2.B.8 The Velocity of Plate Motion

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From the course by University of Illinois at Urbana-Champaign

Planet Earth...and You!

25 ratings

University of Illinois at Urbana-Champaign

Planet Earth...and You!

25 ratings

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.

From the lesson

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

Meet the Instructors

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 we've talked about how the outer layer of the Earth, the lithosphere,

is broken into 15 or 20 or so discrete plates, and

that these plates move relative to one another.

Sometimes moving apart, sometimes moving together, sometimes slipping sideways.

The three different kinds of plate waters.

How fast does this motion occur?

Well, it depends on what your reference frame is.

For example, if you're driving in a car.

And you're driving next to somebody else in a car,

at the same speed down the highway, your relative velocity is zero.

And you could talk to the person out the window,

they wouldn't be moving relative to you.

But of course relative to the ground outside, you're moving.

Now, if you're passing another car, you're travelling faster than that car.

So let's say that this car is going 30 kilometers an hour.

This car is going 60 kilometers an hour,

your relative motion is 30 kilometers an hour.

But relative to a tree, a fixed reference point off to the side of the road,

you're going 60 kilometers an hour.

Geologists make the same distinction between what we call relative

plate motion, which is the motion of one plate relative to it's neighbor or

relative to some other plate, and absolute plate motion, which is plate

motion with respect to a fixed reference frame that's independent of the plate.

Some point say in the interior of the earth that's not moving with the plate.

Now on this map, if you go to a plate boundary like the Mid-Atlantic Ridge,

you see double headed arrows, that's the relative plate motion.

So what we're seeing here, say in the North Atlantic,

is the relative motion of North America, relative to Africa.

And that motion, is about two to three centimeters per year.

That means, that a point on the east coast of the United States, is moving away from

a point, on the west coast of Africa, at about two to three centimeters a year.

That's about that much, it's about the rate that your fingernails grow.

If we were to go to the plate boundary between the Nazca Plate and

the Pacific Plate in the southeastern Pacific,

that plate boundary is called the East Pacific Rise.

It's a spreading ridge like the Mid Atlantic ridge, but

it's different in that it's motion, or the motion occurring along it, is much faster.

In fact, at places, the relative velocity is 18 centimeters per year.

About that far in a given year.

A lot faster than your fingernails grow, but still pretty slow.

You're not going to be able to look at it and say ooh, that plate's moving.

But if you were to measure the two points on opposite sides over the course of

the year, they'd be about that far apart.

So those are relative plate motions.

That means the motion of one plate with respect to its neighbor, and

you can also determine it with respect to other plates on the planet.

But, it's also interesting to understand absolute plate motion,

meaning how fast does a plate move relative someplace in the mantle.

Well, the challenge with that is figuring out what we can use as a reference frame.

And to make a long story short, one of the more visible,

it's not a perfect reference frame, but

one of the more visible reference frames are places called hot spots.

These are places where volcanos happen that are not

necessarily related to activity along a plate boundary.

And they are thought to be associated with rising columns of hot mantle.

We'll talk about that a little bit later.

But in any case, one of those, to give you a sense of it, is Hawaii.

There's Hawaii, out in the middle of the Pacific Plate.

It's not a plate boundary, but it's an active volcano.

So it's called a hot spot.

Now let's take a look a little bit more closely at Hawaii and see how

the geology of the Hawaiian chain can give us insight into absolute plate velocity.

The active volcano of Hawaii is currently on the Big Island.

The other islands of Hawaii are extinct volcanos.

They were active in the past.

And what's most interesting is that the age of the volcanic rocks increases as you

go progressively further to the Northwest, away from the Big Island.

The way geologists explain this now can be seen on this diagram.

What we see is that an island of volcanic rock forms at the top of a mantle plume.

What happens is that rock rises in the mantle plume and

when it gets near the surface, it undergoes melting and

produces molten rock that rises to the surface at the volcano.

The lithosphere plate, shown here in purple,

is moving progressively to the northwest over time.

After a while, it carries the volcano off of the hotspot plume.

And as a consequence, that volcano goes extinct.

Eventually, a new volcano forms above the plume.

With time, that volcano too moves off of the plume and becomes extinct,

and yet another volcano forms above the mantle hot spot.

Now, the result is what we called a hot spot track, and

if you look at a map of the Pacific Ocean floor bathymetry

You can see the track of the Hawaiian hot spot very clearly shown.

Off to the northwest, it merges with another chain

of now submarine volcanic islands, which is called the Emperor Seamount Chain.

The Emperor Seamount Chain is even older.

The oldest of these volcanoes at the northeast end where it's intersecting

the Aleutian Volcanic Arc is about 80 million years old.

The bend in the Hawaiian Emperor Chain

is interpreted to be representative of a change in the direction of motion

of the Pacific plate that happened around 40 million years ago.

Anyway, using what can be called the hot spot reference frame,

or other kind of comparable reference frames.

There are various ways of doing the calculation that are beyond the scope of

this course.

Geologists have come up with calculations of absolute plate motion.

And that is what's represent by these arrows on this map.

And so, relative to a fixed reference frame in the mantle,

you can see North America is moving to the west-southwest.

The pacific plate is moving to the northwest.

So, they're not moving parallel to one another.

And that's why there is relative motion along the plate boundary, so

there are earthquakes along the plate boundary.

You can see that Africa is moving slowly.

Australia is moving faster.

Asia is moving slower.

Now, in more recent time with the advent of GPS,

it's actually possible to measure these motions directly.

Wegener could only dream back in 1915 of being able to observe plate motions.

But they occur so slowly that there was no way that surveying techniques of that era

could possibly reveal plate motions.

But GPS is now so

accurate that you can detect displacements on the order of one millimeter per year,

and with that kind of accuracy, it's possible to actually see plates moving.

So if there's any doubt at all about whether plate tectonics

is happening, forget it.

Plate tectonics is happening and you can see it, and it's recorded by GPS.

Here's an example.

The motion of Turkey.

Turkey is being squeezed to the west as the Arabian plate is

moving northward relative to Asia

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