3.C.6 Eruption Styles and Case Studies

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Planet Earth...and You!

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University of Illinois at Urbana-Champaign

Planet Earth...and You!

26 classificações

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.

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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.C.1 Types of Volcanic Eruptions6:51

3.C.2 Magma and Lava with Gas6:43

3.C.3 Eruption Materials6:07

3.C.4 Pyroclastic Debris4:05

3.C.5 Three Different Volcano Shapes5:41

3.C.6 Eruption Styles and Case Studies4:46

3.C.7 Case Studies of Volcanic Explosions: Pompeii, Present-Day Italy9:37

3.C.8 Super Volcanoes in the Geologic Record7:45

Conheça os instrutores

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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Classically, volcanologists, or geologists who specialize in the study of volcano,

have distinguished among many different kinds of eruptions based on case studies,

based on examples of those different kinds of eruptions.

So as you can see here from this diagram, there are lots of different names.

Names aren't really important per se, but

I'll use them to illustrate different kinds of eruptions.

So let's start by looking at a surtseyan type eruption.

This is characteristic of when a volcano is erupting under water.

And so there's a lot of water around.

The water mixes with the magma and the lava coming out.

It super heats and becomes steam.

And that provides a lot of energy so

the eruption basically explodes out of the water.

And you get these very chaotic, explosive burps of debris and

steam coming out of the surface of the sea.

Another type is a Hawaiian eruption.

We've already talked about those.

Those are the ones that are dominated by fountains of lava that then feed

lava flows that flow down the sides of the volcano.

Sometimes these lava flows can go quite fast near the source.

They may reach 30 kilometers an hour.

Of course, further away, they may only move only a few kilometers an hour.

Icelandic eruptions are also lava dominated, or

effusive eruptions, but they occur when a crack develops.

So you get a curtain of lava erupting along this crack,

rather than having a single fountain coming out of a single source.

Strombolian eruptions, these are ones in which the character of the eruption

is such that it sends out lots of volcanic bombs and glowing cinders into the sky.

So if you take a nighttime photograph of these, you'll see these arching like

curves of fire coming out of the top of the volcano and

then plopping down on the side and glowing and tumbling down the side of the volcano.

Now, we'll get to the really, really explosive, eruptions.

These are the pelean ones.

Mt. Pinatubo in the Philippines was

an example.

And, you can see again, that there's a huge amount of pyroclastic debris, blasted

out of the volcano, in a very short period of time, with huge amounts of energy.

These are very dangerous eruptions.

These are the kind that are most typically associated with large scale

destruction and loss of life.

Now let's look at a simplified cross section

of one of these pelean eruptions more closely.

It allows us to talk a little bit about the nature of this

plume of ash that's coming out of the volcano.

What happens is that when the eruption takes place,

the pressure in the lava drives the lava itself up to some elevation,

maybe at most 50, 100, couple hundred meters.

But then gravity stops that flow and the material starts tumbling down.

In addition, all the very fine fragmental debris, fine pyroclastic debris or

ash dominated material starts blasting up into the sky.

Now some of that debris falls back down onto the flanks of the volcano and

avalanches down the sides of the volcano.

These avalanches are usually called ash flows.

So we have ash flows going down the sides of the volcano

in the wake of one of these explosions.

The rest of the debris, the rest of the ash, gets shot up into the air, but it's

the force of the eruption that sends it so high, rather it's the fact that it's hot.

It warms the air around it.

So you have this turbulent mixture of hot air and hot ash.

And it turns out that,

that mixture is buoyant relative to the surrounding cold air.

So just like the plume of smoke coming out of a chimney,

it rises convectively up to higher levels in the atmosphere.

So it may have enough buoyancy that it carries

all the way up to the stratosphere, several kilometers up the surface,

where it gets picked up by the jet stream and then carried around the planet.

So, in sum, we see that there are really three components

to what's coming out of an explosive or pelean type eruption.

There's the explosively rising column.

There's the ash flows going down the side,

come from the collapsing part of the column.

And then there's the convecting part of the column that goes up to

very high elevations.

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