3.C.4 Pyroclastic Debris

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From the course by University of Illinois at Urbana-Champaign

Planet Earth...and You!

23 оценки

University of Illinois at Urbana-Champaign

Planet Earth...and You!

23 оценки

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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Now I mentioned the word earlier, pyroclastic debris.

Pyroclastic debris Is a general term that includes volcanic ash,

plus other kinds of fragments.

We've seen that some of these fragments are little balls, or pellets of pumice.

But, in some cases, they're other kinds of fragments, as well.

For example,

we're looking at here is a handful of what geologist refer to as lapilli.

These are marble shaped sometimes pea shaped fragments of solid rock.

They can include pellets that are formed by the freezing of little clots

of lava that are ejected during a fountain of lava at an effusive eruption.

Sometimes they're composed of little snowballs of ash that form when the ashes

erupted into a cloud that contains a lot of moisture.

And the ash sticks together because of the moisture and

forms these little balls that then become large enough to fall down.

But the term lapilli is used in either case because it's basically a size

designation.

It refers to pea- to marble-sized fragments of solid volcanic rock.

Another kind of piece of pyroclastic debris is a volcanic bomb.

Volcanic bombs form when a still molten blob of lava gets ejected into the air and

as it falls, it streamlines and develops a fairly smooth surface.

A third type of pyroclastic debris is sometimes called simply blocks.

And these are more irregular, non-streamlined chunks of

either frozen lava or chunks of preexisting volcanic debris that's gotten

ejected during the volcanic eruption, then collects on the surface of the volcano.

Now let's broaden our discussion

of the fragmental material from volcanoes a little broader.

We've seen that there's pyroclastic debris and

that's usually restricted to stuff that's erupted directly from the volcano.

But sometimes that debris mixes with water and then flows somewhere else.

Now in some cases that water comes from the rainfall, the heavy rainfall,

that sometimes is triggered, when ash enters a moist atmosphere.

Sometimes it comes from the melting of glaciers, at the top of tall volcanoes or

the melting of snow, at the top of tall volcanoes.

In any case, when ash and fine grain debris mixes with water,

it forms a muddy like slurry that then rushes down the side of the volcano.

This muddy like slurry is called a lahar and when it finally slows and

accumulates, it will develop another type of volcanic debris.

It's a little different from pyroclastic debris

because it's been transported a little bit with the aid of water.

Sometimes, there's even more water or

the lahar then gets picked up by streams and carried away.

Or ash and other debris that settle down the side of a volcano, gets picked up and

carried away by rivers.

That stuff will be sorted by the action of water, because water will tend to

carry the finer stuff further, and leave the coarser stuff behind.

But, in any case, it will deposit sediment that's Is composed almost entirely of

volcanic debris.

So all this stuff together stream transported sediment, lahars and

pyroclastic debris together are referred to sometimes as volcanoclastic debris.

And it is in fact a major component of what builds up

volcanoes in many environments.

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