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  • When geologist Clarence Dutton first saw the Grand Canyon in 1880, he was spellbound by

  • its colorful walls.

  • The Grand Canyon’s walls are made up of rock layers, each representing a distinct

  • period of Earth’s history.

  • And some of the lower rock beds--deep down in the canyon--look kind of tilted.

  • Dutton knew these layers were made up of very old sediments.

  • Originally, they’d been laid down as horizontal beds, mostly in rivers or shallow seas.

  • Then, the sediments hardened over time and geologic forces pushed some layers upward

  • at an angle.

  • As time passed, the tops of these tilted layers were sheared off by erosion.

  • And later, new layers of sediment--which stayed more or less horizontal--were deposited right

  • on top of them.

  • And he knew every step of this process took time - because you generally don’t get horizontal

  • layers on top of tilted ones unless there’s an age difference between them.

  • In his examinations of the Grand Canyon, Dutton got a firsthand look at what geologists call

  • an unconformity.

  • Basically, that’s a gap in the geological record.

  • It shows that sedimentation didn’t happen continuously, and that there’s an age difference

  • between sets of rock layers...and sometimes that age gap can be millions of years...or

  • more.

  • In 1882, Dutton named this particular breakThe Great Unconformity.”

  • At first, nobody realized just how great it was.

  • Today, we know from radiometric dating that the rocks directly on top of the Great Unconformity

  • were laid down during the Cambrian Period about 500 million years ago.

  • But in some places, the rocks below thisGreat Unconformityare about 1.2 billion years

  • older!

  • That’s… a big gap.

  • I mean, the Earth itself is only about 4.5 billion years old.

  • So at this spot in the Grand Canyon, 25% of the planet’s entire history is gone - those

  • layers just aren’t there anymore.

  • And the Grand Canyon isn’t the only place where this happens.

  • You can see this gap from Siberia to Antarctica--and plenty of spots in between.

  • Scientists are still trying to figure out what caused the Great Unconformity - but they

  • have some ideas.

  • This missing chapter in Earth’s history might be linked to a fracturing supercontinent,

  • out-of-control glaciers, and maybe--just maybe--the diversification of life itself.

  • Sedimentary rock layers are formed by a process called deposition - when eroded geological

  • materials- like sand -are laid down by wind, water, or even ice.

  • They accumulate to build up layers of sediment that later turn into rock.

  • And when youve got a layer of rock that’s distinct from everything above and below it,

  • that’s called a stratum.

  • Pile up a bunch of strata on top of each other--with each level representing one span of time--and

  • together, theyll create a rock sequence.

  • And an unconformity is a boundary dividing two strata with an age difference between

  • them.

  • An unconformity might mean that no new material was deposited at this place for a long time,

  • interrupting the rock sequence.

  • But this isn’t always the case.

  • Usually, unconformities are made when an entire layer gets eroded away somehow.

  • And it turns out that the Grand Canyon’s picturesque walls are full of unconformities.

  • For example, there’s a 150 million-year gap between two strata of limestone: a layer

  • from the Early Carboniferous Period and another one from the Cambrian Period.

  • But that’s nothing compared to the Great Unconformity.

  • It begins under the Tapeats Sandstone, a stratum laid down by the ancient tides of a shallow

  • sea about 508 to 501 million years ago during the Cambrian.

  • Below this, youll find a stretch of 1.75 billion year-old metamorphic rock called the

  • Vishnu Schist, along with tilted rock layers ranging between 1.25 billion and 740 million

  • years old known as the Grand Canyon Supergroup.

  • And figuring out what happened in that gap is the key to explaining the existence of

  • the Great Unconformity.

  • Right now, were living in the Phanerozoic Eon, which began 541 million years ago at

  • the dawn of the Cambrian Period.

  • But before the Phanerozoic, there was the Proterozoic, a much longer Eon that kicked

  • off some 2.5 billion years ago.

  • Yeah, that’s billion with a “b.”

  • The Proterozoic saw the rise and fall of three supercontinents - and the last one is known

  • as Rodinia.

  • Scientists think it formed between 1.3 billion and 900 million years ago.

  • And when it was fully assembled, Rodinia contained just about all the known continents that were

  • around at the time.

  • But supercontinents don’t last forever.

  • Rodinia started breaking apart around 750 million years ago--well before the Cambrian

  • started.

  • And its break-up might be linked to something else that happened late in the Proterozoic.

  • It wouldve been another major event--and a pretty cool one.

  • Literally.

  • Since the 1930s, scientists have been finding evidence that glaciers were common at the

  • equator late in the Proterozoic.

  • Places like Western North America and southern Australia were tropical or subtropical back

  • in those days.

  • And if you look at deposits from the late Proterozoic, youll find a haphazard mixture

  • of sand, mud, gravels, and boulders.

  • Those deposits are called tillites and theyre the material that gets left behind by glaciers.

  • In places that were at low latitudes during the late Proterozoic, geologists often find

  • these tillites sandwiched between limestone beds that probably developed in warm, tropical

  • waters.

  • This finding--along with paleomagnetic data and other clues--gave rise to theSnowball

  • Earthhypothesis, which I’ve talked about before.

  • According to this idea, glaciers once covered all or most of the world’s surface, from

  • the poles all the way down to the equator.

  • And this may have happened more than once in the late Proterozoic.

  • So there couldve been multipleSnowball Earthepisodes--with the first starting

  • around 716 million years ago--and the last one ending just 635 million years ago.

  • And these cold periods might be related to the Great Unconformity - but how depends on

  • who you ask.

  • Either the formation of the Great Unconformity was part of what caused Snowball Earth or

  • Snowball Earth was what formed the Great Unconformity.

  • One study published in 2018 made the case that the Great Unconformity helped set up

  • the start of Snowball Earth.

  • It looked at North America’s Ozark Plateau, where 500 million-year old Cambrian sandstone

  • sits on top of granite that’s 1.4 billion years old.

  • Here, as in the Grand Canyon, the Great Unconformity is plain to see.

  • To date the Great Unconformity, they used uranium isotopes and helium trapped in zircon

  • crystals.

  • From those crystals, they concluded that the area was tectonically uplifted and eroded

  • from about 850 to 680 million years ago.

  • And this coincides with the breakup of Rodinia.

  • Tectonic uplift on a big scale may be a side-effect of fragmenting supercontinents.

  • In this case, the study’s authors think several kilometers of rock were lifted up,

  • only to get whittled down by erosion.

  • Since the Ozarks were really far inland back then, the scientists suspect there was a continent-wide--and

  • maybe even worldwide--outbreak of large-scale erosion.

  • If this really was a global phenomenon, it may be what created the Great Unconformity,

  • eroding away rock layers that now appear as gaps in the geological record.

  • And there’s more.

  • The authors think the erosion of all that Proterozoic bedrock affected the climate,

  • by capturing large quantities of carbon in the Earth and its oceans, keeping it out of

  • the atmosphere.

  • That’s because rainwater often pulls carbon from the atmosphere during the weathering

  • process.

  • Since rainwater is weakly acidic, it dissolves rock and releases ions.

  • These ions can get washed into the ocean where they form calcium carbonate, which is eventually

  • buried, and traps the carbon in rock.

  • And carbon’s a key component of greenhouse gases.

  • So if a lot of carbon didn’t end up in the atmosphere, that might've promoted global

  • cooling--along with other factors like volcanic events and a dimmer sun.

  • And as the world grew colder, glaciers couldve gone unchecked.

  • In other words, welcome to Snowball Earth.

  • So according to this timeline of events, Rodinia’s breakup resulted in lots and lots of erosion.

  • And not only did that erosion create the Great Unconformity, but it also impacted the climate,

  • helping to set the stage for Snowball Earth.

  • But not everyone agrees.

  • A different study, published in 2019, argues the opposite: that Snowball Earth’s glaciers

  • created the Great Unconformity by grinding up the surface of our planet.

  • The glaciers wouldve eroded away a lot of the outer continental crust, dumping it

  • into the oceans.

  • This massive increase of eroded material wouldve also increased the amount of crust being driven

  • down into the Earth’s mantle, where some of it was recycled into fresh magma and sent

  • back toward the surface.

  • And the evidence of this comes - again - from isotopes trapped in zircon crystals.

  • But this time, theyre isotopes of the element hafnium.

  • Some hafnium isotopes are more likely to form at the surface than down in the mantle - and

  • that signature is preserved.

  • This can show where the isotopes came from.

  • And remember, zircons are also used in radiometric dating.

  • So by looking at the ratios of different hafnium isotopes in these zircons and dating the zircons

  • themselves, the researchers figured that the crust erosion and recycling probably happened

  • after Snowball Earth started.

  • And this is because they found a lot of surface hafnium trapped in the zircons - and the zircons

  • formed after Snowball Earth got underway.

  • So if that’s true, the erosion--and all the crust recycling that wouldve gone with

  • it--could not have set up Snowball Earth.

  • Instead, it could have been glaciers that stripped away over a billion yearsworth

  • of rock layers, creating the Great Unconformity.

  • Or at least, that’s the hypothesis.

  • Hopefully, future research will tell us which came first: The Snowball or the Unconformity.

  • But either way the loss of so much material couldve been important for life on earth.

  • Complex life really diversified during the Ediacaran Period--which closed out the Proterozoic--and

  • then life exploded during the Cambrian.

  • Scientists have wondered if this might be related to the Snowball Earth and the Great

  • Unconformity.

  • Some researchers have suggested that when all that crust was destroyed, it fundamentally

  • changed the ocean’s chemistry.

  • The weathering process might have transported a lot of geologic material from land to sea.

  • As a result, scientists think our oceans were filled with calcium, potassium, iron, phosphorus

  • and other vital elements.

  • This couldve revolutionized life on our planet by giving it the chemical building

  • blocks it needed to really get going.

  • One hypothesis even suggests that these new ingredients helped to encourage biomineralization,

  • the process by which living things create minerals for their shells and skeletons.

  • So maybe when Clarence Dutton first saw the Great Unconformity way down in the Grand Canyon,

  • he was actually looking at the calling card of a phenomenon that made his own existence

  • possible.

  • I must give big shoutouts to all these researchers who consulted on this episode!

  • Your input was greatly appreciated.

  • Also grand high fives to this month’s Eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng,

  • Sean Dennis, and Steve!

  • Become an Eonite by pledging your support at patreon.com/eons.

  • And thank you for joining me in the Konstantin Haase Studio.

  • If you like what we do here, subscribe at youtube.com/eons.

When geologist Clarence Dutton first saw the Grand Canyon in 1880, he was spellbound by

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当十亿年消失(When a Billion Years Disappeared)

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    joey joey 發佈於 2021 年 05 月 01 日
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