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Earth didn't always have the land beneath your feet, but what might have caused it to form is a bit of a mystery.

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[ ♪ intro ] The fact that Earth has land  seems like it should be a given.

Like, the sky is blue, cats puke  on the carpet, and Earth has land. But that hasn’t always been the case.

And how all this land formed in the first place… that’s a bit of a mystery. Geologists are still working on the answer, but a study published in 2022  suggests we might have rocks from space to thank for the land beneath our feet! If you drained all the water off the Earth, you’d see that the outer layer  of our planet, the crust, is like a collection of puzzle pieces.

They’re called tectonic plates, and  they sit on a layer called the mantle. Generally, tectonic plates fall  into one of two categories. You have oceanic pslates,  which are usually underwater, and are mostly made of a rock called basalt.

And you have continental plates, which  make up the majority of the land on Earth, and are mostly made of granite. The critical piece here is that  granite is less dense than basalt. This means continental plates are  more buoyant than oceanic plates.

As a result, they’re able to poke  up above sea level and give us land, which is important for our  mostly non-aquatic lifestyles! But about 4.5 billion years ago, shortly after the planet formed,  continental plates didn’t exist. The Earth’s crust was a single  piece of rock, like a shell.

And that shell looked more like an  oceanic plate than a continental one. So, for us to get the world we see  today, continental plates needed to form. And for that, we needed granite.

Today, you can find the right conditions for  making granite at the base of continents, where an oceanic plate slips  underneath a continental one. There, it melts and forms granite. But you can probably see the  chicken and egg problem coming here.

If you need granite to make a continental plate, and granite is formed at the base of continents… how did the first continental plate form? Where did land come from? Generally, researchers believe that if  something caused one spot on an oceanic plate to grow and thicken into a huge mass of basalt, that mass would sit so deep in  the mantle that temperatures would be high enough for its base to melt.

And from there, you’d have the  right conditions to form granite. But what was that something? The standard idea is that plumes of magma came up from the mantle and caused an area of  the crust to grow and thicken from below.

Basically, some of the rocks that had  been on Team Mantle joined Team Crust. But another idea, supported  by a 2022 research paper, is that the culprit may have come from above. The idea is that a series of  enormous asteroid or comet impacts could have fractured Earth’s shell and excavated a huge amount  of material from craters.

Removing all this material would make the  pieces of that shell lighter and more buoyant, relieving pressure on the mantle below. Rock in the mantle would  have then risen up, melted, and thickened the crust. Conveniently, geologists  do have evidence that Earth was hit with a barrage of asteroids at roughly  the same time that the continents emerged. lBut solid evidence that these impacts started  continent formation has been hard to find.

In the last four-plus billion years, much of Earth’s rock has melted and  been recycled into something else. But luckily, a few tough crystals are  still around that preserve a record   of Earth’s earliest days. They’re called zircons.

In their 2022 paper, researchers looked at zircons from a piece of ancient continental  crust in western Australia. Specifically, they looked at how the  amounts of different oxygen isotopes within the zircons changed over time. An isotope is a lighter or  heavier version of an element.

In this case, a tiny percentage of Earth’s oxygen is a little heavier than normal because  the atoms have an extra two neutrons. Heavy oxygen behaves differently  in many reactions and processes , so different kinds of crust or the mantle  have different amounts of these isotopes. That means that measuring the  amount of heavy oxygen in zircons can give you a sense of what kind of  material that crystal formed from.

And the oxygen in these zircons  told a surprising story. Between 3.6 and 3.4 billion years ago, there were sudden drops in how much heavy  oxygen the zircons of the time had inside them. And usually, you see that when  surface water has reacted with rock, not when material in the mantle is rising up.

So, the scientists interpreted  this to mean that around this time, impacts cracked and melted Earth’s crust  and allowed water to flow through it. Then, between 3.4 and 3 billion years ago, zircon crystals formed with  moderate amounts of heavy oxygen. That matches the heavy oxygen amounts that crust has when it forms from  melted material in the mantle.

So, maybe the mantle had risen up by this point, but after an impact got the ball rolling. Finally, 3 billion years ago, the amount of heavy oxygen in the  zircons increased to even higher levels. This indicated that the crust that already  existed was melting and being recycled.

This might mean that a continent  had finally formed by this point, but the team isn’t absolutely sure. In any case, the key detail in these zircons is that the crust seemed to  break before the mantle rose up. And researchers took this as evidence that impacts  from space likely kickstarted continent formation!

In general, impacts often  get a bad rep here on Earth. I mean, look what happened  to the non-avian dinosaurs. But this study provides evidence  that without impacts, those dinos — and all of us — might not have had any land to live on.

That said, it often takes more than one paper to convince every researcher that  a new hypothesis is the best one. So, the team plans to investigate  samples from other parts of the world, to see if there are similar patterns in  ancient continental crust beyond Australia. With every new study, we’ll  get closer to the answer, and to a better understanding of how Earth  became the place we call home, land and all.

Thanks for watching this episode of SciShow Space. If you’re interested in absorbing  even more awesome geology knowledge into your brain sponge you can check out this video on  that period in Earth’s history when the North America tried to rip itself apart. [ ♪ outro ]