3.C.2 Magma and Lava with Gas

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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 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

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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The other aspect, and I've mentioned that the composition of the lava,

specifically the proportion of silica in the lava, is a fundamental control on

it viscosity and therefore a fundamental control on the nature of the eruption.

The other aspect that determines how an eruption behaves is the amount of gas,

or volatile elements that are within the lava.

Now deep in the earth under the high pressures that occur deep in the earth,

volatile elements, specifically water, and carbon dioxide, and some other gases,

are dissolved in the lava.

It's sort of like if you have a bottle of soda you don't see any gas bubbles in

here, because their dissolved within a soda.

But if I shake this up a little bit and release the pressure,

suddenly you see bubbles rising from depth.

So, my point is that at conditions deeper in the Earth,

when the lava is still really magma, it contains dissolved volatiles.

When that magma rises close to the surface into the throat of a volcano and

the pressure is less, those volatiles come out of solution and form bubbles.

That is the gas part of a volcanic eruption.

Now, sometimes there's more gas, sometimes there's less gas.

When you have a lot of gas that changes the behavior of the lava quite a bit.

It can make it seem less viscous.

In any case, when you have a lot of gas present,

under certain conditions, pressure can build up in that gas, because it may

be trapped inside the throat of the volcano by some solid lava above it.

And that pressure can build up until it becomes greater than the strength of

the overlying cap and can trigger a sudden catastrophic explosion.

One other point about the presence of gas is that as gas comes out of solution

it makes the lava overall more buoyant and it makes it rise higher and

faster in the crust, and that helps to bring it out at the surface.

Now, I don't mean to imply that gas only occurs in felsic lavas.

Sometimes gas does occur,

in fact frequently, gas occurs in mafic lavas as well and basaltic lavas.

What happens, though, is that because the lava itself

has such low viscosity the gas bubbles, like in a bottle of soda,

are able to rise to the surface faster than the lava is rising.

When they get to the surface, they burst.

And just like when you pour soda into a glass, the bubbles burst and

get your mouth wet before you drink the soda.

Similarly, the bubbles in a lava burst, and

they eject spatters of lava up into the sky.

And they can produce enough energy to actually cause the lava to fountain out

at the surface.

So for example, what we're seeing here is a lava fountain.

When you see a lava fountain like this,

keep in mind that the magma that is sourcing this lava is rising from below.

As it rises the bubbles are also rising with it,

and sometimes even faster than the lava.

As they expand, they're increasing the pressure within the lava.

So when the lava reaches the surface it's under so much pressure that it squirts out

and ejects into the sky, sometimes 10 to over 100 meters, in some cases.

Also, as the bubbles burst, they send spatters of lava out to the side that,

as we will see, cool into little pellets of solid rock.

So in effect, this is a vigorous eruption, but it's not quite the same as

an explosive eruption that's shooting huge quantities of ash into the sky.

Now, let's go back and talk about what happens in the case of a rhyolitic lava.

Or rhyolitic magma.

Remember the rhyolitic magma is rising from depth

it contains a lot of dissolved gas.

That dissolved gas is coming out of solution, but

because the rhyolitic lava is so

viscus those gas bubbles can't escape, but they're trapped in the lava itself.

So you can imagine, if we look at this little cross sectional sketch of

a volcano, that somewhere at depth, here's the rising magma,

and bubbles are beginning to form within that magma.

But they can't escape, they can't pop out like they do from a basaltic lava, or

perhaps more in your everyday experience,

like the do out of a can of carbonated water.

Now, look at one of these bubbles.

At depth, remember there's magma

rising over the top of this so that this is now at depth beneath the volcano.

That bubble is constrained.

As it rises, still further, that bubble's going to try to expand,

so in other words, this is moving upwards to here.

That bubble's going to try to expand, but

it can't because it's surrounded by solid rock.

So the pressure in that bubble becomes extreme.

And now we've got lots of bubbles around it.

You can imagine that all of these bubbles are under extreme pressure.

Now what happens during an explosive eruption is that, finally,

the pressure in these bubbles gets to be great enough that it fractures

the little grid work of solid rock around it.

And when that happens the whole system collapses.

And suddenly it's like a shotgun explosion.

Suddenly, all that gas plus the pulverized rock ejects out of the volcano.

And you end up with an absolutely immense explosion.

So, it sort of happens in stages.

Bubbles that form at depth, bubbles rise to shallower level and

become under greater pressure.

Finally, the material containing the bubble's fragments, and

with all that pressure, like a shotgun blast, it sends all the material

out the throat of the volcano, into a giant eruption of ash.

We'll get back to the details of the nature of this

ash cloud a little bit later.

So in sum, we see that the behavior of lava depends, first of all,

on the composition of the lava and second of all on the gas content of a lava.

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