean contain subtly different elements and their various forms, known as isotopes.

For example, water in the deep oceans was often not at the surface for many many thousands of years, and has different composition from the water presently on the ocean’s surface. This is important because different water masses contain different amounts of nutrients, redistributing them to different parts of the Earth, changing the potential for life in different places.

Tectonic plates also influence how much of the Sun’s radiation gets reflected back out to space, changing the Earth’s temperature.

How fast tectonic plates move have also varied over time. At different periods in Earth history there were more mid-ocean volcanoes than there are today, creating water movement such as pushing up ocean waters over the continents. At these times, some types of volcanic eruptions were more frequent, pumping more gas into the atmosphere.

Mountain ranges form as tectonic plates collide, which affect oceanic and atmospheric currents as well as exposing rocks to be eroded. This locks up greenhouse gases, and releases nutrients into the ocean.

Understand ancient plate tectonics and we go someway to understanding the ancient Earth system. And the Earth as it is today, and into the future.

The research reported in this article was conducted by a team of researchers from The University of Sydney, The University of Adelaide and Curtin University.

Abstract from Gowanda Research paper on Science Direct

Abstract

Neoproterozoic tectonic geography was dominated by the formation of the supercontinent Rodinia, its break-up and the subsequent amalgamation of Gondwana. The Neoproterozoic was a tumultuous time of Earth history, with large climatic variations, the emergence of complex life and a series of continent-building orogenies of a scale not repeated until the Cenozoic. Here we synthesise available geological and palaeomagnetic data and build the first full-plate, topological model of the Neoproterozoic that maps the evolution of the tectonic plate configurations during this time. Topological models trace evolving plate boundaries and facilitate the evaluation of “plate tectonic rules” such as subduction zone migration through time when building plate models. There is a rich history of subduction zone proxies preserved in the Neoproterozoic geological record, providing good evidence for the existence of continent-margin and intra-oceanic subduction zones through time. These are preserved either as volcanic arc protoliths accreted in continent-continent, or continent-arc collisions, or as the detritus of these volcanic arcs preserved in successor basins. Despite this, we find that the model presented here still predicts less subduction (ca. 90%) than on the modern earth, suggesting that we have produced a conservative model and are likely underestimating the amount of subduction, either due to a simplification of tectonically complex areas, or because of the absence of preservation in the geological record (e.g. ocean-ocean convergence). Furthermore, the reconstruction of plate boundary geometries provides constraints for global-scale earth system parameters, such as the role of volcanism or ridge production on the planet’s icehouse climatic excursion during the Cryogenian. Besides modelling plate boundaries, our model presents some notable departures from previous Rodinia models. We omit India and South China from Rodinia completely, due to long-lived subduction preserved on margins of India and conflicting palaeomagnetic data for the Cryogenian, such that these two cratons act as ‘lonely wanderers’ for much of the Neoproterozoic. We also introduce a Tonian-Cryogenian aged rotation of the Congo-São Francisco Craton relative to Rodinia to better fit palaeomagnetic data and account for thick passive margin sediments along its southern margin during the Tonian. The GPlates files of the model are released to the public and it is our expectation that this model can act as a foundation for future model refinements, the testing of alternative models, as well as providing constraints for both geodynamic and palaeoclimate models.

Full paper available here.

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From Science: “3.77-billion-year-old fossils stake new claim to oldest evidence of life”

Science Magazine

Mar. 1, 2017
Carolyn Gramling

These tubelike structures, formed of an iron ore called hematite, may be microfossils of 3.77-billion-year-old life at ancient hydrothermal vents.

Life on Earth may have originated in the sunless depths of the ocean rather than shallow seas. In a new study, scientists studying 3.77-billion-year-old rocks have found tubelike fossils similar to structures found at hydrothermal vents, which host thriving biological communities. That would make them more than 300 million years older than the most ancient signs of life on Earth—fossilized microbial mats called stromatolites that grew in shallow seas. Other scientists are skeptical about the new claims.

“The authors offer a convincing set of observations that could signify life,” says Kurt Konhauser, a geomicrobiologist at the University of Alberta in Edmonton, Canada, who was not involved in the study. But “at present, I do not see a way in which we will definitively prove ancient life at 3.8 billion years ago.”

When life first emerged on Earth has been an enduring and frustrating mystery. The planet is 4.55 billion years old, but thanks to plate tectonics and the constant recycling of Earth’s crust, only a handful of rock outcrops remain that are older than 3 billion years, including 3.7-billion-year-old formations in Greenland’s Isua Greenstone Belt. And these rocks tend to be twisted up and chemically altered by heat and pressure, making it devilishly difficult to detect unequivocal signs of life.

“It’s a challenge in rocks that have been this messed up,” says Abigail Allwood, a geologist with NASA’s Jet Propulsion Laboratory in Pasadena, California, who was also not involved in the study. “There’s only so much you can do with them.”

Nevertheless, researchers have searched through these most ancient rocks for structural or chemical relics that may have lingered. Last year, for example, scientists reported identifying odd reddish peaks in 3.7-billion-year-old rocks in Greenland that may be the product of stromatolites, though many doubted that interpretation. The best evidence for these fossilized algal mats comes from 3.4-billion-year-old rocks in Australia, generally thought of as the strongest evidence for early life on Earth.

But some scientists think ocean life may have begun earlier—and deeper. In the modern ocean, life thrives in and around the vents that form near seafloor spreading ridges or subduction zones—places where Earth’s tectonic plates are pulling apart or grinding together. The vents spew seawater, superheated by magma in the ocean crust and laden with metal minerals such as iron sulfide. As the water cools, the metals settle out, forming towering spires and chimneys. The mysterious ecosystem that inhabits this sunless, harsh environment includes bacteria and giant tube worms that don’t derive energy from photosynthesis. Such hardy communities, scientists have suggested, may not only have thrived on early Earth, but may also be an analog for life on other planets.

Now, a team led by geochemist Dominic Papineau of University College London and his Ph.D. student Matthew Dodd says it has found clear evidence of such ancient vent life. The clues come from ancient rocks in northern Quebec in Canada that are at least 3.77 billion years old and may be even older than 4 billion years. Dodd examined hair-thin slices of rock from this formation and found intriguing features: tiny tubes composed of an iron oxide called hematite, as well as filaments of hematite that branch out and sometimes terminate into large knobs.

Filaments and tubes are common features in more recent fossils that are attributed to the activity of iron-oxidizing bacteria at seafloor hydrothermal vents. Papineau was initially skeptical. However, he says, “within a year [Dodd] had found so much compelling evidence that I was convinced.”

The team also identified carbonate “rosettes,” tiny concentric rings that contain traces of life’s building blocks including carbon, calcium, and phosphorus; and tiny, round granules of graphite, a form of carbon. Such rosettes and granules had been observed previously in rocks of similar age, but whether they are biological in origin is hotly debated. The rosettes can form nonbiologically from a series of chemical reactions, but Papineau says the rosettes in the new study contain a calcium phosphate mineral called apatite, which strongly suggests the presence of microorganisms. The graphite granules may represent part of a complicated chemical chain reaction mediated by the bacteria, he says. Taken together, the structures and their chemistry point to a biological origin near a submarine hydrothermal vent, the team reports online today in Nature. That would make them among the oldest signs of life on Earth—and, depending on the actual age of the rocks, possibly the oldest.

That doesn’t necessarily mean that life originated in deep waters rather than in shallow seas, Papineau says. “It’s not necessarily mutually exclusive—if we are ready to accept the fact that life diversified very early.” Both the iron-oxidizing bacteria and the photosynthetic cyanobacteria that build stromatolite mats could have evolved from an earlier ancestor, he says.

But researchers like Konhauser remain skeptical of the paper’s conclusion. For example, he says, the observed hematite tubes and filaments are similar to structures associated with iron-oxidizing bacteria, “but of course that does not mean the [3.77-] billion-year-old structures are cells.” Moreover, he notes, if the tubes were formed by iron-oxidizing bacteria, they would need oxygen, in short supply at this early moment in Earth’s history. It implies that photosynthetic bacteria were already around to produce it. But it’s still unclear how oxygen would get down to the depths of early Earth’s ocean. The cyanobacteria that make stromatolites, on the other hand, make oxygen rather than consume it.

The new paper makes “a more detailed case than has been presented previously,” Allwood says. Most previous reports of possible signs of life older than about 3.5 billion years have been questioned, she adds—not because life didn’t exist, but because it’s just so difficult to prove the further back in time you go in the rock record. “There’s still quite a bit of room for doubt.”

See the full article here .

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From Smithsonian: “Researchers Think They’ve Found a Mini Continent in the Indian Ocean”

Smithsonian.com

February 2, 2017
Jason Daley

The beautiful Mauritius island may be hiding a chunk of continent. (Sapsiwai via iStock)

About 200 million years ago, the supercontinent of Gondwana—essentially an an agglomeration of Africa, South America, India, Australia and Antarctica—began slowly ripping apart into the continents recognizable today. But a new study suggests that Gondwana spun out another continent that is now lost beneath the Indian Ocean.

Assemblage of continents, which constitute Gondwana. Image Credit: Griem (2007)

As Alice Klein reports for New Scientist, researchers studying the earth’s crust found that parts of the Indian Ocean’s seafloor had slightly stronger gravitaitonal fields, suggesting that the crust might be thicker there.

The island of Mauritius exhibited this extra oomph, which led Lewis Ashwal, a geologist at the University of the Witwatersrand, South Africa, and his colleagues to propose that the island was sitting atop a sunken chunk of continent.

The researchers studied the geology of the island and rocks spewed out during periods of ancient volcanism. One particular mineral they were looking for are zircons, tough minerals that contains bits of uranium and thorium. The mineral can last billions of years and geologists can use these to acurately date rocks.

The search paid off. The researchers recovered zircons as old as 3 billion years, Ashwal says in a press release. But the island rocks are no older than 9 million years old. The researchers argue that the old rock is evidence that the island is sitting on a much older crust that was once part of a continent. The zircons are remnants of this much older rock and were likely pushed up by volcanic activity. They published their results in the journal Nature Communications.

According to Paul Hetzel at Seeker, researchers had previously discovered zircons on Mauritius’ beaches, but were unable to rule out the possibility that they were brought there by the ocean. The new finding confirms that the zircon comes from the island itself.

Mauritia was likely a small continent, about a quarter the size of Madagascar, reports Klein. As the Indian plate and the Madagascar plate pulled apart, it stretched and broke up the small continent, spreading chunks of it across the Indian Ocean.

One of the 3-billion-year-old zircon crystals discovered on Mauritius (Wits University )

“According to the new results, this break-up did not involve a simple splitting of the ancient super-continent of Gondwana, but rather, a complex splintering took place with fragments of continental crust of variable sizes left adrift within the evolving Indian Ocean basin,” Ashwal says in the press release [phys.org].

Klein reports that other islands in the Indian Ocean, including Cargados Carajos, Laccadive and the Chagos islands might also exist on top of fragments of the continent now dubbed Mauritia.

Surprisingly, this may not be the only lost continent out there. In 2015, researchers at the University of Oslo found evidence that Iceland may sit on top of a sunken slice of crust. And in 2011, researchers found evidence that a micro-continent has existed off the coast of Scotland for about a million years.

See the full article here .

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From Eos: “Understanding Tectonic Processes Following Great Earthquakes”

Eos

12.2.16
Sarah Witman

A building torn in two in Concepción, Chile, following a magnitude 8.8 earthquake in 2010. Credit: hdur, CC BY-NC 2.0

The most powerful and destructive earthquakes (magnitude 8 and higher) happen about once per year. Earthquakes at those magnitudes can do a lot of damage, often causing tsunamis and inflicting devastation across an entire region. The biggest quakes, such as Chile’s 1960 magnitude 9.5 tremor and Japan’s 2011 magnitude 9 event, typically occur in a subduction zone, a section of the Earth’s crust where one massive tectonic plate is sliding beneath another.

The tectonic plates of the world were mapped in 1996, USGS

The pent-up energy of these two plates pressing against one another eventually gives way, creating a colossal earthquake known as a megathrust.

A recent study by Bedford et al. [Journal of Geophysical Research] presents a new approach to analyzing signals detected via satellite in the months following a megathrust earthquake. These signals, related to deformation in the tectonic plates caused by stress and strain, can be used to shed light on the tectonic processes that continue to unfold after disaster has struck.

In many recent megathrust earthquakes, continuous GPS monitoring has allowed scientists to observe a two-part process following each quake: afterslip (slow, quiet movement along the interface of the two plates) and viscoelastic relaxation (the process of malleable, deforming rock adjusting to its new state of stress). However, when modeling GPS data collected in the 4 years after a magnitude 8.8 earthquake struck Chile in 2010, the team of researchers noticed something odd.

A schematic showing the straightening process in action. Viscoelastic relaxation and relocking predictions are subtracted from the curved original signal to leave behind a straight signal that is assumed to be the isolated afterslip signal. The color scale shows the time that has elapsed since the great earthquake. Millimeter scales on the x and y axes denote motions in the east and north horizontal directions, respectively. Credit: Jonathan Bedford

Usually, before an earthquake, the horizontal ground motion of a continental plate is in roughly the same direction as the subducting oceanic plate because the oceanic plate sticks against—and drags along—the continental plate. During the quake, this observed surface motion is reversed when the sticking point between the two plates suddenly slips.

Instead of this usual pattern of horizontal motion due to afterslip and viscoelastic relaxation, however, the researchers saw a distinct curvature in the surface signals. Almost immediately after the quake, the GPS motions started to curve around, starting from roughly the opposite direction of the downgoing plate and shifting toward a direction more in line with the pre-earthquake motion.

The researchers thought that maybe, in addition to afterslip and viscoelastic relaxation, some relocking at the interface of the plates (pumping the brakes, so to speak) might be contributing to the observed curvature. After all, the plate interface must eventually relock in preparation for the next large earthquake. The researchers wondered whether relocking could be a dominant process so soon after a great earthquake. And, if so, how big was its impact relative to the other two processes?

To find out, the researchers applied a novel approach to separating out three different processes: afterslip, viscoelastic relaxation, and plate interface relocking. Their approach, called straightening, assumes that the afterslip motion comes from a nonmigrating afterslip distribution on the plate interface that decays linearly with time. Under these assumptions, the individual contributions of each process can be teased out by finding the combination of relocking and viscoelastic relaxation model predictions that when subtracted from the recorded signal, best reproduces the expected unstraightened afterslip signal features. In other words, they held afterslip to a fixed pattern so that they could vary other parameters to estimate the other components.

Following this method, the researchers discovered that plate interface relocking was indeed the dominant process causing curvature in the signal. Moreover, they were also able to confirm the results of past lab experiments proposing that relocking occurs rapidly, less than a year after an earthquake takes place.

Overall, the study helps provide a more accurate picture of the tectonic processes underlying signals detected after a megathrust earthquake. The researchers hope that in the future their method can be tested at the sites of other megathrust earthquakes, especially those that are well observed by GPS networks.

See the full article here .

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From U Chicago: “Case of Earth’s missing continental crust solved: It sank”

University of Chicago

October 4, 2016
Carla Reiter

Mantle swallowed massive chunk of Eurasia and India, study finds

UChicago scientists have concluded that half the original mass of Eurasia and India disappeared into the Earth’s interior before the two continents began their slow-motion collision approximately 60 million years ago. The participating UChicago scientists are (from left) Miquela Ingalls, doctoral student in geophysical sciences; David Rowley, professor in geophysical sciences; and Albert Colman, assistant professor in geophysical sciences. Rowley holds a rock of the type they believe sank into the interior. Photo by Jean Lachat

How do you make half the mass of two continents disappear? To answer that question, you first need to discover that it’s missing.

That’s what a trio of University of Chicago geoscientists and their collaborator did, and their explanation for where the mass went significantly changes prevailing ideas about what can happen when continents collide. It also has important implications for our understanding of when the continents grew to their present size and how the chemistry of the Earth’s interior has evolved.

The study, published online Sept. 19 in Nature Geoscience, examines the collision of Eurasia and India, which began about 60 million years ago, created the Himalayas and is still in (slow) progress. The scientists computed with unprecedented precision the amount of landmass, or “continental crust,” before and after the collision.

“What we found is that half of the mass that was there 60 million years ago is missing from the earth’s surface today,“ said Miquela Ingalls, a graduate student in geophysical sciences who led the project as part of her doctoral work.

The result was unexpectedly large. After considering all other ways the mass might be accounted for, the researchers concluded that so huge a mass discrepancy could only be explained if the missing chunk had gone back down into the Earth’s mantle—something geoscientists had considered more or less impossible on such a scale.

When tectonic plates come together, something has to give.

The tectonic plates of the world were mapped in 1996, USGS

According to plate tectonic theory, the surface of the Earth comprises a mosaic of about a dozen rigid plates in relative motion. These plates move atop the upper mantle, and plates topped with thicker, more buoyant continental crust ride higher than those topped with thinner oceanic crust. Oceanic crust can dip and slide into the mantle, where it eventually mixes together with the mantle material. But continental crust like that involved in the Eurasia-India collision is less dense, and geologists have long believed that when it meets the mantle, it is pushed back up like a beach ball in water, never mixing back in.

Geology 101 miscreant

“We’re taught in Geology 101 that continental crust is buoyant and can’t descend into the mantle,” Ingalls said. The new results throw that idea out the window.

“We really have significant amounts of crust that have disappeared from the crustal reservoir, and the only place that it can go is into the mantle,” said David Rowley, a professor in geophysical sciences who is one of Ingalls’ advisors and a collaborator on the project. “It used to be thought that the mantle and the crust interacted only in a relatively minor way. This work suggests that, at least in certain circumstances, that’s not true.”

The scientists’ conclusion arose out of meticulous calculations of the amount of mass there before and after the collision, and a careful accounting of all possible ways it could have been distributed. Computing the amount of crust “before” is a contentious problem involving careful dating of the ages of strata and reconstructions of past plate positions, Ingalls said. Previous workers have done similar calculations but have often tried to force the “before” and “after” numbers to balance, “trying to make the system match up with what we think we already know about how tectonics works.”

Ingalls and collaborators made no such assumptions. They used recently revised estimates about plate movements to figure out how large the two plates were at the onset of collision, and synthesized more than 20 years’ worth of data on the geology of various regions of the Earth to calculate how thick the crust would have been.

“By looking at all of the relevant data sets, we’ve been able to say what the mass of the crust was at the beginning of collision,” Rowley said.

Limited options

There were only a few places for the displaced crust to go after the collision: Some was thrust upward, forming the Himalayas, some was eroded and deposited as enormous sedimentary deposits in the oceans, and some was squeezed out the sides of the colliding plates, forming Southeast Asia.

“But accounting for all of these different types of mass loss, we still find that half of the continental crust involved in this collision is missing today,” Ingalls said. “If we’ve accounted for all possible solutions at the surface, it means the remaining mass must have been recycled wholesale into the mantle.”

If large areas of continental crust are recycled back into the mantle, scientists can at last explain some previously puzzling geochemistry. Elements including lead and uranium are periodically erupted from the mantle through volcanic activity. Such elements are relatively abundant in continental crust, but scarce in the mantle. Yet the composition of some mantle-derived rocks indicates that they have been contaminated by continental crust. So how did continental material mix back into the mantle?

“The implication of our work is that, if we’re seeing the India-Asia collision system as an ongoing process over Earth’s history, there has been a continuous mixing of the continental crustal elements back into the mantle,” said Rowley. “And they can then be re-extracted and seen in some of those volcanic materials that come out of the mantle today.”

Funding: National Science Foundation

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From Berkeley via phys.org: “Seismic ‘CT scans’ reveal deep earth dynamics”

UC Berkeley

phys.org

September 23, 2016
Wallace Ravven

A new look 100 miles beneath a massive tectonic plate as it dives under North America has helped clarify the subduction process that generates earthquakes, volcanoes and the rise of the Cascade Range in the Pacific Northwest.

The largest array of seismometers ever deployed on the seafloor, coupled with hundreds of others operating in the continental U.S., has enabled UC Berkeley researchers to essentially create CT scans of the Juan de Fuca plate and part of the earth’s mantle directly below it.

The plate, about the size of the state of Michigan, is grinding under the continent along an 800-mile swath that runs from Northern California to Vancouver Island, known as the Cascadia subduction zone.

The 3-D imaging process, known as seismic tomography, has revealed with unprecedented clarity a huge, buoyant, sausage-shaped region of the upper mantle, or asthenosphere, pressing up on the oceanic plate.

The imaging casts new light on the competing hypotheses about the drivers of plate tectonics, a dynamic earth process that has been studied for more than 50 years but is still poorly understood.

Different evidence has led to three different plate movement scenarios: either the plates are pushed from mid-ocean ridges; or they are pulled from their subducting slabs; or their movement is driven by the drag of the viscous mantle material that lies directly below.

The new research suggests that the third scenario does not apply to the Cascadia subduction zone. Rather, it reveals that a distinct, thin—and difficult to observe—layer separates the plate from the mantle beneath, at least in the Cascadia subduction zone. The layer acts as a kind of berm that the plate rolls over before descending beneath the continent, says UC Berkeley seismologist Richard Allen, leader of the research and co-author of a paper appearing in the Sept. 23 edition of the journal Science.

“What we observe is an accumulation of low-viscosity material between the plate and the mantle. Its composition acts as a lubricant, and decouples the plate’s movement from the mantle below it,” explains Allen, who is director of the Berkeley Seismological Laboratory and professor and chair of Earth and Planetary Science at Berkeley. The plates may move independently of the mantle below, he adds.

The finding, he says, will help refine models of plate tectonic dynamics, aiding the long-range effort to understand the connection between tectonics and earthquakes.

“It is the motion of the plates that causes earthquakes,” Allen says. “Models like this help us understand that linkage so we can be better informed of the coastal hazards.

“First though, we need to learn if what we find here is typical of subduction zones across the planet, or if it is unique for some reason.”

Japan has recently deployed a massive seafloor seismic network to study subduction and earthquakes. Allen hopes to next apply the tomography strategy there. Alaska also beckons.

Lead author on the Science paper is William Hawley, a graduate student in Allen’s lab.

“Plate tectonics is the most fundamental concept explaining the formation of features we see on the earth’s surface,” Hawley says, “but despite the fact that the concept is simple, we still do not know exactly why or how it operates.

“If the asthenosphere acts as a lubricant for tectonic plate movement throughout the planet, it will really change our long-term models of the process”—dynamic changes that occur over a 100 million years.

“Modelers will have to take this lubricating layer into account because it changes the way the mantle and the plates talk to each other.”

Seismic tomography generates 3-D images of the earth’s interior by measuring how differences in shape, density, rock type and temperature affect the path, speed and amplitude of seismic waves traveling through the planet from an earthquake.

Much as in CT scans, computers process differences in energy measured at the receiving end to infer interior 3-D detail. CT scans use X-rays as the energy source, while seismic tomography measures energy from seismic waves.

A dense array of seismometers directly over the region of interest yields the best images and provides the highest resolution of the structures, which can then inform models of the process.

This study used the data from the largest scale ocean-floor deployment to complement the onshore data already available. Together, they generated the best images of the region to date.

The four-year seafloor research effort was made possible by the National Science Foundation’s ambitious $20 million Cascadia Initiative. The NSF aimed to spur greater understanding of plate structure, subduction processes, earthquakes and volcanism by deploying seismometers at 120 sites on the ocean floor, arrayed throughout the 95,000-square-mile Juan de Fuca plate.

Over the four years, the offshore and onshore seismometer array measured thousands of earthquakes throughout the planet, ranging from magnitudes of 5 to about 9 on the Richter scale. The study examined a subset of 321 quakes with magnitudes between about 6 and 7.5.

Grad students and faculty scientists participated in 24 research cruises to deploy the instruments and move them between two swaths of the Juan de Fuca plate. Several of the seismic tomography cruises invited undergraduate students on the two-week trips. On one Berkeley-led cruise aboard the R/V Thomas Thompson, the undergrads dubbed the trip the “Tom Cruise,” and sent daily video blogs.

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From Eos: “Planning for a Subduction Zone Observatory”

Eos
OPINION

5.20.16

By Joan Gomberg, Paul Bodin, Jody Bourgeois, Susan Cashman, Darrel Cowan, Ken Creager, Brendan Crowell, Alison Duvall, Arthur Frankel, Frank Gonzalez, Heidi Houston, Paul Johnson, Harvey Kelsey, Una Miller, Emily Roland, David Schmidt, Lydia Staisch, John Vidale, William Wilcock, and Erin Wirth

The eruption of Alaska’s Augustine Volcano on 27 March 2006, viewed from the M/V Maritime Maid. Credit: Cyrus Read, AVO/USGS

Subduction zones contain many of Earth’s most remarkable geologic structures, from the deepest oceanic trenches to glacier-covered mountains and steaming volcanoes. These environments formed through spectacular events: Nature’s largest earthquakes, tsunamis, and volcanic eruptions are born here.

A scientist investigates dramatic subsidence—in places up to 2 meters—along the northwest coast of Sumatra in January 2005. Here tsunami waves spawned from the Mw9.1 earthquake that shook on 26 December 2004 snapped off tree tops; roots and lower trunks are now submerged in water. Credit: USGS

Great subduction zone earthquakes can cause coastal subsidence in minutes that has impacts comparable to a century or more of climatically driven sea level rise. They can radiate shaking waves that trigger widespread landslides and submarine slope failures and can spawn tsunami waves that reach distant lands. Eruptions can produce fast-moving mudflows as well as high-lofted ash clouds complete with their own lightning systems.

The geologic processes in subduction zones shape surface morphology, couple plate motions to mantle convection, control resource distributions, and even interact with Earth’s climate—volcanic gases can cool the climate, and sediments at subduction margins are important reservoirs for greenhouse gases. In short, the diverse environments in subduction zones are natural laboratories for investigating a wide range of interlinked processes.

The myriad opportunities for collaborative work among scientific disciplines, institutions, and nations have inspired a grassroots movement to create a subduction zone observatory (SZO), now in a nascent stage of planning. Over the past few years, Earth scientists have been discussing options for its geographic scope and structure. For example, should such an observatory focus on one subduction zone or multiple zones? Should it be managed as a center or an umbrella organization? How should resources be balanced between new facilities and funds needed for research?

As part of these discussions, we held a series of seminars focused on SZO topics. A more official and much broader workshop will occur in September 2016. Here we share ideas that have emerged from past talks in an effort to grow the movement, advance community discussion, and help shape a vision.

Why Invest in an SZO Now? Lessons from Tohoku

Scientific focus on subduction zones has increased dramatically in the past decade, accompanying a recognition that large numbers of people are migrating to urban areas that lie within subduction zones. Scientists recognize the hazards inherent in living near tectonically active margins [Bilham, 2009] and are motivated to understand them so that they may provide the best advice to protect communities.

Fig. 1. Fault slip from the Mw9 Tohoku, Japan, earthquake in 2011. Seismic and GPS waveform modeling revealed significant slip in patches with varying characteristics (amplitudes color coded), as if it were a composite of smaller earthquakes. The plate interface (fault) dips to the west. A downdip patch radiated the high-frequency waves that damaged buildings (right map), and the largest slip on an updip patch (left map) generated the enormous tsunami. Modified from Frankel [2013].

Investments in onshore and offshore instrument arrays have enhanced the resolution of our understanding of subduction-related processes. Multifaceted analyses can make monitoring systems and hazard assessments more robust. For example, the Mw9.0 Tohoku earthquake, which shook Japan in 2011, occurred in an area with an abundance of onshore and offshore observations, including geodetic and seismic signals, marine seismic reflection profiles, bathymetry, tsunami wave heights, and novel seafloor acoustic GPS and pressure measurements. Offshore observations provided, for the first time, unambiguous evidence for why the tsunami was so large—they documented several tens of meters of fault slip near the Japan Trench (Figure 1). Rich data sets and multifaceted analyses also revealed that days before the Tohoku earthquake, the plate interface began slipping slowly, with slip propagating toward the initiation point of the Mw9.0 earthquake [Kato et al., 2012].

The 2011 Tohoku earthquake illustrated the potential of an SZO to significantly advance understanding of the processes at work before, during, and after a major event. It also prompted sobering retrospectives on the difficulty of anticipating its enormous size, even after meticulous paleoseismological studies [Sawai et al., 2012], underscoring the need for further research investments that will make subduction zone populations more resilient.

Ideally, an SZO would produce research that is transformational, multidisciplinary, amphibious, and international. As illustrated by the Tohoku earthquake and tsunami, an SZO would enable transformational research by facilitating acquisition and analysis of disparate data with multiple synergistic applications considered from the start. An SZO must be multidisciplinary to understand linkages between processes within subduction zones and the transitions to adjacent tectonic environments.

The tremendous diversity of processes and physical characteristics displayed by the world’s subduction zones begs for a global scope. Although the geographic scope of an SZO remains to be determined, we illustrate key features of an SZO with some examples of broad questions addressed in particular regional studies. Two of the three examples come from the Cascadia subduction zone (Figure 2a), only because it is where most of the authors reside and work.

How do accretionary prism sedimentation, geochemistry, ocean currents, and climate change interact with gas hydrate stability? Subduction margin sediments contain a significant fraction of Earth’s greenhouse gas methane. This methane, derived from sediments of terrestrial origins and organisms at the sea surface, gets bound within the solid crystalline water lattices of gas hydrates. In Cascadia, multichannel seismic reflection profiles reveal a gas hydrate reservoir in a swath along the seafloor where the water depth exceeds 500 meters (depth contour in Figure 2a), above which hydrates decompose. However, seawater warming at these depths threatens to decompose some of this hydrate. This could increase bottom water acidity and lower the water’s oxygen content, which could have dramatic negative effects on organisms and accelerate climate change.

Fig. 2. The Cascadia subduction zone off the west coast of the United States and Canada provides examples of questions a subduction zone observatory could address. Merged bathymetry and topography (shaded) with superposed boundaries (white lines and arrows) illustrate the diversity of environments. (a) Southern and central Cascadia, from Pacific Ocean depths (dark blues) to high Cascade mountains and volcanoes (browns). Processes include formation of an accretionary prism, formation and dissolution of methane hydrates (wiggly white line), consumption of the Juan de Fuca plate at the deformation front (hatchured), and its descent beneath the North American plate. The plate interface is stuck (locked) at shallow depths and slips slowly at greater depths, generating tremors. Deeper, metamorphic changes lead to volcanic arc formation. (b) In northernmost Cascadia, the tectonic activity transitions from subduction to transform boundaries. The red rectangle outlines the rupture plane of the magnitude 7.8 Haida Gwaii earthquake.

How do subduction zones transition to other types of tectonic regimes? At the northern end of the Cascadia subduction zone (Figure 2b), the plate motions change from converging toward one other with associated subduction and thrust faulting along shallowly dipping planes to moving parallel to each other, accommodated by steeply dipping strike-slip faulting, in which most of the movement is horizontal. The Queen Charlotte Fault system is this latter “transform” boundary. The 2012 M7.8 Haida Gwaii earthquake in British Columbia occurred on this system and illustrated the complexity of this transition, with surprisingly many of the characteristics of a subduction zone thrust event.

Recent studies of the Alaska subduction zone demonstrate that during great subduction zone earthquakes, as ruptures approach the seafloor, they sometimes propagate from the plate interface to secondary faults, sometimes also deforming ductile overlying sediments. Uncertainties in seafloor fault displacements remain the most significant unknown in local tsunami hazard assessments [Geist, 2002]. These hazards could be reduced with an SZO monitoring system and seafloor mapping before and after events.

How do subducting slabs and volcanism affect continental crust creation? Processes that generate magmas in subduction zones are the “factories” that create continents, based on the similarity between the compositions of continental crust and volcanic arcs. Numerous hypotheses exist about the processes by which slab materials accrete, subduct, melt, mix, and emerge at crustal depths or extrude from active volcanoes. Testing these hypotheses requires integrated studies spanning the disciplines of chemistry, petrology, fluid dynamics, geology, and seismology, with four-dimensional imaging.

An SZO must be amphibious, and this requires new means of collecting and integrating offshore and onshore observations. Scientists need sustained marine geodetic observations to characterize subduction zone deformation processes over many time scales, from plate motions (decades) to earthquake fault slip (seconds). A geodetic component of an SZO could include high-resolution repeat seafloor mapping, pressure gauges, and novel tools like fiber-optic strain meters. These and other frontier offshore measurements are applicable to multiple scientific questions and practical applications.

A High-Impact, Societally Relevant SZO

The envisioned SZO would contribute to the scientific understanding needed to build societal resilience in the face of natural hazards. It would also serve as an international educational resource. For example, recording earthquake ground motions using densely spaced networks would enable us to properly calibrate the simulations that underlie seismic hazard assessments and building codes (Figure 1).

An SZO should be built collaboratively with those responsible for hazard mitigation and response. Perhaps most important, it could offer opportunities to involve citizens in research, through educating the public and making them stakeholders. For example, local scientists, students, and interested citizenry could be engaged to make biological and survey observations before and after events and to document coastal uplift and subsidence, tsunami inundation, or storm surge impacts (Figure 3). Such engagement could extend understanding of the effects of major natural events in places where we lack prior baselines, and it would fit naturally into educational programs that teach by doing.

Fig. 3. This stray coral boulder was likely transported 230 meters inland by large waves or currents along the Atlantic shore of Anegada, British Virgin Islands. This and other geologic and geomorphic features are being used to assess earthquake and tsunami hazards of the subduction thrust and the adjoining outer rise along the Puerto Rico Trench. Credit: Brian Atwater

A Path Toward an SZO

Moving an SZO from concept to reality will require coordination within and among academic and hazards assessment communities. Individually and as members of professional entities, we must lead, advocate for, and contribute to proposals to governmental and private sector institutions to support, design, and build an SZO. If the Earth science community collaboratively pursues creative strategies, developing an SZO may simultaneously serve to enrich observational tools, improve hazard models, and enhance our basic understanding of one of Earth’s most dynamic environments.

References

Bilham, R. (2009), The seismic future of cities, Bull. Earthquake Eng., 7, 839–887, doi:10.1007/s10518-009-9147-0.

Frankel, A. (2013), Rupture history of the 2011 M9 Tohoku Japan earthquake determined from strong-motion and high-rate GPS recordings: Subevents radiating energy in different frequency bands, Bull. Seismol. Soc. Am., 103, 1290–1306.

Geist, E. L. (2002), Complex earthquake rupture and local tsunamis, J. Geophys. Res., 107(B5), 2086, doi:10.1029/2000JB000139.

Kato, A., K. Obara, T. Igarishi, H. Tsuruoka, S. Nakagawa, and N. Hirata (2012), Propagation of slow slip leading up to the 2011 Mw 9.0 Tohoku-Oki earthquake, Science, 335, 705–708.

Sawai, Y., Y. Namegaya, Y. Okamura, K. Satake, and M. Shishikura (2012), Challenges of anticipating the 2011 Tohoku earthquake and tsunami using coastal geology, Geophys. Res. Lett., 39, L21309, doi:10.1029/2012GL053692.

—Joan Gomberg, U.S. Geological Survey, Seattle, Wash.; and Department of Earth and Space Sciences, University of Washington, Seattle; email: [email protected]; Paul Bodin and Jody Bourgeois, Department of Earth and Space Sciences, University of Washington, Seattle; Susan Cashman, Department of Geology, Humboldt State University, Arcata, Calif.; Darrel Cowan, Ken Creager, Brendan Crowell, and Alison Duvall, Department of Earth and Space Sciences, University of Washington, Seattle; Arthur Frankel, U.S. Geological Survey, Seattle, Wash.; and Department of Earth and Space Sciences, University of Washington, Seattle; Frank Gonzalez and Heidi Houston, Department of Earth and Space Sciences, University of Washington, Seattle; Paul Johnson, School of Oceanography, University of Washington, Seattle; Harvey Kelsey, Department of Geology, Humboldt State University, Arcata, Calif.; Una Miller and Emily Roland, School of Oceanography, University of Washington, Seattle; David Schmidt, Department of Earth and Space Sciences, University of Washington, Seattle; Lydia Staisch, U.S. Geological Survey, Seattle, Wash.; and Department of Earth and Space Sciences, University of Washington, Seattle; John Vidale, Department of Earth and Space Sciences, University of Washington, Seattle; William Wilcock, School of Oceanography, University of Washington, Seattle; and Erin Wirth, Department of Earth and Space Sciences, University of Washington, Seattle

Citation: Gomberg, J., et al. (2016), Planning for a subduction zone observatory, Eos, 97, doi:10.1029/2016EO052635. Published on 20 May 2016.
© 2016. The authors. CC BY-NC 3.0

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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.

From Rice: “Oxygen atmosphere recipe = tectonics + continents + life”

Rice University

Rice-led study offers new answer to why Earth’s atmosphere became oxygenated

May 16, 2016
Jade Boyd

Earth scientists from Rice University, Yale University and the University of Tokyo are offering a new answer to the long-standing question of how our planet acquired its oxygenated atmosphere.

Based on a new model that draws from research in diverse fields including petrology, geodynamics, volcanology and geochemistry, the team’s findings* were published online this week in Nature Geoscience. They suggest that the rise of oxygen in Earth’s atmosphere was an inevitable consequence of the formation of continents in the presence of life and plate tectonics.

“It’s really a very simple idea, but fully understanding it requires a good bit of background about how the Earth works,” said study lead author Cin-Ty Lee, professor of Earth science at Rice. “The analogy I most often use is the leaky bathtub. The level of water in a bathtub is controlled by the rate of water flowing in through the faucet and the efficiency by which water leaks out through the drain. Plants and certain types of bacteria produce oxygen as a byproduct of photosynthesis. This oxygen production is balanced by the sink: reaction of oxygen with iron and sulfur in the Earth’s crust and by back-reaction with organic carbon. For example, we breathe in oxygen and exhale carbon dioxide, essentially removing oxygen from the atmosphere. In short, the story of oxygen in our atmosphere comes down to understanding the sources and sinks, but the 3-billion-year narrative of how this actually unfolded is more complex.”

Lee co-authored the study with Laurence Yeung and Adrian Lenardic, both of Rice, and with Yale’s Ryan McKenzie and the University of Tokyo’s Yusuke Yokoyama. The authors’ explanations are based on a new model that suggests how atmospheric oxygen was added to Earth’s atmosphere at two key times: one about 2 billion years ago and another about 600 million years ago.

Today, some 20 percent of Earth’s atmosphere is free molecular oxygen, or O2. Free oxygen is not bound to another element, as are the oxygen atoms in other atmospheric gases like carbon dioxide and sulfur dioxide. For much of Earth’s 4.5-billion-year history, free oxygen was all but nonexistent in the atmosphere.

“It was not missing because it is rare,” Lee said. “Oxygen is actually one of the most abundant elements on rocky planets like Mars, Venus and Earth. However, it is one of the most chemically reactive elements. It forms strong chemical bonds with many other elements, and as a result, it tends to remain locked away in oxides that are forever entombed in the bowels of the planet — in the form of rocks. In this sense, Earth is no exception to the other planets; almost all of Earth’s oxygen still remains locked away in its deep rocky interior.”

Lee and colleagues showed that around 2.5 billion years ago, the composition of Earth’s continental crust changed fundamentally. Lee said the period, which coincided with the first rise in atmospheric oxygen, was also marked by the appearance of abundant mineral grains known as zircons.

“The presence of zircons is telling,” he said. “Zircons crystallize out of molten rocks with special compositions, and their appearance signifies a profound change from silica-poor to silica-rich volcanism. The relevance to atmospheric composition is that silica-rich rocks have far less iron and sulfur than silica-poor rocks, and iron and sulfur react with oxygen and form a sink for oxygen.

A view of Earth’s atmosphere taken from the International Space Station in 2003. (Photo courtesy of ISS Expedition 7 Crew, EOL, NASA)

“Based on this, we believe the first rise in oxygen may have been due to a substantial reduction in the efficiency of the oxygen sink,” Lee said. “In the bathtub analogy, this is equivalent to partially plugging the drain.”

Lee said the study suggests that the second rise in atmospheric oxygen was related to a change in production — analogous to turning up the flow from the faucet.

“The bathtub analogy is simple and elegant, but there’s an added complication that must be taken into account,” he said. “That is because oxygen production is ultimately tied to the global carbon cycle — the cycling of carbon between the Earth, the biosphere, the atmosphere and oceans.”

Lee said the model showed that Earth’s carbon cycle has never been at a steady state because carbon slowly leaks out as carbon dioxide from Earth’s deep interior to the surface through volcanic activity. Carbon dioxide is one of the key ingredients for photosynthesis.

“On long, geologic timescales, carbon is removed from the atmosphere by the production of condensed forms of carbon, such as organic carbon and minerals called carbonate,” he said. “For most of Earth’s history, most of this carbon has been deposited not in the deep ocean but rather on the margins of continents. The implications are profound because carbon deposited on continents does not return to Earth’s deep interior. Instead, it amplifies carbon inputs into the atmosphere when the continents are subsequently perturbed by volcanism.”

Lee said the team’s model showed that volcanic activity and other geologic inputs of carbon into the atmosphere may have increased with time, and because oxygen production is tied to carbon production, oxygen production also must increase. The model showed that the second rise in atmospheric oxygen had to occur late in Earth’s history.

“Exactly when is model-dependent, but what is clear is that the formation of continental crust naturally leads to two rises in atmospheric oxygen, just as we see in the fossil record,” Lee said.

Exactly what caused the composition of the crust to change during the first oxygenation event remains a mystery, but Lee said the team believes it may have been related to the onset of plate tectonics, where the Earth’s surface, for the first time, became mobile enough to sink back down into Earth’s deep interior.

The tectonic plates of the world were mapped in 1996, USGS.

Lee said the team’s new model is not without controversy. For example, the model predicts that production of carbon dioxide must increase with time, a finding that goes against the conventional wisdom that carbon fluxes and atmospheric carbon dioxide levels have steadily decreased over the last 4 billion years.

“The change in flux described by our model happens over extremely long time periods, and it would be a mistake to think that these processes that are bringing about any of the atmospheric changes are occurring due to anthropomorphic climate change,” he said. “However, our work does suggest that Earth scientists and astrobiologists may need to revisit what we think we know about Earth’s early history.”

This work is the result of an ongoing study of the global carbon cycle funded by the National Science Foundation and administered by Rice University.

[Note mentioned in this article, the activity of cyanobacteria which were the creatures which released the oxygen we breathe.]

*Science paper:
Two-step rise of atmospheric oxygen linked to the growth of continents

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In his 1912 inaugural address, Rice University president Edgar Odell Lovett set forth an ambitious vision for a great research university in Houston, Texas; one dedicated to excellence across the range of human endeavor. With this bold beginning in mind, and with Rice’s centennial approaching, it is time to ask again what we aspire to in a dynamic and shrinking world in which education and the production of knowledge will play an even greater role. What shall our vision be for Rice as we prepare for its second century, and how ought we to advance over the next decade?

This was the fundamental question posed in the Call to Conversation, a document released to the Rice community in summer 2005. The Call to Conversation asked us to reexamine many aspects of our enterprise, from our fundamental mission and aspirations to the manner in which we define and achieve excellence. It identified the pressures of a constantly changing and increasingly competitive landscape; it asked us to assess honestly Rice’s comparative strengths and weaknesses; and it called on us to define strategic priorities for the future, an effort that will be a focus of the next phase of this process.

From livescience: “What If the Supercontinent Pangaea Had Never Broken Up?”

Brought forward 2.6.16
Original date May 13, 2011

Adam Hadhazy

Things would be a little different.

From about 300 million to 200 million years ago, all seven modern continents were mashed together as one landmass, dubbed Pangaea . The continents have since “drifted” apart because of the movements of the Earth’s crust, known as plate tectonics. Some continents have maintained their puzzle piece-like shapes: Look at how eastern South America tucks into western Africa.

The tectonic plates of the world were mapped in the second half of the 20th century.

Life would be: Far less diverse. A prime driver of speciation the development of new species from existing ones is geographical isolation, which leads to the evolution of new traits by subjecting creatures to different selective pressures. Consider, for example, the large island of Madagascar, which broke off from Gondwana, Pangaea’s southern half, 160 million years ago. About nine out of 10 of the plant and mammal species that have evolved on the island are not found anywhere else on the planet, according to Conservation International.

A locked-in Pangaea further constrains life’s possibilities because much of its interior would be arid and hot, said Damian Nance, a professor of geosciences at Ohio University. “Because of Pangaea’s size, moisture-bearing clouds would lose most of their moisture before getting very far inland,” Nance told Life’s Little Mysteries.

Excess mass on a spinning globe shifts away from the poles, so the supercontinent would also become centered on the equator, the warmest part of the planet. Reptiles could deal with such a climate better than most, which is partly why dinosaurs, which emerged during the time the planet’s surface was one giant chunk, thrived before mammals.

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From ANU: “How the Earth’s Pacific plates collapsed”

Australian National University

23 November 2015

Professor Richard Arculus. Image Charles Tambiah and MNF

Scientists drilling into the ocean floor have for the first time found out what happens when one tectonic plate first gets pushed under another.

The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which float on the fluid-like (visco-elastic solid) asthenosphere. The relative fluidity of the asthenosphere allows the tectonic plates to undergo motion in different directions. This map shows 15 of the largest plates. Note that the Indo-Australian Plate may be breaking apart into the Indian and Australian plates, which are shown separately on this map.

The international expedition drilled into the Pacific ocean floor and found distinctive rocks formed when the Pacific tectonic plate changed direction and began to plunge under the Philippine Sea Plate about 50 million years ago.

“It’s a bit like a rugby scrum, with two rows of forwards pushing on each other. Then one side goes down and the other side goes over the top,” said study leader Professor Richard Arculus, from the Research School of Earth Sciences.

“But we never knew what started the scrum collapsing,” said Professor Arculus.

The new knowledge will help scientists understand the huge earthquakes and volcanoes that form where the Earth’s plates collide and one plate gets pushed under the other.

As part of the International Ocean Discovery Program, the team studied the sea floor in 4,700 metres of water in the Amami Sankaku Basin of the north-western Pacific Ocean, near the Izu-Bonin-Mariana Trench, which forms the deepest parts of the Earth’s oceans.

Drilling 1,600 metres into the sea floor, the team recovered rock types from the extensive rifts and big volcanoes that were initiated as one plate bored under the other in a process known as subduction.

“We found rocks low in titanium, but high in scandium and vanadium, so the Earth’s mantle overlying the subducting plate must have been around 1,300 degrees Celsius and perhaps 150 degrees hotter than we expected to find,” Professor Arculus said.

The team found the tectonic scrum collapsed at the south end first and then the Pacific Plate rapidly collapsed 1,000 kilometres northwards in about one million years.

“It’s quite complex. There’s a scissoring motion going on. You’d need skycam to see the 3D nature of it,” Professor Arculus said.

Professor Arculus said that the new knowledge could give insights into the formation of copper and gold deposits that are often formed where plates collide.

The research is published in Nature Geoscience.

ABOUT THE INTERNATIONAL OCEAN DISCOVERY PROGRAM

The International Ocean Discovery Program (IODP) is the world’s largest geoscience research program with 26 countries in its membership.

ANU hosts the office of the Australian and New Zealand IODP Consortium (ANZIC), which includes 15 universities and two government agencies involved in geoscience. The IODP was awarded $10 million in Australian Research Council over five years in the latest round of ARC funding.

More information about ANZIC is available at http://iodp.org.au.

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ANU is a world-leading university in Australia’s capital city, Canberra. Our location points to our unique history, ties to the Australian Government and special standing as a resource for the Australian people.

Our focus on research as an asset, and an approach to education, ensures our graduates are in demand the world-over for their abilities to understand, and apply vision and creativity to addressing complex contemporary challenges.