Tag Archives: crust

Mars’ water didn’t escape; it’s trapped in the red crust

New research from Caltech and JPL suggests that Mars never lost its water — it just drank it up, so to speak.

Digital rendering of Mars. Image credits Kevin Gill via Wikimedia.

Billions of years ago, our red neighbor had an atmosphere and maintained liquid water on its surface. We know this because Mars’ surface is littered with ancient river- and lake beds. The prevailing wisdom today is that once the planet lost its geological activity and thus, its magnetic field, it lost, in turn, its atmosphere and surface water, which were blown away by solar winds.

But new research says that at least the water might still be there. According to the findings, anywhere between 30% to 99% of its original water is trapped in minerals within the Martian crust.

Better red than dry

“Atmospheric escape doesn’t fully explain the data that we have for how much water actually once existed on Mars,” says Caltech PhD candidate Eva Scheller, lead author of the paper.

According to the team, around four billion years ago Mars had enough liquid water to cover its entire surface in an ocean between 100 to 1,500 meters deep. That, they explain, would be roughly equivalent to half the entire volume of the Atlantic Ocean. However, around three billion years ago, Mars looked as it does today — dry as bone. The planet’s low gravitational pull was believed to have allowed this water to escape to space over time under the action of solar winds.

For the study, the team looked at how much water Mars has in all of its forms, as well as the chemical composition of its current atmosphere and crust. They used data beamed back by virtually every Mars rover and orbiter and that we gleaned from meteorites. A particular point of interest for them was to analyze the ratio of deuterium to hydrogen (D/H) isotopes in this water.

The vast majority of water molecules have ‘vanilla’ hydrogen in their molecules — hydrogen atoms with one proton in their nucleus. Around 0.02% of all naturally-occurring water molecules in the Universe, however, include deuterium atoms — “heavy” hydrogen, which has one proton and one neutron at its core — instead.

The value of the D/H ratio in Mars’ atmosphere over time. Image credits L. J. Hallis via Researchgate.

Regular hydrogen is also known as protium and, because of its lower atomic weight, should have an easier time escaping a planet’s gravity into space. But this also means that such a process would increase the D/H ratio in Mars’ current atmosphere (i.e. increase the presence of deuterium above the 0.02% mark), which is something we can check. What the paper argues, however, is that this escape process can’t explain where all the water that’s missing has gone, and the D/H ratio, by itself. Instead, the team proposes that another mechanism worked at the same time: the trapping of water in minerals inside the planet’s crust. Together, the team explains, they could produce the conditions we see today on Mars.

The interaction between water and silicate rocks generates minerals such as clay through a process called (chemical) weathering. These minerals often contain water in their structure. While chemical weathering takes place on both Earth and Mars all the time, Earth is tectonically active, meaning weathered minerals eventually find their way back into the mantle where they’re recycled, which brings the water back out through volcanic eruptions. Since Mars isn’t tectonically active, the water trapped in its crust is no longer being cycled back out.

“Atmospheric escape clearly had a role in water loss, but findings from the last decade of Mars missions have pointed to the fact that there was this huge reservoir of ancient hydrated minerals whose formation certainly decreased water availability over time,” says Ehlmann.

“All of this water was sequestered fairly early on, and then never cycled back out,” adds Scheller.

The team previously used a similar approach to understand how habitability on Mars evolved over time by tracking carbon dioxide, currently the main ingredient of its atmosphere. In the future, they plan to continue examining the processes through which Mars’ water disappeared in their lab, and later expand their research to nitrogen and sulfur-rich minerals. Samples to-be-recovered by the Perseverance rover will help confirm or deny their current hypothesis.

The paper “Long-term drying of Mars by sequestration of ocean-scale volumes of water in the crust” has been published in the journal Science.

Glacier ice.

Ice ages may be caused by tectonic activity in the tropics, new study proposes

New research says that the Earth’s past ice ages may have been caused by tectonic pile-ups in the tropics.

Glacier ice.

A crevasse in a glacier.
Image via Pixabay.

Our planet has braved three major ice ages in the past 540 million years, seeing global temperatures plummet and ice sheets stretching far beyond the poles. Needless to say, these were quite dramatic events for the planet, so researchers are keen to understand what set them off. A new study reports that plate tectonics might be the culprit.

Cold hard plates

“We think that arc-continent collisions at low latitudes are the trigger for global cooling,” says Oliver Jagoutz, an associate professor in MIT’s Department of Earth, Atmospheric, and Planetary Sciences and a co-author of the new study.

“This could occur over 1-5 million square kilometers, which sounds like a lot. But in reality, it’s a very thin strip of Earth, sitting in the right location, that can change the global climate.”

“Arc-continent collisions” is a term that describes the slow, grinding head-butting that takes place when a piece of oceanic crust hits a continent (i.e. continental crust). Generally speaking, oceanic crust (OC) will slip beneath the continental crust (CC) during such collisions, as the former is denser than the latter. Arc-continent collisions are a mainstay of orogen (mountain range) formation, as they cause the edges of CC plates ‘wrinkle up’. But in geology, as is often the case in life, things don’t always go according to plan.

The study reports that the last three major ice ages were preceded by arc-continent collisions in the tropics which exposed tens of thousands of kilometers of oceanic, rather than continental, crust to the atmosphere. The heat and humidity of the tropics then likely triggered a chemical reaction between calcium and magnesium minerals in these rocks and carbon dioxide in the air. This would have scrubbed huge quantities of atmospheric CO2 to form carbonate rocks (such as limestone).

Over time, this led to a global cooling of the climate, setting off the ice ages, they add.

The team tracked the movements of two suture zones (the areas where plates collide) in today’s Himalayan mountains. Both sutures were formed during the same tectonic migrations, they report: one collision 80 million years ago, when the supercontinent Gondwana moved north creating part of Eurasia, and another 50 million years ago. Both collisions occurred near the equator and proceeded global atmospheric cooling events by several million years.

In geological terms, ‘several million years’ is basically the blink of an eye. So, curious to see whether one event caused the other, the team analyzed the rate at which oceanic rocks known as ophiolites can react to CO2 in the tropics. They conclude that, given the location and magnitude of the events that created them, both of the sutures they investigated could have absorbed enough CO2 to cool the atmosphere enough to trigger the subsequent ice ages.

Another interesting find is that the same processes likely led to the end of these ice ages. The fresh oceanic crust progressively lost its ability to scrub CO2 from the air (as the calcium and magnesium minerals transformed into carbonate rocks), allowing the atmosphere to stabilize.

“We showed that this process can start and end glaciation,” Jagoutz says. “Then we wondered, how often does that work? If our hypothesis is correct, we should find that for every time there’s a cooling event, there are a lot of sutures in the tropics.”

The team then expanded their analysis to older ice ages to see whether they were also associated with tropical arc-continent collisions. After compiling the location of major suture zones on Earth from pre-existing literature, they reconstruct their movement and that of the plates which generated them over time using computer simulations.

All in all, the team found three periods over the last 540 million years in which major suture zones (those about 10,000 kilometers in length) were formed in the tropics. Their formation coincided with three major ice ages, they add: one the Late Ordovician (455 to 440 million years ago), one in the Permo-Carboniferous (335 to 280 million years ago), and one in the Cenozoic (35 million years ago to present day). This wasn’t a happy coincidence, either. The team explains that no ice ages or glaciation events occurred during periods when major suture zones formed outside of the tropics.

“We found that every time there was a peak in the suture zone in the tropics, there was a glaciation event,” Jagoutz says. “So every time you get, say, 10,000 kilometers of sutures in the tropics, you get an ice age.”

Jagoutz notes that there is a major suture zone active today in Indonesia. It includes some of the largest bodies of ophiolite rocks in the world today, and Jagoutz says it may prove to be an important resource for absorbing carbon dioxide. The team says that the findings lend some weight to current proposals to grind up these ophiolites in massive quantities and spread them along the equatorial belt in an effort to counteract our CO2 emissions. However, they also point to how such efforts may, in fact, produce additional carbon emissions — and also suggest that such measures may simply take too long to produce results within our lifetimes.

“It’s a challenge to make this process work on human timescales,” Jagoutz says. “The Earth does this in a slow, geological process that has nothing to do with what we do to the Earth today. And it will neither harm us, nor save us.”

The paper “Arc-continent collisions in the tropics set Earth’s climate state” has been published in the journal Science.

World_geologic_provinces

The Earth had continental crust much earlier than thought — potentially life, too

The Earth might have developed its continental crust much earlier than believed, new research reveals. The findings could have major implications for how we think about the evolution of life on our planet.

World_geologic_provinces

Map showing the world’s geologic provinces.
Image credits United States Geological Survey.

Strontium atoms locked in rocks from northern Canada might rewrite the history of life on Earth. According to new research from the University of Chicago, they suggest that continental crust developed hundreds of millions of years earlier than previously assumed.

Crustally fit

“Our evidence, which squares with emerging evidence including rocks in western Australia, suggests that the early Earth was capable of forming continental crust within 350 million years of the formation of the solar system,” says first author Patrick Boehnke.

“This alters the classic view, that the crust was hot, dry and hellish for more than half a billion years after it formed.”

There are two types of crust covering the Earth: oceanic, which is basically solidified magma, and continental, which is less dense and has a different chemical make-up — most notably, a much higher content of silica. We know that all crust starts out as the oceanic kind, and continental crust later develops on top of this. Geologists have been trying to determine how and at what point continental crust first appeared ever since we’ve known there is such a thing as ‘continental crust’.

However, that’s easier asked than answered. Part of the problem is that the Earth’s crust is continuously recycled over geological timescales — it sinks, melts down, and reforms. This also destroys the evidence geologists would need in order to back-track the process of continental crust formation.

Some fragments of these ancient bits of crust can still be found today, embedded into young rocks as flakes of the mineral apatite. But if they’re not perfectly insulated, they will degrade over time through oxidation, interaction with water, or other chemical and mechanical means.

Luckily, some of the younger minerals also include some that are very durable, such as zircons. These are hardy materials, similar to diamonds, that are very weather-resistant. Even better for a geologist with a mission, zircon can be dated.

“Zircons are a geologist’s favorite because these are the only record of the first three to four hundred million years of Earth. Diamonds aren’t forever — zircons are,” Boehnke said.

The team used strontium isotope analysis to date rocks retrieved from sites in Nuvvuagittuq, northern Canada to determine their age and the amount of silica present as it was forming. Because the flakes of rock they recovered were incredibly tiny — about as thick as a strand of spider silk, five microns across — the team had to use chili.

More specifically, they had to use CHILI (all capitalized). This unique instrument, the Chicago Instrument for Laser Ionization, came on-line last year. It uses laser beams that can be tuned to pick out and ionize strontium atoms, allowing the team to count them. The results of this counting process suggested plenty of silica was present when it formed.

The chemical composition of the crust tells us a lot about the state of the Earth at the time — our planet is like one huge chemistry jar, and every component interacts with all others. Crustal composition directly affects the atmosphere, for example, mostly through oxidation effects. It also alters the composition of seawater and dictates what nutrients are available to any potential organisms. The fact that Earth sported continental crust that early, and that is was so chemically similar to that of today, suggests that conditions at the time weren’t that different from those today. That doesn’t mean the continents looked like they do today (because they didn’t) but geochemical conditions should have been pretty similar to those today.

It could also be a sign that fewer meteorites hit Earth at this time than we assumed — these would pummel the planet, making it hard for continental crust to form.

The findings also suggest we need to take a second look at the processes we believe create continental crust: if the team’s findings are true, they need to work much faster than current models assume.

The paper “Potassic, high-silica Hadean crust” has been published in the journal Proceedings of the National Academy of Sciences.

Continental Drift.

What is the Wilson Cycle, builder and slayer of supercontinents?

We all know what tectonics is — but does it happen by accident? Does it have to follow rules? Is there a method to the madness? Actually, there is: a beautiful, epic cycle of death and rebirth named after Canadian geophysicist John T. Wilson.

Continental Drift.

Image via Pixabay.

Geology is, in many ways, a matter of degrees. One of my freshman year professors used to say that nothing “just happens” in geology, that every flake of rock adheres to the same rules that shape whole planets — it’s just a matter of how closely you look. Zoom into a single strand of sand and you’re dealing with petrography — the science of the stuff rocks are made of. Look at the whole beach, and you’re dealing with sedimentology and depositional systems. If you want to know how the beaches got there, you’ll need tectonics.

And while each level of zoom paints a strikingly different picture, they’re all iterations of a single story. A story in which physics, chemistry, sometimes biology, take the leading roles. Their interplay dictates everything, from the intensity of Earth’s magnetic field to the price of gas.

Of these processes, tectonics plays out on the more massive scale of things. Look at a map — it’s tectonics who made it be that way. And it seems to be unique to the Earth. There’re some other planets around there that seem to do it, some very similarly to ours, but the way our blue corner of space goes about tectonics is to the best of our knowledge, unique.

But, being the science-savvy folks that you are, I’m sure you know this already. So what I’d want to talk about today is what you’ll find when you zoom out from tectonics one more time. The rules, if you will, which tectonics has to play by. A guidebook that geologists know as:

The Wilson Cycle

Well if you’re a stickler for rules it’s technically the Supercontinent Cycle, but that’s just a mouthful so I don’t really use the term.

So let’s get down to business. You know how the Earth’s crust is made out of plates that skim on the surface of a molten ocean of lava (the mantle), bumping into one another. Some of them get pushed back down into the mantle where they’re recycled, so have you ever asked yourself — why don’t we run out of plates?

Tectonic plates.

I don’t even own this many regular plates.
Image credits USGS.

Well, the mantle also helpfully supplies new plate material in rift areas — places where existing tectonic plates drift away from one another, so the mantle can reach the surface and harden into rocks. And when they don’t really feel like bumping into or away from each other, they just slide past one another. We call these latter ones “transform boundaries,” while the bits where plates mash together are known as convergent boundaries and those where they move apart are divergent boundaries. Pretty clever, eh?

Now, what John T. Wilson did to warrant us naming the thing after him was to theorize that the motions of these plates aren’t a random hodge-podge, but rather follow a predictable pattern — a cycle. Give them enough time, and these plates will mush together into a single supercontinent. Wait a bit after that, and you’ll see these plates squabble and break up into a lot of tiny continents, more closely resembling what we know today. The really patient (and ridiculously long-lived) will get so see this happening again and again.

One full cycle is estimated to be around 300 to 500 million years old, and the first one likely started somewhere between 3 and 3.2 billion years ago, when the Earth cooled down enough for the rocks we know and love today to harden into crust.

Step one: The Stable Craton

What we consider as being the crust is made up of three large families of rock, two of which form the bulk of it. Geologists also draw a line between oceanic and continental crust. This division stems from a difference in chemical compositions, which translate to physical differences: oceanic crust is generally made up of mafic igneous rocks (which have a lot of heavier minerals such as magnesium and ferric compounds, so they’re denser,) while continental crust largely made up of felsic igneous rocks (richer in lighter minerals such as feldspars and silica.) Both have generous sprinklings of sedimentary and metamorphic rocks.

Being denser, mafic rocks float deeper into the mantle, so they sink below the waterline. Felsic rocks, being less dense, rise above their mafic counterparts and break the waterline, forming land. That’s how they get their names — mafic/oceanic crust naturally bobs below the waterline and forms oceans, felsic/continental crust is light enough to potentially form dry land.

The first step in the Wilson Cycle starts with a single tectonically-stable core (a craton) containing all felsic material surrounded by oceanic crust and, well, the ocean. Since all felsic material is contained here, this core can’t get any lighter or heavier so it stays at a perfect isostatic equilibrium (at a constant level on top of the mantle). There’s no more tectonic movement to form mountains, so erosion has had time to level this continent dead-flat, almost down to the waterline, all over its surface — a state referred to as a peneplane. There’s no volcanic activity and no earthquakes. It’s actually quite uneventful, even for what’s basically a pile of rocks.

Step two: The Rifting

Powered by the heat trapped in our planet’s interior, an incredibly hot and extremely powerful jet of molten material known as a mantle plume shoots upwards from the core and begins to eat into the craton. What happens next resembles a blowtorch melting through a slab of metal, but the material this plume is made of also has a lot of weight and speed, meaning it will also push against the rocks at the surface.

Cratons West Gondwana.

An estimate of how the West Gondwana Craton looked like before it split off in Africa and South America. The WGC itself split off from a larger craton earlier throgh similar processes.
Image credits Woudloper / Wikimedia.

The rocks above the plume start to swell from heat dilation, pushing neighboring rocks aside and forming a dome on the surface. At the same time, the underside of the craton begins to melt — you can see a version of this happening in Yellowstone today. The combined effect is that the crust stretches and thins until it eventually fractures under mechanical stress. The fracturing usually follows three different directions, a triple junction, most often asymmetrical and diverging at odd angles from one another. Molten material from the underside rises up and forms surface vulcanism.

Step three and four: Divergence and Oceanification

These fractures are deep — they go all the way through the crust. Sometimes, as seems to be the case with Yellowstone, plumes just die off or are otherwise blocked in the mantle, and that’s that. But if it keeps going, the fractures can become longer and widen until there’s actually no more crust left in the area, forming a rift valley. Nature can’t have that, so water rushes in to fill the gap and hardens the magma into a ‘transition’ crust, which mixes pre-existing felsic and newly minted mafic rocks. Convection cells created by the plume slowly inch the neighboring slabs of rock apart, meaning more empty space, more oceanic crust formed here, and eventually, you have a full blow ocean on your hands.

So there’s an important thing to keep in mind here: rifting breaks continents apart and creates oceans.

Rifting ocean.

This is how oceans are born.
Image credits Hannes Grobe / Alfred Wegener Institute for Polar and Marine Research.

As these two continents float away from the rift, the new transition and oceanic crust cool down (becoming denser) and sink lower in the mantle. This process also pulls the original continental material from the rifting area that they’re attached to from about 3-5 km above sea level to roughly 14 km below the waterline in 100-110 million years. Some consider this to be a separate step in the process but that’s more of a geological technicality. I’ve mentioned it so you can get an idea of how much heat dilation affects the buoyancy of rocks (and it’s effect in step 1.)

Step Five: Convergence

The more astute of you may be suspecting that the Earth is, in fact, a pretty round object. Now it’s time for our thinking caps to come on.

What happens on one pole of the Earth when there’s a rift actively pumping out new crust on the opposite one for a few million years? Well, it either starts looking like a pug’s face (please be this one) or, or, it can stay round by recycling old crust back into the mantle to make room for the new one. Let’s check.

Earth_MODIS_image.

Damnit, it’s round!
Image credits NASA.

Divergence (with the creation of new crust) will eventually stop as the plume dies off. But it can go on for tens, even hundreds of millions of years, creating a lot of material before that happens. So some old crust needs to crack and subduct into the mantle to make room for all the new stuff. This doesn’t have to happen on the opposite pole, but it has to happen somewhere.

This is the second, closing half of the cycle. As a rule, it’s always oceanic crust that breaks and starts to subduct, since continental crust is just too floaty (in magma that is, not water.) So this subduction zone forms somewhere underwater since that’s where oceanic crust tends to hang around. But wherever it happens you’ll be able to spot it — there’ll be either an island arc nearby, formed by the top plate or a pre-existing continental edge. Part of the subducting crust is dragged down into a trench (about 1-2 km below the normal ocean floor,) and will heat up while it slides into the mantle. At about 120 km deep it’ll begin melting back into magma.

Step six: Re-continentalization

Let this process roll for long enough and all the old oceanic crust will subduct and continents will bump back into each other. There are a few more things taking place between S5 and S6, such as new mountains forming as the cratons get smushed, the creation of new subduction zones, but again, geological technicality.

This new continent will mix pieces of the old craton, bits of transition and oceanic crust (which will tend to sink to the bottom during suture,) and metamorphic rocks it picked up along the way.


In many ways, the second half of the cycle acts like the first one in reverse. It’s often less clear-cut than what I’ve told you about here: new rifts can form opposite the ‘old’ one, fracturing the continents even more. And while the effects of these two halves can become apparent hundreds, thousands of years apart, they take place simultaneously — when there’s new crust being formed, the old crust has to be consumed somewhere else. And it happens all the time. There’s constant creation and destruction of land going on on Earth, each day, right below our feet. Well, right below our feet and a few thousand kilometers out at sea, but you get what I mean.

The Wilson Cycle is the ultimate matchmaker and the end-all homewrecker. Give it enough time, and this process will, through sheer trial and error, bring all the land together, then break it up all over again.

Red Skies.

Silica rains helped form Earth’s crust four and a half billion years ago

Earth’s crust may have been formed in part by atmospheric chemicals which settled on the surface as the planet cooled, McGill University researchers report.

Red Skies.

Image credits David Mark.

We know that about 4.5 billion years ago, a planetoid roughly the same size as today’s Mars slammed into early Earth with enough force to melt the whole thing into a ball of magma. The event was so violent that we believe it led to the formation of the Moon and altered the chemical composition of our planet into the iron-rich Earth we know and love today.

Conventional wisdom holds that following this impact, the Earth gradually cooled down and the outer surface of this ball of lava hardened into a crust — in other words, the rocks on the planet’s surface are igneous in origin. But Don Baker and Kassandra Sofonio, a team of earth scientists from McGill University, say that the event played a direct hand in forming the planet’s modern crust. According to their theory, some of the chemical components we see in the crust today were deposited from the super-heated atmosphere left in the wake of the impact.

Igngaseous

Largely speaking, Earth’s crust comes in two flavors: oceanic and continental. Oceanic crust is the stuff plates are formed of, the rocks that cool from magma at mid-ocean rifts (they are igneous) then get subducted and recycled on the other side of the plate. It’s usually pretty thin and it’s what the ocean floor rests on.

Continental crust is the stuff that we actually live on. These thicker slabs of rock form on top of oceanic crust and reach high enough altitudes (usually) to form continents above water — hence their name. The rocks that go into continental crust can come from many different places, but what’s important now is that more than 90% of these rocks are estimated to be formed from silica-rich minerals, such as feldspar and quartz. Which, as you may have guessed, adds up to a lot of silica.

So where did all this silica-rich crustal material come from? The duo says that the collision 4.5 billion years ago turned the atmosphere into high-temperature steam which dissolved the rocks in the surface into a gaseous solution.

“These dissolved minerals rose to the upper atmosphere and cooled off, and then these silicate materials that were dissolved at the surface would start to separate out and fall back to Earth in what we call a silicate rain,” Baker says.

To test their theory, the team recreated the conditions of early Earth in the lab. They used a mix of bulk silicate materials and water which was enclosed in gold palladium capsules, then heated to 727 degrees Celsius (1340 Fahrenheit) at 100 atm to simulate conditions in the atmosphere about 1 million years after the moon-forming impact.

Using previous work on rock-water interactions at high pressure as a starting point, the team successfully recreated a “surprisingly similar” material to the Earth’s modern crust. The authors believe that following the impact, surface silicate rocks would dissolve and separate, rising to the upper layers of the atmosphere. Here, they cooled off enough to crystallize and fall back to Earth in a “silicate rain.” Sofonio christened the process “aerial metasomatism.”

One surprising implication of the paper is that it could provide researchers with a better understanding of how to spot planets fit for human habitation, or even those that harbor alien life.

“This time in early Earth’s history is still really exciting,” he adds. “A lot of people think that life started very soon after these events that we’re talking about. This is setting up the stages for the Earth being ready to support life.”

The paper “A metasomatic mechanism for the formation of Earth’s earliest evolved crust” has been published in the journal Earth and Planetary Science Letters.

earth mantle layer

Scientists discover another layer in the Earth’s mantle

Most people tend to think of the Earth in terms of crust, mantle and core, and while those are indeed the largest “layers” (you can’t properly call the mantle a layer though), each one of them is made from other, thinner layers. Now, researchers from the University of Utah have identified another one of these thinner layers, 930 miles beneath our feet.

earth mantle layer

An illustration of a slab of rock sinking through the upper mantle above, through the boundary between the upper and lower mantle at 410 miles depth, then stalling and pooling at a depth of 930 miles. Image credits: University of Utah.

“The Earth has many layers, like an onion,” study co-author Dr. Lowell Miyagi, an assistant professor of geology and geophysics at the University of Utah, said in a written statement. “Most layers are defined by the minerals that are present. Essentially, we have discovered a new layer in the Earth. This layer isn’t defined by the minerals present, but by the strength of these minerals.”

Their finding actually has some significant implications. Geophysicists had previously observed that many slabs that are otherwise active and “move around” inside the Earth get stuck at around 930 miles underground – a phenomenon thought to lead to seismic activity and increased volcanic activity. This layer seems to be the cause.

“The result was exciting,” Miyagi said in the statement. “In fact, previous seismic images show that many slabs appear to ‘pool’ around 930 miles, including under Indonesia and South America’s Pacific coast. This observation has puzzled seismologists for quite some time, but in the last year, there is new consensus from seismologists that most slabs pool.”

Miyagi holding a press that houses the diamond anvil, in which minerals can be squeezed at pressures akin to those deep within the Earth.

Oceanic plates are denser and when they collide with continental plates, they typically subduct, triggering earthquakes and volcanism. The subduction process takes place at very long times even geologically, with some estimates putting a 300 million years time stamp for a full subduction. Miyagi and fellow mineral physicist Hauke Marquardt, of Germany’s University of Bayreuth wanted to see why some slabs dive all the way through the mantle, and some stop. This is important because when they stop, they “bump”, potentially creating a deep earthquake.

“Anything that would cause resistance to a slab could potentially cause it to buckle or break higher in the slab, causing a deep earthquake,” Miyagi said.

They used a device a diamond anvil to simulate how the mineral ferropericlase reacts to high pressure – they squeezed thousands of crystals of ferropericlase at pressures up to 960,000 atmospheres. They focused specifically on ferropericlase, a magnesium/iron oxide virtually inexistent on the surface, but one of the main constituents of the lower mantle. They found that the stiffness (or viscosity) of the mineral increased threefold by the time it was subjected to pressure equal to what’s found in the lower mantle (930 miles below Earth’s surface) compared to the pressure at the boundary of the upper and lower mantle (410 miles beneath the surface). The results were quite surprising because it was thought that viscosity varies only slightly in the depths of the mantle.

Miyagi says the stiff upper part of the lower mantle also may explain different magmas seen at two different kinds of seafloor volcanoes and may mean that the lower mantle is hotter than previously believed.

 

Scientists find direct evidence that CO2 heats the Earth’s crust

When we’re talking about CO2 emissions and global warming, we generally mean atmospheric CO2 – where the gas is spewed and generates the greenhouse effect, warming our atmosphere and subsequently, our planet. But a new study conducted by US researchers found that CO2 actually warms the Earth’s crust directly; the more CO2 we emit, the hotter our planet will get.

CO2 pollution warms the Earth’s crust – directly. Image via Pixabay.

The study, funded in part by the U.S. Department of Energy’s Office of Science, utilized data from the NOAA CarbonTracker between 2000 and 2010 (measurement site was in Oklahoma); the tracker took measurements almost everyday, and in total, scientists worked with 8,300 readings altogether. A second research facility in Alaska provided 3,300 measurements during this time span. Despite the two sites being so different in terms of economic development and overall pollution levels, results were very coherent in terms of CO2 pollution; they both indicate that CO2 levels are increasing due to human activity, and furthermore, that it’s heating up our crust.

“We see, for the first time in the field, the amplification of the greenhouse effect because there’s more CO2 in the atmosphere to absorb what the Earth emits in response to incoming solar radiation,” lead author Daniel Feldman of University of California, Berkeley said in a press release. “Numerous studies show rising atmospheric CO2 concentrations, but our study provides the critical link between those concentrations and the addition of energy to the system, or the greenhouse effect.”

The team used highly precise spectroscopic instruments to measure the thermal infrared energy that travels down through the atmosphere to the surface. Using this type of equipment, they can actually detect the unique spectral signature of infrared energy from CO2. In other words, they can see just how much CO2 alone is contributing to warming the crust.

“We measured radiation in the form of infrared energy. Then we controlled for other factors that would impact our measurements, such as a weather system moving through the area,” says Feldman.

They also detected the influence of photosynthesis on the balance of energy at the surface. They found that CO2-related warming drops down in the spring, as plants bloom and they absorb more CO2 from the atmosphere.

It’s not clear exactly how much this radiation contributes to global warming, but it is a clear indication that human-emitted CO2 is warming our planet in more ways than previously estimated.

Journal Reference: D. R. Feldman, W. D. Collins, P. J. Gero, M. S. Torn, E. J. Mlawer & T. R. Shippert. Observational determination of surface radiative forcing by CO2 from 2000 to 2010. Naturedoi:10.1038/nature14240

The Thickest Layer of the Earth

The Earth can be divided into four main layers: the solid crust on the outside, the mantle, the outer core and the inner core. Out of them, the mantle is the thickest layer, while the crust is the thinnest layer.

The Earth’s structure

Artistic depiction of the Earth's structure. Image via Victoria Museum.

Artistic depiction of the Earth’s structure. Image via Victoria Museum.

The Earth’s structure can be defined in several ways, but general, we see the Earth as having a solid crust on the outside, an inner and an outer core, and the mantle in between. The crust’s thickness varies between some 10 km and just over 70 km, having an average of about 40 km. The core has, in total, a radius of 3500 km, but it is generally viewed as two distinct parts:

  • the solid inner core, with a radius of 1220 km
  • the viscous outer core, with a radius of 2300 km

The mantle’s thickness is about 2900 km – so if you consider the Earth’s core as one big thing, then the core is the “thickest layer” (though has a bigger radius is probably a better way of saying it) – but the idea of a separate outer and inner core is generally accepted.

The Mantle – thickness and composition

The mantle comprises about 83% of the Earth’s volume. It is divided into several layers, based on different seismological characteristics (as a matter of fact, much of what we know about the mantle comes from seismological information – more on that later in the article). The upper mantle extends from where the crust ends to about 670 km. Even though this area is regarded as viscous, you can also consider it as formed from rock – a rock called peridotite to be more precise. A peridotite is a dense, coarse-grained igneous rock, consisting mostly of olivine and pyroxene, two minerals only found in igneous rocks.

Peridotite, as seen on the Earth’s surface. Image via Pittsburgh University.

But it gets even more complicated. The crust is divided into tectonics plates, and those tectonic plates are actually thicker than the crust itself, encompassing the top part of the mantle. The crust and that top part of the mantle (going 00 to 200 kilometers below surface, is called the asthenosphere. Scientific studies suggest that this layer has physical properties that are different from the rest of the upper mantle. Namely, the rocks in this part of the mantle are more rigid and brittle because of cooler temperatures and lower pressures.

Below that, there is the lower mantle – ranging from 670 to 2900 kilometers below the Earth’s surface. This is the area with the highest temperatures and biggest pressures, reaching all the way to the outer core.

Mantle Trivia: Even though you can consider the mantle as molten rock or magma, modern research found that the mantle has between 1 and 3 times more water than all the oceans on Earth combined.

How can we study the mantle?

Waves propagating from Earthquakes through the Earth. Image via Brisith Geological Survey.

Waves propagating from Earthquakes through the Earth. Image via Brisith Geological Survey.

Pretty much all the practical geology we do takes places at the crust. All the rock analysis, the drilling… everything we do is done in the crust. The deepest drill ever is some 12 km below the surface… so then how can we know the mantle?

As I said earlier, most of what we know about the mantle comes from seismological studies. When big earthquakes take place, the waves propagate throughout the Earth, carrying with them information from the layers they pass through – including the mantle. Furthermore, modern simulations in the lab showed how minerals likely behave at those temperatures and pressures, and we also have indirect gravitational and magnetic information, as well as studies on magma and crystals found on the surface. But the bulk of the information comes from seismic analysis.

Image via Wiki Commons.

Seismic waves, just like light waves, reflect, refract and diffract when they meet a boundary – that’s how we know where the crust ends and where the mantle begins, and the same goes for the mantle and the core. The waves also behave differently depending on different properties, such as density and temperature.

In the mantle, temperatures range between 500 to 900 °C (932 to 1,652 °F) at the upper boundary with the crust; to over 4,000 °C (7,230 °F) at the boundary with the core. Thanks to the huge temperatures and pressures within the mantle, the rocks within undergo slow, viscous like transformations  there is a convective material circulation in the mantle. How material flows towards the surface (because it is hotter, and therefore less dense) while cooler material goes down. Many believe that this convection actually is the main driver behind plate tectonics.

Mantle convection may be the main driver behind plate tectonics. Image via University of Sydney.

Another interesting fact about the mantle: Earthquakes at the surface are a result of stick-slip faulting; rocks in the mantle can’t fault though, yet they sometimes generate similar earthquakes. It’s not clear why this happens, but several mechanisms have been proposed, including dehydration, thermal runaway, and mineral phase change. This is just a reminder of how little we still know about our planet: we’ve only scratched the surface of the thinnest layer, the crust.

Deep lying bacteria found, reproduce only once in 10.000 years

A surprisingly diverse range of life forms exists deep in the oceanic crust, but they live at an extremely slow pace. Long lived bacteria, which reproduce only once in 10.000 years, have been found in rocks 2.5km below the ocean floor, rocks which are 100 million years old. Viruses and fungi have also been found in the same conditions.

iodp1

Aside from its intrinsic value, the discovery raises some significant questions, regarding how life can persist under such extreme conditions of temperature, pressure, and apparent lack of nutrients. Scientists from the Integrated Ocean Drilling Program have announced the findings at the Goldschmidt conference, in Florence, Italy.

It’s not the first time the Integrated Ocean Drilling Program has come up with exciting results – in 2012, they set a new record for scientific ocean drilling, and in March this year, they reported the first case of bacteria living in the oceanic crust. Now, Fumio Inagaki of the Japan Agency for Marine-Earth Science and Technology explained that the microbes exist in very low concentrations – around 1000 in every teaspoon of sample, compared to the billions or trillions which you would get in the same amount of surface material.

iodp2

Just as interesting, they found that not only do viruses also exist at these depths, but they significantly outnumber the bacteria – 10 to 1, and even more as you go deeper. This offers some important information on what we know on viruses.

“We’re pushing the boundaries of what we understand about how viruses cycle on Earth elsewhere, by studying them in the deep biosphere” Dr Beth Orcutt of Bigelow Laboratory for Ocean Sciences in Maine, US, explained.

Alive… or just undead?

The characteristics of these specimens make researchers question if they even are alive.

“One of the biggest mysteries of life below the sea floor is that although there are microbes down there it’s really hard to understand how they have enough energy to live and how incredibly slowly they are growing.

“The deeper we look, the deeper we are still finding cells, and the discussion now is where is the limit? Is it going to be depth, is it going to be temperature? Where is the limit from there being life to there being no life?”

They are reproducing so rarely that it’s very much unlike anything science has encountered so far.

“The other question we have is that even though we are finding cells, is it really true to call it alive when it’s doubling every thousands of years? It’s almost like a zombie state,” Dr Orcutt commented.

A reproductive cycle of 10.000 years is indeed a few magnitudes of order higher than anything we know, but these microbial communities which inhabit the deep earth are alive by every definition, and they may very well change what we think about life itself.

Understanding magma in the mantle: rocks melt at greater depth than previously thought

Magma forms much deeper than geologists previously believed, according to a new study conducted by Rice University.

Magma and Crust

 

mantle

The group led by geologist Rajdeep Dasgupta put very small samples of peridotite under very large pressures, to find out if the rock can liquify, at least in small amounts, as deep as 250 km beneath the ocean floor. Peridotite is the dominant rock of the Earth’s mantle above a depth of about 400 km, and it was previously believed that the mineral doesn’t liquify above that depth.

mantle structure

The Rice team focused on mantle beneath oceans because that is where the crust is typically formed; in a much-simplified version, the silicate melts (magma) rise with the convective currents, cool as they reach the crust and then solidify to create new crust. The starting point for melting has long been thought to be at 70 kilometres beneath the seafloor.

 

Geophysics

 

In order to determine the mantle’s density and properties, geophysicists used seismic information; seismic waves from earthquakes travel the globe much like sound waves, and by measuring their speed, we can estimate some the medium’s properties. These waves travel faster in solids (especially in denser solids), and slower in liquids; the first questions arose here.

Seismologists have observed anomalies in their velocity data as deep as 200 kilometers beneath the ocean floor,” Dasgupta said. “Based on our work, we show that trace amounts of magma are generated at this depth, which would potentially explain that.”

seismic wave

So it didn’t really add up – and this wasn’t the only clue. Geophysicists have also struggled to explain the bulk electrical conductivity of the oceanic mantle – something which was observed but couldn’t really be figured out.

“The magma at such depths has a high enough amount of dissolved carbon dioxide that its conductivity is very high,” Dasgupta said. “As a consequence, we can explain the conductivity of the mantle, which we knew was very high but always struggled to explain.”

The thing is, we cannot really dig down to the mantle – this is miles away from happening, both figuratively and literally, so we have to rely on indirect measurements (seismology, electric measurements, etc), lab experiments and surface extrapolations. Another interesting they found in this experiment was that carbonated rocks melt at significantly lower temperatures than non-carbonated.

“This deep melting makes the silicate differentiation of the planet much more efficient than previously thought,” Dasgupta said. “Not only that, this deep magma is the main agent to bring all the key ingredients for life — water and carbon — to the surface of the Earth.”

Volcanic windows

However, Dasgupta believes that volcanic rocks are the key to understanding our planet’s mantle.

“Our field of research is called experimental petrology,” he said. “We have all the necessary tools to simulate very high pressures (up to nearly 750,000 pounds per square inch for these experiments) and temperatures. We can subject small amounts of rock samples to these conditions and see what happens.”

A surfaced volcanic rock - peridotite

A surfaced volcanic rock – peridotite

To subject the rocks to these hellish conditions, they use massive hydraulic presses.

“When rocks come from deep in the mantle to shallower depths, they cross a certain boundary called the solidus, where rocks begin to undergo partial melting and produce magmas,” Dasgupta said. “Scientists knew the effect of a trace amount of carbon dioxide or water would be to lower this boundary, but our new estimation made it 150-180 kilometers deeper from the known depth of 70 kilometers,” he said.

These findings have major implications for all planetary sciences:

“What we are now saying is that with just a trace of carbon dioxide in the mantle, melting can begin as deep as around 200 kilometers. And when we incorporate the effect of trace water, the magma generation depth becomes at least 250 kilometers. This does not generate a large amount, but we show the extent of magma generation is larger than previously thought and, as a consequence, it has the capacity to affect geophysical and geochemical properties of the planet as a whole.”

The paper will be published this week in Nature