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  • Earth has land.

  • You know thatthe odds are pretty good you're on some of it right now.

  • But here's a weird thing to think about: It's possible that land didn't always exist.

  • And technically-speaking, it doesn't /have/ to.

  • I mean, if you were just to simply smooth out the Earth's crust, the oceans contain enough water to

  • cover the planet in a sea more than two kilometers deep.

  • Sowhy does land exist?

  • Why is it so varied, with all those mountains and valleys and flat plains?

  • Andhere's a fun one: Could anything ever get rid of land on Earth?

  • To answer those questions, which god knows I want to do, you need to travel back more

  • than four billion years.

  • Billions of years ago, Earth started as a cloud of dust and grains left over from the Sun's formation.

  • Then, over time, those pieces slowly balled together into proto-Earth.

  • That ball was made of all kinds of elements.

  • And as it aged, the denser ones, like iron, sank towards the center of the ball to become

  • Earth's core, while lighter ones stayed towards the outside.

  • Eventually, the planet separated into the layers we know today: the inner and outer

  • core, the gooey mantle, and the crust.

  • These days, there are two main kinds of crustcontinental and oceanicand they're

  • made of different ingredients.

  • Oceanic crust tends to be mostly a type of

  • rock called basalt and contains more heavy compounds.

  • And continental crustwhich generally makes up landtends to be mostly granite

  • and contains more relatively light compounds.

  • But the composition of the /early/ crust and how it changed in Earth's first billion

  • years or so is pretty hard to pin down.

  • Like a lot of Earth's early history, we just don't have muchor in some cases /any/ — physical evidence.

  • It's pretty much all been recycled and destroyed by now.

  • So there are a lot of interpretations.

  • But mostly, models seem to start with a crust that would more resemble oceanic crust today,

  • with continental crust slowly growing over time.

  • The exact date when the first continental crust appeared is one of the big questions in geoscience.

  • Some models say it started growing almost immediately; others say it didn't really get

  • going until about 3.6 billion years ago.

  • But there's something potentially really interesting hidden in there.

  • Because, depending on which of these models is right, early Earth might have been a /water world./

  • We think the oceans had to have existed by around 3.8 billion years ago.

  • That's based on evidence like ancient pillow lavas dated to around that time, which only

  • form when lava flows into water.

  • So if continental crust hadn't formed by then, there would have been a point in time

  • at which the Earth was, indeed, an ocean worldwhere land did not exist.

  • So, like, don't take land for granted!

  • We could all be fish!

  • As for why continental crust started forming, there are a couple of ideas.

  • One of the most well-studied relies on the movement of tectonic plates, the big slabs

  • that make up Earth's crust.

  • And it goes like this:

  • At some point, the idea says, the crust started to form into these giant plates, possibly

  • thanks to massive magma plumes from deep within the Earth.

  • and as the plates started pushing against each other, some of them began sliding down towards

  • the mantle in a process called subduction.

  • And as that happened, the increased heat near and in the mantle began to heat the rock.

  • But since rock isn't completely homogenous, it's not like it all melted at once.

  • Instead, different chemicals started to liquify at different rates, and the rock /separated/

  • in a process known as partial meltingwith some areas being denser, and others less dense.

  • Over time, this process repeated, and we ended up with new oceanic crust and the first continental crust material.

  • That material was brought up through volcanic eruptions, which then built up into small

  • volcanic islands above the ocean.

  • The very first land!

  • Peeking its little head above the water!

  • And as more volcanoes erupted and material got scraped off subducting plates, these islands

  • would have grown over time into larger continents.

  • Of course, like I said earlier, studying the beginning of Earth's history is hard, so

  • not all scientists agree that subduction was necessary to build the first continents.

  • Like, in 2012, one group proposed something a little morelike, ooze-y.

  • They got this idea while looking at rocks

  • from the Isua Greenstone Belt in Greenland, which are more than 3.5 billion years old.

  • They compared the amount of trace elements found in those rocks to amounts we'd expect

  • to see if they were formed by subduction And they concluded that this ancient crust may

  • not have needed to get /all/ the way down into the mantle via subduction to melt and reform.

  • Instead, it might have kind of /oozed/ up as rocks melted higher up, in the crust.

  • So, no subduction zone needed.

  • No matter how this occurred, though, eventually the Earth /did/ get its first continent.

  • Based on various pieces of evidence, some

  • researchers have proposed that this continent, which they call Vaalbara, was made of rocks

  • that are today found in Southern Africa and Australia.

  • While others favor Ur, a land mass made up of what

  • would today be parts of India, Madagascar, and Australia.

  • In any case, land happened.

  • And so far as we can tell, Earth has had it ever since.

  • Since the time of Ur and Vaalbara, plate tectonics and other forces have kept continents above

  • water and made them even craggier.

  • These days, new continental crust is still being formed and destroyed at subduction zones.

  • And plate collisions have also pushed up mountains, like in the Himalayas, making the Earth even less smooth.

  • Meanwhile, erosion and other processes have also played a part, with wind and rain carving

  • canyons, arches, and other amazing landscapes.

  • So, no matter how it got here, the land hasn't been unchanging and still.

  • It's continually shaped, changed, and even sometimes destroyed or completely hidden by

  • forces of nature.

  • And that makes you wonder: If all these forces are still at play, reshaping the landscape all the time

  • Well, could those forces ever make land disappear?

  • Well the good news is, continental crust is usually fairly stable.

  • It's mostly the oceanic stuff that subducts and is recycled when plates collide.

  • And today, the Earth has reached more-or-less equilibrium between the amount of crust made and the amount of crust lost.

  • But some models have suggested that the amount of continental crust /has/ actually decreased

  • from some ancient peak.

  • And a 2016 paper suggested that when India hit Asia, a substantial portion of the continental

  • crustlike, /half/ of itended up being forced down into the mantle.

  • Like, oh, bye-bye, land! Like, there you go!

  • So it /is/ possible to destroy continental crust on a large scale.

  • But even then, land will probably never disappear entirely.

  • Like I said, Earth seems to have reached a sort of equilibrium between crust made and crust lost.

  • And we also have plate tectonics working to push parts of the ground higher and higher

  • above sea level all the timeso even if some sort of catastrophic flooding happened,

  • that wouldn't be the end of dry land.

  • Even if plate tectonics stopped /altogether/ — which for the record, is /really/ unlikely,

  • since plate tectonics is powered by heat from Earth's core, and that's not cooling down

  • any time soonEarth still wouldn't become perfectly spherical.

  • Scientists at Caltech noted that while erosion might wear the mountains down into hills,

  • there would still be other processes.

  • Things like meteorite impacts could still happen, which could create large dents in

  • Earth's surfacelittle rings of land that could stick above water.

  • Volcanoes would still exist, toobecause although many are powered by magma from those

  • all-important subduction zones, they can also exist far away from plate edges, like the

  • hotspot under Hawai'i.

  • In those places, you don't need a subduction zone.

  • Instead, magma plumes in the mantle are hot enough to melt their way up through the crust.

  • In fact, while Earth is the only planet with active tectonic plates, volcanoes like this

  • have created land on other worlds, too.

  • Like, even though it's dry now, Mars used

  • to have a huge ocean.

  • But it still had dry landin part, thanks to things like Olympus Mons, its gigantic, now-extinct volcano.

  • So, even if Earth was a water world billions of years ago, the odds of that happening again

  • are pretty slim.

  • Which is great news!

  • Because while we probably haven't always had land as we know it, the fact that it /does/

  • existwell, has shaped basically everything about our species and also millions of others.

  • And combined with the awesome forces of plate tectonics, erosion, and other geology processes,

  • we've ended up with the vast and beautiful array of geography we have today.

  • Thanks for watching this episode of SciShow!

  • And as always, a huge thank-you to our patrons on Patreon.

  • We are very glad to have you, and if you haven't already, feel free to stop by the Discord

  • and say hello.

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B1 US crust earth continental land mantle plate

Why Is There Land?

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    joey joey posted on 2021/05/19
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