Placeholder Image

字幕表 動画を再生する

  • Imagine a world covered in ice.

  • Estimates vary, but some scientists think at the poles, it could reach negative 130

  • degrees celsius.

  • And there was no escaping the cold even at the equator, where temperatures would have

  • dipped below 0 degrees.

  • Sheets of ice coat both land and sea, and beneath them, the world is quiet and relatively

  • still.

  • It may sound like some far-off planet, but that's what our own planet once looked like.

  • And actually, it happened twice during a pair of episodes of intense glaciation between

  • 716 million and 635 million years ago.

  • These global freezes occurred within that period of geologic time known as the Cryogenian,

  • orTime of Ice.”

  • But most people refer to this chapter in our history simply as Snowball Earth.

  • So how did this happen?

  • How did the world become covered in ice?

  • And most importantly for us, why did the planet eventually thaw again?

  • Strangely enough, for both questions, the answer lies in volcanoes.

  • The evidence for snowball earth is written on every continent today.

  • Since the early 1900's, scientists have been finding clues all over the world, in

  • the form of dropstones.

  • These are rocks and pebbles that were picked up by glaciers as they moved across the land.

  • And once the glaciers met the seas, icebergs broke off and floated away, carrying the rocks

  • with them.

  • When the ice melted, the stones dropped into the ocean.

  • These dropstones show up in ancient marine formations all over our planet.

  • And while the continents have shifted since the Cryogenian, scientists have been able

  • to reconstruct the original positions of those ocean sediments using magnetic particles preserved

  • in the formations themselves.

  • These particles record the direction of the North Pole, which tells us where on the planet

  • the dropstones originally fell into the sediment.

  • And when you reconstruct where these dropstones were deposited, you can see that they stretched

  • from the poles to the tropics.

  • Which means ice did too.

  • Now we know that this extensive glaciation actually happened twice between 716 and 635

  • million years ago.

  • The first episode started 716 million years ago, and lasted for about 36 million years.

  • And the second lasted from about 650 to about 635 million years ago.

  • Now, there have been glaciers on our planet beforein fact, we have some nowbut

  • what makes these two periods so interesting is the extent of that ice.

  • After all, today the tropics are pretty warm – a balmy 31* C in the afternoon, which

  • is awfully warm if you're trying to freeze over an ocean.

  • So how did our lovely temperate world get cold enough to freeze?

  • Well, at first, scientists thought: If there's evidence of ice having been at the equator,

  • then maybe the equator wasn't actually at the equator.

  • Maybe earth had been tipped over on its side at some pointwhich would've made the

  • equator part of the poles.

  • That's how weird it was to find evidence of ice in the tropics: Scientists thought

  • it was more likely that Earth fell over than that the equatorial oceans had frozen.

  • But we now know that the evidence is too widespread for a change in Earth's tilt to explain

  • it.

  • In fact, the evidence is so complete that it's likely that almost all of earth froze

  • over, including both the equator AND the poles.

  • Because, in addition to dropstones, more evidence has been found, in the form of carbonate rock.

  • This rock is created when other rocks on the continents weather and break down to form

  • ions, which eventually make their way into the water.

  • When those ions attach to dissolved CO2, they join together to form carbonate.

  • And studies of ocean sediments all over the world have found that, during parts of the

  • Cryogenian, these carbonate rocks disappear.

  • Because, when the world was covered in ice, almost no weathering took place on land, so

  • carbonates became really rare.

  • But when the ice started to melt, weathering resumed -- and huge deposits of carbonates

  • began to form again.

  • Most geologists think that the absence and reappearance of these rocks is a sign that

  • earth was mostly to completely covered in ice.

  • But while that makes sense to the geologists, it doesn't make sense to some biologists.

  • Life had existed on Earth for over a billion years by the time the Cryogenian started.

  • And organisms like photosynthetic cyanobacteria, and even animal life like sponges, had evolved

  • before the ice sheets grew.

  • Which raises the question of how early life could have survived under the ice.

  • Some scientists have suggested that there must have been a fair amount of open, unfrozen

  • water at the equator for life to persist.

  • This model is called Slushball Earth, but it doesn't line up with all of the geological

  • evidence.

  • So yet another hypothesis is that there was ice everywhere, but that it was thin enough

  • in places for light to shine through and to allow photosynthetic life to survive.

  • Studies of modern cyanobacteria in Antarctica suggest that life may even have thrived on

  • top of the ice sheets themselves.

  • But whether it was thick ice, or thin ice, the ice was abundant.

  • So, then, why did these massive glaciations actually happen in the first place?

  • Well, the most popular theory is that our planet's thermostat justfailed.

  • That thermostat is the Carbon Cyclethe swapping back and forth of carbon between

  • the atmosphere and the earth's crust.

  • And it starts with volcanoes, which, over the course of thousands to millions of years,

  • gradually emit CO2 into the atmosphere, where it helps keep the world warm.

  • But CO2 levels are kept in check, because that gas gets stored in carbonate rocks during

  • the process of weathering.

  • So volcanic emissions and rock-weathering are the two counterbalances that keep earth

  • not too hot, and not too cold.

  • But in the Cryogenian, an early supercontinent known as Rodinia messed with the thermostat

  • by breaking up.

  • Breaking up is hard to do and rocks usually do it pretty violently.

  • But the breakup of Rodinia was especially intense, because it pumped out a lot of a volcanic

  • rock known as basalt.

  • And basalt is really, really good at soaking up CO2 in the process of weathering.

  • Plus, Rodinia was sitting at the equator at the time, where it was warmer and wetter,

  • which weathered the rock even faster.

  • So scientists think that this could have thrown off the carbon cycle, soaking up CO2 faster

  • than volcanoes could release it.

  • And there was another contributing factor: the sun.

  • During the Cryogenian, the sun was actually about 7% dimmer than it is today.

  • That doesn't sound like a lot, but it was enough that, once the levels of CO2 dropped,

  • it was so cold that the glaciers started to grow.

  • And in the last few years, scientists have discovered yet another driving force behind

  • this phenomenon: a truly massive and spectacular eruption that took place 18 million years

  • before the glaciation even started.

  • Today, the remains of that eruption are known the Franklin Large Igneous Province: more

  • than a thousand square kilometers of basalt lava that cover the Canadian Arctic.

  • But what sets these rocks apart from others is that they were full of another planet-cooling

  • gas: sulfur.

  • When you pump sulfur into the air, it cools the earthbut normally, it doesn't do

  • it for long.

  • Sulfur dioxide interacts with water in the atmosphere and forms acid rain, typically

  • leaving the atmosphere within a couple of years.

  • But these eruptions weren't made by your standard volcanoes.

  • Instead they sprayed out huge jets of lava called fire fountains, which could have erupted

  • for years, spraying plumes of sulfur gases up to 12 kilometers into the atmosphere.

  • And that high above Earth's surface, near the stratosphere, sulfur dioxide would take

  • a lot longer to break down and rain out.

  • So, low CO2 levels let things cool down, and a dimmer sun didn't help.

  • Then suddenly, 716 million years ago, vast amounts of sulfur dioxide may have been a

  • final blow to earth's thermostat - and ice began to form.

  • The second glaciation may have had similar causes, but it isn't as well dated or understood

  • as the first.

  • But for both episodes, the real problem came when the ice started to grow.

  • Ice reflects more light than water does, which makes the world cooler, which makes more ice

  • grow, which makes the world even coolerand so on.

  • This feedback loop is called a runaway icehouse effect.

  • And scientists who have modeled this process found that, once our planet had ice below

  • about 30 degrees latitudethe latitude of Modern-Day New Orleansthe growing

  • ice was basically unstoppable.

  • So why are we not still stuck on a world that's basically ... Hoth?

  • Because of our old friend carbon dioxide.

  • Rodinia didn't stop splitting apart just because it was covered with ice.

  • As it kept breaking up, volcanoes kept forming and releasing CO2 into the atmosphere.

  • But this time, because the planet's rocks were mostly locked beneath ice sheets, they

  • weren't able to absorb all of that greenhouse gas.

  • So instead, it began to build up in the air.

  • It took almost 50 million years for enough CO2 to melt the first round of glaciers, and

  • about 10 to 15 million years to melt the second.

  • Between the two glaciations, Rodinia continued to break up near the equator - which is why

  • the thermostat broke twice during the Cryogenian.

  • But by the end of the Cryogenian, Rodinia was largely in the southern hemisphere, and

  • had stopped splitting so dramatically, so the thermostat could re-set itself.

  • Once most of the ice had melted by about 635 million years ago, the warmer oceans suddenly

  • began to fill with animal life.

  • The period that immediately followed the Cryogenian -- known as the Ediacaran period -- is full

  • of some strange and varied forms, descendants of the survivors of snowball earth.

  • But animal life itself didn't actually first evolve in the Ediacaran.

  • Molecular clock analyses suggest the most recent common ancestor of all animal life

  • lived long before that -- some 800 million years ago.

  • Which means that somehow, animal life actually lived through Snowball earth.

  • How?

  • Well, the earliest animals were practically unkillableand it turns out, they not

  • only survived snowball earth, they helped change oceans for the better.

  • But that's a story for another time.

  • So come back soon to learn all about the enterprising, trail blazing, and nearly indestructible animals

  • that clung to life throughout the snowballs: the sponges.

  • Thanks to this month's Eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng, and Steve.

  • If you'd like to join them and our other patrons in supporting what we do here, then

  • go to patreon.com/eons and make your pledge!

  • And if you want to join us for more adventures in deep time, just go to youtube.com/eons

  • and subscribe.

  • Thanks for joining me today in the Konstantin Haase studio, and if you'd like to learn

  • more about the very deep past, then watchThe Search For the Earliest Life.”

Imagine a world covered in ice.

字幕と単語

動画の操作 ここで「動画」の調整と「字幕」の表示を設定することができます

B1 中級

How Volcanoes Froze the Earth (Twice)

  • 1 0
    joey joey に公開 2021 年 05 月 01 日
動画の中の単語