From today, the Earth is around 60 million years older — and so is the moon

Work presented today at the Goldschmidt Geochemistry Conference in Sacramento, California shows that the timing of the giant impact between Earth’s ancestor and a planet-sized body occurred around 40 million years after the start of solar system formation. This means that the final stage of Earth’s formation is around 60 million years older than previously thought.

Geochemists from the University of Lorraine in Nancy, France have discovered an isotopic signal which indicates that previous age estimates for both the Earth and the Moon are underestimates. Looking back into “deep time” it becomes more difficult to put a date on early Earth events. In part this is because there is little “classical geology” dating from the time of the formation of the Earth – no rock layers, etc. So geochemists have had to rely on other methods to estimate early Earth events. One of the standard methods is measuring the changes in the proportions of different gases (isotopes) which survive from the early Earth.

Guillaume Avice and Bernard Marty analysed xenon gas found in South African and Australian quartz, which had been dated to 3.4 and 2.7 billion years respectively. The gas sealed in this quartz is preserved as in a “time capsule”, allowing Avice and Marty to compare the current isotopic ratios of xenon, with those which existed billions of years ago. Recalibrating dating techniques using the ancient gas allowed them to refine the estimate of when the earth began to form. This allows them to calculate that the Moon-forming impact is around 60 million years (+/- 20 m. y.) older than had been thought.

Previously, the time of formation of the Earth’ s atmosphere had been estimated at around 100 million years after the solar system formation. As the atmosphere would not have survived the Moon-forming impact, this revision puts the age up to 40 million years after the solar sytem formation (so around 60 million years older than previously thought).

According to Guillaume Avice:

“It is not possible to give an exact date for the formation of the Earth*. What this work does is to show that the Earth is older than we thought, by around 60 m.

“The composition of the gases we are looking at changes according the conditions they are found in, which of course depend on the major events in Earth’s history. The gas sealed in these quartz samples has been handed down to us in a sort of “time capsule”. We are using standard methods to compute the age of the Earth, but having access to these ancient samples gives us new data, and allows us to refine the measurement”.

The xenon gas signals allow us to calculate when the atmosphere was being formed, which was probably at the time the Earth collided with a planet-sized body, leading to the formation of the Moon. Our results mean that both the Earth and the Moon are older than we had thought”.

Bernard Marty added

“This might seem a small difference, but it is important. These differences set time boundaries on how the planets evolved, especially through the major collisions in deep time which shaped the solar system”

A journey through Cuba’s culture and geology

Few destinations capture the imagination like Cuba; a forbidden fruit to U.S. citizens since the 1960s. Recently, 14 earth scientists from the U.S.-based Association for Women Geoscientists travelled there to explore its geology and culture.

The expedition is chronicled in the August issue of EARTH Magazine. While Cuba is an intriguing destination as an actor on the global political stage, its geological history captures events that tell scientists even more about the history of the planet.

While there, the scientists studied rocks that captured the extra-terrestrial impact attributed to the demise of the dinosaurs – including shocked quartz and tsunami deposits. The scientists also learned about how local limestone was used to build forts intended to protect Cuba’s harbors from pirate attacks. Their guide even took them to sites that represent the breakup of the supercontinent Pangaea. The rocks observed in Cuba have been shown to be closely related to the Mediterranean.

Any earth scientist would agree the geologic history contained on this island is astounding. More importantly, these scientists visited Cuba to experience UNESCO World Heritage sites, and share in “people-to-people” experiences between two cultures that continue to be divided. Read more about the geological diversity of Cuba, including miles of underground cave networks and risks posed by a San Andreas-like fault at: http://bit.ly/152DT0u.

Don’t miss other exciting stories this month’s issue of Earth available at the Digital Newsstand: http://www.earthmagazine.org/digital. Read about the improvements scientists are making in hurricane forecasts, water challenges faced by a tropical paradise, and the discovery of sauropod embryos in southern China.

Ancient trapped water explains Earth’s first ice age

The North Pole area, Pilbara, Western Australia, where the samples came from. -  University of Manchester
The North Pole area, Pilbara, Western Australia, where the samples came from. – University of Manchester

Tiny bubbles of water found in quartz grains in Australia may hold the key to understanding what caused the Earth’s first ice age, say scientists.

The Anglo-French study, published in the journal Nature, analysed the amount of ancient atmospheric argon gas (Ar) isotopes dissolved in the bubbles and found levels were very different to those in the air we breathe today.

The researchers say their findings help explain why Earth didn’t suffer its first ice age until 2.5 billion years ago, despite the Sun’s rays being weaker during the early years of our planet’s formation.

“The water samples come from the Pilbara region in north-west Australia and were originally heated during an eruption of pillow basalt lavas, probably in a lake or lagoon environment,” said author Dr Ray Burgess, from the University of Manchester’s School of Earth, Atmospheric and Environmental Sciences.

“Evidence from the geological record indicates that the first major glaciations on Earth occurred about 2.5 billion years ago, and yet the energy of the Sun was 20 per cent weaker prior to, and during, this period, so all water on Earth should already have been frozen.

“This is something that has baffled scientists for years but our findings provide a possible explanation.”

The study, done in collaboration with the CRPG-CNRS, University of Lorraine and the Institut de Physique du Globe de Paris, revealed that the ratio of two argon isotopes – 40Ar, formed by the decay of potassium (40K) with a half-life of 1.25 billion years, and 36Ar – was much lower than present-day levels. This finding can only be explained by the gradual release of 40Ar from rocks and magma into the atmosphere throughout Earth’s history.

The team used the argon isotope ratio to estimate how the continents have grown over geological time and found that the volume of continental crust 3.5 billion years ago was already well-established being roughly half what it is today.

Dr Burgess said: “High levels of the greenhouse gas carbon dioxide in the early atmosphere – in the order of several percent – which would have helped retain the Sun’s heat, has been suggested as the reason why the Earth did not freeze over sooner, but just how this level was reduced has been unexplained, until now.

“The continents are a key player in the Earth’s carbon cycle because carbon dioxide in the atmosphere dissolves in water to form acid rain. The carbon dioxide removed from the atmosphere by this process is stabilised in carbonate rocks such as limestone and if a substantial volume of continental crust was established, as revealed by our study, then the acid weathering of this early crust would efficiently reduce the carbon dioxide levels in the atmosphere to lower global temperatures and lead to the first major ice age.

He added: “The signs of the Earth’s evolution in the distant past are extremely tenuous, only fragments of highly weathered and altered rocks exists from this time, and for the most part, the evidence is indirect. To find an actual sample of ancient atmospheric argon is remarkable and represents a breakthrough in understanding environmental conditions on Earth before life existed.”

Slow earthquakes: It’s all in the rock mechanics

Earthquakes that last minutes rather than seconds are a relatively recent discovery, according to an international team of seismologists. Researchers have been aware of these slow earthquakes, only for the past five to 10 years because of new tools and new observations, but these tools may explain the triggering of some normal earthquakes and could help in earthquake prediction.

“New technology has shown us that faults do not just fail in a sudden earthquake or by stable creep,” said Demian M. Saffer, professor of geoscience, Penn State. “We now know that earthquakes with anomalous low frequencies — slow earthquakes — and slow slip events that take weeks to occur exist.”

These new observations have put a big wrinkle into our thinking about how faults work, according to the researchers who also include Chris Marone, professor of geosciences, Penn State; Matt J. Ikari, recent Ph.D. recipient, and Achim J. Kopf, former Penn State postdoctural fellow, both now at the University of Bremen, Germany. So far, no one has explained the processes that cause slow earthquakes.

The researchers thought that the behavior had to be related to the type of rock in the fault, believing that clay minerals are important in this slip behavior to see how the rocks reacted. Ikari performed laboratory experiments using natural samples from drilling done offshore of Japan in a place where slow earthquakes occur. The samples came from the Integrated Ocean Drilling Program, an international collaborative. The researchers reported their results recently in Nature Geoscience.

These samples are made up of ocean sediment that is mostly clay with a little quartz.

“Usually, when you shear clay-rich fault rocks in the laboratory in the way rocks are sheared in a fault, as the speed increases, the rocks become stronger and self arrests the movement,” said Saffer. “Matt noticed another behavior. Initially the rocks reacted as expected, but these clays got weaker as they slid further. They initially became slightly stronger as the slip rate increased, but then, over the long run, they became weaker.

The laboratory experiments that produced the largest effect closely matched the velocity at which slow earthquakes occur in nature. The researchers also found that water content in the clays influenced how the shear occurred.

“From the physics of earthquake nucleation based on the laboratory experiments we would predict the size of the patch of fault that breaks at tens of meters,” said Saffer. “The consistent result for the rates of slip and the velocity of slip in the lab are interesting. Lots of things point in the direction for this to be the solution.”

The researchers worry about slow earthquakes because there is evidence that swarms of low frequency events can trigger large earthquake events. In Japan, a combination of broadband seismometers and global positioning system devices can monitor slow earthquakes.

For the Japanese and others in earthquake prone areas, a few days of foreknowledge of a potential earthquake hazard could be valuable and save lives.

For slow slip events, collecting natural samples for laboratory experiments is more difficult because the faults where these take place are very deep. Only off the north shore of New Zealand is there a fault that can be sampled. Saffer is currently working to arrange a drilling expedition to that fault.

Viscous cycle: Quartz is key to plate tectonics

Quartz may play a major role in the movements of continents, known as plate tectonics. -  USGS
Quartz may play a major role in the movements of continents, known as plate tectonics. – USGS

More than 40 years ago, pioneering tectonic geophysicist J. Tuzo Wilson published a paper in the journal Nature describing how ocean basins opened and closed along North America’s eastern seaboard.

His observations, dubbed “The Wilson Tectonic Cycle,” suggested the process occurred many times during Earth’s long history, most recently causing the giant supercontinent Pangaea to split into today’s seven continents.

Wilson’s ideas were central to the so-called Plate Tectonic Revolution, the foundation of contemporary theories for processes underlying mountain-building and earthquakes.

Since his 1967 paper, additional studies have confirmed that large-scale deformation of continents repeatedly occurs in some regions but not others, though the reasons why remain poorly understood.

Now, new findings by Utah State University geophysicist Tony Lowry and colleague Marta Pérez-Gussinyé of Royal Holloway, University of London, shed surprising light on these restless rock cycles.

“It all begins with quartz,” says Lowry, who published results of the team’s recent study in the March 17 issue of Nature.

The scientists describe a new approach to measuring properties of the deep crust.

It reveals quartz’s key role in initiating the churning chain of events that cause Earth’s surface to crack, wrinkle, fold and stretch into mountains, plains and valleys.

“If you’ve ever traveled westward from the Midwest’s Great Plains toward the Rocky Mountains, you may have wondered why the flat plains suddenly rise into steep peaks at a particular spot,” Lowry says.

“It turns out that the crust beneath the plains has almost no quartz in it, whereas the Rockies are very quartz-rich.”

He thinks that those belts of quartz could be the catalyst that sets the mountain-building rock cycle in motion.

“Earthquakes, mountain-building and other expressions of continental tectonics depend on how rocks flow in response to stress,” says Lowry.

“We know that tectonics is a response to the effects of gravity, but we know less about rock flow properties and how they change from one location to another.”

Wilson’s theories provide an important clue, Lowry says, as scientists have long observed that mountain belts and rift zones have formed again and again at the same locations over long periods of time.

But why?

“Over the last few decades, we’ve learned that high temperatures, water and abundant quartz are all critical factors in making rocks flow more easily,” Lowry says. “Until now, we haven’t had the tools to measure these factors and answer long-standing questions.”

Since 2002, the National Science Foundation (NSF)-funded Earthscope Transportable Array of seismic stations across the western United States has provided remote sensing data about the continent’s rock properties.

“We’ve combined Earthscope data with other geophysical measurements of gravity and surface heat flow in an entirely new way, one that allows us to separate the effects of temperature, water and quartz in the crust,” Lowry says.

Earthscope measurements enabled the team to estimate the thickness, along with the seismic velocity ratio, of continental crust in the American West.

“This intriguing study provides new insights into the processes driving large-scale continental deformation and dynamics,” says Greg Anderson, NSF program director for EarthScope. “These are key to understanding the assembly and evolution of continents.”

Seismic velocity describes how quickly sound waves and shear waves travel through rock, offering clues to its temperature and composition.

“Seismic velocities are sensitive to both temperature and rock type,” Lowry says.

“But if the velocities are combined as a ratio, the temperature dependence drops out. We found that the velocity ratio was especially sensitive to quartz abundance.”

Even after separating out the effects of temperature, the scientists found that a low seismic velocity ratio, indicating weak, quartz-rich crust, systematically occurred in the same place as high lower-crustal temperatures modeled independently from surface heat flow.

“That was a surprise,” he says. “We think this indicates a feedback cycle, where quartz starts the ball rolling.”

If temperature and water are the same, Lowry says, rock flow will focus where the quartz is located because that’s the only weak link.

Once the flow starts, the movement of rock carries heat with it and that efficient movement of heat raises temperature, resulting in weakening of crust.

“Rock, when it warms up, is forced to release water that’s otherwise chemically bound in crystals,” he says.

Water further weakens the crust, which increasingly focuses the deformation in a specific area.