28 million years ago, ancient Colorado exploded.
Superheated ash and pumice erupted outwards with enough power to leave behind a gaping
caldera more than 8 times the size of Denver.
Clouds of hot ash blanketed the landscape, creating deposits that are still over 300
meters thick today.
Eventually the ash cooled and became a rock known as ignimbrite - a Latin word that
means "stormcloud of fire.”
And this particular ignimbrite records an eruption that spewed out twice as much material
as Yellowstone's biggest eruption.
That's almost 5000 times as much ash as Mt.
St Helens’ 1980 eruption.
It’s known as the Fish Canyon Tuff.
And while by today's standards it seems inconceivably
huge, it was one of many in a "swarm"
of supervolcanoes that blanketed North America with ash for millions of years.
In fact, the western US is pockmarked with similarly-devastating craters that formed
from about 36 million to 18 million years ago.
This time period is known to geologists as the Mid-Tertiary Ignimbrite Flare-up.
And these large scale eruptions were happening as often as every 10,000 years apart, which
would be an absolutely insane
pace for a devastating problem.
Except… it wasn't devastating.
There was no big extinction event, and there were no smaller ones, either.
In fact, some of North America's biggest and most continuous fossil records were preserved
during this time period, like the John Day Fossil Beds in Oregon.
So how is it possible to have both cataclysmic eruptions… but no real cataclysm?
Today, North America’s volcanoes mostly sit along its Pacific coast.
This so-called Ring of Fire is formed by plate tectonics, with volcanoes popping up where
oceanic plates - like the Pacific plate - are being slowly shoved under a continental plate.
Now, the actual 'meeting place' for these plates is out in the ocean, but the volcanoes
are further inland.
This is because volcanoes don't form above the spot where the two plates collide.
Instead, they develop where the oceanic plate has sunk deep enough to melt from the increased
heat of the Earth’s mantle.
As the plate melts, hot magma rises, and eventually
volcanoes form to release some of that pressure.
And this same process was taking place back in the Eocene epoch.
An old oceanic plate called the Farallon Plate was sliding underneath the North American
continental plate, and that had been going on since at least the Jurassic period.
But there was one big difference.
See, oceanic plates are really heavy compared to continental plates.
That's why when the two collide, the oceanic plate always loses.
It goes under and sinks fairly fast, turning into magma, continuing the volcanic cycle.
So the oceanic Farallon plate should have been sinking.
But, it wasn't - or at least, it wasn't sinking very fast.
It was just kind of bumping up against the bottom of the continental plate, so it didn’t melt much.
And without much molten magma, the few volcanoes that did form were pretty small.
Instead, the scraping of the Farallon plate caused it to be shoved into the continent,
forming the wrinkles that we now call the Rocky Mountains.
Which is important - because when the Farallon plate finally gave up and started to sink,
around 50 million years ago, it was doing so under a much tougher crust.
As the plate finally sank, a whole bunch of new magma rushed in to fill the gaps.
But the continental plate was much thicker than normal, which meant there was no easy
outlet for the new magma.
Instead of quickly erupting upwards, the magma
had to melt its way through the thick continental cap.
And you might think that slowing down magma is a good thing, but it can actually make
eruptions more explosive.
Because, as the magma slows, it melts the plate around it, which changes its composition.
See, continental plates are made of different material than oceanic ones - they have a lot
more silica in them.
Silica makes for a sticky, thick magma.
And if a magma contains gas bubbles, silica will prevent them from exiting the magma body.
So these higher-silica magmas build up pressure and pressurized gasses.
And when they finally reach the surface, they
don't just gently puke out lava, they sort of… explode.
Because the Farallon plate was pretty big, it left a lot of room for magma as it finally
sank, which meant by 33 million years ago
these silica-rich eruptions started happening a lot.
Geologists have identified over 230 different eruptions in North America during this period.
And at least 25 of them were supervolcanic.
Together, these eruptions produced enough material that, if it was spread out, it could
blanket the entire United States 55 meters deep – deeper than a 10 story building!
And while some of that was lava, most of it was massive clouds of hot ash and pumice known
as pyroclastic flows - which, when they solidify, form ignimbrites.
Pyroclastic flows are incredibly dangerous.
They can move as fast as 200 meters per second
and reach temperatures of hundreds of degrees Celsius.
They behave like fluids, and when they're going fast enough, they can jump over barriers
like canyon walls as high as 600 meters!
Nowhere is this process more obvious and terrifying
than in the Fish Canyon Tuff.
The eruption released more than 5000 cubic kilometers of ash - which is why, in many
areas, the deposit is still tens of meters thick or more.
Not every eruption during the Ignimbrite flare-up was quite this big - but even an eruption
a fifth this size is bigger than any that has happened in more than 25,000 years.
Being buried by more than a 10 story building's worth of hot ash seems…
unsurvivable, especially when it's happening frequently.
Yet most North American animals and plants survived.
So the question is… how?
To answer that, we need to look at how animals survive volcanic eruptions today.
Take Mt. St. Helens in Washington State, for instance,
a much smaller eruption than the ash flows
of the Ignimbrite Flare-up, but one that has been extensively studied.
Mt. St. Helens has shown us how some animals can
survive these eruptions… surprisingly easily, it turns out!
Firstly, while ash can travel farther and faster than lava can, it's actually easier
to deal with: because you can dig through it once it's cooled.
So one survival strategy is to hide out in your burrow during the eruption and then poke
your head up through the ash afterwards.
We see this at Mount St Helens, where both pocket gophers and western toads were hiding
in their holes and came back out once the ash had cooled.
The western toads may have even been hibernating when the eruption actually happened.
And animals in lakes have specific strategies too!
Explosive eruptions often send debris and trees into lakes further downhill.
But an aquatic animal can hide deep enough in the water to not be affected.
This is especially true if the volcano erupts when it’s still cold, because animals are
safe under the ice and avoid debris.
This is what we see with some aquatic salamanders at Mt. St. Helens, which were tucked safely in the
muddy lake bottoms beneath ice floes and rode the eruption out.
What Mt. St. Helens and other modern eruptions have
shown us is that mammals who can’t hide
underground, outrun the eruptions, or wait out a food shortage are the most impacted.
For example, Mt. St. Helens is estimated to
have killed about 7000 big game animals, like deer and bears
But larger animals have larger ranges.
Though individuals near the eruption will die, other populations of that species will
survive - and once the plants return, the big animals will, too.
And plants grow really quickly in ash deposits.
Unlike lava, ash breaks down very easily - and it makes for increased nutrient-rich soils.
Ash also helps preserve moisture in the soil,
which is important in dry places - like the Western US.
Ultimately, ash deposits quickly regain their plants and animals.
In fact, studies found that the animals at Mt. St. Helens had completely recovered within
about 5 years of the eruption - life, uh, finds a way.
Which is why the ancient supervolcanic eruptions didn't cause mass extinction.
Life is too resilient and the properties of
ash are especially good at helping ecosystems recover.
And not only did life survive these supervolcanoes,
it actually survived IN them.
These massive eruptions left craters that filled with water and these lake systems preserved
amazing fossils – like the Creede Formation, a lake deposit that formed in the caldera
that birthed the Fish Canyon Tuff.
This formation is known for its amazing insect and plant fossils.
There's even a fossil bird feather that is so well-preserved you can see color banding in it!
The supervolcanoes formed fossils indirectly, too.
Ash and volcanic mudslides dammed up rivers to form lake fossil sites, like the Florissant
Formation, which preserves beautiful trees and plants - and even delicate mosses.
So, far from destroying life at the time,
the Ignimbrite Flare-Up nurtured healthy ecosystems
and eventually preserved them well enough
for people to study, some 30 million years down the line.
It turns out life can adapt and carry on against
incredible odds - no matter how much ash the Earth throws our way.
Have you checked out our posters yet?
Take a journey through millions of years of natural history without ever leaving your
room with our classic deep time poster and our “saga of the supercontinents” poster.
They are available in the link in the description!
We really lava this month’s Eontologists!
Gale Brown, Juan M., Jacksy Weiss, Melanie
Lam Carnevale, Annie & Eric Higgins, John Davison Ng, Jake Hart, and Colton.
Become an Eonite at patreon.com/eons and you can get fun perks like submitting a joke for
us to read, like this one from Andrew and Sarah:
Andrew: Did you know that Phosphatherium was an ancient elephant ancestor that had no trunk?
Sarah: No trunk? How did it smell?
Andrew: Terrible!
That was pretty good!
Phosphatherium, Phosphatherium, cool. And as always thank you for joining me
in the Adam Lowe studio. Subscribe at youtube.com/eons for more epic epochs.
I keep wanting to say magma like Austin Powers, MAGMA...
[flashback]...oceans made of magma...
[present day]...every time I read the word.
Magma...
All right.