continents

A Study Tells the Truth About How the First Continents Formed. It’s Not What You Thought.

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Turns out there was a lot of burying and melting going on at the beginning of the world.

world map with paper cut effect on blue background

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  • Scientists know that the world’s continents emerged during the Archean eon, which stretched from about 4 to 2.5 billion years ago. But how they formed is a tougher question.
  • A new study from the University of British Columbia reveals that magmatism of Earth’s current crust’s precursor formed three rocks—tonalite, trondhjemite and granodiorite, known collectively as TTGs—that likely formed the first continents.
  • This idea would rule out that meteorites or subduction zones were required to form TTGs and, subsequently, continents.

The formation of the Earth’s continents occurred during a fiery afterbirth known as the Archean Eon, which stretched from 4 billion to 2.5 billion years ago. It was in this bubbling cauldron of magmatic activity that geologic forces created the world’s first continents. While geologists have narrowed down the “when” of continental formation, the “how” is still a pretty big mystery.

That’s because the origin story of Earth’s continents is a bit of a chicken-or-egg question: Did the first continents form as the result of primordial subduction zones, or did existing ancient land masses produce the first subduction zones? In a new paper published last week in the journal Nature Communications, scientists from the University of British Columbia tipped the scales in the direction of the latter.

Three types of granitoid rocks are central to this geologic origin story—tonalite, trondhjemite, and granodiorite—which together are known as TTG. Evidence of Archean TTG rock can be found throughout the world, and even in your home, as varieties of this type of rock can be found in kitchen countertops. But exactly where did this early continental rock come from?

“We tracked a specific set of trace elements that aren’t affected by alteration and pristinely preserve signatures from the original magma that made new TTG crust,” lead author Matthijs Smit, an associate professor the University of British Columbia’s Department of Earth, Ocean and Atmospheric Sciences, said in a press statement. “These elements allowed us to look back through the chemical changes that TTG magmas go through and trace the melt compositions back to their initial state and source.”

Smit’s team discovered that TTG rocks (and other rocks associated with them) formed from a “slow burial, thickening, and melting of precursor crust.” This ancient proto-crust likely resembled the oceanic plateaus of today, and because this crust kept being buried, the base had no choice but to melt. And voilà: TTGs were created, which then formed the Earth’s first continents.

According to the scientists, the discovery of this standalone “intra-crustal” formation disproves the idea that TTGs could’ve formed from the geologic crust-making factories known today as subduction zones. So if subduction zones are the chicken and TTG magmatism is the egg, it turns out the egg came first (which also happens to be true of chickens).

“We show that these things may actually not be directly related,” Smit said. “The recognition of the type of source rock makes this leap possible and also takes away the need to have other mechanisms, such as meteorite impact, to explain the growth of the first real continents.”

Massive asteroid may have kickstarted the movement of continents

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Earth was still a violent place shortly after life began, with regular impactors arriving from space. For the first time, scientists have modelled the effects of one such violent event – the strike of a giant asteroid. The effects were so catastrophic that, along with the large earthquakes and tsunamis it created, this asteroid may have also set continents into motion.

That is probably an underestimate.

The asteroid to blame for this event would have been at least 37km in diameter, which is roughly four times the size of the asteroid that is alleged to have caused the death of dinosaurs. It would have hit the surface of the Earth at the speed of about 72,000kmph and created a 500km-wide crater.

At the time of the event, about 3.26 billion years ago, such an impact would have caused 10.8 magnitude earthquakes – roughly 100 times the size of the 2011 Japanese earthquake, which is among the biggest in recent history. The impact would have thrown vapourised rock into the atmosphere, which would have encircled the globe before condensing and falling back to the surface. During the debris re-entry, the temperature of the atmosphere would have increased and the heat wave would have caused the upper oceans to boil.

Donald Lowe and Norman Sleep at Stanford University, who published their research in the journalGeochemistry, Geophysics, Geosystems, were able to say all this based on tiny, spherical rocks found in the Barberton greenstone belt in South Africa. These rocks are the only remnants of the cataclysmic event.

According to Simon Redfern at the University of Cambridge, there are two reasons why Lowe and Sleep were able to find these rocks. First, the Barberton greenstone belt is located on a craton, which is the oldest and most stable part of the crust. Second, at the time of the event, this area was at the bottom of the ocean with ongoing volcanic activity. The tiny rocks, after having been thrown into the atmosphere, cooling, and falling to the bottom of the ocean, then ended up trapped in the fractures created by volcanic activity.

This impact may have been among the last few major impacts from the Late Heavy Bombardment period between 3 and 4 billion years ago. The evidence of most of these impacts has been lost because of erosion and the movement of the Earth’s crust, which recycles the surface over geological time.

However, despite providing such rich details about the impact, Lowe and Sleep are not able to pinpoint the location of the impact. It would be within thousands of kilometres of the Barberton greenstone system, but that is about all they can say. The exact location may not be that important, Lowe argued: “With this study, we are trying to understand the forces that shaped our planet early in its evolution and the environments in which life evolved.”

One of the most intriguing suggestions the authors make is that this three-billion-year-old impact may have initiated the the movement of tectonic plates, which created the continents that we observe on the planet.

The continents ride on plates that make up Earth’s thin crust; the crust sits on top of the mantle, which is above a core of liquid iron and nickel. The heat trapped in the mantle creates convection, whichpushes against the overlying plates.

All the rocky planets in our solar system – Mercury, Venus, Earth and Mars – have the same internal structure. But only Earth’s crust shows signs of plate motion.

A possible reason why Earth has moving plates may be to do with the heat trapped in the mantle. Other planets may not have as much heat trapped when they formed, which means the convection may not be strong enough to move the plates.

However, according to Redfern: “Even with a hot mantle you would need something to destabilise the crust.” And it is possible that an asteroid impact of this magnitude could have achieved that.

The Conversation