The World Before Plate Tectonics

Around 2 billion years ago, Earth was a very different place.

Pretty much everything was more extreme than it is today, from ocean chemistry, to slimy

life on land, to the position of landmasses. Things were generally… a little weird.

The oceans would have been inhospitable to most animal life today, because they were

very low in oxygen but really high in sulfur.

Meanwhile, on land, there wasn’t much life to speak of, except for microbial organisms,

like cyanobacteria and possibly lichens.

Even the temperature inside the Earth was more extreme than it is today.

But the world for these early living things was also kind-of...boring.

The climate was remarkably stable; there wasn’t much glacial activity, and the sun was 5 to

18% less powerful than it is today.

Even the rocks under the microbes were fairly stable.

Modern plate tectonics, the movement of rock plates on top of Earth’s mantle, hadn’t

gotten started yet.

The mantle was just too hot, so it bound the continental plates together into a supercontinent.

And the plates were flimsy and thin.

Because this time in Earth’s history was so stable, geologists once called it the Boring

Billion - the billion years from about 1.8 billion to 800 million years ago.

But the fact is, this period was anything but boring. In fact, it set the stage for

our modern version of plate tectonics - and probably for the rise of life as we know it.

Today, Earth’s plate tectonics cycle is active.

The plates move around, forming mountains as they smash together, shallow seas when

they rift apart, and even volcanoes when they slide under each other.

Plate movement is responsible for some of Earth’s most noticeable features, including

the Himalayas, the East African Rift valley, and the Pacific Ring of Fire.

They’re so important that it’s almost hard to imagine what the planet looked like

before plate tectonics.

To figure that out, we have to go back to when Earth was first settling down from its

formation, back at the end of the Archean Eon, almost 3 billion years ago.

Unfortunately for geologists, a lot of the rock record has been continuously recycled

and destroyed through subduction, where one slab of rock slides under another and dives

down into the mantle, where it melts.

That means evidence of the oldest rocks is really limited, because they’ve been ripped

apart, smashed together, and eroded away.

But the Archean rocks that did manage to survive on Earth’s surface are spread out

all over the world, in places like North America, Australia, Africa, and Eastern Europe - and

some are more than 3 billion years old!

And that presented a puzzle to geologists who wanted to find out when modern plate tectonics

started, and what it looked like.

Since subduction is one of the main drivers behind today’s plate tectonics, they looked

for evidence of subduction as a sign of the beginning of ancient plate tectonics.

So scientists slowly pieced together evidence from the few surviving rock slabs, as well

as computer models, to start to understand the Archean.

You see, geologists can look at minerals in a rock to figure out how deep a piece of land

went into Earth’s interior, and how hot it got.

And they found that starting around 2.78 billion years ago, in the late Archean, there is solid

evidence that Earth’s lithosphere was busy.

There were supercontinents breaking apart and forming, mountain-building episodes, and

metamorphism, where rocks are transformed by high heat and pressure.

Now, that certainly sounds a lot like plate tectonics, but does it really count as the

first example of plate tectonics? Well, geologists have opinions about that.

It didn’t work the same way as the modern version of plate tectonics because Earth’s

mantle was hotter than it is today -- about 250 degrees celsius hotter.

The mantle reached peak temperatures in the Archean, and has been slowly cooling since

then. 2.8 billion years ago, the mantle was still holding on to more heat than it is today.

Those high mantle temperatures made the crust thin, weak, and easy to deform - like cookies

straight out of the oven. The metamorphic rock record shows that if there was subduction

into the mantle, the sinking crust stayed pretty shallow, unlike today.

It was a different “flavor” of subduction. That’s why some geologists don’t think

this really counts as the first evidence of plate tectonics.

But these early movements of the lithosphere helped separate the crust into plates.

Those plates crammed together in one spot, leading to the formation of the supercontinent

Nuna by 1.8 billion years ago.

Welcome to the so-called Boring Billion.

Remember, all of this was over a billion years before the Cambrian explosion. So with the

exception of some microbes, there was no life on land: it was confined to the ocean.

And the ocean was very different than it is now.

2 billion years ago, it’s likely that most of the ocean was very low in oxygen.

But what it did have in abundance was hydrogen sulfide. And by 1.6 billion years ago that

combination created a condition known as euxinia, which is toxic to most eukaryotes -- organisms

that have an enclosed nucleus in each cell.

But the prokaryotes, simpler life forms that don’t have an enclosed nucleus, made the

best of their strange environment.

Microscopic life forms like archaea were perfectly happy. And so were bacteria, like cyanobacteria,

which were photosynthetic and could metabolize the abundant sulfur in the ocean.

Also there were purple and green sulfur bacteria, which are brightly-colored photosynthetic

microbes that can form squishy mats in aquatic environments.

They sound like they belong in a scifi movie, but they’re very real, and still around

today.

So most of the life found in the Boring Billion was prokaryotic. And with all this sulfur

in the environment, researchers think this period was the stinkiest time on Earth!

But to geobiologists - scientists who study the interactions between the biosphere and

Earth’s physical processes - this period was never boring, because it marked the beginning

of complex eukaryotic life.

For example, in China there are rock formations that contain fossils of eukaryotes that date

back 1.7 billion to 1.4 billion years ago!

The majority of these organisms lived in water, including protists and other early eukaryotes,

So life on the early supercontinent, with limited plate movement, was doing okay. But

it was still all microbes and slimy mats, trying to survive in sulfur-rich water.

So how did we get from there to here, or even to the Cambrian explosion?

Well, life needed Earth to shake things up.

And that didn’t happen in the Boring Billion, at least not enough to make a huge difference.

The old flavor of softer, squishier plate tectonics continued, with minor, shallow subduction

around Nuna and the next supercontinent, Rodinia.

Plates on the outside of the supercontinent were mostly stagnant, but started sinking

into the mantle, which was beginning to cool.

Then, probably no later than 750 million years ago, during the breakup of Rodinia,

the cooler mantle meant that the plates weren’t continuously melting and sticking together.

Separate slabs of rock could interact, forming rift valleys and subduction zones.

Some geologists say this was the beginning of modern plate tectonics, because they have

clear evidence of deep subduction.

For example, they’ve found metamorphic minerals that could only have formed at high pressure,

deep in Earth’s mantle.

And that would’ve been impossible under the earlier type of plate tectonics, when

the plates stayed shallower and softer.

So, experts are still debating whether the earlier period of subduction really “counts”

as the beginning of plate tectonics. And some say that there was even older evidence of

tectonic movement, as far back as 4 billion years ago!

But there’s one thing that they all agree on: plate tectonics helped shape the planet

into the habitable world we know today.

As supercontinents like Rodinia broke up into separate plates, the slabs jostled each other,

smashing together or moving apart.

When two plates separate from each other, oceanic ridges form - underwater mountain

ranges where hot magma constantly comes out and cools, becoming part of the plates. This

process is called seafloor spreading.

Today, seafloor spreading is occuring in several ocean basins, and oceanic ridges are some

of the best places to find hydrothermal activity, where water interacts with hot, fresh, ocean

crust.

These vents are hotspots for biodiversity, especially because they are so rich in iron

and silica, important fertilizers for many life forms.

And moving plates can constantly create new

habitats and destroy others, which promotes rapid diversification of life.

Researchers have shown that biodiversity increases really fast when there’s more continental

fragmentation.

Plus, the arrangement of the continents can impact ocean circulation, climate, carbon

cycling, and many of the other processes that help shape life on Earth.

So it turns out that the Boring Billion wasn’t really boring at all! Earth was just settling

down and getting ready for its next big move.

And today, we’re the only planet known to have this type of plate tectonics.

So, while the movement of continental plates can be destructive, it might’ve also been

pretty important for pushing life past the squishy microbe stage to create the lush,

complex diversity of living things that we know today.

If you’re a fan of Eons, then you’ll love the new PBS three-part natural history series:

Prehistoric Road Trip!

Get ready for an epic adventure through dinosaur country to discover the mysterious creatures

and bizarre ecosystems that have shaped Earth as we know it. With popular YouTube personality

Emily Graslie as host and guide, you’ll travel thousands of miles to visit some of

the most active and dynamic fossil sites in the world.

Prehistoric Road Trip premieres Wednesdays, June 17th-July 1st at 10/9c. Streaming is

available across platforms, including pbs.org and the video app – find out more at the

link in the description.

No-so-boring high fives to this month’s Eontologists: Lucan Curtis-Mahoney, Sean Dennis,

Jake Hart, Jon Davison Ng, Patrick Seifert, and Steve! Become an Eonite at pateron.com/eons

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for more adventures in deep time.