Week 3 Lab: Volcanoes! Lab Introduction

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Del curso dictado por University of Illinois at Urbana-Champaign

Planet Earth...and You!

26 calificaciones

University of Illinois at Urbana-Champaign

Planet Earth...and You!

26 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.

Week 3 Lab: Volcanoes! Lab Introduction 8:46

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

[SOUND]

[MUSIC]

In this video I'm going to talk about our volcano lab, and

give you some introduction so that you can get started on the work of this lab.

In the lecture videos, Dr.

Marshak talked about different kinds of volcanic eruptions.

In this particular lab, we're going to be looking at one specific type of eruption,

and that is explosive eruptions called ash falls.

Geologists refer to ash fall eruptions when a volcano erupts a huge cloud of

ash which rises sometimes very high into the atmosphere and

then slowly drifts down like snow and covers the landscape in a layer of ash.

Typically, these volcanoes will alternate eruptions of ash and eruptions of lava,

and so you get alternate layers of lava and ash making up the body of the volcano.

We call these kinds of volcanoes stratovolcanoes and

that word comes from layers.

One of the specific eruptions we are going to looking at in this lab and

working with was the eruption of Mr Vesuvius in Italy.

That eruption occurred in the year 79 of the Common Era and we

actually have a lot of information about that, even though it occurred so long ago.

First of all, it's estimated it killed around 20,000 people.

It buried at least four roman towns as listed here on the map,

Herculaneum, Oplontis, Stabiae, and Pompeii.

The eruption was an extended one, because the ash fall lasted for a couple of days.

You can see on the map how the winds carried ash off towards the southeast.

After that two days of ash fall, there was then a more serious eruption

of what we called pyroclastic debris, which is kind of a mixture of hot gas and

ash that rushes down the side of the mountain at high speed.

How do we know these things?

Well, geologists have done a lot of studies at Vesuvius,

and in this picture you can see the sorts of observations geologists make.

We drill cores in various places and

look at the layers down into the depth of the cores, and then you try to match up

the layers you see in one place with the layers that you see in another place, and

if you do that in enough places you can build up a story of the eruption.

The Roman town of Pompeii was completely buried by the ash fall eruption.

And it was lost for hundreds of years.

Rediscovered in 1700s, I believe, and in years since then,

archaeologists have excavated the entire town of Pompeii.

Here's a screen shot from Google Earth showing you Mount Vesuvius today.

Mount Vesuvius is basically in the middle of that picture and you can see that it's

completely surrounded by the city of Naples, Italy and its suburbs.

The population of the Naples metropolitan area is roughly 4 million.

It has a very high population density, over 8000 people per square kilometer.

So you can see or

you can imagine that if Vesuvius were to erupt again as it did back then,

that there would be many more than 20,000 people effected by that eruption.

As a consequence geologists are very carefully monitoring Mount Vesuvius

all the time.

And city officials and others are engaged in making emergency plans.

Second ash fall eruption that we're going to be talking

about here is the eruption of Mount St. Helens.

This occurred on May 18th, 1980.

So within living memory at least some people.

So let me briefly go through the sequence of events that led up

to the eruption of Mount St. Helens.

Unlike the eruption of Vesuvius 2,000 years ago, we have lots of

details about how the eruption of Mount St, Helens unfolded through time.

I'll start a couple months before the eruption in March of 1980, where

geologists began to notice an increase in small earthquakes around the volcano.

And the side, one side began to bulge upwards and outwards.

Both of those observations are interpreted in terms of magma beginning to

rise from underneath the mountain and working it's way towards the surface.

The red line marks the original side of the mountain.

And you can see on the right hand side of the photo how the mountainside has swelled

upwards and outwards almost 100 meters from it's original position.

Geologists were watching this mountain very carefully because the rocks that make

up Mount St. Helens are of a composition called felsic.

And lavas with a felsic composition tend to produce very explosive eruptions.

Fast forward then, from March to May of 1980.

These earthquakes continued, the bulge kept growing.

On May 18th, at about 8:30 in the morning,

there were a couple of slightly larger earthquakes, most of them had

the previous earthquakes had been on the order of maybe a magnitude three or four.

There were a couple that were magnitude five.

Those earthquakes shook loose the rocks that made up

the bulge on the mountainside.

Those rocks began to slide downhill and that triggered a big landslide

where essentially the entire bulging side of the mountain went downhill.

In turn, the removal of all that rock

released the pressure on the liquid rock inside the volcano and

at approximately 8:32 in the morning, the eruption actually began.

It started with a blast of ash vertically upwards, and

ash reached a height of approximately 12 kilometers in the atmosphere.

At the same time, or virtually the same time,

there was another blast that came sideways out of the mountain.

And that blast knocked down forests for kilometers away from the mountain.

The ash was carried buy the winds all the way across the continent of

North America until it reached the Atlantic ocean.

By around ten in the morning on May 18th, the eruption was well underway.

Remember the picture I showed you from beforehand,

how the top of the mountain was covered with ice and snow.

Well, you combine ice and snow with hot ash and all that ice and snow melts.

Now you've got water that mixes with the volcanic ash to make mudflows.

The geological term for volcanic mud flows is lahar but

I'll just continue talking about mudflows.

Those mudflows went surging down the valley's all around the mountain and

caused severe flooding in downstream areas.

A mixture of volcanic ash and water is very thick and

it's much more powerful than just plain water.

It's able to move huge boulders as you can see in this picture.

That boulder in the middle of the picture was not there prior to the eruption and

mudflows.

Also look at the trees in background of the image and

you can see the high water mark from the mud left on the tree trunks.

So this was a really large and powerful flood down many of these rivers.

The mud flows extended for tens of kilometers from the mountain.

And, in fact, probably more property damage occurred as a result

of the mud flows than of the actual ash flow of the eruption itself.

As you work your way through this lab,

you're going to be comparing these two eruptions.

Both Vesuvius and Mount St. Helens are stratovolcanoes.

They had similar types of eruptions.

Because we have so much detail about Mount St.

Helens, we can use what we learned from that eruption in order to make

predictions about what could happen in the future around Mount Vesuvius.

[MUSIC]

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