The Plate Tectonics Revolution: Crash Course Geography #19

Summary

The script from 'Crash Course Geography' delves into the theory of plate tectonics, a pivotal concept in earth science that reshaped our understanding of the planet's structure and dynamics. It traces the journey from early speculations about landmass shapes to the comprehensive theory that explains continental drift, mountain formation, and seismic activities. Alizé Carrère narrates the scientific evolution, from Alfred Wegener's hypothesis to the discovery of seafloor spreading and magnetic evidence, culminating in the recognition of tectonic plates' movement and their role in shaping Earth's landscapes and geological phenomena.

Takeaways

• 🌏 The 1960s revolutionized our understanding of Earth's structure and behavior through the theory of plate tectonics.
• 🧩 The theory of plate tectonics was developed over many years by piecing together various scientific findings, not by a single individual.
• 🏞 The lithosphere, which includes the crust and uppermost mantle, is broken into tectonic plates that move independently.
• 🌐 Alfred Wegener proposed the idea of continental drift and the existence of a supercontinent called Pangaea in 1912.
• 🌊 Bruce Heezen and Marie Tharp's mapping of the Atlantic Ocean floor revealed the Mid-Atlantic Ridge and other features, challenging previous assumptions.
• 🔥 Harry Hess proposed seafloor spreading, describing the seafloor as a conveyor belt of magma and new crust formation.
• 🧲 Paleomagnetism provided evidence for seafloor spreading, with magnetic patterns in rocks recording Earth's magnetic field reversals.
• 🌋 Subduction zones, where one tectonic plate is pushed under another, were discovered through seismic monitoring during the Cold War.
• 🌈 Plate tectonics is a unifying theory in earth science, influencing all aspects of our understanding of the planet.
• 🌍 The theory describes how the Earth's continents, oceans, mountains, and valleys were formed through the movement of tectonic plates.
• 🛰️ Modern technology, including satellite imagery, continues to refine our understanding of plate tectonics and its effects on Earth's surface.

Q & A

What major shift in understanding occurred in the 1960s regarding the Earth's structure and behavior?

In the 1960s, the theory of plate tectonics was developed, which revolutionized our understanding of the Earth's structure and behavior, explaining the formation of continents, oceans, mountains, valleys, volcanoes, and the occurrence of earthquakes.

What is the lithosphere and how is it related to plate tectonics?

The lithosphere is the rocky outer part of the Earth that includes the crust and the uppermost part of the mantle. It is broken into tectonic or lithospheric plates that move independently of each other, which is the basis of plate tectonics.

What evidence did Alfred Wegener use to support the idea of continental drift?

Alfred Wegener used the spatial distribution of fossils, location of rock types, and trends of mountain ranges as evidence to support his hypothesis of continental drift, suggesting that the Earth's land was once a single supercontinent called Pangaea.

What was the significance of the physiographic map of the Atlantic Ocean floor published in 1957?

The physiographic map of the Atlantic Ocean floor revealed that the ocean floor had mountains, valleys, and deep trenches, contrary to the previously held belief that it was a flat featureless plain. This discovery was crucial in supporting the idea of continental drift and plate tectonics.

What is the concept of seafloor spreading as proposed by Harry Hess?

Seafloor spreading is the concept that new seafloor is formed on either side of the mid-oceanic ridges as magma flows out and pushes away the old seafloor. This process was proposed by Harry Hess and is a key component of plate tectonics.

How did the paleomagnetism evidence support the theory of seafloor spreading?

Paleomagnetism evidence, which records the Earth's magnetic field reversals in rocks, showed a magnetic barcode of black and white stripes around the mid-ocean ridges, indicating that the seafloor was built as magma on either side spilled out and spread laterally, supporting the theory of seafloor spreading.

What is the significance of the discovery of deep earthquakes beneath ocean trenches?

The discovery of deep earthquakes beneath ocean trenches revealed the process of subduction, where the denser oceanic plate is pushed under the lighter continental plate, forming a subduction zone and a trench. This process is a critical part of the plate tectonics theory.

What are the three types of plate boundaries and how do they differ?

The three types of plate boundaries are divergent, convergent, and transform. Divergent boundaries occur where plates move away from each other, allowing magma to well up and create new crust. Convergent boundaries occur where plates move towards each other, with one plate being subducted beneath the other, causing earthquakes and volcanic activity. Transform boundaries occur where plates slide past each other horizontally, often causing earthquakes due to built-up stress.

How do the movements of tectonic plates relate to the formation of mountain ranges like the Himalayas?

The Himalayas were formed by the collision of two continental plates, the Indian plate and the Eurasian plate. When these plates converge, neither is subducted due to their similar densities, resulting in the compression and folding of the crust, which pushes up to form large mountain ranges.

What is the Pacific Ring of Fire and why is it significant in the context of plate tectonics?

The Pacific Ring of Fire, also known as the Circum-Pacific Belt, is an area surrounding the basin of the Pacific Ocean where a large number of volcanoes and earthquakes occur. It is significant in plate tectonics because it is the result of the movement and interaction of tectonic plates, particularly along convergent and transform boundaries.

How has the understanding of plate tectonics evolved since its initial proposal, and what are some current areas of research?

Since its initial proposal, the understanding of plate tectonics has been fine-tuned with advancements in technology and scientific knowledge. Current areas of research include understanding the causes of the Earth's outer shell cracking apart, the beginnings of crust recycling, and comparisons with other planets like Venus to explore why Earth has plate tectonics while Venus does not.

Outlines

00:00

🌏 The Birth of Plate Tectonics Theory

This paragraph introduces the revolutionary theory of plate tectonics that reshaped our understanding of Earth's structure and behavior. It highlights the collective effort of scientists over many years to piece together the puzzle of Earth's broken outer shell, which rises from and falls back into the mantle, recycling Earth's material. The theory explains the formation of continents, oceans, mountains, valleys, volcanoes, and earthquakes. It is compared to other grand unifying theories in biology and physics and is described as being 4.5 billion years in the making. The script also touches on the early speculations about Earth's landmasses fitting together like a jigsaw puzzle and the concept of the lithosphere. The paragraph concludes with the introduction of Alfred Wegener's hypothesis of continental drift and the supercontinent Pangaea, supported by evidence from fossils, rock types, and mountain ranges.

05:04

🌊 Oceanic Discoveries and Plate Movements

This section delves into the mid-20th-century discoveries that led to the revival and eventual acceptance of the continental drift theory. It discusses the creation of the first physiographic map of the Atlantic Ocean floor by Bruce Heezen and Marie Tharp, revealing the presence of the Mid-Atlantic Ridge and other oceanic features. The paragraph explains Harry Hess's conveyor belt hypothesis of seafloor spreading, where new seafloor is created at mid-ocean ridges and old seafloor is recycled in the mantle. It also covers the evidence supporting this theory, found in the magnetic stripes recorded in the seafloor rocks due to Earth's magnetic field reversals. The paragraph further explains how these findings provided the missing evidence for Wegener's theory, demonstrating the movement of Earth's landmasses and the youth of the seafloors.

10:06

🏔️ Subduction, Mountain Building, and Earth's Dynamic Processes

This paragraph explores the process of subduction, where oceanic crust is pushed under continental crust, forming deep trenches and subduction zones. It discusses how observations from a global seismic surveillance system led to the understanding of deep earthquakes and the visualization of these processes. The paragraph also describes how precise mathematical calculations and new computing power allowed geophysicists to predict tectonic plate movements and fit coastlines together, leading to the formulation of the plate tectonics theory. It explains the movement of the seven major plates and micro plates across the asthenosphere and the geological activities that occur at plate boundaries, such as the Pacific Ring of Fire. The paragraph concludes with the ongoing exploration of plate tectonics, the debate over landmass formation, and the comparison of Earth's tectonics with that of Venus.

🌱 The Impact of Tectonics on Life's Evolution

The final paragraph discusses the emerging connections between deep Earth processes and the evolution of complex life. It suggests that continental collisions and mountain-building events may have provided significant nutrient pulses to the biosphere during critical evolutionary periods, such as the Cambrian explosion. The paragraph emphasizes that the understanding of plate tectonics is still evolving, with scientists continuing to explore its implications. It concludes by inviting viewers to join the exploration in future episodes of Crash Course Geography, which focuses on how volcanic and tectonic activity shapes our landscapes.

Keywords

💡Plate Tectonics

Plate tectonics is a scientific theory that describes the large-scale motion of Earth's lithosphere. It explains the movement of the Earth's crust and the formation of various geological features such as mountains, valleys, and volcanoes. The theory is central to understanding the video's theme as it is the 'grand unifying theory' that has reshaped our view of Earth's structure and behavior over the past 4.5 billion years. The script mentions how the theory was developed over many years, integrating various pieces of evidence to explain the Earth's broken outer shell and its recycling process.

💡Lithosphere

The lithosphere is the rigid, outermost layer of the Earth, comprising the crust and the uppermost part of the mantle. It is a key concept in the video as it is the layer that is broken into tectonic plates, which are the subject of plate tectonics. The script refers to the lithosphere when discussing the Earth's landmasses and how they fit together like a jigsaw puzzle, indicating the historical connection between continents.

💡Continental Drift

Continental drift is the hypothesis that the Earth's continents move across the planet's surface over geological time. Proposed by Alfred Wegener in 1912, it was initially met with skepticism but later became a foundational concept of plate tectonics. The script uses the term to describe the process by which the supercontinent Pangaea broke apart, leading to the current distribution of landmasses.

💡Pangaea

Pangaea, meaning 'all earth' in Greek, refers to the supercontinent that existed during the late Paleozoic and early Mesozoic eras. The concept is integral to the video's narrative as it represents the starting point of continental drift, with the script explaining how Pangaea eventually broke apart into the continents we know today.

💡Mid-Atlantic Ridge

The Mid-Atlantic Ridge is a massive underwater mountain range that runs down the center of the Atlantic Ocean. It is a critical piece of evidence for plate tectonics, as discussed in the script, and is associated with the seafloor spreading hypothesis. The ridge is where new oceanic crust is formed through volcanic activity, contributing to the movement of tectonic plates.

💡Seafloor Spreading

Seafloor spreading is the process by which new oceanic crust is formed at mid-ocean ridges and then moves away from the ridge as more material is added. It is a key mechanism in plate tectonics and is highlighted in the script as the process that provides evidence for the movement of Earth's landmasses, supporting Alfred Wegener's theory of continental drift.

💡Paleomagnetism

Paleomagnetism is the study of the record of Earth's magnetic field as preserved in rocks. The script mentions paleomagnetism as a crucial line of evidence for plate tectonics, as the magnetic stripes on the seafloor, created by periodic reversals of Earth's magnetic field, provide a historical record that supports the theory of seafloor spreading.

💡Subduction

Subduction is the process where one tectonic plate moves under another and is forced to descend into the mantle. It is a key concept in the video as it explains how the Earth's crust is recycled. The script describes subduction zones, such as the Peru Chile Trench, where the oceanic plate is pulled beneath the continental plate, leading to earthquakes and volcanic activity.

💡Plate Boundaries

Plate boundaries are the edges of tectonic plates, where they interact with each other. The script discusses three types of plate boundaries: divergent, where plates move apart; convergent, where plates move towards each other; and transform, where plates slide past each other. These boundaries are critical to understanding the Earth's geological activity, including earthquakes and volcanic eruptions.

💡Pacific Ring of Fire

The Pacific Ring of Fire, also known as the Circum-Pacific Belt, is a region around the Pacific Ocean where a large number of earthquakes and volcanic eruptions occur. The term is used in the script to illustrate the areas of high geological activity, which are a direct result of plate tectonics, particularly at convergent and subduction zones.

💡Continental Shield

A continental shield is a part of the continental crust that is ancient, stable, and typically composed of igneous and metamorphic rocks. The script refers to continental shields as the nuclei from which continents 'grow', suggesting that they are the foundation upon which other crustal fragments are added through processes like collision.

Highlights

The 1960s revolution in understanding Earth's structure and behavior through the theory of plate tectonics.

The theory of plate tectonics as a unifying concept in Earth science, akin to Darwin's theory of evolution and Einstein's theory of relativity.

The lithosphere is broken into tectonic plates that move independently, a radical idea when proposed.

Alfred Wegener's hypothesis of continental drift and the supercontinent Pangaea, supported by fossil and rock evidence.

Post-World War II oceanographic evidence revived the idea of drifting continents, including the discovery of the Mid-Atlantic Ridge.

Bruce Heezen and Marie Tharp's mapping of the Atlantic Ocean floor, revealing underwater mountains and valleys.

Harry Hess's hypothesis of seafloor spreading as a conveyor belt of new ocean crust formation and old crust recycling.

The discovery of magnetic stripe evidence on the seafloor, supporting the theory of seafloor spreading.

The global seismic surveillance system's unintended role in observing deep earthquakes and the process of crust subduction.

The formation of the theory of plate tectonics through the integration of ocean mapping, paleomagnetism, and crust subduction.

The movement of tectonic plates at a rate similar to human fingernail growth, across the asthenosphere.

The dynamic geological activity at plate boundaries, including the Pacific Ring of Fire and its associated earthquakes and volcanoes.

The three types of plate boundaries: divergent, convergent, and transform, with examples such as the East Pacific Rise and San Andreas Fault.

The formation of mountain ranges like the Himalayas due to continental plate collision and compression.

The ongoing refinement of plate tectonics theory and current explorations into the origins of Earth's crustal recycling.

The use of modern satellite technology to detect plate motion and the broader implications of plate tectonics for the evolution of life on Earth.

Transcripts

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 planet – how 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, meaning “all earth” in 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 theory” in 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 continents “grow” from 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

complex life are emerging.

Continental collisions and mountain building events may have supplied large pulses of nutrients

to the biosphere during key moments of evolution -- like during the Cambrian explosion of 500

million years ago.

So that quiet plate tectonics revolution that changed our understanding of the Earth is

kinda still happening.

And we’ll keep exploring these revelations next time when we look at how volcanic and

tectonic activity shapes the landscapes we call home.

Thanks for watching this episode of Crash Course Geography which is filmed at the Team

Sandoval Pierce Studio and was made with the help of all these nice people.

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