Tag Archives: Oceanic

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.

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.


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.