3.E.1 Predicting Volcanic Eruptions

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Del curso dictado por University of Illinois at Urbana-Champaign

Planet Earth...and You!

25 calificaciones

University of Illinois at Urbana-Champaign

Planet Earth...and You!

25 calificaciones

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.

De la lección

Week 3: Volcanoes!

This week, you will learn how and where rocks can melt, and what happens when molten material of various compositions bursts out of the ground. The lecture videos also cover different types of eruptions, as well as the rocks and mountains produced by them. In the lab, you will study details about the 1980 eruption of Mount St. Helens and the eruption of Mount Vesuvius in the year 79. The discussion forum gives you the opportunity to weigh risks to people living on or near volcanoes and what can be done to minimize damage and loss of life. The weekly assignment provides a place for you to share your own experiences with volcanoes or eruptions or, if you have never been near a volcano, your thoughts about such events.

3.E.1 Predicting Volcanic Eruptions7:08

3.E.2 Dealing with Volcanic Hazards6:30

Conoce a los instructores

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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>> What we're going to focus on today is the question of whether or

not it's possible to determine if a volcano is

going to erupt in the not too distant future.

And if so, is there anything we can do about it?

Can we somehow prevent the eruption?

Can we somehow prevent damage from it?

Or can we take steps that will prevent the volcanic eruption from affecting people?

The first thing is, are there any kind of observations that we can make

that will hint that a volcano is becoming active and is about to erupt?

And the answer is fortunately, yes.

Specifically, we can make a variety of observations,

ranging from measuring ground deformation, to detecting the gas that's coming

out of the volcano, to measuring the heat that's being generated from the volcano,

that can lead us to the conclusion that a volcano is becoming more active, and

then an eruption may be imminent.

So let's look at each of these different sources of data separately,

try to understand what they're telling us, and

try to understand how the measurements are being made.

First of all, think about what happens when magma from deep below the surface of

the earth starts to rise, and starts to enter the throat of the volcano.

It's molten rock, it's filling cracks, and it's pushing the cracks open.

And what it's doing, as a consequence, is it's causing the volcano to inflate.

You're familiar with inflating something.

You inflate a balloon.

You inflate a tire.

The surfaces of a balloon start to move apart

as you fill the area inside the balloon with air.

Thus likewise, the surfaces of a volcano are going to start to move apart,

as you fill the interior of the volcano with magma.

In other words, the surface of the volcano can start to tilt.

We can measure that tilting using instruments, and if we see that tilting

happening, we have a clue that magma is starting to fill the interior.

How do you do that?

Well, there's a classic kind of instrument called a tiltmeter that's basically

a water level that's connected by a pipe to a water level at some distance away.

And so when one part of that system or

one of those detectors moves up relative to the other the water level changes.

And we can observe that using the gauge that's on the instrument.

Of course, these days we do these instruments electronically.

There's a bubble level in the interior of the tiltmeter, and

as the tiltmeter starts to change its orientation, electrodes are able

to measure the motion of that bubble and make a record of the tilt of the volcano.

These instruments are extremely accurate.

For example, a volcanologist can detect the tilt to

the surface of a volcano of only 0.00006 degrees.

So what does that mean?

Well, that's like being able to detect a change in

distance the thickness of a coin, over a distance of about a kilometer.

The changes in shape of a volcano can also be detected using satellite imagery.

What it's measuring, using radar, is the distance between the satellite and

the volcano.

So you can imagine a satellite goes around the earth.

It measures the distance to the ground, continues around its orbit, comes back and

sometime later, makes exactly the same measurement.

Well, it's able to tell by just a few wavelengths of the radar beam,

whether the volcanic surface has gone up or down relative to where it was before.

And it can represent that information as a series of color bands.

When the color bands are closer together, there's been more motion.

When the color bands are further apart, there's been less motion.

So you're able to get an image,

by looking at the volcano, of whether it's surface is going up or down.

Of course, another way of doing this is by simply using GPS

over a longer period of time.

Because GPS is now able to detect displacements on the order of a millimeter

over a period of time, and

that allows you to again, be able to see the motion of the volcanic surface.

We can also use another kind of measurement to detect the injection of

magma, and that's the heat flow that's being admitted by the volcano.

Remember, magma is really hot.

And so when the magma gets into the volcano, it starts conducting heat

into its surroundings, and that heat will radiate off the side of the volcano.

We can measure that heat and measure the rate at which heat is being emitted,

and that's called the heat flow.

An increase in heat flow may be a clue that the magma's getting

into the interior of the volcano and may be in a position to start erupting.

Another bit of information that tells us that magma is getting into the interior of

the volcano, that is the gas output of the volcano.

We've talked earlier about how gas is dissolved into the magma.

Well, as that magma comes up to the surface,

the pressure acting on it is less, and so the gas starts to form bubbles.

And those bubbles will be released into the atmosphere and

come out the volcanic vent.

But we can measure, using instruments, the concentration of gases coming out, and

that's telling us that there's more magma coming in to the volcano.

Finally, we can use seismometers to detect microseismicity, meaning very,

very tiny earth quakes that are happening due to the cracking and

breaking of rock within the volcano.

Now why does this happen?

As the magma pushes into cracks that may already exist,

it's going to push the walls of the cracks apart.

And it's also going to cause the cracks themselves to propagate

into unfractured rock.

In other words, it's going to cause rock to start to crack.

That cracking generates vibrations, and

we can record those vibrations in the form of earthquakes with a seismometer.

So what happens is that we can compare the background seismicity of a volcanic area,

sort of infrequent earthquakes that might be happening under normal circumstances,

to an increase in seismic activity that happens

when the volcano may be about to erupt.

Look at these two photographs.

One shows Mount Saint Helens, or a seismic record of Mount Saint Helens over a day,

some time before the catastrophic 1980 eruption, and

the other is over a time period very close to the timing of the eruption.

You can see that there's almost continuous volcanic activity.

Partly that's due to the continuous cracking of rocks

as magma was pushing into the interior of the volcano.

Part of it's actually due to bursting of bubbles and

other activity within the magma itself.

But then the level of seismic

activity is a clue that

the volcano is about to erupt.

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