From The New York Times: “The Earth’s Shell Has Cracked, and We’re Drifting on the Pieces”

From The New York Times

Dec. 18, 2018
Natalie Angier

Plate tectonics helped make our planet stable and habitable. But the slow shifting of continents is still a mysterious process.

The San Andreas fault in the Carrizo Plain in California. The fault line forms the boundary between the Pacific and the North American plates. Credit Peter Menzel/Science Source

The theory of plate tectonics is one of the great scientific advances of our age, right up there with Darwin’s theory of evolution and Einstein’s theory of relativity.

The idea that Earth’s outer shell is broken up into giant puzzle pieces, or plates, all gliding atop a kind of conveyor belt of hot, weak rock — here rising up from the underlying mantle, there plunging back into it — explains much about the structure and behavior of our home planet: the mountains and ocean canyons, the earthquakes and volcanoes, the very composition of the air we breathe.

Yet success is no guarantee against a midlife crisis, and so it is that half a century after the basic mechanisms of plate tectonics were first elucidated, geologists are confronting surprising gaps in their understanding of a concept that is truly the bedrock of their profession.

They are sparring over when, exactly, the whole movable plate system began. Is it nearly as ancient as the planet itself — that is, roughly 4.5 billion years old — or a youthful one billion years, or somewhere in between?

They are asking what caused the shell to crack apart in the first place, and how the industrious recycling of Earth’s crust began.

They are comparing Earth with its sister planet, Venus. The two worlds are roughly the same size and built of similar rocky material, yet Earth has plate tectonics and Venus does not. Scientists want to know why.

“In the 1960s and 70s, when people came up with the notion of plate tectonics, they didn’t think about what it was like in the distant past,” said Jun Korenaga, a geophysicist at Yale University.

“People were so busy trying to prove plate tectonics by looking at the present situation, or were caught up applying the concept to problems in their own field. The origin issue is a much more recent debate.”

Researchers also are exploring the link between plate tectonics and the evolution of complex life. Fortuitously timed continental collisions and mountain smackdowns may well have supplied crucial nutrients at key moments of biological inventiveness, like the legendary Cambrian explosion of 500 million years ago, when the ancestors of modern life-forms appeared.

“The connection between deep Earth processes and Earth surface biology hasn’t been thought about too clearly in the past, but that’s changing fast,” said Aubrey Zerkle, a geochemist at the University of St. Andrews in Scotland.

It’s increasingly obvious that “you need plate tectonics to sustain life,” Dr. Zerkle added. “If there wasn’t a way of recycling material between mantle and crust, all these elements that are crucial to life, like carbon, nitrogen, phosphorus and oxygen, would get tied up in rocks and stay there.”

The origin and implications of plate tectonics were the subject of a recent meeting and themed issue of Philosophical Transactions of the Royal Society.

Researchers said that pinning down when and how Earth’s vivid geological machinations arose will do more than flesh out our understanding of our home base. The answers could well guide our search for life and habitable planets beyond the solar system.

Robert Stern, a geoscientist at the University of Texas at Dallas, argues that if we’re looking for another planet to colonize, we want to avoid ones with signs of plate tectonic activity. Those are the places where life is likely to have evolved beyond the “single cell or worm stage, and we don’t want to fight another technological civilization for their planet.”

“A relatively benign way for the Earth to lose heat”

Mount Singabung erupting in Indonesia in October 2014. Plate tectonics “allows Earth to maintain a stabler and more benign environment overall,” explained one scientist. Credit Dedy Sahputra/European Pressphoto Agency

The idea that continents are not fixed but rather peregrinate around the globe dates back several centuries, when mapmakers began noticing the complementarity of various land masses — for example, the way the northeast bulge of South America looks as though it could fit snugly in the cupped palm of the southwest coast of Africa.

But it wasn’t until the mid-twentieth century that the generic notion of “continental drift” was transformed into a full-bodied theory, complete with evidence of a subterranean engine driving these continental odysseys.

Geologists determined that Earth’s outer layer is broken into eight or nine large segments and five or six smaller ones, a mix of relatively thin, dense oceanic plates riding low and thicker, lighter continental plates bobbing high.

At large fissures on the ocean floor, melting rock from the underlying mantle rises up, adding to the oceanic plates. At other fracture points in the crust, oceanic plates are diving back inside, or subducting, their mass devoured in the mantle’s hot belly.

The high-riding continental plates are likewise jostled by the magmatic activity below, skating around at an average pace of one or two inches a year, sometimes crashing together to form, say, the Himalayan mountain chain, or pulling apart at Africa’s Great Rift Valley.

All this convective bubbling up and recycling between crust and mantle, this creative destruction and reconstruction of parts — “tectonic” comes from the Greek word for build — is Earth’s way of following the second law of thermodynamics. The movement shakes off into the frigidity of space the vast internal heat that the planet has stored since its violent formation.

And while shifting, crumbling plates may seem inherently unreliable, a poor foundation on which to raise a family, the end result is a surprising degree of stability. “Plate tectonics is a relatively benign way for Earth to lose heat,” said Peter Cawood, an Earth scientist at Monash University in Australia.

“You get what are catastrophic events in localized areas, in earthquakes and tsunamis,” he added. “But the mechanism allows Earth to maintain a stabler and more benign environment overall.”

Sulfuric gas in the Afar Triple Junction in Ethiopia, at the top of the Great Rift Valley. Three tectonic plates meet at this spot: the Arabian plate and two African plates, Nubian and Somali. Credit Massimo Rumi/Barcroft Media, via Getty Images

Unfortunately for geologists, the very nature of plate tectonics obscures its biography. Oceanic crust, where the telltale mantle exchange zones are located, is recycled through the upwelling and subducting pipeline every 200 million years or so, which means hard evidence of tectonic origins was destroyed long ago.

Continental crust is older, and rocks dating back more than 4 billion years have been identified in places like Jack Hills, Australia. But continental plates float above the subductive fray, revealing little of the system’s origins.

Nevertheless, geoscientists are doing their best with extant rocks, models and laboratory experiments to sketch out possible tectonic timelines. Dr. Korenaga and his colleagues have proposed that plate tectonics began very early, right after Earth’s crust solidified from its initial magmatic state.

“That is when the conditions would have been easiest for plate tectonics to get started,” he said. At that point, he said, most of the water on Earth — delivered by comets — would still be on the surface, with little of it having found its way into the mantle. The heat convecting up through the mantle would exert a stronger force on dry rocks than on rocks that were lubricated.

At the same time, the surface water would make it easier for the hot, twisting rocks beneath to crack the surface lid apart, rather as a sprinkling of water from the faucet eases the task of popping ice cubes from a tray. The cracking open of the surface lid, Dr. Korenaga said, is key to getting the all-mighty subduction engine started. With subduction established, water, like oceanic crust, would cycle between Earth’s surface and mantle.

Water is constantly recycled between the mantle and crust

A map of tectonic plates in the Indian Ocean based on data showing seafloor gravity anomalies. The red areas show areas where gravity is stronger, largely aligning with underwater ridges, seamounts and plate edges. Credit Joshua Stevens, Sandwell, D. et al., NASA

On the opposite end of the origins debate is Dr. Stern, who argues that plate tectonics is a mere billion years old or less, and that Earth spent its first 3.5 billion years with a simple “single lid” as its outer shell: a crust riddled with volcanoes and other means of heat ventilation, but no moving plates, no subduction, no recycling between inside and out.

As evidence of the youthfulness of the plate regimen, Dr. Stern points to two classes of rocks: ophiolites and blueschist.

Ophiolites are pieces of oceanic crust atop bits of underlying mantle that have made their way onto land and thus have escaped the relentless recycling of oceanic crust. Recent research has shown that ophiolites are not just any slice of oceanic crust, Dr. Stern said, but rather were formed by the forces of subduction.

Similarly, blueschists are rocks that are fashioned under very high pressure but low temperatures, and “the only place you can do that is in a subduction zone,” Dr. Stern said.

Nearly all ophiolites are less than a billion years old, he added, while the most ancient blueschists, found in China, are just 800 million years old. No ophiolites, no blueschists, no evidence of subduction or plate tectonics.

Most geologists opt for a middle ground. “Science is a democratic process,” said Michael Brown, a geologist at the University of Maryland and an editor of the themed issue, “and the prevailing view is that Earth started to exhibit behaviors that look like plate tectonics 2.5 to 3 billion years ago.”

Significantly, that chronology decouples plate tectonics from the origin of life on Earth: evidence of the earliest single-celled organisms dates back more than 3.6 billion years. Nevertheless, scientists view plate tectonics as vital to the sustained evolution of that primordial life.

In Iceland, a visible fault between the North American and Eurasian plates, which are pulling away from each other at a rate of about an inch a year. Credit Universal History Archive/UIG, via Getty Images

Plate tectonic activity did not just help to stabilize Earth’s heat management system. The movement kept a steady supply of water shuttling between mantle and crust, rather than gradually evaporating from the surface.

It blocked the dangerous buildup of greenhouse gases in the atmosphere by sucking excess carbon from the ocean and subducting it underground. It shook up mountains and pulverized rocks, freeing up essential minerals and nutrients like phosphorus, oxygen and nitrogen for use in the growing carnival of life.

Dr. Zerkle discerns a link between geological and biological high drama: “It’s been suggested that time periods of supercontinental cycles — when small continents smash together to make large supercontinents, and those supercontinents then rip apart into smaller continents again — could have put large pulses of nutrients into the biosphere and allowed organisms to really take off.”

Plate tectonics also built the right playing fields for Darwinian games.

“Think about what drives evolution,” Dr. Stern said. “It’s isolation and competition. You need to break continents and continental shelves apart, and separate one ocean from another, for speciation to occur.”

Life is always falling apart, on the rocks — and a good thing, too.

See the full article here .

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From UC Santa Barbara: “Under the Sea”

From UC Santa Barbara

June 5, 2018
Jeff Mitchell

Earth scientist Zach Eilon plumbs the depths of the Pacific Ocean to learn more about plate tectonics.

The Pacific ORCA science party on board the research vessel Kilo Moana; UCSB’s Zach Eilon is seventh from left. Photo Credit: Courtesy Zach Eilon

Watchstanders processing data in the vessel’s computer lab spot an underwater volcano that has never before been imaged. Photo Credit: Courtesy Zach Eilon

Preparing to deploy an Ocean Bottom Seismometer (OBS) at sunset. Photo Credit: Courtesy Zach Eilon

Preparing to test all the OBS communication devices, temporarily housed in the “rosette”, sitting beneath the A-frame; the yellow packages on deck are the OBS instruments, awaiting deployment. Photo Credit: Courtesy Zach Eilon

Voyaging across a vast swath of the Pacific Ocean to learn more about how the Earth’s tectonic plates work, scientist Zach​​ Eilon was assisted along the way by friendly deep-sea denizen SpongeBob SquarePants.

No, the beloved animated character wasn’t really there, but SpongeBob was the nickname Eilon, a UC Santa Barbara assistant professor of earth sciences, gave the sophisticated instrument that played a key role in his research.

Otherwise known as ocean bottom seismometers, or OBS’s, these instruments are sensitive enough to detect earthquakes on the other side of the world.

While the seismometers themselves sit on the seafloor, they are attached to a bright yellow flotation package — hence, the SpongeBob comparison — and are about a meter in width. The packages are affixed to a plastic base containing complex electronics.

Eilon and collaborators carefully placed 30 of them on the ocean floor about 2,000 miles southeast of Hawaii during their recent Pacific ORCA (Pacific OBS Research into Convecting Asthenosphere) expedition aboard the U.S. Navy research vessel Kilo Moana.

U.S. Navy research vessel Kilo Moana

The trip and the experiment were part of an ongoing and high-profile international effort, on which UCSB is one of three lead institutions in the U.S., to seismically instrument the Pacific Ocean.

Oceanic plates make up 70 percent of the Earth’s surface and offer important windows into the Earth’s mantle, Eilon said, yet they are largely unexplored due to the obvious challenge of putting sensitive electronics three miles beneath the sea surface. The earth science community has identified several unanswered questions regarding the thermal structure of oceanic plates, the significance of volcanism in the middle of oceanic plates and how the convecting mantle beneath the plates controls their movements.

Undulations in the gravity field and unexplained shallowing of the ocean floors hint that small-scale convection may be occurring beneath the oceanic plates, but this remains unconfirmed, according to Eilon. The new experiment could help prove it.

“Our little instruments will sit on the ocean floor for approximately 15 months, recording earthquakes around the world,” he said. “When we return to retrieve them next year they’ll hold seismic data in their memory banks that could change the way in which we understand the oceanic plates. That understanding is pretty significant, considering that these plates make up about 70 percent of our planet’s surface.”

When they are recovered in July 2019, the OBS units are expected to provide data that allows Eilon and his collaborators to make 3-D images of the oceanic tectonic plates – a bit like taking a CAT-scan of the Earth. Of particular interest is the mysterious asthenosphere, the zone of Earth’s mantle lying beneath the lithosphere (the tectonic plate) and believed to be much hotter and more fluid than rocks closer to the surface. The asthenosphere extends from about 60 miles to about 250 miles below Earth’s surface.

Once ready for deployment, the weighted instrument packages are designed to carefully sink upright to the seafloor. When the science party returns to the site, the ship will send an acoustic signal down to the individual science packages, commanding them to release the weight holding them down, allowing the buoyant yellow “SpongeBob” portion of the device to slowly float them to the surface, he explained.

Once on the surface, the ship’s crew will home in on the package (which has a light, flag, and radio so the scientists can locate it) and lift it from the sea. From there the science team will commence the process of downloading the seismic data which are detailed records of the ocean floor vibrations. Turning these wiggles into 3D images is the result of highly complex computer processing and mathematics.

Eilon said that in addition to giving researchers a better idea of how the Earth’s tectonic plates work, the data is expected to provide important information about geologic hazards.

“By improving our understanding of interactions between plates, the data we collect should improve our ability to forecast earthquakes and volcanic eruptions,” he said, “which I hope will help authorities save lives when these events occur.”

Eilon, along with co-principal investigator Jim Gaherty of Columbia University, led the expedition’s diverse 14-member science team (drawn from 11 institutions across three continents). The $4-million research project is supported by the National Science Foundation.

See the full article here .

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The University of California, Santa Barbara (commonly referred to as UC Santa Barbara or UCSB) is a public research university and one of the 10 general campuses of the University of California system. Founded in 1891 as an independent teachers’ college, UCSB joined the University of California system in 1944 and is the third-oldest general-education campus in the system. The university is a comprehensive doctoral university and is organized into five colleges offering 87 undergraduate degrees and 55 graduate degrees. In 2012, UCSB was ranked 41st among “National Universities” and 10th among public universities by U.S. News & World Report. UCSB houses twelve national research centers, including the renowned Kavli Institute for Theoretical Physics.

From Astrobiology Magazine: “Did plate tectonics set the stage for life on Earth?”

From Astrobiology Magazine

Jun 7, 2018
Lisa Kaspin-Powell

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

A new study suggests that rapid cooling within the Earth’s mantle through plate tectonics played a major role in the development of the first life forms, which in turn led to the oxygenation of the Earth’s atmosphere. The study was published in the March 2018 issue of Earth and Planetary Science Letters.

Scientists at the University of Adelaide and Curtin University in Australia, and the University of California at Riverside, California, USA, gathered and analyzed data on igneous rocks from geological and geochemical data repositories in Australia, Canada, New Zealand, Sweden and the United States. They found that over the 4.5 billion years of the Earth’s development, rocks rich in phosphorus accumulated in the Earth’s crust. They then looked at the relationship of this accumulation with that of oxygen in the atmosphere.

Phosphorus is essential for life as we know it. Phosphates, which are compounds containing phosphorus and oxygen, are part of the backbones of DNA and RNA as well as the membranes of cells, and help control cell growth and function.

To find out how the level of phosphorus in the Earth’s crust has increased over time, the scientists studied how rock formed as the Earth’s mantle cooled. They performed modeling to find out how mantle-derived rocks changed composition as a consequence of the long-term cooling of the mantle.

Their results suggest that during an early, hotter period in Earth’s history – the Archaean period between four and 2.5 billion years ago – there was a larger amount of molten mantle. Phosphorus would have been too dilute in these rocks. However, over time, the Earth cooled sufficiently, aided by the onset of plate tectonics, in which the colder outer crust of the planet is subducted back into the hot mantle. With this cooling, partial mantle melts became smaller.

As Dr. Grant Cox, an earth scientist at the University of Adelaide and a co-author of the study, explains, the result is that “phosphorus will be concentrated in small percentage melts, so as the mantle cools, the amount of melt you extract is smaller but that melt will have higher concentrations of phosphorus in it.”

A cross section of the Earth, showing the exterior crust, the molten mantle beneath it and the core at the center of the planet. Image credit: NASA/JPL-Université Paris Diderot – Institut de Physique du Globe de Paris.

Phosphorus’ role in the oxidation of Earth

The phosphorus was concentrated and crystallized into a mineral called apatite, which became part of the igneous rocks that were created from the cooled mantle. Eventually, these rocks reached the Earth’s surface and formed a large proportion of the crust. When phosphorus minerals derived from the crust mixed with the water in lakes, rivers and oceans, apatite broke down into phosphates, which became available for development and nourishment of primitive life.

The scientists estimated the mixing of elements from the Earth’s crust with seawater over time. They found that higher levels of bio-essential elements parallel major increases in the oxygenation of the Earth’s atmosphere: the Great Oxidation Event (GOE) 2.4 billion years ago, and the Neoproterozoic Oxygen Event, 800 million years ago, after which oxygen levels were presumed to be high enough to support multicellular life.

Even before the GOE, from approximately 3.5 to 2.5 billion years ago, some of the earliest life forms possibly generated oxygen through photosynthesis. However, during that time, most of this oxygen reacted with iron and sulfur in igneous rocks. To understand how these reactions affected oxygen levels in the atmosphere over a period of four billion years, the scientists measured the amounts of sulfur and iron in igneous rocks, and figured out how much oxygen had reacted. They compared all of these events with changes in levels of atmospheric oxygen. The scientists found that decreases in sulfur and iron along with increases in phosphorus paralleled the Great Oxidation Event and the Neoproterozoic Oxygen Event.

An explosion of life

All of these events support a scenario in which the cooling of the Earth’s mantle led to the increase of phosphorus-rich rocks in the Earth’s crust. These rocks then mixed with the oceans, where phosphorus-containing minerals broke down and leached into the water. Once phosphorus levels in seawater were high enough, primitive life forms thrived and their numbers increased, so they could generate enough oxygen that most of it reached the atmosphere. Oxygen reached levels sufficient to support multicellular life.

Dr. Peter Cawood, a geologist at Monash University inMelbourne, Australia, comments to Astrobiology Magazine that, “it’s intriguing to think that the [oxygen] on which we depend for life owes its ultimate origin to secular decreases in mantle temperature, which are thought to have decreased from some 1,550 degrees Celsius some three billion years ago to around 1,350 degrees Celsius today.”

Could a similar scenario be playing out on a possible exo-Earth? With the Kepler discoveries of a growing number of possibly Earth-like planets, could any of these support life? Cawood suggests that the finding is potentially significant for the development of aerobic life (i.e. life that evolves in an oxygen-rich environment) on exoplanets. “This is provided that [phosphorus] within the igneous rocks on the surface of the planet is undergoing weathering to ensure its bio-availability,” says Cawood. “Significantly, the phosphorus content of igneous rocks is highest in those rocks low in silica [rocks formed by rapid cooling] and rocks of this composition dominate the crusts of Venus and Mars and likely also on exoplanets.”

Cox concludes by saying that, “This relationship [between rising oxygen levels and mantle cooling] has implications for any terrestrial planet. All planets will cool, and those with efficient plate tectonic convection will cool more rapidly. We are left concluding that the speed of such cooling may affect the rate and pattern of biological evolution on any potentially habitable planet.”

The research was supported by the NASA Astrobiology Institute (NAI) element of the NASA Astrobiology Program, as well as the National Science Foundation Frontiers in Earth System Dynamics Program and the Australian Research Council.

See the full article here .

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From Eos: “Seafloor Activity Sheds Light on Plate Tectonics”

AGU

Eos

27 October 2017
Sarah Witman

Seafloor topography under the Atlantic Ocean. Credit: ETOPO1/NOAA

Much like the way humans constantly generate new skin cells, the bottom of the ocean regularly forms fresh layers of seafloor. Volcanic activity causes the seafloor to spread along oceanic ridges, forming new areas of crust and mantle. After being generated, this new oceanic lithosphere cools down and contracts by up to 3% of its own volume. This contraction can trigger oceanic earthquakes.

The basic mechanics of tectonic plates—the massive, constantly shifting puzzle pieces that make up the Earth’s surface—are fairly well understood.

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

However, scientists cannot accurately predict how much the oceanic lithosphere will contract horizontally during the process described above.

Sasajima and Ito studied this thermal contraction [Tectonics]by examining stress released by oceanic earthquakes over the past 55 years in newly formed sections of oceanic lithosphere (approximately 5–15 million years old). They also simulated this activity using mathematical models.

The team found a distinct difference in two components of the released stress: one parallel to the ridge and another perpendicular to the ridge (i.e., in the seafloor spreading direction). Namely, the ridge-parallel components experienced 6 times as much extensional stress release, whereas the spreading components endured 8 times as much compressional stress release.

In their numerical simulation, the researchers found that young oceanic lithosphere hardly ever contracts in the ridge-parallel direction. At most, it would do so only a quarter of the times that it would contract in the spreading direction. They concluded that because the layer of mantle underneath the lithosphere (the asthenosphere) is weak (low viscosity) and also because oceanic ridges are relatively weak, the young oceanic lithosphere is able to contract more freely in the spreading direction.

This study provides critical insight into the driving and resisting forces underlying plate tectonics, one of the greatest physical phenomena in our world. (Tectonics, https://doi.org/10.1002/2017TC004680, 2017)

See the full article here .

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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 Curtin: “New research ‘rocks’ long-held geological theory”

Curtin University

28 February 2017 [Just appeared in social media, better late than never]

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New research into ancient rocks in Western Australia contradicts the commonly held belief that Earth’s first stable continents were formed in a plate tectonic setting.

The Curtin University-led paper, Earth’s first stable continents did not form by subduction, was published today in Nature.

Dr Tim Johnson, from The Institute for Geoscience Research (TIGeR) and the Department of Applied Geology at the Curtin WA School of Mines, explained that the geodynamic environment in which Earth’s first stable continents formed remained controversial.

“Uniformitarianism is the precept in geology that the processes we can observe happening today are those that have operated throughout Earth’s history,” Dr Johnson said.

“Many geologists have subscribed to the uniformitarian view that the first continental crust formed in subduction zones via modern-style plate tectonics.

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

“Some, however, believe that alternative (non-uniformitarian) processes were involved – our research supports the latter.”

Dr Johnson said examples of truly ancient continental crust – rocks formed around 3.5-4 billion years ago, in the early stages of the Archaean Eon – could still be found today, in places like Australia, South Africa, India, North America and Europe.

“Most exposed areas of Archaean crust include a specific type of granitic rocks known as TTGs,” [Tonalite-Trondhjemite-Granodiorite] Dr Johnson said.

“These rocks were formed by partial melting of a low magnesium basaltic source, and have a trace element signature that resemble crust produced in modern subduction settings.

“My previous research came to a similar conclusion, that plate tectonic processes are not required for the formation of the earliest continents, and that other mechanisms are plausible. We wanted to explore the hypothesis further, leading to our current findings.”

Samples were selected from the low magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup in the East Pilbara Terrane, Western Australia. These rocks, amongst the oldest basaltic lavas on Earth, have previously been shown to contain a trace element composition consistent with source rocks for TTGs .

“Through phase equilibria modelling of the Coucal basalts, we confirmed their suitability as TTG parents, suggesting they were produced by melting in a high geothermal gradient environment. By contrast, many researchers maintain that TTGs were formed in subduction zones, which are characterised by very low geothermal gradients,” Dr Johnson said.

“Additionally, the trace element signature of the Coucal basalts indicates that they were derived from an earlier generation of mafic rocks, suggesting this signature was inherited from an ancestral lineage.

“This leads us to believe that a protracted, multistage process, in combination with high geothermal gradients, was required for the production and stabilisation of the first continents. These results are not consistent with formation of TTGs in subduction zones, but rather favour their production near the base of thick basaltic plateaux in the early Archaean.”

See the full article here .

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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.

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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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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 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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