2.B.5 Three Types of Plate Boundaries: Transform Plate Boundary

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来自 University of Illinois at Urbana-Champaign 的课程

Planet Earth...and You!

28 个评分

University of Illinois at Urbana-Champaign

Planet Earth...and You!

28 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

[MUSIC]

So far, we've talked about two of the three types of plate boundaries,

divergent plate boundaries and convergent plate boundaries.

That leaves the third type which we refer to as a transform plate boundary.

It is a plate boundary where one plate slides past one another.

Now, think about it for a second.

If this is the motion that is taking place,

we're not creating new plate material as we do a diversion boundary, and

we're not destroying old plate material or

subducting old plate material, as we have at the conversion plate boundary.

We're simply having one plate slide past another.

And if you remember our description of faults, a fault on which there's

horizontal motion, such as shown, is called a strike-slip fault.

So basically, transform plate boundaries are big strike-slip faults.

Now, where do they occur?

Well, I was a little bit careful when I talked about

all earthquakes at divergent boundaries being associated with normal faulting.

Because it turns out that in general

divergent plate boundaries are not just a continuous band of normal faults.

If we look carefully, we can see the mid-Atlantic ridge.

And we see that it's not just a single curving but

it's broken by east west trending breaks.

Those are the transform faults.

We can label a few of them here.

So the little red segments represent the spreading ridges and

the yellow segments represent the transform faults.

Now, not all transform faults cut across ocean floor.

Not all of them are associated with steps between ridges.

Here's an example of probably one of the most famous transform faults on

the planet, San Andreas fault, California.

It extends from a spreading ridge in the Gulf of California

just south of the US-Mexican border,

northwestward across much of California to where it connects with another spreading

ridge off the coast of northern California and southern Oregon.

There's a little plate between the west coast of the United States and

the Pacific plate, and that little plate is called the Juan de Fuca plate.

There's a chain of volcanoes along the west coast of Oregon and Washington and

a little bit of into Northern California called the Cascade Volcanic Arc.

And that is the Continental Volcanic Arc due to the subduction

of the Juan de Fuca plate beneath North America.

The great earthquakes that happen in California that were responsible, for

example, for the 1906 distraction of San Francisco.

And even bigger earthquake that happened in the area that is now Los Angeles back

or near the area that is now Los Angeles back in 1857 are due

to the stripe slip displacements along the San Andreas fault and related faults.

Notice the sense of slip on the San Andreas fault

is that it is a right lateral fault.

And that means that the Pacific plate is moving to the north,

relative to North American along that fault.

Now because the San Andreas fault for

much of its life is on land, you can stand on it, or near it.

And you can have for example, it's not really just a hairline, but

you can pretty much out one leg on North America, and one leg on the Pacific.

That portion of California that's to the west of the San Andreas Fault is moving

north at about six centimeters a year relative to the plate on the eastern side.

Now all that motion isn't happening at once, because when earthquakes occur,

it's not a continuous, smooth motion.

But, it builds up for awhile and then suddenly that.

Things build up for awhile and then suddenly slip and so

it is a lurch type motion.

We call it stick slip, and so when I say that the plates to the west of

the San Andreas fault is moving six centimeters per year,

that's on average, averaged over many years.

Another major strike-slip fault that cuts across

land is the Alpine fault in New Zealand that cuts across the south island.

Motion on the Alpine fault is not completely strike-slip.

There is some uplift along it, and that has led to the development

of the Southern Alps in the south island of New Zealand.

You've seen that the theory of plate tectonics is a revolutionary idea

that explains many, many earth phenomena.

We've seen already that it explains why most earthquakes occur,

it explains why volcanoes occur.

And we can even use it to understand perhaps subtle differences

such as why different kinds of earthquakes happen at different locations.

[MUSIC]

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