2.B.4 Three Types of Plate Boundaries: Convergent Plate Boundary

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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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The second type of plate boundary, so

called conversion plate boundaries where two plates are moving together.

Another term that geologists use for them, are use for

these boundaries is subduction zone.

Because, what happens is as I mention they just don’t push together like that.

Rather one plate sinks and heads back into the mantel beneath the other plate.

And that process is called subduction, now when subduction takes place,

one plate shears against the other, as the plate bends and

starts to subduct, it breaks.

And that breaking, causes generation of some earthquakes,

also as the subducting plate shears and

slides against the base of the overriding plate, it also generates earthquakes.

And as we'll see later, earthquakes can develop in the descending plate

even deeper in the Earth, but the net result

is that we get lots of seismic activity at a conversion plate boundary.

And that shearing creates reverse faults, and

those slip on those reverse faults generates the earthquakes.

Of convergent plate boundaries, so we have this process called subduction

that takes place on the convergent plate boundary, we'll see later that when

the sinking slab or what geologist refer to as the downgoing slap

reaches depth inside the mantle, it warms up sufficiently.

That the process of melting is triggered, it's not actually that the plate melts.

We'll see that what happens is that gasses come off of the plate, and

they cause the overlying mantle to melt.

But, are point is that melting often accompanies the process or

always accompanies the process of subduction.

And as a result, a chain of volcanoes forms above the downgoing slab,

that chain forms on the edge of the overriding slab.

We call that chain of volcanoes a volcanic arc,

if an oceanic slab is going underneath another oceanic slab,

then the volcanic arc that's formed is a chain of islands called an island arc.

If the chain of volcanoes forms along the edge of a continent,

then it's called a continental volcanic arc.

So, let's take a little voyage, a little field trip to the west coast of

South America and look at a map on Google Earth that shows South America.

And what we see is the Pacific Ocean floor,

it's moving to the east relative to the continent of South America,

which is part of the South Atlantic Plate moving towards the west.

The Pacific Ocean floor sinks underneath the continent, and

that generates a chain of volcanoes along the Andes, it also creates

a long linear, very deep portion of the ocean floor that's called a trench.

In this particular case it's called the Peru-Chile trench, so

where we have subduction taking place, it's marked topographically by a trench

on the sea floor and geologically by a volcanic arch on the over-riding plate.

And then of course,

all the earthquakes that happen because two plates are shearing past one another.

Here's an example of an island arc on the western side of the Pacific Ocean floor,

where the Pacific Ocean floor is sliding back into the mantle or

being subducted beneath the Philippine plate.

It creates a very deep, trench called the Mariana trench and

a chain of volcanoes called the Mariana island arc.

The Mariana trench includes the deepest parts of the ocean floor,

because there's no continent nearby to shed sediment and to fill it up,

and also the ocean flow is very old and fairly low elevation to start with.

So this is where the deepest part of the ocean floor is there are places in

the Mariana trench where the ocean floor is about 11 kilometers deep,

as opposed to most of the ocean where it's only about four to five kilometers deep.

Now, we've already noticed

that lots of earthquakes occur at convergent plate boundaries.

All you need to do is look at a map of epicenters to see the seismic belts

that are associated with convergent plate boundaries,

such as in the Peru Chili trench and at the Mariana trench.

If we now look instead at a cross-section showing the depth of earthquake foci,

we see that there are some shallow earthquakes

due to the faulting that happens where subducting plate bends,

or where it sheers against the base of the overriding plate, but in addition there

are some intermediate and deep earthquakes in the downgoing slab.

They actually occur within the subducted plate down to great depth.

Down as far as maybe 600 kilometers or so into the mantel, and

it's now recognized that these earthquakes can be used.

To track the location of the plate as it sinks back down into the mantle,

the discovery of that was attributed to two seismologists Wadati and

Benioff, and that's sometimes called the Wadati-Benioff zone,

or the Benioff-Wadati zone depending on where you're from.

In recent years, new seismological techniques

have actually allowed geologists to image the position of the downgoing plate.

Because the downgoing plate tends to be cooler earthquake waves travel through it

more quickly than they do through the surrounding warmer as thinnest prior and

thus this shows up as a seismic velocity anomaly.

Basically, it means that the earthquake waves travel at a different rate

than what they would travel at if the plate were not as cool as it is.

Using these techniques, geologists are able not only to see the plate down

to 600 kilometers, or so where there are also earthquakes, but

in some cases, contract the plates even deeper into the mantel.

In effect we can see that in the deeper part of the earth,

in the lower mantle there is a grave yard of old plates that have accumulating over

a long period of geologic time.

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