2.B.2 Earth's Internal Layers and the Concept of a Plate

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来自 University of Illinois at Urbana-Champaign 的课程

Planet Earth...and You!

25 个评分

University of Illinois at Urbana-Champaign

Planet Earth...and You!

25 个评分

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.

从本节课中

Week 2: 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.

2.B.1 Discovering Plate Tectonics4:56

2.B.2 Earth's Internal Layers and the Concept of a Plate7:07

2.B.3 Three Types of Plate Boundaries: Divergent Plate Boundary6:20

2.B.4 Three Types of Plate Boundaries: Convergent Plate Boundary6:25

2.B.5 Three Types of Plate Boundaries: Transform Plate Boundary5:05

2.B.6 Continental Rifts and Continental Collisions5:16

2.B.7 Intraplate Earthquakes6:04

2.B.8 The Velocity of Plate Motion7:45

2.B.9 Plate Driving Mechanisms7:02

与讲师见面

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

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So let's look at a cutaway of the planet.

Geologists recognize, or seismologist recognize

that the internal portion of the Earth has distinct layers.

The outer layer is called the crust, that's what we live on.

Beneath that, is the greatest volume of the planet, a layer called the mantle.

And then in the center of the earth is a dense ball called the core.

So again, the nature of the earth the internal structure of the earth is one

in which the planet is somewhat like an onion.

Now, modern studies have shown fairly accurately

where the boundaries between these layers are.

And I won't go into the details of how these are determined.

But for our purposes, we can recognize that the outer layer,

the crust, is really quite thin.

Beneath oceans it's only on the order of about five to ten kilometers thick.

On average about seven kilometers thick.

That's really not very far at all.

You can run that distance in under an hour easily.

Beneath continents it can be thicker.

It ranges, the thinnest is maybe on the order of 25 kilometers

the thickest is maybe on the order of 70 kilometers.

But most of continental area is the crust is on

the order of 35, 38 kilometers thick.

Again not very thick indeed.

if you compare to the overall diameter of of the earth which is 6371 kilometers,

there abouts.

Beneath that is the mantle.

Geologists divide the mantle into two layers, the outer mantle and

the inner mantle.

The combined thickness of these two is about 2885 kilometers.

The boundary between the two lies at a depth of around 660.

At a fairly discrete boundary at the base of the mantle,

suddenly things change radically, and we enter another realm called the core.

The core really differs from the rest of the earth in that the core is

composed primarily of an alloy of metallic iron.

Whereas the mantle and the crust are composed primarily of rocks.

Now, I mentioned that the core can be separated into an inner core and

outer core.

The inner core is solid iron alloy and the outer core is liquid iron alloy.

In fact it's the circulation of the iron alloy in the outer core

that generates the magnetic field that we have on this planet.

Now using techniques again that are a little bit beyond our scope here,

seismologist have been able to come up with a model for

how earthquake velocity changes or earthquake wave velocity changes

as a function of depth as you go into the earth.

And you can see that gradually, the velocity increases as you

go from the crust into the upper mantle and down to the base of the mantle.

And then there's a sudden decrease in velocity when you enter the iron core and

then the velocity increases again as you go down towards the center of the earth.

So it's these sudden jumps in the velocity of seismic waves,

which seismologist refer to as seismic discontinuities that define precisely

where the boundaries are between the different layers inside the earth.

Now, so far, we've been talking about the layering of the earth from two angles.

We've said that the boundaries are defined by sudden jumps in velocity,

and we can also say that those sudden jumps in velocity

represent changes in composition, fundamental changes in composition.

So the crust has a different composition from the mantel, and

the mantel has a different composition when compared to the core.

Specifically, the crust, well beneath the oceans,

it's composed primarily of a dark gray rock called basalt.

Beneath continents, on average, it has the composition of a lighter colored rock

called granite, and then in the mantle, it's composed of a very dense,

very dark rock called peridotite.

Basically, if we use those three rock names we're accounting for

most of the volume of the rock on this planet.

And then as i mentioned earlier, as soon as we cross the core mantle boundary and

get into the core, we're in a realm where everything is composed of iron alloy.

You can imagine a soft material that's like molasses molasses or

honey, and if you push on it, it will flow.

You can imagine in contrast, a rigid material, say like a block of wood or

a block of wood or a block of rock at the surface of the earth.

If you push on it, it's too hard to flow.

It doesn't flow.

It's too hard to flow.

So we can also make distinctions between the nature of the interior of the earth

by looking at the mechanical behavior.

Sometimes we use the fancier term, rheological behavior.

How a material responds to a push or pull, or

in other words how a material responds to a stress.

And we make the fundamental distinction between plastic materials,

which are able to flow, even if they flow very,

very slowly, and rigid materials which cannot flow.

So if we take that distinction we find that we can divide the uppermost part

of the earth into two different layers that behave fundamentally differently.

The outer layer, is called the lithosphere.

Lithos from the Latin for rock.

The lithosphere includes all of the crust, but

it also includes the uppermost part of the underlying mantle.

That's because the uppermost part of the mantle is the cooler part, where the rock,

even though it's a different composition and everything,

the main point is that that in the upper most part of the mantle is cold enough.

It's still pretty hot.

It's actually really hot, so you'd burn yourself if you touched it, but

it's cool enough so that it's not able to float easily.

Beneath that, at a boundary that's effectively defined by a temperature.

On the order of 1,280 degrees centigrade.

At temperatures greater than that, the peritotite of the mantle, is able to flow.

And, we mark that boundary, as the boundary between the lithosphere above,

and another layer, called the asthenosphere, below.

The asthenosphere, is the layer of the mantle,

that is able to flow relatively easily.

We'll be seeing that the lithosphere is what makes up the plates

in plate tectonics.

So, understanding the concept of lithosphere is very important.

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