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  • In the 1960s a quiet revolution took place that shifted our entire understanding of how

  • the Earth works.

  • Like many revolutions throughout history, it's not a single idea that came from a

  • single person.

  • But eventually we pulled all that science together to create the theory of plate tectonics

  • which explains the structure and behavior of our home planethow we got continents

  • and oceans, mountains and valleys, volcanoes and earthquakes.

  • It took many scientists many years to put together all the puzzle pieces and tell the

  • story of how Earth's broken outer shell rises from the mantle and falls back in.

  • Kind of like a dance of creative destruction and reconstruction that recycles earth material

  • between the crust and mantle.

  • And like Darwin's theory of evolution in biology and Einstein's attempts at a theory

  • of relativity in physics, plate tectonics influences everything we know about earth science.

  • It's the grand unifying theory, and it was four and a half billion years in the making.

  • I'm Alizé Carrère, and this is Crash Course Geography.

  • INTRO

  • Once we started getting a complete picture of the globe in the 16th century, we started

  • speculating about the shape of the Earth's landmasses and if they once fit together like a jigsaw.

  • Today we refer to these landmasses as part of the lithosphere, the rocky outer part of

  • the Earth that includes the crust and the uppermost part of the mantle.

  • And we know it's broken into tectonic or lithospheric plates that each move independently

  • of each other.

  • But even though Africa and South America look like they fit together pretty neatly, all

  • Earth's land being linked together long ago was a pretty radical theory.

  • It would take over 300 years for astronomer and meteorologist Alfred Wegener to propose

  • the idea again in 1912 and back it up with evidence.

  • Using the spatial distribution of fossils, location of rock types, and trends of mountain

  • ranges -- among other evidence -- he hypothesized that approximately 225 to 300 million years

  • ago, the Earth's land was a single supercontinent called Pangaea, meaningall earthin Greek.

  • It broke apart in a process called continental drift and eventually led to the landmass distribution

  • we have now.

  • Which was an outrageous proposal to the rest of the scientific community -- or, you know,

  • geologists who were upset that a non-geologist was horning in on their turf.

  • And to be fair, despite all his evidence, Wegener didn't have an explanation for the

  • energy needed to break apart huge chunks of continents and plough them through the oceans.

  • It wouldn't be until after the Second World War when new evidence emerged from the depths

  • of the oceans that the idea of drifting continents was reactivated.

  • In 1957 the first physiographic map showing the physical features of the Atlantic ocean

  • floor was published by two geologists Bruce Heezen and Marie Tharp.

  • Using a type of sonar device called a continuous echo sounder, Heezen collected bathymetric

  • soundings, which are different depth measurements.

  • This was the early 1950s and Tharp couldn't go on research expeditions because she was

  • a woman, so she converted the raw data into maps that revealed how the ocean would look

  • if drained of water.

  • The centerpiece of their map was a vast mid Atlantic mountain spine crisscrossed by huge

  • fracture lines, called the Mid-Atlantic Ridge.

  • Heezen and Tharp focused first on the Atlantic, but it's just one of several mid-oceanic

  • ridges that extend for over 60,000 kilometers across different oceans.

  • Heezen and Tharp changed how we thought about the Earth because the ocean floor wasn't

  • just a flat featureless plain like we'd assumed, but had mountains, valleys, and even

  • deep trenches which are the deepest feature on the planet.

  • Then in 1960 Harry Hess, a geologist who had collected vast amounts of ocean data when

  • he captained a ship equipped with an echo sounder during the Second World War, proposed

  • that even more was happening on the seafloor.

  • Magma spills out from the fracture lines of the mid-oceanic ridges.

  • So Hess proposed that the seafloor was kind of like a giant conveyor belt: new seafloor

  • was formed on either side of the ridges as magma flowed out and pushed away the old seafloor.

  • And when it finally reached the distant trenches, the old ocean crust was cooled and dragged

  • down into the mantle and recycled.

  • Which was another radical theory.

  • Though geologist and oceanographer Robert Dietz published a similar idea he called the

  • spreading seafloor theoryin 1961.

  • But the evidence was actually recorded in the seafloor itself and published a few years

  • later in 1963 by geologists and geophysicists Fred Vine and Drummond Matthews.

  • You see, the Earth's magnetic field reverses periodically and this is recorded in rocks

  • that contain iron as part of the Earth's paleomagnetism.

  • When magma cools and crystallizes, the alignment of the magnetic field is locked in place in

  • the magnetic particles of the rocks.

  • So each time the Earth's magnetic field flipped, the magma erupting at the mid-ocean

  • ridges recorded the opposite polarity to the previous batch.

  • The result is a magnetic barcode of black and white stripes that mark where polarity

  • changed, arranged symmetrically around the ridge.

  • That means Hess and Dietz were really onto something, and mid-oceanic ridges were built

  • as magma on either side spilled out and spread laterally.

  • This seafloor spreading pushes the seafloor away in both directions and with it, the Earth's landmasses.

  • Which meant we finally had the evidence Wegener was missing in 1912 for how the Earth's

  • landmasses were moving.

  • Thanks to seafloor spreading, all of the Earth's seafloors are quite young (for rocks).

  • In fact, determining the age of the basalt rock in the seafloor confirmed the matching

  • patterns of magnetic histories.

  • As for the crust being destroyed in the vast ocean trenches, it turns out that's exactly

  • what happens too.

  • In the late 50s and early 60s during the Cold War, when nuclear test bans were being negotiated

  • between the USA and USSR, a global seismic surveillance system was created to monitor

  • underground blasts.

  • Which also happened to provide North American seismologists with observations of deep earthquakes

  • more than 700 kilometers beneath ocean trenches.

  • These observations were later used to visualize a thick slab of Pacific ocean floor that was

  • being pushed under the edge of a different slab of Earth's crust and consumed into

  • the mantle in a process called subduction.

  • Because the oceanic crust has a greater density than the continental crust, the thinner denser

  • oceanic plate dives beneath the lighter, thicker and more buoyant continental crust.

  • This forms a subduction zone and what we see on the surface is a giant trench.

  • So with ocean mapping, seafloor spreading, paleomagnetism, and crust subduction becoming

  • confirmed pieces of scientific knowledge, the world was now on the brink of a revolution.

  • The final piece of evidence needed to produce a grand unifying theory of earth science came

  • from precise mathematical calculations combined with new computing power.

  • With these techniques, geophysicists were able to calculate how the coastlines of the

  • Americas, Africa, and Europe best fit together and predict how the tectonic plates moved.

  • And with that, a revolution unfolded in earth science.

  • Leading the charge was a bunch of mostly young, unknown scientists working in a handful of

  • institutions in North America, Britain, and Europe.

  • Fueled by the technology, money, and military in those places, they peered into the depths

  • of the oceans and fundamentally changed the way we understand the Earth.

  • Years of independent research and disparate discoveries finally described the structure

  • of the Earth's surface and led to a map of the world divided into moving plates and

  • created the theory of plate tectonics.

  • Moving on average as fast as our fingernails grow, the seven major plates and a scattering

  • of micro plates glide across the weaker, hot, plasticy section of the mantle called the

  • asthenosphere.

  • And we know now that where these plates meet are dynamic places where much of the planet's

  • geological action happens, like earthquakes and volcanic activity.

  • Like the area surrounding the basin of the Pacific Ocean is known as the Pacific Ring

  • of Fire or the Circum-Pacific Belt because approximately 75 percent of all volcanoes

  • are dotted around it, and 90 percent of earthquakes occur along its path.

  • At the edges of the Ring of Fire, the plates come together in 3 different types of boundaries.

  • On the eastern side, the seafloor spreads from the mid-ocean ridge called the East Pacific

  • Rise that runs along the eastern edge of the Pacific plate from near Antarctica all the

  • way to North America.

  • It's a divergent plate boundary, or place where plates are moving away from each other,

  • with the Nazca plate moving east and the Pacific plate moving northwest.

  • Along divergent plates magma can well up and the seafloor regenerates and spreads.

  • But farther south and west of South America, the Peru Chile Trench marks the subduction

  • zone where the denser oceanic Nazca plate collides with and is pulled beneath the lighter

  • continental South American plate creating a convergent plate boundary.

  • As the Nazca plate is dragged down, enormous friction produces major earthquakes and hundreds

  • of meters of sediments are carried down into the deep trenches.

  • As the sediments melt, they turn into magma which migrates up into the overriding plate.

  • And where it reaches the surface, we get a volcano.

  • Like the Andes formed from plates colliding along a convergent plate boundary and have

  • many volcanoes.

  • Then circling north again we find the San Andreas Fault along the west coast of North America.

  • It lies on a transform boundary, where the North American plate, moving roughly southwest,

  • is sliding horizontally past the Pacific Plate moving northwest.

  • Where the plates touch, they can get stuck and stress builds up as the rest of the plate

  • continues to move.

  • The stress causes rocks to break, suddenly lurching the plates forward and causing earthquakes.

  • So plates are moving away from each other, moving towards each other, and sliding past

  • each other, but there's one more type of boundary that's not very common along the Ring of Fire.

  • When continental crust converges with oceanic crust, the ocean crust usually gets subducted.

  • But when two continental plates collide, neither plate is subducted.

  • The collision compresses the crust, folding and pushing up huge mountain ranges.

  • Like the Himalayas -- they sit where the Indian plate is converging with the Eurasian plate.

  • So even as we speak, the structure of the Earth is changing as plates move all over

  • the world.

  • In the years following the revolution, the plate tectonics theory has been fine-tuned.

  • And while we know a lot about how new ocean floor is created, how landmasses form is also

  • being debated.

  • We think continentsgrowfrom a nucleus of ancient and stable igneous and metamorphic rocks.

  • And where those rocks are exposed at the surface is called a continental shield.

  • And fragments of the crust that might originally have been offshore island arcs, undersea volcanoes,

  • or islands like New Zealand or Madagascar, are added to the main continent by collision.

  • Today in 2021, we continue to explore plate tectonics.

  • Plate motion is detected by satellites like the European Sentinel series which record

  • changes in Earth's surface down to the millimeter.

  • The story of fragmented lithospheric plates moving around the Earth's surface has had

  • many twists and turns, but it's not over.

  • Scientists want to know what caused the outer shell to crack apart in the first place and

  • how the recycling of the crust began.

  • And they're even comparing Earth's plate tectonics with Venus and asking why Earth

  • has plate tectonics and Venus doesn't.

  • As we learn more, crucial connections between deep earth processes and the evolution of