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  • {♫Intro♫}

  • The event that killed the dinosaurs might

  • be the most famous

  • mass extinction ever.

  • And scientists are pretty sure they've identified

  • the scene of the crime:

  • a meteor crater in the Gulf of Mexico.

  • But that event sixty-six million years ago

  • is neither the only one in Earth's history,

  • nor the worst.

  • Not by a long shot.

  • Scientists studying a remote swath of land

  • in Russia have pieced together clues about

  • an extinction event

  • that makes the demise

  • of the dinosaurs look tame.

  • The extinction event in question happened

  • at the end of the Permian period, which was

  • a geologic span of time that ran from about

  • 299 million to 252 million years ago.

  • This was before the dinosaurs, but not so

  • far back that life on Earth would have seemed

  • totally alien. There were no birds or mammals,

  • but there were reptiles, amphibians, ferns,

  • and insects.

  • Granted, some Permian creatures -- like Dimetrodon

  • look pretty strange to us now, but you'd

  • still be able to walk around and see plants

  • and animals you could put a name to.

  • But at the end of the Permian, a lot of life

  • suddenly went extinct. Like, a lot.

  • Within no more than two hundred thousand years,

  • most of the marine and terrestrial species

  • on Earth vanished.

  • Estimates vary, but more than 90% of all species

  • may have disappeared.

  • It would take about 8 to 9 million years for

  • Earth's ecosystems to really recover. This

  • extinction was so dramatic

  • that scientists have nicknamed it

  • The Great Dying”.

  • There have been a handful of hypotheses over

  • the years to explain this massive die-off.

  • Some researchers have proposed another big

  • asteroid, like the one that would kill off

  • the dinosaurs.

  • But one place on Earth seems to stand out

  • as the most likely scene of the crime: a massive

  • volcanic region in Russia known as

  • the Siberian Traps.

  • The rock underneath this area of Siberia is

  • relatively stable --

  • it's not on a tectonic plate boundary,

  • like many volcanoes are.

  • But about the same time as the Great Dying,

  • a massive pulse of magma from deep inside

  • the Earth

  • known as a mantle plume

  • melted through the crust.

  • The plume triggered the biggest volcanic eruptions

  • ever known, which may have lasted more than

  • a million years,

  • and covered an area

  • bigger than Greenland in lava.

  • After the initial eruptions, there was a quiet

  • period for a while, followed by one big, final

  • burst of activity.

  • These eruptions happened at pretty much the

  • same time as the mass extinction, and scientists

  • think this volcanic activity is a pretty likely

  • explanation for what happened.

  • The eruptions could have spewed massive amounts

  • of carbon dioxide and methane into the Earth's

  • atmosphere, changing the chemistry

  • of the Earth's air and water.

  • The oceans, for instance, would have abruptly

  • become more acidic and as much as ten degrees

  • warmer --

  • and would have lost

  • almost all of their oxygen.

  • If that seems bad, you're right!

  • But there's still some details to work out.

  • Sure, we found a site of massive volcanic activity,

  • and it's about the right age.

  • But the timeline actually isn't quite right

  • for the initial burst of eruptions

  • at the Siberian Traps

  • to have caused the Great Dying.

  • By the time the extinction event started,

  • the traps had already been erupting

  • for about three hundred thousand years and

  • had spewed out

  • two-thirds of their magma.

  • Also, this kind of massive eruption has happened

  • in other times and in other places. Some of

  • those are associated with mass extinctions,

  • but not all.

  • So does that let the Siberian Traps

  • off the hook?

  • One leading hypothesis, published in 2009,

  • proposes that it was volcanic activity but,

  • remarkably, it might have not been those

  • huge initial eruptions.

  • Remember that quiet period? During that period,

  • magma was still flowing, but instead of erupting

  • dramatically, it spread underground, intruding

  • in between layers of existing rock to form

  • what are known as sills.

  • Visible today in places like river banks,

  • the sills look kind of like the frosting in

  • between the layers of a giant cake.

  • As the magma spread out, it heated and altered

  • the surrounding rocks in a process known as

  • contact metamorphism.

  • These existing rocks, put downs by millions

  • of years of geologic activity, contained quite

  • a bit of carbon, and even included coal beds.

  • The intruding magma would have effectively

  • cookedall these rocks, creating and

  • releasing greenhouse gases like

  • carbon dioxide and methane.

  • They would also have released a type of chemical

  • called halocarbons, which would have stripped

  • the ozone layer, just like CFCs in more recent years.

  • And researchers have calculated that the contact

  • metamorphism generated four to nine times

  • more carbon dioxide than the lava itself contained.

  • In 2017, researchers took a closer look

  • at the start of this quiet, deadly period.

  • And they were able to pin the timing down

  • to line up with the onset of the mass extinction,

  • thanks to very precise radiometric dating,

  • which uses the steady decay of radioactive

  • elements like a clock

  • to estimate the age of rocks and other materials.

  • This research would explain why it took

  • 300,000 years for the extinction

  • to start. The big eruptions, though dramatic,

  • may not have brought up enough carbon dioxide

  • by themselves, nor did they cook as much of

  • the underground rock as the sills.

  • And all that carbon dioxide, once released

  • from the cooking rocks, would be enough to

  • change the Earth's atmosphere,

  • leading to warmer temperatures

  • and acidic, de-oxygenated oceans.

  • And from there to the most staggering loss

  • of life our planet has ever seen.

  • This may be the best smoking-gun evidence

  • we ever get to tie the Great Dying to a place

  • on Earth that you could actually theoretically visit.

  • And it's a bunch of volcanic rock

  • in Siberia.

  • Thanks for watching this episode of SciShow,

  • and thanks to our channel members for your support.

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  • {♫Outro♫}

{♫Intro♫}

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B1 中級

シベリアの罠。2億5千万年前の犯罪現場 (The Siberian Traps: A 250 Million Year Old Crime Scene)

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    林宜悉 に公開 2021 年 01 月 14 日
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