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2.B.1 Discovering Plate Tectonics

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2.B.1 Discovering Plate Tectonics

In the early twentieth century, publication of the hypothesis on continental drift caused an uproar that soon died down. Data collected in mid-century led geologists to reconsider the idea that continents could move. During the 1960s and 1970s, old ideas were reworked into what is now called the theory of plate tectonics. As we will see, this robust theory encompasses many geological phenomena that appear to be unrelated at first glance: earthquakes and volcanoes, but also ice ages, fossils, and mountains. Today, plate tectonics provides an overarching framework for interpreting the Earth. We study its details in Week 2, but we will return to this theory again and again throughout the rest of this course.

Earthquakes, volcanoes, mountain building, ice ages, landslides, floods, life evolution, plate motions—all of these phenomena have interacted over the vast expanses of deep time to sculpt the dynamic planet that we live on today. Planet Earth presents an overview of several aspects of our home, from a geological perspective. We begin with earthquakes—what they are, what causes them, what effects they have, and what we can do about them. We will emphasize that plate tectonics—the grand unifying theory of geology—explains how the map of our planet's surface has changed radically over geologic time, and why present-day geologic activity—including a variety of devastating natural disasters such as earthquakes—occur where they do. We consider volcanoes, types of eruptions, and typical rocks found there. Finally, we will delve into the processes that produce the energy and mineral resources that modern society depends on, to help understand the context of the environment and sustainability challenges that we will face in the future.

Impartido por:

Dr. Stephen Marshak
Professor and Director of the School of Earth, Society, and Environment
Dr. Eileen Herrstrom
Lecturer

Transcripción

[MUSIC]

What we're going to do today, is basically introduce the theory of plate tectonics,

sort of a mini course on the theory of plate tectonics.

We'll see, that when you take Wegener's concept of continental drift,

add to it, Hess's concept of seafloor spreading, add to that,

all the new data that was discovered during the interval of the 1930s

through the 1960s or so, what comes out is a grand unifying theory.

Now known as, the theory of plate tectonics which we will see,

explains a huge variety of Earth phenomena.

But to start this, we want to review a couple key points about earthquakes.

Because we'll see that,

the nature of earthquakes, was really key to the understanding, or

the development of the theory, of plate tectonics in the first place.

So, remember that when an earthquake occurs,

it usually occurs on a fault, meaning a fracture on which there's sliding,

and that fault is usually underground,

sometimes faults extend all the way to the surface, sometimes faults don't.

The area the fault that slips, and generates the earthquake energy, is called

the focus of the earthquake, or sometimes called the hypocenter of the earthquake.

But when we look at a map of the planet, or when we look at a globe,

we can't actually see those points, because they're underground somewhere.

So what geologists, or seismologists tend to do, is plot a point

on the surface of the Earth called, the epicenter of the earthquake.

So when we see a map, that shows the distribution of earthquakes,

we are looking at what's called the epicenter.

One of the amazing discoveries, that took place in the middle part of the 20th

century, back in the 1940s, and 50s, was the realization that

earthquakes don't really happen randomly around the surface of the globe.

It's not like they're scatter shot,

or buck shot from a shotgun that's spread over a huge area.

In fact, earthquakes cluster in distinct zones, or belts, and

these are called seismic belts, or seismic zones, and

when these are plotted on a map they outline very specific areas of the planet.

So we're looking at a map of the Earth here, and

we're seeing that Earth quakes occur in a belt along the west coast of

South America, they occur along the west coast of Mexico

in portions of the West coast of North America along the coast of Alaska.

In fact, they occur all the way around the Pacific,

as if the Pacific Ocean is seismically active around it's entire edge,

but that's not the only place they occur.

Many earthquakes that people didn't really realize existed,

because they're occurring in the floor of the ocean, out in the middle of the ocean.

Once people learned how to recognize,

or identify the location of earthquakes, they began to realize that there were

belts of seismicity that ran down the interiors of the ocean, for example,

there's one belt that runs down the middle of the Atlantic Ocean.

There's another belt that cuts across the southeastern corner of the Pacific Ocean,

and there's one that runs down the middle of the Indian Ocean.

Also of course,

there are earthquakes that span the major active mountain ranges on the planet.

So for example many earthquakes to curl on the Himalayas, and

earthquakes occur in the Mediterranean region, again,

the point is, that earthquakes do not occur randomly.

The majority of earthquakes occur in distinct belts,

called seismic belts, now as, I mentioned,

until the 1960s, seismologists had no real idea why this happened.

There were some speculations but nothing that really held together as a good idea,

but then between 1960, and 1968,

not one person, but a group, really, of several geologists,

probably about a dozen or so, can really be considered the major contributors

of the idea came up with the idea of the theory of plate tectonics.

And in a nut shell, what plate tectonics says, is that the outer

shell of the Earth, and we'll see that it's a layer called the lithosphere.

That this outer shell is broken into pieces along distinct boundaries, and

that all the movements that generate earthquakes are by and

large concentrated along these boundaries.

So to understand this, we need to step back a little bit, and

first talk about the internal structure of the Earth, because remember, I just said

that there's an outer layer, so if there's an outer layer, is there an inner zone?

We have to understand a little bit about, how the Earth is structured to understand

what this term lithosphere means, and why it's the place where earthquakes happen.

[MUSIC]

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