The World Before Plate Tectonics

Aug 10, 2023

The World Before Plate Tectonics

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There was a time in Earth’s history that was so stable, geologists once called it the Boring Billion. 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.

Thanks to Fabrizio de Rossi for the excellent supercontinent reconstructions: https://www.facebook.com/ArtofFabrici…

This video features a map by the USGS as well as this Paleogeographic Map: Scotese, C.R., 2019. Plate Tectonics, Paleogeography, and Ice Ages, YouTube video: • Scotese Plate Tectonics Paleogeograph… .

Produced in collaboration with PBS Digital Studios: / pbsdigitalstudios

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Content

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

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

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

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

20.359 -> very low in oxygen but really high in sulfur.

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

28.8 -> like cyanobacteria and possibly lichens.

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

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

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

46.74 -> 18% less powerful than it is today.

49.42 -> Even the rocks under the microbes were fairly stable.

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

57.18 -> gotten started yet.

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

63.74 -> And the plates were flimsy and thin.

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

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

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

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

87.58 -> Today, Earth’s plate tectonics cycle is active.

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

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

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

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

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

113.18 -> before plate tectonics.

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

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

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

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

133.56 -> down into the mantle, where it melts.

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

140.23 -> apart, smashed together, and eroded away.

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

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

152.84 -> some are more than 3 billion years old!

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

160.93 -> started, and what it looked like.

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

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

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

177.04 -> as computer models, to start to understand the Archean.

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

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

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

194.76 -> evidence that Earth’s lithosphere was busy.

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

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

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

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

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

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

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

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

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

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

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

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

255.06 -> this really counts as the first evidence of plate tectonics.

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

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

267.72 -> Nuna by 1.8 billion years ago.

270.96 -> Welcome to the so-called Boring Billion.

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

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

285.58 -> And the ocean was very different than it is now.

288.16 -> 2 billion years ago, it’s likely that most of the ocean was very low in oxygen.

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

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

304.56 -> that have an enclosed nucleus in each cell.

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

311.86 -> best of their strange environment.

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

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

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

328.96 -> microbes that can form squishy mats in aquatic environments.

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

337.54 -> today.

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

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

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

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

357.88 -> of complex eukaryotic life.

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

365.1 -> back 1.7 billion to 1.4 billion years ago!

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

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

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

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

388.86 -> Well, life needed Earth to shake things up.

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

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

402 -> around Nuna and the next supercontinent, Rodinia.

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

409.4 -> into the mantle, which was beginning to cool.

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

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

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

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

431.699 -> clear evidence of deep subduction.

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

438.84 -> deep in Earth’s mantle.

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

443.919 -> the plates stayed shallower and softer.

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

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

455.31 -> tectonic movement, as far back as 4 billion years ago!

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

463.59 -> into the habitable world we know today.

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

471.199 -> smashing together or moving apart.

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

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

483.281 -> process is called seafloor spreading.

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

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

496.711 -> crust.

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

503.22 -> and silica, important fertilizers for many life forms.

506.569 -> And moving plates can constantly create new

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

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

518.209 -> fragmentation.

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

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

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

532.86 -> down and getting ready for its next big move.

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

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

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

550.76 -> complex diversity of living things that we know today.

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

562.79 -> Prehistoric Road Trip!

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

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

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

579.089 -> the most active and dynamic fossil sites in the world.

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

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

595.009 -> link in the description.

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

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

608.939 -> and get exclusive access to our Discord and the Eonites Only Podcast - hosted by me!

614.16 -> And thank you for joining me today in the Konstantin Haase studio. Subscribe at youtube.com/eons

619.489 -> for more adventures in deep time.

Source: https://www.youtube.com/watch?v=DI6SemRT2iY