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For the last hundred and fifty years or so,
geologists have been trying to wrap their heads around the mystery of missing time.
In some places, the geologic record just seems to jump by over billion years, a phenomenon ominously called The Great Unconformity.
But now, a team of researchers may have finally figured out what happened.
In a paper published last week in the Proceedings of the National Academy of Sciences,
they lay out some pretty convincing evidence that massive glaciers ripped away over a dozen vertical kilometers of rock
during a period of Earth’s history many call “Snowball Earth”.
The Great Unconformity is so called because, well, it’s a really big unconformity.
That’s a technical term in geology which
refers to places where the aging of rock layers suddenly jumps.
Usually, the slow deposition of sediment from normal wear and tear to rocks leads to layers of rocks that get older as you dig deeper.
But back in 1869, a geologist named John Powell was going through the Grand Canyon and found that over a billion years of geologic record
from just before the diversity of life on Earth exploded was somehow completely missing.
It came to be known as Powell’s Great Unconformity, and soon, geologists discovered it wasn’t isolated to the American Southwest.
In all, we’re talking around ten billion
cubic kilometers of earth just… gone.
That much misplaced sediment represents a lot of unaccounted time—almost a quarter of Earth’s history—which makes scientists
all the more motivated to uncover what happened.
And there are really only two possibilities:
either something made it so basically no new sediment was deposited over those millennia, or there was suddenly massive amounts of erosion which scraped away the geologic record.
So, as geologists do, the researchers looked for evidence of what happened in the rocks above and below the unconformity.
Above the missing layers sits the Tapeats
Sandstone, which was forged in the Cambrian period about 525 million years ago,
while below is what’s known as the Vishnu Schist, which is over 1.6 billion years old.
One thing that stands out is how the layers of rocks above have a much greater volume per unit of time than the ones below.
You’d expect the lower layers to be squished since they’ve been under more pressure for longer, but they’re even more squished than they should be.
And that’s more consistent with the erosion hypothesis, as they could have been flattened by the weight of the rocks now missing.
But no one could quite explain where all that removed Earth went.
So, the international research team looked at teeny tiny crystals of a mineral called zircon.
These are among the first solid things to
form when molten, liquid rocks, or magma, starts to cool,
and they’re very tough, so they tend to survive major geologic processes that destroy other minerals.
More importantly, they capture the surrounding geochemical conditions when they form.
And scientists can date these crystals by
looking at different variants or isotopes of uranium
a radioactive element that decays very, very slowly.
In this study, the team also looked at isotopes of oxygen and hafnium, a silvery metal, in about 30,000 zircon crystals.
The ratios of these are different enough between the continental crust that forms dry land and the oceanic crust that forms the seabed
that it should be clear if either contributed significantly to the magma the crystals came from.
And their results suggested that a whole bunch of the Earth’s land—as much as 14 vertical kilometers—
was dumped into the oceans and then pulled underground and recycled into magma through the process of subduction.
That lines up quite well with the idea that
about 650 million years ago, the entire planet, or the majority of it, was covered in ice.
It’s a hypothesis referred to as “Snowball
Earth,” and while it was once considered kind of ridiculous, lately, more and more
evidence has come to support it.
And the researchers in this study added to that evidence by looking at old impact craters.
They noted that there should be more intact craters after the global glaciation event than before,
as the massive ice sheets would have scraped away shallower craters.
Sure enough, there are plenty of impact sites from the Cambrian onward, but very few from before,
all of which are many kilometers deep.
And combined with the zircon crystal data, they make a strong case that giant glaciers really did scour the earth some 650 million years ago.
There are still a few kinks to be worked out, of course.
One of the biggest is that the Earth would
have warmed up millions of years before the unconformity ends,
so it’s not clear why there aren’t layers from that timeframe.
The authors think the ice may have simply
left nothing to be eroded, and it took time for new land to form, but more data would
be needed to support this claim.
Other scientists think the erosion came before the snowball, and in fact triggered the global winter conditions.
So there’s still more research to be done.
What’s perhaps most intriguing about these findings, though, if true, is how they might explain the explosion of life that came after.
A while ago, some scientists suggested the massive environmental changes from all that erosion might have been responsible for
the huge bump in biodiversity known as the Cambrian Explosion.
And the researchers in this paper note that in addition to adding minerals to the water, the gouges left by the glaciers
could have filled to become fertile, shallow seas where marine life could flourish.
They treat the hypothesis skeptically, of
course, as they should, since their study doesn’t provide any direct evidence that
it’s true.
But I like it!
On a gut level, there’s just something kind
of poetic about the idea that the coldest, harshest winter in Earth’s history may have been essential for the incredible diversity of life we see today to evolve.
Thanks for watching this episode of SciShow News!
And a special thank you to our President of Space, Matthew Brant.
We really appreciate your continued support, Matthew!
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