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, or “Time 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
before – in fact, we have some now – but 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 point – which 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 just … failed. That thermostat is the Carbon Cycle – the
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 earth – but 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 cooler – and
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 latitude – the latitude
of Modern-Day New Orleans - the 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
unkillable – and 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 watch
“The Search For the Earliest Life.”
