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Earth History: How Continents Were Recycled

22 Aug 2017


Science Daily




Researchers from Germany and Switzerland have used computer simulations to analyse how plate tectonics have evolved on Earth over the last three billion years. They show that tectonic processes have changed in the course of the time, and demonstrate how those changes contributed to the formation and destruction of continents. The model reconstructs how present-day continents, oceans and the atmosphere may have evolved.


Priyadarshi Chowdhury and Prof Dr Sumit Chakraborty from Ruhr-Universität Bochum, together with Prof Dr Taras Gerya from the Swiss Federal Institute of Technology in Zürich (ETH), present their work in the journal Nature Geosciences.


Hotly disputed: when did plate tectonics emerge?


The Earth formed approximately four and a half billion years ago. There was a phase -- perhaps even several -- when it was mainly composed of molten rock. As it cooled, solid rock and the Earth's crust formed. Generally speaking, there are two types of crust on Earth: a lighter continental crust that is rich in silicon and constitutes the dry land above sea level, and a denser oceanic crust where water gathers in the form of large oceans. "These properties render the Earth habitable," says Sumit Chakraborty. "We haven't found anything comparable anywhere else in the universe."

Even though the young Earth did have continents and oceans, there were initially perhaps no plates and, consequently, no plate tectonics. The question when they emerged is much disputed. The Earth's crust slowly assumed its present dynamic form: in some places the plates go into the mantle; in other places new plates form from the hot material that rises from the interior of the Earth.

Also, the question when plate tectonics first emerged is not the only one that remains unanswered; it is also unclear whether that process has always been the same and whether continents last forever or are recycled. These are the questions that the German-Swiss research team investigated. Their new thermomechanical computer model supports the growing notion that perhaps plate tectonics was already operating approximately three billion years ago. More uniquely, the study demonstrates how the Earth's earliest continental crust -- richer in iron and magnesium -- was destroyed some two or three billion years ago and how the present continental crust -- richer in silicon -- formed from it.


Continental recycling is the order of the day


On the young Earth, continents were recycled all the time. Continental recycling still takes place today when two continents collide, but it progresses more slowly and in a different manner than it used to. "Over time, the continental crust became prone to preservation during continent-continent collision," says Priyadarshi Chowdhury. On the old, still hot Earth, thin layers peeled off from the Earth's crust whereas on the present-day Earth, chunks of the continental crust break off in the collision zones, i.e. in places where one plate moves under another.

The researchers assume that the destruction of the early iron-magnesium rich continental crust was crucial for the formation of the silicon-rich continents and that it was the reason why these continents could rise above sea level to a larger extent. "These changes to the continental character might have contributed to the Great Oxygenation Event on Earth -- and, consequently, to the origin of life as we know it," suspects Chowdhury.

Story Source:


Provided by Ruhr-Universitaet-Bochum.

Journal Reference:

  1. Priyadarshi Chowdhury, Taras Gerya, Sumit Chakraborty. Emergence of silicic continents as the lower crust peels off on a hot plate-tectonic Earth. Nature Geoscience, 2017; DOI: 10.1038/ngeo3010
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Ancient Earth's Hot Interior Created a Graveyard of 'Continental' Slabs

22 Aug2017


Science Daily


Plate tectonics has shaped the Earth's surface for billions of years: Continents and oceanic crust have pushed and pulled on each other, continually rearranging the planet's façade. As two massive plates collide, one can give way and slide under the other in a process called subduction. The subducted slab then slips down through the Earth's viscous mantle, like a flat stone through a pool of honey.


For the most part, today's subducting slabs can only sink so far, to about 670 kilometers below the surface, before the mantle's makeup turns from a honey-like consistency, to that of paste -- too dense for most slabs to penetrate further. Scientists have suspected that this density filter existed in the mantle for most of Earth's history.

Now, however, geologists at MIT have found that this density boundary was much less pronounced in the ancient Earth's mantle, 3 billion years ago. In a paper published in Earth and Planetary Science Letters, the researchers note that the ancient Earth harbored a mantle that was as much as 200 degrees Celsius hotter than it is today -- temperatures that may have brewed up more uniform, less dense material throughout the entire mantle layer.

The researchers also found that, compared with today's rocky material, the ancient crust was composed of much denser stuff, enriched in iron and magnesium. The combination of a hotter mantle and denser rocks likely caused subducting plates to sink all the way to the bottom of the mantle, 2,800 kilometers below the surface, forming a "graveyard" of slabs atop the Earth's core.

Their results paint a very different picture of subduction than what occurs today, and suggests that the Earth's ancient mantle was much more efficient in drawing down pieces of the planet's crust.

"We find that around 3 billion years ago, subducted slabs would have remained more dense than the surrounding mantle, even in the transition zone, and there's no reason from a buoyancy standpoint why slabs should get stuck there. Instead, they should always sink through, which is a much less common case today," says lead author Benjamin Klein, a graduate student in MIT's Department of Earth, Atmospheric and Planetary Sciences (EAPS). "This seems to suggest there was a big change going back in Earth's history in terms of how mantle convection and plate tectonic processes would have happened."

Klein's co-authors are Oliver Jagoutz, associate professor in EAPS, and Mark Behn of the Woods Hole Oceanographic Institution.


Temperature difference


"There's this open question as to when plate tectonics really started in Earth's history," Klein says. "There's general consensus that it was probably going on back at least 3 billion years ago. This is also when most models suggest the Earth was at its hottest."

Around 3 billion years ago, the mantle was probably about 150-200 C warmer than it is today. Klein, Jagoutz, and Behn investigated whether hotter temperatures in the Earth's interior made a difference in how tectonic plates, once subducted, were transported through the mantle.

"Our work started as this thought experiment to say, if we know temperatures were much hotter, how might that have modulated what the tectonics looked like, without changing it wholesale?" Klein says. "Because the debate before was this binary argument: Either there was plate tectonics, or there wasn't, and we're suggesting there's more room in between."


A "density flip"


The team carried out its analysis, making the assumption that plate tectonics was indeed shaping the Earth's surface 3 billion years ago. They looked to compare the density of subducting slabs at that time with the density of the surrounding mantle, the difference of which would determine how far slabs would have sunk.

To estimate the density of ancient slabs, Klein compiled a large dataset of more than 1,400 previously analyzed samples of both modern rocks and komatiites -- classic rock types that were around 3 billion years ago but are no longer produced today. These rocks contain a higher amount of dense iron and magnesium compared to today's oceanic crust. Klein used the composition of each rock sample to calculate the density of a typical subducting slab, for both the modern day and 3 billion years ago.

He then estimated the average temperature of a modern versus an ancient subducting slab, relative to the temperature of the surrounding mantle. He reasoned that the distance a slab sinks depends on not only its density but also its temperature relative to the mantle: The colder an object is relative to its surroundings, the faster and further it should sink.

The team used a thermodynamic model to determine the density profile of each subducting slab, or how its density changes as it sinks through the mantle, given the mantle's temperature, which they took from others' estimates and a model of the slab's temperature. From these calculations, they determined the depth at which each slab would become less dense than the surrounding mantle.

At this point, they hypothesized that a "density flip" should occur, such that a slab should not be able to sink past this boundary.

"There seems to be this critical filter and control on the movement of slabs and therefore convection of the mantle," Klein says.


A final resting place


The team found that their estimates for where this boundary occurs in the modern mantle -- about 670 kilometers below the surface -- agreed with actual measurements taken of this transition zone today, confirming that their method may also accurately estimate the ancient Earth.

"Today, when slabs enter the mantle, they are denser than the ambient mantle in the upper and lower mantle, but in this transition zone, the densities flip," Klein says. "So within this small layer, the slabs are less dense than the mantle, and are happy to stay there, almost floating and stagnant."

For the ancient Earth, 3 billion years ago, the researchers found that, because the ancient mantle was so much hotter than today, and the slabs much denser, a density flip would not have occurred. Instead, subducting slabs would have sunk straight to the bottom of the mantle, establishing their final resting place just above the Earth's core.

Jagoutz says the results suggest that sometime between 3 billion years ago and today, as the Earth's interior cooled, the mantle switched from a one-layer convection system, in which slabs flowed freely from upper to lower layers of the mantle, to a two-layer configuration, where slabs had a harder time penetrating through to the lower mantle.

"This shows that when a planet starts to cool down, this boundary, even though it's always there, becomes a significantly more profound density filter," Jagoutz says. "We don't know what will happen in the future, but in theory, it's possible the Earth goes from one dominant regime of one-layer convection, to two. And that's part of the evolution of the entire Earth."

This research was funded, in part, by the National Science Foundation.

Story Source:

Provided by MIT. Original written by Jennifer Chu. Note: Content may be edited for style and length.

Journal Reference:

  1. Benjamin Z. Klein, Oliver Jagoutz, Mark D. Behn. Archean crustal compositions promote full mantle convection. Earth and Planetary Science Letters, 2017; 474: 516 DOI: 10.1016/j.epsl.2017.07.003
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World’s Largest Volcano Now Named TAMU

5 Sep 2013

Texas A&M Today





Texas Aggies like to think their school is among the world’s biggest movers and shakers, and now science has confirmed it. An oceanographer has uncovered the world’s largest volcano in the Pacific Ocean ““ about the size of New Mexico ““ and has named it for Texas A&M University.

William Sager, who spent 29 years working at Texas A&M in the College of Geosciences and was holder of the Jane and R. Ken Williams ’45 Chair in Ocean Drilling Science, Technology and Education, first began studying the volcano about 20 years ago. He named it Tamu Massif ““ Tamu for the abbreviation of Texas A&M University, while massif is the French word for “massive” and a scientific term for a large mountain mass.

Will Sager (left of center, white polo shirt) lead author on the Nature Geoscience study confirming Tamu Massif as the world's largest volcano, waits to inspect a core sample drilled on Integrated Ocean Drilling Program ship.

Will Sager (left of center, white polo shirt) lead author on the Nature Geoscience study confirming Tamu Massif as the world’s largest volcano, waits to inspect a core sample drilled on Integrated Ocean Drilling Program ship. (Photo courtesy of IODP)

It is believed to be the largest single volcano ever discovered on Earth and may rival some of the giants found on Mars. Sager, who recently joined the faculty of the University of Houston, and team members from Oregon State, Yale, the University of Hawaii and the United Kingdom, have published their findings in the current issue of Nature Geoscience.

The project was funded by the National Science Foundation, both through direct grants and through its Integrated Ocean Drilling Program (IODP) that is headquartered at Texas A&M.

Sager and the team examined a large underwater area in the northwest Pacific known as the Shatsky Rise, located about 1,000 miles east of Japan. Sager had found that the plateau contained three enormous mounds.

“We got tired of referring to them as the one on the left, the one on the right and the big one,” Sager recalls.   He dubbed the largest one Tamu after Texas A&M, and hence the school is now in the volcano business.

“We knew it was big, but we had no idea it was one large volcano,” he adds.

“Our final calculations have determined it is about 120,000 square miles in area, or about the size of the state of New Mexico, making it by far the largest ever discovered on Earth. It rivals in size some of the largest volcanoes in the solar system, such as Olympus Mons on Mars.”

Olympus Mons, the largest volcano on Mars, is so big that it can be seen with many common backyard telescopes.

a 3-d map of the Tamu Massif formation

A 3-d map of the Tamu Massif formation. (Photo courtesy of IODP)

The largest active volcano on Earth is Mauna Loa in Hawaii, which has erupted off and on for the past 700,000 years.   But it is about 2,000 square miles in size, a tiny fraction of Tamu Massif.

Tamu Massif is believed to be about 145 million years old, and it became inactive within a few million years after it was formed, Sager says.

Its top lies about 6,500 feet below the ocean surface, while much of its base is believed to be in waters that are almost four miles deep.

“What is unusual about the volcano is its slope ““ it’s not high, but very wide, so the flank slopes are very gradual,” Sager explains. “In fact, if you were standing on its flank, you would have trouble telling which way is downhill. We know that it is a single immense volcano constructed from massive lava flows that emanated from the center of the volcano to form a broad, shield-like shape.

“Its shape is different from any other sub-marine volcano found on Earth, and it’s very possible it can give us some clues about how massive volcanoes can form. An immense amount of magma came from the center, and this magma had to have come from the Earth’s mantle. So this is important information for geologists trying to understand how the Earth’s interior works.”

Tamu Massif follows a long line of locations named after Texas A&M or people associated with it. These include many topographic features in the Gulf of Mexico or Atlantic seaboard, including Antoine Bank, named for Texas A&M researcher John Antoine; Geyer Mound and Geyer Bank, named for Texas A&M researcher Richard Geyer; McGrail Bank, named after Texas A&M oceanographer David McGrail; Tamu Basin, Tamu Bank and Tamu Dome, all named for the school; Bryant Canyon, named after oceanographer William Bryant; Rudder Basin and Reveille Basin, named for former president James Earl Rudder and the   school’s collie mascot; Gyre Basin, named for a former Texas A&M research ship; and Applebaum Bank, named for Texas A&M researcher Bruce Applebaum.




About Research at Texas A&M University: As one of the world’s leading research institutions, Texas A&M is in the vanguard in making significant contributions to the storehouse of knowledge, including that of science and technology. Research conducted at Texas A&M represents total annual expenditures of more than $776 million. That research creates new knowledge that provides basic, fundamental and applied contributions resulting in many cases in economic benefits to the state, nation and world.



Aggie Geology



The world's biggest volcano is a magnetic mix-up

Five weeks of mapping at sea suggests two possible origins for the underwater Tamu Massif.

==> Nature

Edited by jamessavik
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Long-lost continent found submerged deep under Indian Ocean


New Scientist

Mauritius sits on an ancient continent


Mauritius sits on part of an ancient continent


By Alice Klein

An ancient continent that was once sandwiched between India and Madagascar now lies scattered on the bottom of the Indian Ocean.

The first clues to the continent’s existence came when some parts of the Indian Ocean were found to have stronger gravitational fields than others, indicating thicker crusts. One theory was that chunks of land had sunk and become attached to the ocean crust below.

Mauritius was one place with a powerful gravitational pull. In 2013, Lewis Ashwal at the University of the Witwatersrand in South Africa and his colleagues proposed that the volcanic island was sitting on a piece of old, sunken continent.

Although Mauritius is only 8 million years old, some zircon crystals on the island’s beaches are almost 2 billion years old.

Volcanic eruptions may have ejected the zircon from ancient rock below.

Now, Ashwal and his team have found zircon crystals in Mauritius that are up to 3 billion years old. Through detailed analyses they have reconstructed the geological history of the lost continent, which they named Mauritia.

The break-up

Until about 85 million years ago, Mauritia was a small continent — about a quarter of the size of Madagascar — nestled between India and Madagascar, which were much closer than they are today. Then, India and Madagascar began to move apart, and Mauritia started to stretch and break up.

“It’s like plasticine: when continents are stretched they become thinner and split apart,” says Martin Van Kranendonk at the University of New South Wales in Australia. “It’s these thin pieces that sink below the ocean.”

There is evidence that other volcanic islands in the Indian Ocean, including the Cargados Carajos, Laccadive and Chagos islands, also sit on fragments of Mauritia.

More and more remnants of other old continents are being uncovered, says Alan Collins at the University of Adelaide in Australia.

Several pieces have recently been found off Western Australia and underneath Iceland, he says. “It’s only now as we explore more of the deep oceans that we’re finding all these bits of ancient continents around the place.”

Journal reference: Nature Communications, DOI: 10.1038/ncomms14086

Edited by jamessavik
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Geologists Find Largest Exposed Fault on Earth

Nov 29, 2016 

An international team of geologists from the Australian National University and Royal Holloway University of London has for the first time documented the Banda Detachment fault in eastern Indonesia and worked out how it formed. The research is published in the journal Geology.

The Banda Detachment fault beneath the Weber Deep basin. A - cross section through eastern Banda arc, cut parallel to grooves on fault surfaces and proposed direction of rollback; geometry of proto-Banda Sea slab is inferred from earthquake hypocenter locations catalogued by International Seismological Centre Online Bulletin; KSZ - Kawa shear zone. B - enlargement of Banda detachment showing schematically the configuration of over-riding continental allochthons (dark red); red triangles represent volcanoes. Image credit: Jonathan M. Pownall et al, doi: 10.1130/G38051.1.

The Banda Detachment fault beneath the Weber Deep basin. A – cross section through eastern Banda arc, cut parallel to grooves on fault surfaces and proposed direction of rollback; geometry of proto-Banda Sea slab is inferred from earthquake hypocenter locations catalogued by International Seismological Centre Online Bulletin; KSZ – Kawa shear zone. B – enlargement of Banda detachment showing schematically the configuration of over-riding continental allochthons (dark red); red triangles represent volcanoes. Image credit: Jonathan M. Pownall et al, doi: 10.1130/G38051.1.

“The find will help researchers assess dangers of future tsunamis in the area, which is part of the Ring of Fire – an area around the Pacific Ocean basin known for earthquakes and volcanic eruptions,” said lead author Dr. Jonathan Pownall, from the Australian National University.

“The abyss has been known for 90 years but until now no one has been able to explain how it got so deep.”

“Our research found that a 4.3-mile (7 km) deep abyss beneath the Banda Sea off eastern Indonesia was formed by extension along what might be Earth’s largest-identified exposed fault plane.”

By analyzing high-resolution maps of the Banda Sea floor, Dr. Pownall and co-authors found the rocks flooring the seas are cut by hundreds of straight parallel scars.

These wounds show that a piece of crust bigger than Belgium or Tasmania must have been ripped apart by 74.5 miles (120 km) of extension along a low-angle crack, or detachment fault, to form the present-day ocean-floor depression.

“This fault, the Banda Detachment, represents a rip in the ocean floor exposed over 14.8 million acres (60,000 sq. km),” Dr. Pownall said.

“The discovery will help explain how one of the Earth’s deepest sea areas became so deep.”

“This was the first time the fault has been seen and documented by researchers,” said co-author Prof. Gordon Lister, also from the Australian National University.

“We had made a good argument for the existence of this fault we named the Banda Detachment based on the bathymetry data and on knowledge of the regional geology.”

“I was stunned to see the hypothesized fault plane, this time not on a computer screen, but poking above the waves,” Dr. Pownall said.

“Rocks immediately below the fault include those brought up from the mantle. This demonstrates the extreme amount of extension that must have taken place as the oceanic crust was thinned, in some places to zero.”

According to the team, the discovery of the Banda Detachment fault would help assesses dangers of future tsunamis and earthquakes.

“In a region of extreme tsunami risk, knowledge of major faults such as the Banda Detachment, which could make big earthquakes when they slip, is fundamental to being able to properly assess tectonic hazards,” Dr. Pownall said.


Jonathan M. Pownall et al. 2016. Rolling open Earth’s deepest forearc basin. Geology 44 (11): 947-950; doi: 10.1130/G38051.1



I find this to be really interesting because Indonesia is home to some of the worlds most volatile and dangerous volcanoes including Toba, Tambora and Krakatoa. -JS

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Why the Giant Mexican Earthquake Happened

Thursday night's quake, near an undersea crust collision zone, was the strongest to hit Mexico in a century

  • By Josh Fischman 2017 Sep 08
  • Scientific American
  • CCBBA7D0-382D-4189-81F5BF977DBF64C6.png?
    The 8.1 magnitude quake happened between the line of the Middle America Trench and Mexico's southern coast. Credit: Image from U.S.G.S.

    Late Thursday night the biggest earthquake to hit Mexico in 100 years shook the country—and a large part of the globe. The magnitude 8.1 temblor was centered just off the southern end of Mexico’s Pacific coast. It was stronger than the 1985 quake that killed thousands of people in Mexico City. Last night’s quake took 32 lives, according to news reports, and the toll may rise.


    The quake happened about 54 miles offshore of the southern state of Chiapas, just to the east of an undersea geological feature called the Middle America Trench. Here, several parts of the planet’s crust are colliding. This quake, however, probably occurred within one of those crust slabs, not at the junction, according to the U.S. Geological Survey’s Earthquake Hazards Program. The slab fractured about 43 miles below the surface, which is deeper than the trench zone.


    The shaking could be detected by seismic instruments across North America and in Asia. It was profound in Mexico City, more than 450 miles away, where warning sirens split the night, buildings shivered, and streetlights and stone monuments swayed back and forth. The noticeable motion in the city is because the earthquake waves were amplified by the loose ground beneath the metropolis, noted Jascha Polet, a seismologist at California Polytechnic State University in Pomona. The city is built on sediments from an ancient lake bed. 

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Scientists Closing in on the Dawn of Plate Tectonics

The massive slabs of Earth’s crust might have started their journey more than 3.5 billion years ago

Scientists now think early Earth contained light-colored rocks, like the granite within Yosemite’s Half Dome. Such rocks likely formed via plate tectonics. Credit: David Iliff Wikimedia (CC-BY-SA 3.0)

Geologists think early Earth may have looked much like Iceland—where jet-black lava fields extend as far as the eye can see, inky mountainsides rise steeply above the clouds and stark black-sand beaches outline the land.

But over time the world gradually became less bleak. Today Earth also harbors light-colored rocks, like the granite that composes Half Dome in Yosemite National Park. But scientists remain uncertain as to when the world started to transition from the one that looked like Iceland to that which we know today.

A new study published Thursday in Science suggests the shift transpired more than 3.5 billion years ago. Not only does the finding tell scientists the color of the world’s early beaches, it might help them understand when tectonic plates—the interlocking slabs of crust that fit together like puzzle pieces far beneath our feet—started to wake up and shuffle around. That is because the lighter-colored rocks, known as felsic rocks, are actually dark, or mafic, rocks “reincarnated.” In short, felsic rocks form when mafic ones are pushed deep inside Earth—possibly when one tectonic plate slips under another in a process called subduction. Given that light-colored felsic rocks were abundant billions of years ago, plate tectonics had likely already kicked into action.

In order to reach that conclusion, Nicolas Greber, a geologist at the University of Chicago, and his colleagues analyzed 78 different layers of sediment to pin down the ratio of felsic to mafic rocks. This was not as simple as counting light versus dark stones (both had long ago eroded into tiny particles). Instead, Greber’s team looked at titanium. Although the metallic element is present in both types of rock, the proportion of its isotopes (chemically identical atoms with the same number of protons but a different number of neutrons) shifts as the rock changes from mafic to felsic. Suppose you mix something that turns out both salty and sweet, Greber says. An analysis like this gives you “an idea of how much salt you added and how much sugar you added.” He had expected the earliest sediments in his sample, which date back 3.5 billion years, would be composed mostly of mafic particles. But to his surprise, roughly half of the particles locked within were felsic.

Assuming those rocks formed within subduction zones, that means tectonic plates were already on the go by that time—a conclusion that just might help solve an age-old mystery: the birth date of plate tectonics. Scientists have long argued over the precise date these crustal plates started to rouse from their slumber, with estimates ranging from one billion to 4.2 billion years ago. That range is far too large if scientists want to understand the evolution of early Earth. Shifting plates have the ability to dramatically reshape the planet by sculpting ocean basins and thrusting up mountain ranges. They also alter the composition of the atmosphere and oceans. This would have affected the supply of nutrients available to the fledgling life on our young planet.

With such a vast time range involved, it is easy to see why scientists cannot agree on a firm date. Paul Tackley, a geophysicist at the Swiss Federal Institute of Technology, disagrees with the latest interpretation. He contends felsic rocks can form anytime mafic rocks sink deep within Earth—and not only along subduction zones. In fact, he argues this process can occur on a motionless plate. Should a volcano erupt, for example, the newly released lava will push down on mafic rocks until they become so deeply buried that they melt under the high subterranean pressures and temperatures, transforming into felsic rocks.

Although Greber agrees felsic rocks can certainly form like this, he argues such a high felsic ratio cannot be explained by Tackley’s rock-sinking explanation alone. Take Iceland, for example—because the island is far from any subduction zones high numbers of light-colored rocks simply do not form—hence the island’s endless black lava fields and black-sand beaches. So Greber argues the high ratios of light-colored rocks discovered in his old sediments can only mean plate tectonics began early in our planet’s history. But 3.5 billion years is just a lower limit. In the future he hopes to find even older rocks, allowing him to pinpoint an exact birth date.

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  • 1 month later...
Nov. 2, 2017- NASA/JPL press release

Warm Air Helped Make 2017 Ozone Hole Smallest Since 1988

Measurements from satellites this year showed the hole in Earth’s ozone layer that forms over Antarctica each September was the smallest observed since 1988, scientists from NASA and NOAA announced today.

This year’s ozone hole was similar in area to the hole in 1988, about 1 million miles smaller than in 2016. Although scientists predict the ozone hole will continue to shrink, this year’s smaller ozone hole had more to do with weather conditions than human intervention.
Credits: NASA's Goddard Space Flight Center/Kathryn Mersmann


According to NASA, the ozone hole reached its peak extent on Sept. 11, covering an area about two and a half times the size of the United States – 7.6 million square miles in extent - and then declined through the remainder of September and into October. NOAA ground- and balloon-based measurements also showed the least amount of ozone depletion above the continent during the peak of the ozone depletion cycle since 1988. NOAA and NASA collaborate to monitor the growth and recovery of the ozone hole every year.


“The Antarctic ozone hole was exceptionally weak this year,” said Paul A. Newman, chief scientist for Earth Sciences at NASA's Goddard Space Flight Center in Greenbelt, Maryland. “This is what we would expect to see given the weather conditions in the Antarctic stratosphere.”


The smaller ozone hole in 2017 was strongly influenced by an unstable and warmer Antarctic vortex – the stratospheric low pressure system that rotates clockwise in the atmosphere above Antarctica. This helped minimize polar stratospheric cloud formation in the lower stratosphere. The formation and persistence of these clouds are important first steps leading to the chlorine- and bromine-catalyzed reactions that destroy ozone, scientists said. These Antarctic conditions resemble those found in the Arctic, where ozone depletion is much less severe.

max hole
Ozone depletion occurs in cold temperatures, so the ozone hole reaches its annual maximum in September or October, at the end of winter in the Southern Hemisphere.
Credits: NASA/NASA Ozone Watch/Katy Mersmann


In 2016, warmer stratospheric temperatures also constrained the growth of the ozone hole. Last year, the ozone hole reached a maximum 8.9 million square miles, 2 million square miles less than in 2015. The average area of these daily ozone hole maximums observed since 1991 has been roughly 10 million square miles.  


Although warmer-than-average stratospheric weather conditions have reduced ozone depletion during the past two years, the current ozone hole area is still large because levels of ozone-depleting substances like chlorine and bromine remain high enough to produce significant ozone loss.


Scientists said the smaller ozone hole extent in 2016 and 2017 is due to natural variability and not a signal of rapid healing.


First detected in 1985, the Antarctic ozone hole forms during the Southern Hemisphere’s late winter as the returning sun’s rays catalyze reactions involving man-made, chemically active forms of chlorine and bromine. These reactions destroy ozone molecules.


Thirty years ago, the international community signed the Montreal Protocol on Substances that Deplete the Ozone Layer and began regulating ozone-depleting compounds. The ozone hole over Antarctica is expected to gradually become less severe as chlorofluorocarbons—chlorine-containing synthetic compounds once frequently used as refrigerants – continue to decline. Scientists expect the Antarctic ozone hole to recover back to 1980 levels around 2070.


Ozone is a molecule comprised of three oxygen atoms that occurs naturally in small amounts. In the stratosphere, roughly 7 to 25 miles above Earth’s surface, the ozone layer acts like sunscreen, shielding the planet from potentially harmful ultraviolet radiation that can cause skin cancer and cataracts, suppress immune systems and also damage plants. Closer to the ground, ozone can also be created by photochemical reactions between the sun and pollution from vehicle emissions and other sources, forming harmful smog.


Although warmer-than-average stratospheric weather conditions have reduced ozone depletion during the past two years, the current ozone hole area is still large compared to the 1980s, when the depletion of the ozone layer above Antarctica was first detected. This is because levels of ozone-depleting substances like chlorine and bromine remain high enough to produce significant ozone loss.

At its peak on Sept. 11, 2017, the ozone hole extended across an area nearly two and a half times the size of the continental United States. The purple and blue colors are areas with the least ozone.
Credits: NASA/NASA Ozone Watch/Katy Mersmann


NASA and NOAA monitor the ozone hole via three complementary instrumental methods. Satellites, like NASA’s Aura satellite and NASA-NOAA Suomi National Polar-orbiting Partnership satellite measure ozone from space. The Aura satellite’s Microwave Limb Sounder  also measures certain chlorine-containing gases, providing estimates of total chlorine levels.


NOAA scientists monitor the thickness of the ozone layer and its vertical distribution above the South Pole station by regularly releasing weather balloons carrying ozone-measuring “sondes” up to 21 miles in altitude, and with a ground-based instrument called a Dobson spectrophotometer.


The Dobson spectrophotometer measures the total amount of ozone in a column extending from Earth’s surface to the edge of space in Dobson Units, defined as the number of ozone molecules that would be required to create a layer of pure ozone 0.01 millimeters thick at a temperature of 32 degrees Fahrenheit at an atmospheric pressure equivalent to Earth’s surface.


This year, the ozone concentration reached a minimum over the South Pole of 136 Dobson Units on September 25— the highest minimum seen since 1988. During the 1960s, before the Antarctic ozone hole occurred, average ozone concentrations above the South Pole ranged from 250 to 350 Dobson units. Earth's ozone layer averages 300 to 500 Dobson units, which is equivalent to about 3 millimeters, or about the same as two pennies stacked one on top of the other.


"In the past, we've always seen ozone at some stratospheric altitudes go to zero by the end of September," said Bryan Johnson, NOAA atmospheric chemist. "This year our balloon measurements showed the ozone loss rate stalled by the middle of September and ozone levels never reached zero."



Last Updated: Nov. 3, 2017
Editor: Sara Blumberg
I have to wonder if there is some linkage to the solar minimum (that brought us 3 very ugly hurricanes) and our planets geomagnetic field. The evidence is statistical at this point but as yet no causative agent is apparent. This is where science happens- on the edge of what is understood and mysteries that nature still holds.
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