Earth has land. You know that — the odds are pretty goodyou’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’scrust, the oceans contain enough water to cover the planet in a sea more than two kilometersdeep. So… why does land exist? Why is it so varied, with all those mountainsand valleys and flat plains? And — here’s a fun one: Could anythingever get rid of land on Earth? To answer those questions, which god knowsI want to do, you need to travel back more than four billion years.
Billions of years ago, Earth started as acloud of dust and grains left over from the Sun’s formation. Then, over time, those pieces slowly balledtogether 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 stayedtowards the outside. Eventually, the planet separated into thelayers we know today: the inner and outer core, the gooey mantle, and the crust.
These days, there are two main kinds of crust— continental and oceanic — and they’re made of different ingredients. Oceanic crust tends to be mostly a type of rock called basalt and contains more heavycompounds. And continental crust — which generallymakes up land — tends to be mostly granite and contains more relatively light compounds. But the composition of the /early/ crust andhow it changed in Earth’s first billion years or so is pretty hard to pin down. Like a lot of Earth’s early history, wejust don’t have much — or in some cases /any/ — physical evidence.
It’s pretty much all been recycled and destroyedby now. So there are a lot of interpretations. But mostly, models seem to start with a crustthat would more resemble oceanic crust today, with continental crust slowly growing overtime. The exact date when the first continentalcrust appeared is one of the big questions in geoscience. Some models say it started growing almostimmediately; others say it didn’t really get going until about 3.6 billion years ago. But there’s something potentially reallyinteresting hidden in there. Because, depending on which of these modelsis right, early Earth might have been a /water world./ We think the oceans had to have existed byaround 3.8 billion years ago.
That’s based on evidence like ancient pillowlavas dated to around that time, which only form when lava flows into water. So if continental crust hadn’t formed bythen, there would have been a point in time at which the Earth was, indeed, an ocean world— where land did not exist. So, like, don’t take landfor 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 themovement 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 startedto form into these giant plates, possibly thanks to massive magma plumes from deep withinthe Earth. and as the plates started pushing against eachother, some of them began sliding down towards the mantle in a process called subduction. And as that happened, the increased heat nearand 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 liquifyat different rates, and the rock /separated/ in a process known as partial melting — withsome areas being denser, and others less dense.
Over time, this process repeated, and we endedup with new oceanic crust and the first continental crust material. That material was brought up through volcaniceruptions, 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 materialgot scraped off subducting plates, these islands would have grown over time into larger continents. Of course, like I said earlier, studying thebeginning of Earth’s history is hard, so not all scientists agree that subduction wasnecessary to build the first continents. Like, in 2012, one group proposed somethinga little more… like, 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 elementsfound in those rocks to amounts we’d expect to see if they were formed by subduction Andthey concluded that this ancient crust may not have needed to get /all/ the way downinto the mantle via subduction to melt and reform. Instead, it might have kind of /oozed/ upas rocks melted higher up, in the crust. So, no subduction zone needed. No matter how this occurred, though, eventuallythe 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 andAustralia. 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 itever since. Since the time of Ur and Vaalbara, plate tectonicsand other forces have kept continents above water and made them even craggier.
These days, new continental crust is stillbeing 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 havealso played a part, with wind and rain carving canyons, arches, and other amazing landscapes. So, no matter how it got here, the land hasn’tbeen unchanging and still. It’s continually shaped, changed, and evensometimes destroyed or completely hidden by forces of nature. And that makes you wonder: If all these forcesare 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 usuallyfairly stable.
It’s mostly the oceanic stuff that subductsand is recycled when plates collide. And today, the Earth has reached more-or-lessequilibrium between the amount of crust made and the amount of crust lost. But some models have suggested that the amountof continental crust /has/ actually decreased from some ancient peak. And a 2016 paper suggested that when Indiahit Asia, a substantial portion of the continental crust — like, /half/ of it — ended upbeing forced down into the mantle. Like, oh, bye-bye, land! Like, there you go! So it /is/ possible to destroy continentalcrust on a large scale. But even then, land will probably never disappearentirely.
Like I said, Earth seems to have reached asort of equilibrium between crust made and crust lost. And we also have plate tectonics working topush parts of the ground higher and higher above sea level all the time — so even ifsome 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 fromEarth’s core, and that’s not cooling down any time soon — Earth still wouldn’t becomeperfectly spherical.
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Scientists at Caltech noted that while erosionmight wear the mountains down into hills, there would still be other processes. Things like meteorite impacts could stillhappen, which could create large dents in Earth’s surface — little rings of landthat could stick above water. Volcanoes would still exist, too — becausealthough many are powered by magma from those all-important subduction zones, they can alsoexist far away from plate edges, like the hotspot under Hawai’i. In those places, you don’t need a subductionzone. Instead, magma plumes in the mantle are hotenough to melt their way up through the crust.
In fact, while Earth is the only planet withactive 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 land — in part, thanksto things like Olympus Mons, its gigantic, now-extinct volcano. So, even if Earth was a water world billionsof years ago, the odds of that happening again are pretty slim. Which is great news! Because while we probably haven’t alwayshad land as we know it, the fact that it /does/ exist — well, has shaped basically everythingabout our species and also millions of others. And combined with the awesome forces of platetectonics, erosion, and other geology processes, we’ve ended up with the vast and beautifularray of geography we have today.
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