Tag Archives: granite

Researchers find Earth’s oldest rocks — and they were probably created by meteorites

Scientists have found 4.02 billion-year-old rocks, which they believe to be the oldest rocks on the planet’s surface. These rocks, they explain, were likely created by meteorites.

The year is 4 billion BC — and the Earth is a hellish place. The planet doesn’t really have an atmosphere to shield it from meteorite impacts, and the planetary surface is still in its nascent days. As far as we know, no living creatures can inhabit Earth under these conditions — but this doesn’t mean the Earth is inactive. Geologically, lots of things are happening, and researchers have found strong evidence of this.

Australian geologists investigated the Slave Craton formation in Northern Canada, north of Yellowknife and the Great Slave Lake. This is one of the oldest and most stable tectonic structures on our planet, which makes it an ideal place to search for rocks from that period. In particular, researchers have focused on some areas near the Acasta River, discovering 4.02 billion-year-old silica-rich felsic rocks.

[panel style=”panel-default” title=”Felsic and Mafic” footer=””]Igneous rocks, which formed through the slow cooling of magmatic rocks, are typically split into two main groups: felsic, and mafic.

Felsic rocks are relatively rich in elements that form feldspar and quartz, contrasting with the mafic rocks, which are relatively rich in magnesium and iron. The term “felsic” combines the words “feldspar” and “silica”, while the word “mafic” combines “magnesium” and “ferric.”

Generally speaking, felsic rocks tend to be whiter or lighter than mafic rocks, which are generally dark and black.[/panel]

Scientists have long known that the Acasta rocks are very different from the vast majority of felsic rocks, such as granites. Now, in a new study, researchers have shown how these rocks may have formed — and it involves meteorites.

“Our modelling shows that the Acasta River rocks derived from the melting of pre-existing iron-rich basaltic rock, which formed the uppermost layers of crust on the primitive Earth”, said team leader Tim Johnson, from Curtin University, Perth.

They found that the Acasta River rocks were produced by partial melting of previously mafic rocks. This process happened at very low pressures, indicating that they were close to the surface. But there’s not much reason why the 900°C temperatures needed to melt the rocks would have been reached near the surface. Researchers suspect there was a drastic event, most likely a meteorite bombardment, that caused this rise in temperature.

“We estimate that rocks within the uppermost 3km of mafic crust would have been melted in producing the rocks we see today. We think that these ancient felsic rocks would have been very common, but the passage of 4 billion years, and the development of plate tectonics, means that almost nothing remains,” Johnson continues.

“We believe that these rocks may be the only surviving remnants of a barrage of extraterrestial impacts which characterized the first 600 million years of Earth History”.

The study has been published in Nature Geoscience and will be presented at the Goldschmidt conference on the 14th of August.

The World’s Deepest Hole Lies Beneath this Rusty Metal Cap – The Kola Superdeep Borehole

Would you believe me if I told you that under this rusty, abandoned metal cap there lies the deepest hole ever dug by mankind? That beneath this metal seal, which measures only 9 inches in diameter, there are 12,262 meters (40,230 ft) of nothingness? You might have your doubts — but hear me out.

A journey to the center of the Earth

During the Cold War, the race for space took all the headlines, but the digging race was equally competitive (digging boreholes, that is). This is the Kola Superdeep Borehole – a project funded by the USSR and then Russia between 1970 and 1994. In that period, geologists and geophysicists had only indirect evidence as to what was going on in the Earth’s crust, and superdeep boreholes provided much-needed information for a better understanding of the underlying geology by utilizing direct observation. Even to this day, information gathered by this project is still being analyzed and interpreted.

Granites… granites everywhere

The first surprise they encountered was the lack of the so-called “basaltic layer” at about 7 km deep. Previously, the best geological information about the deeper parts of the crust came from analyzing seismic waves, and the waves suggested a discontinuity — basically, they were expecting to find granites, and as they went deeper, basalts. But much to everybody’s surprise, when they went deeper, they actually found… more granites. As it turns out, the seismic discontinuity was caused by the metamorphosis of the granites, not by basalts.

Water?!

A photograph depicting the operation of the drilling — not the best quality, but you get the picture.

As if that wasn’t enough, between 3 and 6 km deep, they also found water. By the knowledge we had back then, water simply shouldn’t have existed at those depth – and yet, there it was. Now we understand that even deep granites can get fractured, and those fractures can get filled with water. Technically speaking the water is just hydrgen and oxygem atoms squeezed out by the enormous pressures caused by the depth – and trapped in impermeable “layers” of rock.

Boiling with hydrogen

Researchers also reported the extraction of mud, which was “boiling with hydrogen” – such large quantities of hydrogen at these depths were completely unexpected.

Life — deep inside the Earth?

Without a doubt, the biggest surprise was the discovery of life: microscopic plankton fossils in rocks over two billion years old, found four miles beneath the surface. These “microfossils” represented about 24 ancient species and were encased in organic compounds which somehow survived the extreme pressures and temperatures so far beneath the Earth’s surface. This raised numerous questions about the potential survival of life forms at impressive depths.

Now, research has shown that life can exist even in oceanic crust, and even macroscopic life was found at over 1 km deep, but at the time, finding those fossils came as a shocker.

Abandoned, but not forgotten

Now, the Kola Superdeep Borehole is all but abandoned. At depths in excess of about 10,000 feet, researchers started to notice that the temperature increased faster than expected, and the first problems started to occur.  In 1983, the drill passed 12,000 m (39,000 ft), and drilling was stopped for about a year to “celebrate” the event. I have no idea why they would stop for a year to “celebrate”, but this idle period probably contributed to the breakdown in September 1984: after drilling to 12,066 m (39,587 ft), a 5,000 m (16,000 ft) section of the drill string twisted off and was left in the hole. Drilling was later restarted from 7,000 m (23,000 ft).

The drill bit used in the digging process (one of them). The nearby town of Zapolyarny holds the Kola Core Repository, which displays rock samples obtained during the drilling operation.

However, temperatures continued to grow more than the expected values, and by the time the hole reached its maximum length, the temperature was a whopping 180 °C (356 °F) instead of expected 100 °C (212 °F). The drill bit could no longer work at such temperatures, and drilling was stopped in 1992.

The entire project was closed down in 2005 due to (you’ve guessed it) of lack of funding. All the drilling and research equipment was scrapped and while data produced by the Kola drilling project continues to be analyzed, the site itself has been abandoned since 2008; the hole was welded shut by the metal cap we still see today, as if to seal off any devils or mysteries that might lurk beneath.

You can visit the now-abandoned site but, unfortunately, you won’t be able to peek through the hole that, to this day, is the deepest hole dug below the surface.

 

 

Evidence of granite found on Mars – Red Planet geology more complex than previously thought

Geologists have now found the most compelling evidence of granites on Mars – something which prompts more complex theories about the geology and tectonic activity on the Red Planet.

Granites and basalts

Basalt and Granite. Credits: Rice University.

Granites are igneous rocks, pretty common on the surface of Earth. It is often called a ‘felsic’ (white rock) – because it is very rich in so-called white minerals, such as quartz or feldspar. It is contrasted with mafic rocks (for example basalt), which are relatively richer in magnesium and iron. Now, large amounts of feldspar have been found in a Martian volcano. Interestingly enough, minerals commonly found in basalts are completely absent from that area; considering how basalts are almost ubiquitous on Mars, this initially came as a shock, but now, geologists have come up with a theory to explain this.

Granite, or its eruptive equivalent, rhyolite, is often found on Earth in tectonically active regions such as subduction zones. However, since Mars isn’t tectonically active, there are no subduction zones there, so there has to be a different cause. The team studying the case concluded that prolonged magmatic activity on Mars can also produce these granitic compositions on very large scales.

“We’re providing the most compelling evidence to date that Mars has granitic rocks,” said James Wray, an assistant professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology and the study’s lead author.

Red Planet geology

A 'spectral window' into the Martian geology - bright magenta outcrops have a distinctive feldspar-rich composition. (Credit: NASA/JPL/JHUAPL/MSSS)

A ‘spectral window’ into the Martian geology – bright magenta outcrops have a distinctive feldspar-rich composition. (Credit: NASA/JPL/JHUAPL/MSSS)

For many years, the geology of Mars has been considered to be very simplistic, consisting of mostly one single type of rock: basalt – a common extrusive igneous (volcanic) rock formed from the rapid cooling of basaltic lava exposed at or very near the surface. The dark rock can also be found on Earth in many volcanically active areas, such as Hawaii or Iceland for example.

But earlier this year, the Mars Curiosity started to cast some doubt on those beliefs, when it reported finding soils with a composition similar to granite. No one really knew what to make of this discovery, but since it appeared to be very localized, it was just considered a local anomaly. However, this new research analyzed things at a much larger scale, using remote sensing techniques with infrared spectroscopy to survey a large volcano on Mars that was active for billions of years. The volcano is perfect for this type of study, because it is dust free (a true rarity on Mars) – some of the fastest-moving sand dunes on Mars sweep away any would-be dust particles on this volcano.

Much to the delight of researchers, the limitations of the remote sensing technology were an advantage in this case:

“Using the kind of infrared spectroscopic technique we were using, you shouldn’t really be able to detect feldspar minerals, unless there’s really, really a lot of feldspar and very little of the dark minerals that you get in basalt,” Wray said.

Separating the white and the black

So we have an island of white feldspar amidst an ocean of black basalt – how did it form?

When you have magma in the subsurface, it cools off very, very slowly. In a tectonically inactive planet like Mars, this process can be very stable. While the magma slowly cools off, low density melt separates from high density crystals, and if the conditions are just right, this process can take place for billions of years, leading to the creation of granitic rocks, as computer simulations showed.

“We think some of the volcanoes on Mars were sporadically active for billions of years,” Wray said. “It seems plausible that in a volcano you could get enough iterations of that reprocessing that you could form something like granite.”

While we are trying to figure out the existence (or lack of it) of life on Mars, this is another wake-up call, showing just how little we understand about the geologic processes on the Red Planet – which ultimately govern the appearance of life. Anyway, the geology of Mars just got a lot more interesting.

Journal Reference:

  1. James J. Wray, Sarah T. Hansen, Josef Dufek, Gregg A. Swayze, Scott L. Murchie, Frank P. Seelos, John R. Skok, Rossman P. Irwin, Mark S. Ghiorso. Prolonged magmatic activity on Mars inferred from the detection of felsic rocksNature Geoscience, 2013; DOI: 10.1038/NGEO1994

Ancient, long-lost continent found under the Indian Ocean

Evidence of drowned remnants of an ancient microcontinent have been found in sand grains from the beaches of a small Indian Ocean island, according to a new research.

Zircons and volcanoes

mauritia

This evidence was found in Mauritius, a volcanic island 900 kilometres east of Madagascar which serves as an exotic destination for many tourists. Basaltic rocks from the island have been dated to approximately 9 million years ago, but now, an international research team analyzed the beaches and found fragments of zircon that are much older, between 600 million and 2 billion years old.

Bjørn Jamtveit, a geologist at the University of Oslo explained that the zircons had crystallized within granites or other acidic igneous rocks (basalts being basic, non acidic). He believes that rocks containing these minerals came from a long-submerged landmass that was once wedged between India and Madagascar in a prehistoric supercontinent known as Rodinia; geologically recent volcanic eruptions brought the rocks up to the surface, where they were eroded, resulting in the shards they picked up. Most of the rocks were melted by the high temperatures, but some grains of zircons survived and were frozen into the lavas, rolling towards the Mauritian surface.

“When lavas moved through continental material on the way towards the surface, they picked up a few rocks containing zircon,” study co-author Bjørn Jamtveit, a geologist at the University of Oslo in Norway, explained in an email.

rodinia

The tectonic plates are mobile in geologic time – the surface of the Earth didn’t always look like this. As a matter of fact, the further down you go on the time scale, the more different it looks like. According to plate tectonic reconstructions, Rodinia existed between 1.1 billion and 750 million years ago; virtually all of the Earth’s landmass was concentrated in this single supercontinent which started to split 3/4 billion years ago.

The study also analyzed the gravity field and as it turns out, something really interesting happened to the remains of Rodinia in that area. As India and Madagascar began to drift apart some 85 million years ago, the landmass just sinked, Atlantis style. The cause was tectonic rifting and sea-floor spreading sending the Indian subcontinent surging northeast, sinking the fragments of Mauritia (how the researchers named this microcontinent).

The variations in the gravitational field observed in some areas in Mauritius, the Seychelles, and the Maldives is pretty much a smoking gun suggesting a thick crust supporting the long-lost continent theory, with the continent being “tucked” under the Indian Ocean.

gravity

A non-geologic accident?

The only weak point, is that the study, thorough as it is, relies mostly on those zircons; couldn’t they be just some sort of non-geologic accident?

“There’s no obvious local source for these zircons,” says Conall Mac Niocaill, a geologist at the University of Oxford, UK, who was not involved in the research.

It also doesn’t look like they were brought there by winds.

“There’s a remote possibility that they were wind blown, but they’re probably too large to have done so,” adds Robert Duncan, a marine geologist at Oregon State University in Corvallis.

Also, the samples were picked up from remote sites, where it’s quite unlikely that humans would have brought them there. However, Jérôme Dyment, a geologist at the Paris Institute of Earth Physics in France, is not convinced. He believes that a number of non-geologic processes could have brought the minerals there, as part of ship ballast or modern construction material for example.

“Extraordinary claims require extraordinary evidence, which are not given by the authors so far,” said Dyment, who did not participate in the research. “Finding zircons in sand is one thing, finding them within a rock is another one … Finding the enclave of deep rocks that, according to the author’s inference, bring them to the surface during an eruption would be much more convincing evidence.”

He makes an even more convincing argument, explaining that if remains of such a continent were to exist, evidence for its existence should have been found as part of an ongoing experiment that installed deep-sea seismometers to investigate Earth’s mantle around Réunion Island, which is situated about 200 kilometers from Mauritius.

So is this compelling evidence, or is it more of an educated assertion? But Conall Mac Niocaill, a geologist at the University of Oxford in the U.K. who was also not involved in the study, is spot on: “the lines of evidence are, individually, only suggestive, but collectively they add up to a compelling story.”, he says. Particularly, the geophysic (gravimetric) evidence is highly consistent with the researchers’ claims. All in all, it paints a consistent picture which makes sense in a tectonic context, but as almost always in geology, you can’t just draw a line and say “This is so”; one thing’s for sure though: oceanic basins worldwide may very well host similarly submerged remains of “ghost continents”.

Via Nature Geoscience